safety of food crops on land contaminated with trace elements

13

49

ReviewReceived: 3 September 2009 Revised: 20 August 2010 Accepted: 22 September 2010 Published online in Wiley Online Library: 28 March 2011

(wileyonlinelibrary.com) DOI 10.1002/jsfa.4355

Safety of food crops on land contaminatedwith trace elementsBal Ram Singh,a∗ Satish K Gupta,b Hassan Azaizeh,c Stefan Shilev,d

Damien Sudre,e Won Yong Song,e Enrico Martinoiae and Michel Menchf

Abstract

Contamination of agricultural soils with trace elements (TEs) through municipal and industrial wastes, atmospheric depositionand fertilisers is a matter of great global concern. Since TE accumulation in edible plant parts depends on soil characteristics,plant genotype and agricultural practices, those soil- and plant-specific options that restrict the entry of harmful TEs into thefood chain to protect human and animal health are reviewed. Soil options such as in situ stabilisation of TEs in soils, changes inphysicochemical parameters, fertiliser management, element interactions and agronomic practices reduce TE uptake by foodcrops. Furthermore, phytoremediation and solubilisation as alternative techniques to reduce TE concentrations in soils arealso discussed. Among plant options, selection of species and cultivars, metabolic processes and microbial transformations inthe rhizosphere can potentially affect TE uptake and distribution in plants. For this purpose, genetic variations are exploitedto select cultivars with low uptake potential, especially low-cadmium accumulator wheat and rice cultivars. The microbialreduction of elements and transformations in the rhizosphere are other key players in the cycling of TEs that may offer thebasis for a wide range of innovative biotechnological processes. It is thus concluded that appropriate combination of soil- andplant-specific options can minimise TE transfer to the food chain.c© 2011 Society of Chemical Industry

Keywords: cultivars; food safety; immobilisation; land contamination; metal transport; microbial transformation; molecular process;plant species; phytoremediation; rhizosphere; trace element

INTRODUCTIONThe production of quality food depends on the availability offertile, uncontaminated soil, on an adequate supply of moistureand nutrients and on the biological functioning of the agro-ecosystem. In addition to inputs of trace elements (metalsand metalloids) through fertilisers, pesticides and atmosphericdeposition, inadvertent use of municipal and industrial wastes andrecycling of animal manures, especially pig slurries, containingessential nutrients but also significant levels of potentiallytoxic trace elements (TEs) are major sources increasing thecontamination of agricultural soils and thus a matter of growingglobal concern.

Some TEs, e.g. copper (Cu), molybdenum (Mo) and zinc (Zn),are essential for plant growth and human and animal nutritionbut can create phytotoxicity and/or zootoxicity concern whenaccumulated in excessive concentrations in soil and plants.Other TEs such as cadmium (Cd), arsenic (As), mercury (Hg)and lead (Pb), not essential for either plants or humans andanimals, pose risks when they enter the food chain. Althoughcontaminants in food and fodder do not induce quick death,they cause chronic health effects. Therefore the ability ofcontaminants to release TEs should be properly regulatedthrough agronomic, biochemical and physical processes anddecontamination programmes.

The potential uptake of TEs by roots, from either anthropogenicor geochemical sources, depends on their total concentration insoil, soil solution and exchangeable forms. Levels of contaminationby TEs in soil, the actual risk, agronomic regions at major risk, theTE levels in soil that would trigger soil management measures,

crops more sensitive to TE pollution (e.g. leafy vegetables, carrots,rice and durum wheat in the case of Cd accumulation) andelements of major concern (Cd, As, Hg and Pb) are detailedelsewhere.1 – 18 Metal inputs differ among EU countries butremain within the upper maximum limits stipulated in the EUDirective. Likewise, loadings in the USA, Canada and Australasia aredifferent. Hooda18 provides an overview of the different regulatorylimits.

The chemical behaviour of TEs varies from soil to soil and isinfluenced by soil properties such as pH, organic matter (OM)

∗ Correspondence to: Bal Ram Singh, Department of Plant and EnvironmentalSciences, Norwegian University of Life Sciences (UMB), PO Box 5003, N-1432,Ås, Norway. E-mail: [email protected]

a Department of Plant and Environmental Sciences, Norwegian University of LifeSciences (UMB), PO Box 5003, N-1432, Ås, Norway

b Research Center Agroscope Reckenholz-Tanikon ART, Reckenholzstrasse 1991,CH-8046 Zurich, Switzerland

c Institute of Applied Research (affiliated with University of Haifa), The GalileeSociety, PO Box 437, Shefa Amr 20200, Israel

d Department of Microbiology and Environmental Biotechnology, AgriculturalUniversity, Plovdiv, Bulgaria

e Department of Plant Biology, University of Zurich, Zollikestrasse 107, CH-8008Zurich, Switzerland

f UMR BIOGECO INRA 1202, University of Bordeaux 1, Bat. B8 RdC Est, Avenue desFacultes, F-33405 Talence, France

J Sci Food Agric 2011; 91: 1349–1366 www.soci.org c© 2011 Society of Chemical Industry

13

50

www.soci.org BR Singh et al.

content, clay and amorphous hydrous oxide contents and cationexchange capacity.1,19 However, TE accumulation and transportin harvested and edible parts of plants also depend on bioticfactors. Those factors affecting the uptake of TEs are crop speciesand cultivar, root activity, rooting pattern and rhizosphere-associated micro-organisms.20,21 For example, crop Cd is notsimply a function of total soil Cd but also depends on soil (e.g.Cd-binding strength, soil pH and soil chloride) and plant (e.g.plant species and cultivar and preceding crop) factors.1,10,17,22,23

Cases of Cd contamination of agricultural products caused byhistorical smelter emissions have been reported from Kempen,an area in eastern Flanders, Belgium and the adjoining part ofthe Netherlands, and from rural areas around Evin-Malmaison,Auby and Mortagne-du-Nord in northern France.2,24 Based on asoil survey from 11 provinces in China, 13 330 ha of farmland hasbeen contaminated by Cd, which has created an increasing healthproblem in China.25 People who are nutritionally marginal withrespect to Zn, iron (Fe) and calcium (Ca) are at higher risk ofCd disease than those who are nutritionally adequate. A urinarylevel of 1–2 µg Cd L−1 is associated with increased risk of bonedemineralisation and fracture, and 2–4 µg Cd L−1 with pre-clinicalkidney damage.26

Contaminated soils pose a serious threat to healthy foodproduction and hence their remediation is required. Remediationof contaminated soils using conventional clean-up technologiesis expensive and not feasible for large agricultural areas. WhereTEs can be tightly bound to the soil constituents and are notbioavailable as evidenced by bioavailability bioassays and othertoxicity assays, exhaustive clean-up of soils may not be necessaryas the contaminants may not pose a risk to end users. Thismanagement option is termed ‘risk-based land management’and is considered attractive as it may save millions of euros inremediation costs.

The uptake and distribution of TEs, especially Cd, differ widelyamong plant species and cultivars.20,25,27 – 29 This could partlybe related to differences in the ability of plants to control themovement of TEs from the xylem into the phloem and via thephloem into the seeds.30,31 Therefore plant breeding and selectioncan be an important tool to reduce potentially harmful TEs suchas Cd in food crops.22 Similarly, TE uptake in edible plant parts

can be restricted from soil to root by reduction in influx acrossroots, accumulation in vacuoles and flux back to non-edible plantparts.

Plants and micro-organisms exude a variety of inorganic andorganic substances that may alter soil pH and directly influence TEavailability via solubilisation and complexation. Factors influencingTE fractionation and bioavailability in soil include root-induced pHchanges, TE binding by root exudates,32 root-induced microbialactivities21 and root depletion as a consequence of plant uptake.The processes affecting rhizosphere pH involve the evolution ofCO2, the release of root exudates, the excretion or reabsorptionof H+ or HCO−

3 and the microbial production of organic acids.33

The microbial reduction of elements is gaining interest becausesuch transformations can play crucial roles in the cycling of bothinorganic and organic compounds in a range of environments. Ifharnessed, it may offer the basis for a wide range of innovativebiotechnological processes.

In spite of our increased knowledge on TEs in soil/plant/humansystems, the complex web of geochemical and biologicalinteractions limits the prediction of TE bioavailability for plantuptake and subsequent assimilation in humans and animals. Thisreview focuses on soil- and plant-specific options restricting theentry of harmful TEs into the food chain and thus protectinghuman and animal health. Among soil-specific options, emphasisis placed on in situ TE stabilisation in soils by sorbing agents (e.g.mineral oxides, manures and organic materials, phosphates, clays,etc.), changes in physicochemical parameters (e.g. addition ofalkaline materials, coal fly ashes, etc.), fertiliser management,element interactions and agronomic practices (crop rotationand tillage).1,34 – 36 Among plant-specific options, selection ofspecies and cultivars, metabolic processes (e.g. metal bindingby proteins, detoxification by glutathione or amino acids) andmicrobial transformations in the rhizosphere affecting TE uptakeand distribution in plants are discussed. Although TEs such asCd, Hg, Pb, As and selenium (Se) are of major concern withrespect to the human food chain, Cd is given major focus herebecause its labile pool is relatively important in soils facilitatingplant uptake and Cd is extremely hazardous to animal and humanhealth.

Table 1. Different categories of remediation techniques for soils polluted by trace elements (adapted from Gupta et al.38)

1. In situ gentle 2. In situ harsh 3. In situ harshSoil use-restrictive measures Soil-destructive measures

1.1. Stabilisation 1.2. Decontamination (in situ) (ex situ or in situ)

1.1.1. Increase of soil pH by liming 1.2.1. Controlled and targetedmobilisation (with natural andsynthetic acidifying and complexingagents)

2.1. Land use ban andlimitation

3.1. Incineration

1.1.2. Increase of binding capacity (byclays, oxides, zeolites or clean wastecontaining clay and oxides, e.g.gravel sludge)

1.2.2. Capture of mobilised metals (withplants or natural and syntheticcaptors)

2.2. Land use change 3.2. Deposition (ofcontaminated layersstripped off)

1.1.3. Plants 1.2.3. Harvesting of metal-loadedcaptors

2.3. Seal and close 3.3. Thermal treatment

1.1.4. Micro-organisms 3.4. Electromigration

3.5. Soil washing

3.6. Vitrification

3.7. Deep ploughing

3.8. Dilution

wileyonlinelibrary.com/jsfa c© 2011 Society of Chemical Industry J Sci Food Agric 2011; 91: 1349–1366

13

51

Safety of food crops on land contaminated with trace elements www.soci.org

g.v. = guide valuefor soluble heavymetal content(Verordnung über.Schadstoffe im Boden(VSBo), 1986)

MobilisationImmobilisation No treatment

Capture andsubsequentharvest

Hea

vy m

etal

cont

ents

g.v.

Treatment withorganic acids

Treatment with limeor binding agents

Hea

vy m

etal

cont

ents

g.v.

Totalcontent

Solublecontent

Hea

vy m

etal

cont

ents

g.v.

Totalcontent

Solublecontent

Totalcontent

Solublecontent

Figure 1. Effect of remediation treatments on mobilisation/immobilisation of metals (adopted from Alloway41).

SITE-SPECIFIC MANAGEMENTOF CONTAMINATED SOILS/SITESRisk reduction at contaminated sites is principally carried out byvarious soil management techniques. These are mainly dividedinto two categories, stabilisation and decontamination (Table 1).The choice of the principal category is mainly based on different sitefactors such as soil type, the nature and distribution of pollutionas well as the severity of the hazard, current land use, soil pHand clean-up goals.37 With knowledge of these major points thedecision can be taken whether stabilisation or a decontaminationprocedure is preferable. Knowledge of current land use will revealwhether or not changes are needed. If the pollutants should bestabilised, the pH of the soil makes it clear whether liming oranother stabilisation technique should be applied. When the finalgoal is complete decontamination of the soil, further investigationsare necessary to determine the appropriate decontaminationtechnique. Up to now, most in situ remediation techniques are stillat an experimental stage and are not adapted to a large spectrumof soil types or different pollutants.38

In the following sections, known and possible new techniquesare critically evaluated and presented in detail. Our concept ofgentle remediation is not restricted to either stabilisation ordecontamination. In a remediation process, stabilisation may beonly the first step in reducing the hazard, allowing time fordetailed investigations to optimise subsequent stabilisation anddecontamination.38

Soil-specific approaches (agronomic management)In situ stabilisation through different binding agentsIn contrast to organic pollutants, TEs are not subjected to decom-position processes and hence may cause persistent contamination.To manage TE-contaminated soils, various remediation techniquesare proposed (Table 1). Most techniques are aimed at protectinghumans, animals and the environment from exposure to hazardsby removing the source or interrupting the pollutant linkages.Two major categories of remediation and clean-up techniquescan be distinguished: those that enable the long-term restorationand preservation of soil fertility, so-called ‘gentle’ remediationtechniques, and ‘harsh’ clean-up techniques that primarily aimat eliminating human health risks. Most harsh techniques impairbiological activity or destroy the physical structure of soil.

Gentle remediation techniques are required where large areasof low-TE-contaminated agricultural land need to be remediated.The main principle behind in situ stabilisation is to render TEsunavailable or inactive.35,39,40 In situ stabilisation can reduce themobile and bioavailable TE fractions in soil (Fig. 1), prevent theirmigration into ground water, limit the uptake of TEs by plantsand thus reduce their toxicity to plants.41 – 43 In situ stabilisationincreases the sorption capacity of the soil matrix by addition ofagents such as clay minerals, Fe, manganese (Mn), titanium (Ti)and aluminium (Al) oxides, phosphates, OM, etc. or decreases theconcentrations of dissolved pollutants by changing soil parameterssuch as pH (e.g. dolomitic limestone, coal fly ashes, etc.) and redoxpotential (Eh).

Liming. Liming can reduce the mobility of TEs such as Cd, nickel(Ni) and Zn.35,42,44 Its effect on Cu solubility and plant uptakeis more complicated owing to the formation of complexes withsoluble organic substances after liming.45 Although liming hasproved to be efficient for minimising risks that TE pollutants poseby entering the food chain, its effects can vary considerably withTE, soil conditions, plant growth and, especially, root distributionin the soil. Liming also provides only a transitory solution to thepollution problem. In light textured soils, over-liming may alsodecrease the availability of essential micronutrients such as Znand Fe.

The ‘soluble’ TE fraction in soils can be reduced by increasing pH(e.g. liming, alkaline fertilisation) or cation-binding capacity (e.g.addition of clay minerals or gravel sludge).46,47 Liming decreasesthe NH4NO3-extractable fraction of TEs (e.g. Cd) in soil and reducedthe uptake of Cd, Ni and Zn in wheat and carrot crops grown innaturally metal-rich soils (alum shale soils) in Norway47 (Table 2).Excessive application of sewage sludge and pig manure leadsto the accumulation of potentially toxic elements in soil, butliming reduced the solubility and plant uptake of Cd and Zn.27

However, the reduction was higher in control plots than in sludge-treated plots, showing the interaction of TEs with soil pH, OM,root distribution and the rhizosphere. In contrast to the generaldecrease in TE concentrations after liming, some studies foundincreased Cd concentration in crops.48,49 The Ca2+ added throughlime may desorb surface-bound Cd2+ into soil solution, makingit available to plants. Calcium inhibits Cd2+ sorption to the soil

J Sci Food Agric 2011; 91: 1349–1366 c© 2011 Society of Chemical Industry wileyonlinelibrary.com/jsfa

13

52


Table 2. Trace element concentrations (mg kg−1) in wheat grainafter liming to different pH levels in a naturally metal-rich moraine soil(adapted from Singh et al.47)

Soil pH Cd Ni Zn Cu

5.5 1.34 1.39 47.8 4.6

6.5 0.55 0.83 41.9 3.2

7.0 0.55 0.76 31.6 4.0

7.5 0.52 0.84 27.3 4.3

LSD0.05 0.31 0.36 12.4 2.2

surface, but the mechanism can have a significant effect only if thepH-induced increase in sorption of Cd2+ with liming is less thanthe Ca-induced desorption of Cd.19,50

Trace element-binding materials. Binding agents that increasesorption capacity in the soil matrix include chelates, ion exchangeresins and natural materials such as organic substances or clayminerals.51 Out of 20 different additives tested in batch and columnexperiments, zeolite combined with ferrous sulfate was effectivein immobilising Cd in various soils.52 Alkaline fly ashes reducedmetal availability and decreased TE uptake in maize.53 Anothercoal fly ash called beringite, which is a modified aluminosilicatefrom the fluidised bed burning of coal mined in Belgium, hasalso been tested.54,55 In field experiments, beringite additionto a soil polluted with 6000 mg Zn, 30 mg Cd and 500 mg Cukg−1 enabled the re-establishment of plant growth and protectedthe polluted area from erosion. Addition of beringite at 5% toa Zn-contaminated soil reduced the foliar Zn concentration inbean (Phaseolus vulgaris) from 350 mg kg−1 in the untreated soilto 146 mg kg−1 in the beringite-treated soil.56 Similar beneficialeffects of beringite (2.5 and 5% additions) were obtained on plantgrowth and Cu uptake by bean (P. vulgaris L.) and maize (Zea maysL.) respectively grown in a Cu-contaminated soil (250 mg kg−1)from coffee orchards in Tanzania.57 Increased soil pH and Cusorption are the suggested mechanisms.

The potential of montmorillonite (MMT), Al-MMT and gravelsludge to immobilise TEs in agricultural soils was investigated.58

In batch experiments, both Al-MMT and MMT were effective inimmobilising Zn and Cd. Zinc is specifically bound on Al-MMT andin time becomes increasingly incorporated into the Al hydroxidecoating. No specific Zn sorption occurs on MMT. Cadmium isbound on MMT and Al-MMT non-specifically by cation exchangeprocesses.

In a pot experiment, zeolites reduced TE uptake, but thereduction was partially caused by the pH increase resultingfrom zeolite addition.59 Regarding TE leaching, the effluent TEconcentration was 50% lower in zeolite-treated columns than inCaCO3-treated soils. Some adverse or subsequent effects suchas immobilisation of nutrients (phosphorus (P) and Mn) mayoccur.60 An effect on foliar Ca, magnesium (Mg), potassium(K) and P concentrations in bean was reported in stabilised Cu-contaminated soil.45 Remobilisation by soil acidification has notyet been investigated and, similar to remediation with liming, thelong-term behaviour of immobilised metals in different soils isgenerally unknown and the database of long-term field studies isweak.61

Trace element reactions with iron and manganese oxides. Sorptionis an important chemical process that regulates TE partitioning

between solution and solid phases in soils. Iron and Mnoxide minerals are important sinks for TEs in soils62,63 andresidual-amended soils.64,65 Hydrous ferric oxide decreases soil-extractable Cd, but reduced Cd uptake by plants could not bemeasured.66 Root exudates may dissolve hydrous ferric oxide,thus making Fe available. Other Fe-bearing products such as Fe-rich adsorbents67,68 and zerovalent Fe grit35,36,40 have also beenstudied. A combination of zerovalent Fe grit and beringite wasfound to be very efficient for stabilising metals and As.40,55,60,69

However, long-term evaluation of an As/Zn-contaminated spoilshowed that As concentrations in leachates percolated from theremediated spoils were higher than those from the unamendedsoil.70 Manganese oxides are important adsorbents and one ofthe most reactive forms is synthetic birnessite (sometimes calledδ-MnO2).71 In a study on immobilising capacity it was found thatbirnessite exhibited the best potential for reducing Cd and Pbuptake by plants as compared with other additives (hydrous ferricoxide, basic slag, beringite and lime).51 However, under reducingconditions, Mn oxides may be reduced to Mn2+, which can betoxic for organisms.

Solubilisation of trace elements by ligands to enhance plant uptakeTo enhance TE uptake by the use of chelators, the followingsteps are necessary: TEs must be (1) dissolved from the solid,(2) transported to the plant roots, (3) absorbed by the roots and(4) translocated within plants to the above-ground parts. Thesolubilisation process must be carried out with caution in orderto avoid loss of TEs by leaching to ground water but at the sametime provide an optimum concentration of soluble TEs in theroot zone available for removal by plants.72,73 This concentrationmust maximise plant uptake but not induce growth reduction.To maintain such optimum concentration during the vegetativeperiod, it might be necessary to add the amendment several timesat a low dose. The optimum time span between two treatmentswill depend on the degradation rate of the applied ligand. In orderto minimise leaching, chelator application should be restricted tothe root zone.

The formation of soluble metal complexes may not necessarilylead to enhanced metal phytoavailability. While some authorshave reported that the use of chelators such as ethylene diaminetetraacetate (EDTA) increases metal uptake by plants,74 othersdid not observe an enhancement but rather a reduction in metaluptake by plants.75 Furthermore, the addition of chelators mayhave undesired side effects such as increasing metal toxicity andthe risk of metal leaching to deeper soil layers or ground water.

The influence of four natural organic agents (citric, oxalic,phthalic and salicylic acids) and three synthetic organic agents(EDTA, nitrilotriacetate (NTA) and diethylene triamine pentaac-etate (DTPA)) on metal solubilisation in soils was studied in batchexperiments.72 The experiments were performed with soils fromtwo agricultural sites in northern Switzerland contaminated withZn, Cu, Cd and, at one site, Pb. The efficiency of the chelatorswas far better than that of the natural organic agents. Despitesubstantial differences in stability constant, there were no signif-icant differences among NTA, EDTA and DTPA in their ability toextract metals from the two soils. Because of the high degree ofbiodegradability of NTA in soils, it was chosen for use in furtherphytoextraction experiments.

Pot and field experiments were conducted to investigate theeffectiveness of NTA (chelator) and elemental sulfur (S) (agent tolower soil pH) on metal solubilisation and uptake by Nicotianatabacum and Z. mays.73 Potential harmful side effects such as


13

53


metal leaching to deeper soil layers were also studied. ElementalS addition at 100 mmol kg−1 increased dissolved Zn and Cdconcentrations about tenfold in calcareous soil and up to 30-foldin acidic soil by decreasing the soil pH by about 1–1.5 units. Noeffect on soil pH was observed in NTA treatments. NTA application(0.5 mmol kg−1 in calcareous soil and 0.25 mmol kg−1 in acidicsoil) increased soluble Zn, Cd and Cu about 100-, 19- and 20-foldrespectively in calcareous soil and about 13-, two- and fourfoldrespectively in acidic soil. Dissolved Pb was increased by NTA up to50-fold in acidic soil. The solubilising effects lasted for only 7 daysand then decreased rapidly within 20 days to almost initial values.

In general, metal solubilisation treatments increased NaNO3-extractable Zn, Cd, Cu and Pb concentrations in soils.73 However,this did not translate into an equivalent increase in metal uptakeby plants, although in nutrient solution experiments a muchhigher increase in Cu uptake and translocation into shoots couldbe observed. The lower efficiency in the soil is attributed to theshort duration of the solubilising effects. TE leaching was notinvestigated in these experiments.

Phytoremediation. Decontamination techniques include the useof hyperaccumulator or high-biomass crops that accumulatehigh TE levels in shoots and thus can remove TEs fromcontaminated soils.76 Plants represent a more environmentallycompatible and less expensive method of site restoration, throughextraction, degradation or fixation of pollutants, compared withphysicochemical and engineering options, even though thetimescale required to reach the fixed end-points is a limitingfactor.

To overcome the limitations of phytoextraction techniques,several options are taken. Efforts are made to increase growthof hyperaccumulators by crossbreeding them with related plantsthat produce more biomass77 or using molecular mechanismsand genes leading to hyperaccumulation in tolerant species.78

Additionally, attempts are made to improve the metal uptakecapability of high-biomass plants by somaclonal variation orchemical mutagenesis and selection techniques.79 Furthermore,some plants such as Salix can decrease the metal concentration insoils. For example, in a field study in Sweden, growing Salix priorto a wheat crop decreased the Cd concentration significantly inthe soil as well as in the wheat grain.80

A different option is to increase metal phytoavailability in thesoil. Factors primarily controlling metal phytoavailability in soilsare soil pH, cation exchange capacity and/or OM content.81 Forincreasing phytoavailability, two major ways are investigated:artificial soil acidification and solubilisation by addition of ligands,in particular chelators. Citric acid and hydrochloric acid displayeffects, forming complexes and decreasing soil pH. For bothsubstances an enhancement of element uptake by plants hasbeen reported.82,83

Once mobilised in the rhizosphere, mineral elements andcontaminants need to be taken up into the root. For example,the Zn hyperaccumulator Thlaspi caerulescens over-expresses aZIP family root plasma membrane transporter. In the closelyrelated non-accumulator species Thlaspi arvense, high externalconcentration suppresses expression of this Zn transporter,indicating that metal regulation of gene expression is altered inthe hyperaccumulator. One hypothesis is that key genes necessaryto cope with and translocate potentially toxic TEs are up-regulatedin hyperaccumulators. However, those genes also exist in sensitivespecies but are not expressed in the appropriate tissues or at asufficient level.

Fertiliser management and trace element interactionsFertilisers, especially phosphates, contain TE impurities and canresult in excessive TE levels in soils. Many examples of TEaccumulation in soils, especially of Cd in long-term fertilised soils,have been reported worldwide.84 – 87 However, increased total soilCd is not always reflected in increased Cd concentration in foodcrops, because fertilisers can affect TE root uptake by changing soilpH, ionic strength in the soil solution and plant growth parameters,e.g. root distribution, rhizosphere conditions and shoot yield. Thetype and amount of fertiliser used and interactions betweenTEs and major nutrients (nitrogen (N), P and S) and among TEsthemselves (e.g. Zn–Cd and Fe–Cd) play an important role in TEuptake by crops.88 High Fe nutrition caused a marked reductionin Cd content in both leaves and roots. Iron content in plantswas lower under high Cd (5 mmol L−1) stress than under low Cd(<1 mmol L−1) stress. Cadmium stress affects the uptake of Fe, Cuand Zn.89

Interaction with major nutrients. Changes in TEs in food cropsin relation to N fertilisation are difficult to evaluate, because Nsupply affects crop yield as well as soil reaction. Grain Cd inwheat increased with increasing N fertilisation rate, except forurea.90 Nitrogen and Cd concentrations in wheat grain in Swedenshowed a positive correlation.91,92 Each 10 kg N ha−1 additionalapplication increased grain Cd by 1–3 µg kg−1.92 The relativeincrease in Cd concentration as a function of N rate varied from 6to 14% across sites and cultivars when the N rate was increasedfrom 145 to 175 kg ha−1. The increased concentration of Ca2+ dueto Ca nitrate application may increase ion exchange Cd in the soilsolution as well as wheat grain Cd.

Phosphate fertilisation and Cd in soils are often positivelycorrelated.88,93 – 95 Total P and Cd were linearly related in pasturesoils receiving long-term input of superphosphate.95 In Norway,although long-term use of P fertiliser increased total soil Cdconcentration, it did not necessarily enhance Cd uptake byplants.96 Different P sources may affect Cd uptake differently.Cadmium uptake by rape and oats in greenhouse experimentswas higher with single superphosphate than with NPK.97 Thesingle superphosphate contained 12% S, and acidification due toits application could have enhanced the solubility and availabilityof Cd. However, the P source had only a small effect on theCd concentration in potato tubers.98 Although K application canincrease Cd uptake by food crops, this may be associated more withthe accompanying anion of the salt. Application of KCl increasedgrain Cd in barley.99 This uptake may be associated with increasedsoil solution concentration of Cd via formation of CdCl2−n

n ionpairs.100

Interaction with micronutrients. The Zn and Cd interaction iswidely studied, because these metals behave chemically in asimilar way and coexist in contamination sources. The effect ofZn fertilisation will depend on the Zn status of soils and plants,and significant effects were found in Zn-deficient soils.101 On a Zn-deficient soil, Zn application up to 100 kg ha−1 reduced Cd uptakeby potato tubers by 20%.88 Also, Cd competes with Cu for plantuptake.102 Increases in seed Zn concentration, whether caused bysoil Zn status, P fertilisation or application of Zn fertiliser, result indecreased Cd concentration.103 A 20 mg Zn kg−1 soil applicationwith P decreased seed/grain Cd by 42% for flax and 65% for durumwheat and Cd translocation to seed/grain by 20% for flax and 34%for durum wheat.104 An antagonistic effect of Zn on Cd root uptake


13

54


and translocation to seed/grain in both crops was evaluated ingrowth chamber studies. Increasing Cd application to Zn-deficientplants tends to decrease plant Zn concentrations, whereas, inplants with adequate Zn supply, Zn concentrations are either notaffected or increased by Cd.105 Durum wheat is more sensitiveto Zn deficiency and Cd toxicity than bread wheat. It has beenhypothesised that Zn protects plants from Cd toxicity by improvingtheir defence against oxidative stress and by competing with Cdfor binding to critical cell constituents.105 Cadmium decreased by11–90% in wheat when Zn was applied at 15 mg kg−1 soil.106

Because Zn concentrations in grain cereals are too low to meet thenutritional requirement of humans, biofortification of cereal grainZn would be an issue.30 Evidence for other interactions, notablywith silicon (Si), is emerging. In maize seedlings, Si alleviated Cdtoxicity as revealed by some antioxidant enzyme activities,107 themost prominent effects being found in the roots. Silicon alsoinduced Cd resistance in rice108 and affected wheat grain Cd.109

Salinity and chloride and sulfate ions. Chloride (Cl) forms com-plexes with TEs, especially Cd, and hence Cl supplied eitherthrough fertilisers (e.g. KCl) or in irrigation water can increaseCd uptake by crops.82,110,111 In Australia, 70% of the variation inCd concentration in potato tubers across 80 sites was causedby salinity and Cl supply through irrigation water.110 Similarly,increasing salinity increased concentrations of Cd species (Cd2+,CdCl+, CdHCO3+ and CdCl0) in soil solution but decreased totaland free Zn2+ concentrations in soil solution and its concentrationin wheat shoots.106 Uptake of the CdSO4 ion pair by Swiss chardwas found to be as efficient as that of free Cd2+ from nutrientsolution, but little or no increase in Cd uptake was observed whenSO2−

4 was applied to soils.112

Tillage systems and crop rotationLimited information on the effect of tillage practices, i.e.conventional versus reduced or no tillage, on TE concentrationsin food crops makes it difficult to draw any definite conclusions.Wheat grain grown under direct drilling contained higher Cd levelscompared with reduced tillage or conventional cultivation.88,90

However, EDTA-extractable Cd in soils is not affected by tillagepractices. No tillage may cause stratification of TEs, i.e. Cu, Fe,Mn and Zn, because of crop residue and OM accumulation onthe surface.113 Deep tillage could be an effective technique inconditions where surface soil is enriched with TEs, because it maylead to dilution of elements by blending of surface and subsurfacesoils.

Rhizosphere effects of plants in crop rotation may affect theavailability of TEs to subsequent crops. Lupins (Lupinus L.) areknown to release citric acid, leading to soil acidification andconsequently increased mobility of elements in the soil. Grain Cdin wheat was found to be higher after lupin culture.90 The increasein Cd concentration may be partially, but not solely, attributed toacidification by this legume and consequent Cd mobilisation foruptake by the subsequent crop. Cropping systems such as rotationand intercropping may have numerous advantages in terms ofincreasing the availability of micronutrients, including Zn. In aChinese peanut/maize intercropping example, phytosiderophore(PS) excretion by maize into the rhizosphere played an importantrole in improving Fe nutrition of peanut intercropped withmaize.114 Enhanced PS release by plants may mobilise Zn inthe soil and enhance Zn uptake.115

Plant-specific approachesSelection and breeding of plants with low uptake potentialSince crop species and cultivars differ in their genetic tendency totake up TEs, selection and breeding of crops for low uptakepotential open up new opportunities to minimise harmfulelements in the food chain. Large genetic differences in kernelCd concentration were found among 200 sunflower genotypes.116

The average Cd concentration of the five lowest genotypes wasfourfold lower than that of the five highest genotypes. Similarly,49 rice cultivars grow under simulated upland conditions on Cd-contaminated soils differed in Cd concentration.117 Differences inrice grain Cd among cultivars were much higher than in rootsand stems.118 Wheat variety trials across Australia showed geneticdifferences in grain Cd among cultivars.119 Low-Cd cultivars tendedto have similar pedigrees, indicating the potential for selectinglines with low Cd concentration. Differences among plant cultivarsin secretion of low-molecular-weight (LMW) organic acids mayinfluence root uptake of Cd.120 Root Cd uptake may restricttranslocation to stems, leaves, fruits and grains.121,122 Low-Cd ricecultivars retained more Cd in roots and translocated less to grainsthan high-Cd cultivars.123 Differences in Cd concentration in durumwheat cultivars were attributed to differences in translocation fromthe root to the shoot and within the shoot rather than to differencesin root uptake.124 High cation exchange capacity of roots can causehigh grain Cd in wheat.21

In spite of genetic variations in Cd uptake by cultivars, limitedefforts have been made to use selection or breeding to reduce Cdin crops in the past. Greater emphasis is now placed on finding low-Cd cultivars of grain crops such as wheat,20 durum wheat,22,125,126

rice122,123 and soybean.123 Cadmium uptake by maize in the maturestage showed a significant genetic variation.127 Studies have alsobeen made on rapeseed128 and lettuce.129

A crossing programme by Clarke et al.130 developed near-isogenic high/low grain Cd concentration from five durum wheatcrosses. Each high/low pair was genetically uniform except forthe Cd concentration trait. The average grain Cd concentrationwas about 2.5 times greater for the high than for the low isolines(Table 3).126 The low-Cd uptake trait had no effect on yield, proteincontent and kernel yield. The low-Cd uptake trait also had noconsistent effect on grain concentrations of other TEs, but someindication that the low-Cd trait may also be associated with low Znaccumulation under Zn-deficient conditions was found in solutionculture experiments.131 However, in field experiments, low Zn andCu supply resulted in higher wheat grain Cd.132

Even though cultivar selection can efficiently reduce Cdconcentration in food crops (phytoexclusion), there are manyconstraints in utilising this option. It is a time-consuming process,and the low-Cd trait of a cultivar must meet the requirementsof acceptable yield, agronomic suitability, quality and diseaseresistance.22 In wheat, grain Zn varied by a factor of 1.6 acrosscultivars, which is lower than for grain Cd (factor of 2–4 dependingon year).133 Furthermore, both low- and high-Cd cultivars will beinfluenced by soil type, management practice and yearly climaticconditions.20,87 Therefore combining management practices anduse of low-Cd cultivars would be more effective in reducing Cdmovement into the food chain.22

Molecular and physiological aspects of trace element transportIn plants, even at low concentrations, Cd accumulation can causeserious damage, e.g. leaf chlorosis and necrosis, and affect growthand development. Other Cd effects also occur in plants, such as


13

55


Table 3. Grain Cd concentration (mg kg−1) in durum isogenic pairsand parents grown in varying environments (adapted from Grantet al.22)

Genotypea Casselton Regina Swift current Stewart valley

8982-SF-L 0.09 0.11 0.95 0.05

8982-SF-H 0.21 0.33 0.11 0.15

8982-TL-L 0.10 0.11 0.04 0.08

8982-TL-H 0.28 0.34 0.11 0.18

W9260-BC-L 0.14 0.19 0.10 0.03

W9260-BC-H 0.30 0.32 0.14 0.07

W9261-BG-L 0.10 0.14 0.05 0.13

W9261-BG-H 0.27 0.27 0.10 0.23

W9262-339A-L 0.08 0.10 0.03 0.20

W9262-339A-H 0.23 0.30 0.11 0.07

Contrast H vs Lb ∗∗ ∗∗ ∗∗ ∗∗

a Isogenic line designations ending in ‘L’ and ‘H’ indicate low- andhigh-Cd accumulators respectively.b Significance of high vs low isolines: ∗∗ P < 0.01.

breakdown of the photosynthetic apparatus, reduced respiration,indirect production of reactive oxygen species, DNA interaction,replacement of Zn and Fe as prosthetic groups and interaction withthiols. To limit Cd toxicity, plants have developed various strategiessuch as exclusion, formation of complexes, compartmentalisationand sequestration.134

Different strategies for limiting potentially toxic TE uptake inedible plant parts can be envisaged at various levels:

(1) restriction of TE movement to roots by mycorrhizas, bindingmetals to the cell wall and root exudates;

(2) reduction of influx across the root plasma membrane or activeefflux into the apoplast and finally to the soil;

(3) increased TE chelation in the cytosol by various ligands,activated TE transport and accumulation in vacuoles to fixTEs in non-edible plant parts, modulation of long-distance TEtransport in order to either reduce the TE transport to edibleplant parts or increase the flux back to non-edible plant parts(Fig. 2).

Cadmium uptake and role of iron-regulated transporters. Epider-mal cells constitute the main barrier between soil and plant.Transporters that are not specific enough to recognise only oneof the required micronutrients but also recognise and transportnon-essential TEs probably play a central role in plant survival aswell as in the human diet. Cd uptake is mediated by transporters orchannels for other divalent cations.135 In particular, several of theZn- and Fe-transporting ZIP (ZRT, IRT-like protein) gene productstransport Cd with a wide range of affinities.136,137 One of the firstmembers identified in this family was IRT1, an Arabidopsis cationtransporter expressed in the roots of Fe-deficient plants.138 AtIRT1is essential for Fe acquisition from the soil in non-grass plantssuch as Arabidopsis thaliana but also in rice, which as a strategy IIplant takes up Fe both as Fe2+ and Fe phytosiderophore.139 – 142

IRT1 is able to transport several divalent metal ions, including Cd,cobalt (Co), Mn and Zn.143 Under Fe-deficient conditions, strategyI plants acidify the soil through the activation of a specific plasmamembrane H+-ATPase localised in root epidermal cells, potentiallyencoded by the AHA2 gene in Arabidopsis.144 Consequently, Fesolubility increases and Fe3+ is reduced by a specific reductase

in order to be converted into the transportable Fe2+ form. Fe3+reductase activity is probably the best studied among the differentplasma membrane reductases.145 FRO2 is the enzyme responsiblefor the plasma membrane Fe(III) reductase activity that is inducedunder Fe deficiency in Arabidopsis roots.146 In Arabidopsis, FRO2 isregulated both transcriptionally and post-transcriptionally.

Plants grown under Fe deficiency accumulate a variety ofcations, including Cd2+.147 This is directly linked to incomplete IRT1selectivity. Under Fe deficiency the ratio of Fe2+ to Cd2+ is changedin favour of Cd2+ and, as a consequence, proportionally more Cd2+is taken up. AtIRT1-over-expressing A. thaliana plants under thecontrol of the 35S promoter accumulate larger amounts of Cd thanwild-type plants, rendering them hypersensitive, which evidencesthe role of IRT1 in Cd uptake.148 Heterologous expression of IRT1in Saccharomyces cerevisiae previously indicated its contributionto Cd2+ uptake.143 IRT1 and FRO2 expression is repressed byCd.126 In contrast to Fe, Cd(II) cannot change its redox state,so it is unlikely that FRO2 exhibits a function in Cd uptake. Ahypothesis is that FRO2 and IRT1 form a complex that is onlystable when both proteins are present in the membrane.149

Other ZIP family members may also contribute to Cd uptake,although to a lower degree. Heterologous expression of AtZIP1,AtZIP2, AtZIP3 and TcZNT1 in S. cerevisiae shows that Zn2+ uptakeactivity is partially blocked by Cd2+.150 In yeast, these transportersmediate high-affinity Zn2+ uptake and low-affinity Cd2+ uptake,151

suggesting the contribution of other ZIP transporters to Cd2+uptake. Additionally, ZIP transporters such as AhZIP9 and AhZIP6,which are also present in the shoot, could be involved in Cdroot-to-shoot transport and xylem unloading as well.

Iron is an essential micronutrient with a limited labile poolin many soils. Therefore many studies have been carried outto understand the transport mechanisms and regulation ofIRT1.152,153 Detailed studies of the ZIP proteins expressing differentmutated forms in yeast demonstrate that some of the residuesare important for substrate recognition and transport activity.154

For example, the mutated strain expressing IRT1 in both D100Aand E103A is less sensitive to Cd than either single mutant andtransports Zn but not Fe or Mn. Plaza et al.155 expressed two AtIRT1homologues from two different ecotypes of the hyperaccumulatorT. caerulescens and showed that the two gene products conferreddifferent Cd sensitivities to yeast. Detailed knowledge about IRT1would allow the development or selection of plants that take up Femore specifically, exclude non-essential metals and have normalor increased contents of essential minerals in their edible parts. Incontrast, ferritin over-expression can enhance Cd uptake.156

ATP-binding cassette transporters involved in heavy metal tolerance.ATP-binding cassette (ABC) transporters are one of the largestfamilies of proteins in living organisms ranging from bacteriato humans.157 ABC proteins are defined by the presence of anATP-binding cassette, and several are highly conserved.158,159 Themajority of ABC genes encode membrane-bound proteins thatparticipate in the transport of a wide range of molecules acrossmembranes.160 – 163

Various types of ABC transporter are involved in TE, particularlyCd, resistance processes. Two yeast ABC transporters sequestermetals into vacuoles. ScYCF1 is an ABC transporter of S. cerevisiaethat contributes to Cd resistance by pumping glutathione-conjugated Cd into the vacuole.164 In contrast, SpHMT1, a half-sizeABC transporter of Schizosaccharomyces pombe, is a vacuolarphytochelatin-Cd complex transporter.165 Compared with wild-type plants, YCF1-over-expressing A. thaliana plants exhibit


13

56


Vacuole

ATP

ATP

ATP

ADP+Pi

ADP+Pi

ADP+Pi

CAX

Nramp

ABC

transporter?

ABCtransporter

?

HMA

ZIP

Cd2+ Cd2+

Cd2+

Cd2+

Cd2+

?-Cd/Cd2+

Cd2+Cd2+

Cd2+

Cd2+

Cd2+

Cd2+

PC-Cd

PC-CdGS2-Cd

PC-Cd ?

Xylem

Ca2+

Channels?

?-Cd

?-Cd

H+

Cd2+GS2-Cd

PC synthase

PC-Cd(=LMW)GS2-Cd

HMW

?

?

?

S2

2GSH

Cd2+

2GSH

?

?-Cd

PC-Cd/GS2-CdGS2-Cd

PC-Cd

GS2-Cd

Figure 2. Schematic representation of Cd uptake and translocation in plant roots. Cd2+ is taken up into the plant by ZIP transporters (IRT1, ZIP1–4and ZNT1,2 are good candidates) and possibly by Ca2+ channels. In the cytosol the main part of Cd is chelated with GSH to form bisglutathionato-Cdcomplexes (GS2-Cd) and other unknown molecules (?-Cd). GS2-Cd can interact with PC synthase, resulting in the formation of PC-Cd (LMW) complexes.Chelation and/or sequestration processes by ferritins, metallothioneins and small molecules such as citrate are postulated in plants, but they are notmentioned here. Also, the presence of free Cd ions is supposed to be very limited owing to the physiological conditions in the cytosol. Furthermore,Cd2+ may also interact with Ca2+-binding proteins. For detoxification, Cd2+ or Cd conjugates could be remobilised from the cytoplasm into the apoplastby ABC transporters (PDR8) or sequestered into the vacuole. For the latter, two different pathways are postulated: GS2-Cd and/or LMW complexes arehypothesised to be transported into the vacuole by ABC transporters not yet characterised, or Cd2+ could be sequestered into the vacuole by Cd2+/H+antiporters (AtCAX2 and AtCAX4 exhibit the highest Cd(II) transport activity). At least a part of vacuolar Cd is bound in HMW and in yet unidentifiedcomplexes (?-Cd). However, a fraction of vacuolar Cd can be remobilised into the cytosol by Nramp transporters, which are up-regulated under Festarvation (Nramp1,3,4). The efficiency of the sequestration and exclusion processes determines the amount of Cd that will be transferred to the aerialparts of the plant. Loading of the xylem with Cd occurs by HMA-type plasma membrane efflux pumps (HMA2,4), and an efflux of PC-Cd or GS2-Cdcomplexes from the cytosol to the xylem sap is possibly mediated by an unknown transporter. In the xylem, Cd is bound to yet unknown ligands (?-Cd).Similar mechanisms are occurring in aerial parts of the plant, where some of the transporters described above or their homologues have similar functions.Phloem loading and unloading of Cd(II) is postulated but is only poorly described.

increased Cd and Pb levels in shoots.166 Additionally, vacuoles fromtransgenic plants exhibit higher bis-glutathionate-Cd transportactivity. The plant ABC transporters AtMRP3167 and AtATM3168

have been suggested to transport Cd2+. AtMRP3 partially restoresCd resistance when expressed in the ycf1 mutant,169 whilethe mitochondrial ABC transporter AtATM3 confers Cd and Pbresistance when over-expressed in Arabidopsis168 and is alsoimplicated in Fe homeostasis.170 Gaillard et al.171 suggest thatAtMRP6 is part of a cluster of ABC transporters involved in metaltolerance.

The pleiotropic drug resistance (PDR) subfamily is found only inyeast and plants.172 All yeast and plant PDR proteins that have beenlocalised so far reside in the plasma membrane and are regulatedby various stimuli.173 Several members of this gene family arealso regulated by essential or non-essential metals. NtPDR3 isan Fe-deficient inducible ABC transporter in N. tabacum.174 Theexpression of OsPDR9 is markedly induced by Zn and Cd in rice,175

while AtPDR12 contributes to Pb resistance in A. thaliana.176 Basedon membrane localisation and mutant analysis, Pb resistance isrelated to AtPDR12. Over-expression of this gene reduces the Pbcontent, suggesting that AtPDR12 functions as a Pb(II) or Pb(II)-chelate extruder at the root level. Particularly interesting is thefate of PDR8. This PDR is involved in pathogen resistance177,178

and is also implicated in Cd2+ and Pb2+ resistance.179 Plants

over-expressing AtPDR8 are resistant to Cd2+ and have reducedshoot and root Cd contents compared with wild-type, knockoutor silencing plants. The strong expression in the root epidermalcells is probably a main reason for the decreased amount ofCd in AtPDR8-over-expressing plants. Consequently, AtPDR8 mayconfer Cd2+ resistance by extruding Cd2+ or Cd conjugates fromthe cytosol back to the soil. Moreover, AtPDR8 decreases the Cdcontent more in the shoots than in the roots, which may bedue to the root/shoot barrier, which allows only limited transferof Cd2+ to the shoots. A similar observation was made whenthe heavy metal-pumping ATPase ZntA was expressed under thecontrol of the 35S promoter, which also conferred increased Cd2+

tolerance and reduced Cd2+ content in Arabidopsis plants.180 ThatAtPDR8 extrudes Cd2+ across the plasma membrane appears quiteunique, since, so far, no similar observation has been reported forany organism. Mechanisms involving AtPDR8 may have a practicalimpact, since they allow one either to search for plant varietieswith constitutively high expression of the AtPDR8 homologue orto produce AtPDR8-over-expressing plants to reduce plant Cdcontent.

Trace element chelation. Within the cytosol the free concentrationof most metal ions is very low. Metal ions entering root cellsbind to functional groups that act as metal chelators. These can


13

57


be either LMW organic compounds or macromolecules such asproteins. Detoxification in plants and other organisms usuallyoccurs through chelating compounds such as metallothioneins,phytochelatins, amino acids such as histidine, organic acidssuch as citrate and malate, nicotianamine and its derivatives,the phytosiderophores.181 – 184 Methallothioneins are the mostcommon metal chelators in the cytoplasm of plant and animalcells. They are small cysteine-rich proteins that bind a variety ofTEs and play a major role in plant metal homeostasis.181

Phytochelatins (PCs) are by far the most important Cd chelatorsin plant cells and therefore play an important role in Cddetoxification. They are LMW compounds synthesised from thetripeptide glutathione and chelate TEs by complexing them totheir thiol group. PC synthase is ectopically expressed in plant cellsand activated by binding Cd or other toxic metals/metalloids. Inmost plants, PC synthase is not up-regulated after exposure toCd; however, in some plants, PC synthesis is induced by activationof the enzyme as well as by increasing its expression level.185

Following inactivation of PC synthase, plants suffer from severeCd2+ hypersensitivity, as this heavy metal is no longer efficientlycomplexed within the cytosol. As an example, the mutant lineof Arabidopsis cad1-3 that is deficient in PC synthase showsa severe loss of Cd tolerance.186 PCs also provide protectionagainst arsenate, but it is uncertain whether they contributeto tolerance against other toxic metals/metalloids. The PC-Cdcomplexes formed are subsequently transported into the vacuole(see below).

Cadmium transport at vacuolar membrane.

CAX transporters. The vacuole is supposed to be a main siteof Cd2+ accumulation, and tonoplast cation/H+ antiporters areconsidered as one of the systems for translocation of Cd from thecytoplasm to the vacuole, where it is thought to be sequestered.187

Even though PC-Cd complexes (LMW) are transported into thevacuole by HMT1, a half-size ABC transporter from S. pombe,165,188

and PCs are also produced by plants, no protein responsible forthe same activity has been identified so far in plants.

CAX transporters were originally identified as vacuolar Ca2+/H+

antiporters. They contain 11 predicted α-helices and severalconserved histidine residues.189 In plants, vacuolar Cd2+/H+

antiport activity has been demonstrated,190 and at least oneof the CAX transporters catalyses the exchange of Cd2+ and othercations with protons at the vacuolar membrane.191 AlthoughArabidopsis cation exchanger genes catalyse the exchange ofCa2+ with protons, they do not appear to encode ion-specifictransporters, and modification of a single amino acid (His338) bysite-directed mutagenesis increases Cd selectivity of the strongCa transporter sCAX1 (N-terminal truncation of CAX1 resulting inconstitutive CAX1 activity).192 Comparison of seven CAX genes inN. tabacum cv. KY14 indicated that all transport Cd2+, Ca2+, Zn2+

and Mn2+ to varying degrees but that CAX4 and CAX2 exhibithigh Cd2+ transport activity and selectivity in root tonoplastvesicles.193 AtCAX2- or AtCAX4-over-expressing tobacco plantsunder the control of different promoters are more Cd-tolerant andaccumulate more Cd in the roots compared with control plants. Incontrast, shoot Cd did not differ in seedlings of transgenic and wild-type plants grown in hydroponic culture in the presence of 0.02or 3 µmol L−1 Cd2+. The lower leaves of mature plants expressingAtCAX2 or AtCAX4 under the control of two different root-selectivepromoters grown in the field (no Cd2+ amendment to the soil)

accumulated less Cd than the respective controls.194 Korenkovet al.193 suggested that CAX antiporters are not negativelyimpacted by high Cd and that supplementation of tonoplastwith AtCAX compensates somewhat for reduced tonoplast protonpumping and leakage, thereby resulting in sufficient vacuolarCd sequestration to provide higher tolerance. CAX2 and CAX4expression affects the root-to-shoot Cd distribution, and theamount of Cd taken up has a great impact on this distribution.These results assume that CAX transporters contribute to vacuolarCd sequestration and that this vacuolar mechanism in root cellsmight reduce Cd2+ translocation to the shoot. Since the substratespecificity of these transporters can be easily altered, CAX genesmay be a target for increasing the vacuolar metal sink in the rootin order to decrease shoot Cd content.

NRAMP transporters. Natural resistance-associated macro-phage proteins (NRAMPs) are a ubiquitous family of metal trans-porters present in bacteria, fungi, plants and animals.195 TheArabidopsis genome contains seven members of the NRAMP fam-ily. Three NRAMPs are implicated in Fe transport.196,197 Basedon their ability to complement an Fe uptake mutant in yeast,AtNRAMP1, AtNRAMP3 and AtNRAMP4 are characterised as Fetransporters.198,199 These genes also confer Cd uptake activitywhen expressed in S. cerevisiae. NRAMP4 and NRAMP3 are localisedin the vacuolar membrane.200 In contrast, AtNRAMP1 has a plastidtargeting sequence. Over-expression of AtNRAMP1 increases thetolerance of plants to excessive Fe concentrations, suggestinga role in Fe distribution rather than plastidic Fe uptake.166 LikeIRT1, AtNRAMP3 transports Cd2+ and is up-regulated under Fedeficiency.154 AtNRAMP3 over-expression down-regulates the pri-mary Fe uptake system, IRT1 and FRO2.199 This suggests that theover-expression of AtNRAMP3 increases Fe levels in the cytosol,thereby down-regulating Fe deficiency-induced genes such asFRO2 and IRT1. In A. thaliana it also results in Cd2+ hypersen-sitivity. AtNRAMP3 and AtNRAMP4 exhibit redundant functions,because the single mutants do not have obvious growth defects.In contrast, the double mutant is sensitive to Fe depletion andthe phenotype is correlated with the level of Fe depletion. Doublemutants are no longer able to mobilise Fe stores from the vacuoleearly in development. This is a main constraint during germi-nation, since the young plantlet depends on internal Fe stores.Results suggest that Cd can be remobilised from the vacuole intothe cytosol via NRAMP transporters. Plants that translocate Feduring seed germination will also necessarily remobilise storedCd. A sufficient Fe supply during seed production may lower Cdtranslocation during the vegetative period. Alternatively, a betterunderstanding of substrate recognition by NRAMPs may allowthe engineering of NRAMPs that will be more specific for Fe2+,decreasing the translocation of vacuolar Cd2+.

Translocation of cadmium from root to shoot. The activity ofmetal-sequestering pathways in root cells likely plays a key rolein determining the rate of translocation to the aerial parts of theplant.

Heavy metal-transporting P1b-ATPases. Long-distance trans-port of inorganic nutrients plays a crucial role in plant developmentand metal distribution. This is the only way to deliver metals takenup by the root to the shoot. In the shoot, part of the nutrientsand metals may be remobilised during further growth then during


13

58


senescence for seed production. The understanding of this pro-cess, which is mediated by different transporters, can be useful toselect or engineer plants that have the capacity to accumulate orreduce non-essential metals in different tissues.

Heavy metal-transporting P1b-ATPases (HMAs) are membraneproteins that use ATP to drive metal transport across biologicalmembranes against their electrochemical gradient.201,202 InArabidopsis, three members of the HMA family, AtHMA2, AtHMA3and AtHMA4, are classified as Zn2+-ATPases. AtHMA4 was thefirst member of this group to be cloned and characterised.203 Itconfers Cd2+ resistance in yeast and rescues the Zn deficiencyof the Escherichia coli zntA mutant, suggesting a role in Zn2+and Cd2+ transport. AtHMA3 confers Cd2+ and Pb2+ tolerance to�ycf1 yeast cells, being apparently located in the yeast vacuolemembrane when fused with green fluorescence protein.204 ThusAtHMA3 likely participates in the vacuolar storage of Cd in plants.Interestingly, plants over-expressing AtHMA3 have improvedtolerance to Cd, Co, Pb and Zn, and Cd accumulation increasesby about two- to three-fold in AtHMA3-over-expressing plantscompared with wild-type plants.205 While hma2, hma3 and hma4single mutants do not show any obvious growth defects whengrown in soil, hma2 and hma4 double mutants exhibit a drasticphenotype. These plants are chlorotic and fail to set seeds.206

HMA2 mutant plants accumulate exclusively more Zn and Cd thanwild-type plants, while HMA4 mutant plants accumulate more Znand Cd in the roots but less Zn and Cd in the leaves.207 This impaireddistribution is more pronounced in HMA2 and HMA4 doublemutants. Addition of excess Zn to the growth medium suppressesthe growth defect, despite the fact that these double mutantsstill have consistently lower levels of Zn in the aerial portionsof the plant. Additionally, plants over-expressing HMA4 have anincreased tolerance to both Zn2+ and Cd2+. HMA4 and HMA2would mediate the efflux of Zn across the plasma membrane,resulting in xylem loading. Promoter GUS fusions show that thesegenes are expressed in the vascular bundles of roots and shoots.HMA2 and HMA4 reside in the plasma membrane206,207 and, apartfrom their main function as Zn translocators to the shoot, they mayalso transport Cd2+ to the shoot. This hypothesis is confirmed inthe Zn/Cd hyperaccumulator Arabidopsis halleri. In this plant thehyperaccumulation of both metals depends on the metal pumpHMA4, which is highly expressed owing to a triplication of HMA4and altered cis-regulatory elements. As for NRAMPs, plants requireHMA4 homologues, since Zn has to be transferred to the shoot.However, it may be possible to screen for plants where the Zn/Cdratio is altered and find elements in the HMA4 protein that affectthe specificity of this transporter. Further studies on HMA4 willalso reveal amino acids important for this specificity and allow theengineering of plants that have a reduced Cd translocation rate.

Plant root and rhizosphere effectsMicrobial transformation in soil and plants. The microbial re-duction of elements has attracted recent interest because suchtransformations can play crucial roles in the cycling of both inor-ganic and organic species in a range of environments. If harnessed,it may offer the basis for a wide range of innovative biotechnolog-ical processes. Under certain conditions, however, microbial metalreduction can also mobilise non-essential metals with potentiallyharmful effects on human health.208

The mobilisation of As from sediments to drinking water andto plants constitutes a major toxic hazard to millions of peoplein Bangladesh and West Bengal. Islam et al.209 detected the roleof indigenous metal-reducing bacteria in the formation of toxic,

mobile As(III) in sediments through the use of a microcosm-basedstudy. The addition of acetate to anaerobic sediments, as a proxyfor OM and a potential electron donor for metal reduction, resultedin stimulation of the microbial reduction of Fe(III), followed by As(V)reduction and release of As(III). These results suggest that eitherdirect enzymatic microbial reduction of As(V) by Fe(III)-reducingbacteria or indirect mechanisms associated with the reductionof Fe(III) oxides could be important mechanisms for As releasein these sediments, with the involvement of Geobacter speciesimplicated in these transformations. Although Geobacter specieshave not been reported to reduce As(V), these organisms do havethe physiological capacity to reduce a wide range of metals andmetalloids,208,210 via a battery of c-type cytochromes,211 whilethe existence of an As resistance operon, including a gene fora putative arsenate reductase (arsC), was reported for Geobactersulfurreducens.211 The potential of G. sulfurreducens to mobilisevia direct enzymatic reduction and indirect mechanisms linked toFe(III) reduction has been studied.209,212

Although the full environmental relevance of TE transformationprocesses has only recently become apparent, rapid advances inthe understanding of these important biotransformations havebeen made. However, we still have much to learn about theprecise mechanisms involved and the full impact of such reactionson a range of biogeochemical cycles. Given the availability ofgenomic sequences for key metal-reducing micro-organisms, newpost-genomic and proteomic approaches and the possibility ofcombining these tools with advanced techniques from otherbranches of science and technology are required.

Plant root interactions and rhizosphere effects. The study ofTE uptake by plants, however, requires knowledge of theprocesses by which metals and metalloids are transferred to plantroots, including the rhizosphere processes, especially to basemanipulation.213,214 Micro-organisms may affect TE bioavailabilitythrough their influence on (1) the growth and morphology of roots,(2) the physiology and development of plants, (3) the fractionationof TEs and (4) the root uptake process. Understanding the role ofplant–microbe–soil interactions in governing nutrient availabilityin the rhizosphere will enhance the economic and environmentalsustainability of crop production.

Root exudates selectively influence the growth of micro-organisms that colonise the rhizosphere by altering the soilchemistry in the root vicinity and by serving as selective growthsubstrates for soil micro-organisms. The latter in turn influence thecomposition and quantity of various root exudate componentsthrough their effects on root cell leakage, cell metabolism andplant nutrition. Based on differences in root exudation andrhizodeposition in different root zones, rhizosphere microbialcommunities can vary in structure and species composition invarious root locations or in relation to soil type, plant species,nutritional status, age, stress, disease and other environmentalfactors.215 – 217

Cadmium accumulation varies between cultivars of durumwheat (Triticum turgidum var. durum), and LMW organic acids(LMWOAs) produced at the soil/root interface may control theavailability and uptake of Cd by these plants.218 No water-extractable LMWOAs were identified in the bulk soil, indicatingthe importance of microbe–plant interactions in TE accumulation.The total amount of LMWOAs in the rhizosphere soil of thehigh-Cd accumulator wheat cultivar was greater than that in therhizosphere soil of the low-Cd accumulator wheat cultivar in all


13

59


three soils tested, resulting in increased Cd uptake by the high-Cdaccumulator.

Rhizobacteria can play an essential role in the resistance of plantsto stress induced by some TEs. The inoculation of rape and brownmustard plants with rhizobacteria enhances the resistance of theplants to Ni, Pb, Zn and Cd.219 Seed treatment with TE-resistantrhizobacteria strains such as Azospirillum lipoferum, Arthrobactermysorens, Agrobacterium radiobacter and Flavobacterium sp.improves the growth of barley plants and the nutrient uptakefrom Pb- and Cd-contaminated soil under laboratory and fieldconditions.220 Seed treatment also prevents the accumulation ofPb and Cd in barley plants, thereby mitigating the toxic effect ofboth metals on the plants.

Root exudation of organic compounds contributes to increasednutrient availability in the rhizosphere. The regulation of thecomplete exudation process and the underlying genetics need tobe further elucidated. Fully understanding the interactions amongroot exudation, indigenous rhizosphere micro-organisms and TEavailability is crucial for crop production.

Microbial plant growth promotion in contaminated soils. Theapplication of bioinoculants such as arbuscular-mycorrhizalfungi and/or plant growth-promoting rhizobacteria such asAzospirillum, Agrobacterium, Pseudomonas and several Bacillusspecies is an environmentally friendly, energy-efficient andeconomically viable option for reclaiming soils and increasingbiomass production.221,222 The inoculation of bacterial strainsproducing exopolysaccharides enables plants to withstand theinitial effects of excessive TE exposure and osmotic stresses,but it also benefits the inoculated plants in terms of increasedexploitation of soil nutrients. By providing an increased amountof rhizodeposits in the soil, bioinoculants assist in initiating soilmicrobial activities.223

Plant growth-promoting rhizobacteria (PGPRs) associated withplant roots exert beneficial effects on plant growth and nutritionthrough a number of mechanisms such as N2 fixation, productionof phytohormones and siderophores and transformation ofnutrient elements when they are either applied to seeds orincorporated into the soil.224,225 Also, some rhizobacteria canexude compounds such as antibiotics, soluble phosphates,indoleacetic acid (IAA), siderophores and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, which increase bioavailabilityand facilitate root absorption of nutrients such as Fe or non-essential elements such as Cd and Pb226 and enhance thetolerance of host plants by promoting plant growth.227,228 IAAproduced by rhizobacteria is believed to play an important roleas a phytohormone, influencing many cellular plant processes.The biosynthesis of auxins and their excretion into the soilcontribute most to the bacterial plant growth-promoting effect.229

Fluorescent pseudomonads produce siderophores, pyoverdines,which are available in both homologous and heterologous uptakesystems.230 These pseudomonads are LMW Fe chelators that arereleased under Fe-limited conditions in their surroundings, possesshigh binding affinity and specificity for Fe(III) and facilitate itstransport into bacterial cells.231 All these bacterial characteristicssupport symbiotic interactions in the rhizosphere zone for themutual benefit of plants and microbes.

Some PGPRs, i.e. free-living soil bacteria that are involved ina beneficial association with plants, contain the enzyme ACCdeaminase,232 which can cleave the plant ethylene precursor ACCand lower the level of ethylene in a developing or stressed plantdue to excessive TE exposure. PGPRs containing ACC deaminase

may ensure that the ethylene level does not impair root growth233

and, by facilitating the formation of longer roots, may enhanceseedling survival and plant root growth. PGPRs stimulate rootgrowth of various crop plants, including sunflower and maize.219

The bacteria utilise the ammonia evolved from ACC as an Nsource and thereby decrease ACC within the plant.234 PGPRs usedifferent mechanisms to suppress the development of plant rootpathogens.235

REGULATORY CONTROL OF TRACE ELEMENTENTRY TO FOOD CHAINMost European soil protection and soil contamination legislationwas promulgated in the 1990s. In a case of suspected contamina-tion, soil investigation mostly follows a stepwise approach startingwith a preliminary investigation, followed by an in-depth investi-gation and finally remediation. The main purposes of regulatoryvalues for TEs are to reduce their accumulation rate in Europeanagricultural soils on the one hand and to grow consumable cropsthat do not pose adverse and unacceptable risks to animal andhuman health on the other hand. The establishment of controlvalues is complicated, however, because (1) soil organisms differin their sensitivity to metals, (2) exposure pathways differ for differ-ent TEs and (3) the properties of soil and materials (e.g. fertilisers,biosolids, etc.) contaminated with TEs influence the degree ofexposure (bioavailability) of metals and metalloids in soil and thechemical nature of diffent TEs. The precautionary option adoptedin Scandinavian countries is to maintain the status quo in termsof metal concentrations in soil, implying that input must notexceed output of metals. This other side of sustainability is noteasily achievable. Furthermore, the feasibility of this option can berestricted in many countries because of the presence of contam-ination sources yet to be minimised and for economic reasons.The alternative EU option is to regulate TE concentrations in soilto levels that will maintain environmental health for agriculturalpurposes and also avoid any off-site impacts due to movementof contaminants to interlinked ecosystems such as water, air, etc.It is strongly based on observed TE impacts. However, the majorconsideration for regulatory authorities is that contamination ofagricultural soils by TEs is irreversible.

Soil standards in most countries should trigger gentle and hardactions. Differences in selected software model, parameter values(standards) and selected human toxicological and ecotoxicologicalcriteria are the reason for the substantial variation in soil standardgeneric values and clean-up standards for TEs from country tocountry.3 Toxicity thresholds based on free metal ion activity varymore than those expressed on the basis of total soil metal; thelatter are explained but not predicted using the concept of thebiotic ligand model and rise in line with the cation exchangecapacity and contaminant aging in the soil.17 In Switzerland andGermany there are two types of generic value, one based on totallevel and the other based on neutral salt-extractable level (derivedfrom adverse effects on plant and soil organisms). The soil genericvalues should be in conjunction with the crop values and viceversa. This will help in achieving model input–output.

Most generic soil standard values differ widely and do notaccount for the interfacial interaction of contaminants. Sometimesthese values are selected on a political basis rather than ascientific basis. Furthermore, they should be used as a basis forprevention rather than complete cure. The generic values shouldsafeguard the growth of very sensitive organisms. Efforts shouldbe made to harmonise the selection and basis of development


13

60


of generic values. Appreciable efforts in this direction havebeen made by ISO236 and the EU Joint Research Centre.237

Many single background concentrations have been defined fora particular country that could give rise to either overestimation orunderestimation of metal contamination and the associated riskfor a particular soil. Regional guidelines for TEs in soils, accountingfor soil type and subjacent rock stone, are proposed to betterassess soil contamination.238 – 241

For a sustainable soil quality able to grow healthy plants, seriousefforts to improve the quality of agricultural inputs as well as airquality, water quality and soil organisms are needed.

The total weekly intake (TWI) of 7 µg Cd kg−1 body weight (BW)set by the Joint FAO/WHO Expert Committee on Food Additives(JECFA) in 1988 and reaffirmed in 1995 is used by the EU. However,in January 2009 the EFSA’s Panel on Contaminants in the FoodChain reduced this TWI to 2.5 µg Cd kg−1 BW.242 Risk assessorswould consider this new value, so this could affect decisions oningredients grown in contaminated areas.

Grains, vegetables, pulses and nuts as well as meat are frequentlyconsumed foods that may come with a high Cd content. Otherfoods, e.g. fish, chocolate, mushroom and dietary supplements,may also have a high Cd content but are eaten less often. Thenew TWI of 2.5 µg Cd kg−1 BW set by the EFSA panel is basedon studies investigating levels of Cd in urine and levels of beta-2-microglobulin, a protein that indicates kidney function, and datatranslation to actual dietary exposure.242 This TWI considers earlyindicators of a change in kidney function and not its damage.Therefore, even though exposure should be reduced, the risk ofactual kidney damage from exceeding the TWI is very low. Dataon Cd in food in 20 countries and consumption data have beenreviewed. High Cd exposure reached 3.0 µg kg−1 BW per week(average 2.3 µg Cd kg−1 BW), but vegetarians could eat as muchas 5.4 µg Cd kg−1 BW per week. Children tend to eat more foodper kg BW and could exceed the TWI.

The Food Standards Agency’s Committee on Toxicity ofChemicals in Food, Consumer Products and the Environment(COT) surveyed 24 elements in 2006, including metals in samplesof 20 different food groups bought in 24 randomly selected UKtowns, to estimate the dietary exposure of these elements for UKconsumers.245 There were no specific health concerns associatedwith the findings, which showed that levels of most of the elementsin UK food groups were the same as or lower than in a previoustotal diet study conducted in 2000. COT made the followingrecommendations: (1) future research should include informationon Al and its different forms, barium (e.g. in nuts) and Mn (e.g.in beverages) in foods and how bioavailable they are; (2) largevariability in Al in foods should be ‘clarified’ and attention shouldbe paid to whether this represents an increasing trend; (3) effortsshould continue to reduce dietary exposure to inorganic As andto Pb.243

SUMMARY AND CONCLUSIONSTrace elements, especially Cd, in the human food chain are of majorconcern and thus restricting their entry into the food chain andprotecting human and animal health is an important challenge foragronomists, microbiologists, plant biologists and physiochemists.The in situ stabilisation of TEs in soils by amendment with lime,OM, phosphates, mineral oxides, etc. can reduce the bioavailableTE fractions in soils and their entry into food crops; however, theeffects depend on soil conditions, plant species and managementpractices. Similarly, element interactions affect TEs in food crops.

Zinc application reduces Cd uptake by 40 and 60% in flaxand durum wheat respectively, and Cd translocation to theseed/grain in both crops (>30%). Grain Cd increases when wheatis grown after lupin, so a proper crop rotation is essential tominimise Cd uptake. Although decontamination techniques suchas phytoremediation by the use of hyperaccumulators or high-biomass crops and solubilisation by ligands (e.g. ion exchangeresins, natural and synthetic chelators) provide alternative optionsfor reducing TE transfer to food crops, they suffer from severallimitations. Phytoremediation is a slow and long-term process toachieve the remediation objectives, whereas solubilisation mayhave undesired side effects such as increasing the toxicity of TEsand their leaching to deeper soil layers or ground water.

Selection and breeding of crops for their low TE uptake potential(phytoexclusion) can minimise non-essential elements in thefood chain. Large genetic variations in Cd uptake exist amongcultivars, e.g. low-Cd rice cultivars retain more Cd in the root andtranslocate less to the grain than high-Cd cultivars. Phytoexclusionhas many constraints, one being the time-consuming processof plant selection, because excluder cultivars must also meetthe requirements of suitable yield, agronomic suitability, quality,disease resistance, etc.

Knowledge of the identity of transporters for various TEs isincreasing through the use of molecular techniques. HoweverTE–ligand speciation and TE transport in plants are dynamicprocesses varying across tissues, subcellular compartments,developmental stages and plant species. The increased availabilityof gene deletion mutants or plants over- or under-expressingcertain key genes or chimeric genes under the control ofdifferent promoters will provide evidence in relation to Cdtranslocation and accumulation. Finding the genes responsiblefor low Cd content in edible plant parts is a target of futureplant-breeding programmes. Root exudates contribute to nutrientavailability in the rhizosphere. The rhizodeposition regulation andunderlying genetics need further insight. A full understanding ofthe interactions between root exudation, indigenous rhizospheremicro-organisms and TE availability is crucial for crop production.Bioengineering the rhizosphere by adding beneficial micro-organisms will require an understanding of microbe–microbeand microbe–plant interactions, enabling introduced micro-organisms to show full activity in the targeted rhizosphere toimprove crop production and yields. In spite of the complexweb of geochemical and biological interactions determining thebioavailability of TEs in soil–plant–human systems, we concludethat soil- and plant-specific options must act in synergy to reduceTE transfer to the food chain.

ACKNOWLEDGEMENTSThis review is a part of the activities of Working Group 3 (WG3)of COST Action 859 and is coauthored by many members of thisWG. We would like to thank COST 859 and its leadership forencouragement and support in the preparation of this review.

REFERENCES1 McLaughlin MJ and Singh BR (eds), Cadmium in Soils and Plants.

Kluwer Academic, Dordrecht (1999).2 Nawrot T, Plusquin M, Hogervorst J, Roels HA, Celis H, Thijs L, et al,

Environmental exposure to cadmium and risk of cancer: aprospective population-based study. Lancet Oncol 7:119–126(2006).


13

61


3 Provoost J, Cornelis C and Swartjes F, Comparison of soil clean-upstandards for trace elements between countries: why do theydiffer? J Soils Sediments 6:173–181 (2006).

4 European Environment Agency, Progress in management of con-taminated sites. European Environment- state and outlook 2010 -Report CSI 015, EEA, Copenhagen (2007).

5 [Online]. Available: http://themes.eea.europa.eu/IMS/IMS/ISpecs/ISpecification20041007131746/IAssessment1152619898983/view content [16 April 2010].

6 Kabata-Pendias A and Mukherjee AB, Trace Elements from Soil toHuman. Springer, Berlin (2007).

7 Blacksmith Institute, The World’s Worst Polluted Places. The Top Ten (ofthe Dirty Thirty). Blacksmith Institute, New York, NY (2007).

8 De Temmerman LO, Hoenig M and Scokart PO, Determination of‘normal’ levels and upper limit values of trace elements in soils.J Plant Nutr Soil Sci 147:687–694 (2007).

9 De Vries W, Romkens PFAM and Schutze G, Critical soil con-centrations of cadmium, lead, and mercury in view of health effectson humans and animals. Rev Environ Contam Toxicol 191:91–130(2007).

10 Warne M, Rayment G, Brent P, Drew N, Klim E, McLaughlin M, et al,Final report of the National Cadmium Management Commit-tee, CSIRO. [Online]. (2007). Available: http://www.cadmium-management.org.au/documents/NCMC-Final-Report-web.pdf[20 April 2010].

11 Bech J and Bini C, Trace elements in soils: baseline levels andimbalance. J Geochem Explor 96:vii–viii (2008).

12 Alvarez-Ayuso E, Cadmium in soil–plant systems: an overview. IntJ Environ Pollut 33:275–291 (2008).

13 De Vries W, Romkens PFAM and Bonten LTC, Spatially explicitintegrated risk assessment of present soil concentrations ofcadmium, lead, copper and zinc in the Netherlands. Water AirSoil Pollut 191:199–215 (2008).

14 Lado LR, Hengl T and Reuter HI, Heavy metals in European soils:a geostatistical analysis of the FOREGS geochemical database.Geoderma 148:189–199 (2008).

15 Posch M and de Vries W, Dynamic modelling of metals – time scalesand target loads. Environ Model Software 24:86–95 (2009).

16 Gay R and Korre A, Accounting for pH heterogeneity and variabilityin modelling human health risks from cadmium in contaminatedland. Sci Total Environ 407:4231–4237 (2009).

17 Smolders E, Oorts K, Van Sprang P, Schoeters I, Janssen CR,McGrath SP, et al, Toxicity of trace metals in soils as affectedby soil type and aging after contamination: using calibratedbioavailability models to set ecological soil standards. EnvironToxicol Chem 28:1633–1642 (2009).

18 Hooda P, Trace Elements in Soils. Wiley, Chichester (2010).19 Baize D, Bellanger L and Tomassone R, Relationships between

concentrations of trace metals in wheat grains and soil. AgronSustain Dev 29:297–312 (2009).

20 Mench M, Winkel B, Baize D and Bodet JM, French bread wheatcultivars differ in grain Cd concentrations, in -Omics Approachesand Agricultural Management: Driving Forces to Improve FoodQuality and Safety?, ed. by Bouchardon JL, Faure O and Leclerc JC.Ecole Nationale Superieure des Mines, Saint-Etienne, France,pp. 89–90 (2006).

21 Greger M and Landberg T, Role of rhizosphere mechanisms in Cduptake by various wheat cultivars. Plant Soil 312:195–205 (2008).

22 Grant CA, Clarke JM, Duguid S and Chaney RL, Selection andbreeding of plant cultivars to minimize cadmium concentration.Sci Total Environ 390:301–310 (2008).

23 Reeves PG and Chaney RL, Bioavailability as an issue in riskassessment and management of food cadmium: a review. SciTotal Environ 398:13–19 (2008).

24 Mench M, Chartier S, Girardi S, Solda P, Van Oort F and Baize D,Exposition de vegetauxaux elements-traces, evaluation etgestion des risques pour la surete des aliments d’originevegetale – exemple de la zone agricole de Mortagne-du-Nord, inContaminations Metalliques des Agrosystemes et Ecosystemes Peri-industriels, ed. by Cambier Ph, Schvartz C and van Oort F. EditionsQuae, Versailles, pp. 85–116 (2009).

25 Liu W, Zhou Q, An J, Sun Y and Liu R, Variations in cadmiumaccumulation among Chinese cabbage cultivars and screeningfor Cd-safe cultivars. J Hazard Mater 173:737–743 (2010).

26 Louekari K, Makela-Kurtto R and Jousilahti P, Health risks associatedwith predicted increase of cadmium in cultivated soils and in thediet. Environ Model Assess 13:517–525 (2007).

27 Grant CA, Bailey LD, McLaughlin MJ and Singh BR, Managementfactors which influence cadmium concentration in crops, inCadmium in Soils and Plants, ed. by McLaughlin MJ and Singh BR.Kluwer Academic, Dordrecht. pp. 151–198 (1999).

28 Hocking PJ and McLaughlin MJ, Genotypic variation in cadmiumaccumulation by seed of linseed, and comparison with seeds ofsome other crop species. Aust J Agric Res 51:427–433 (2000).

29 Clarke JM, Norvell WA, Clarke FR and Buckley WT, Concentration ofcadmium and other elements in the grain of near-isogenic durumlines. Can J Plant Sci 82:27–33 (2002).

30 Cakmak I, Enrichment of cereal grains with zinc: agronomic or geneticbiofortification? Plant Soil 302:1–17 (2008).

31 Hart JJ, Welch RM, Norvell WA and Kochian LV, Characterization ofcadmium uptake, translocation and storage in near-isogenic linesof durum wheat that differ in grain cadmium concentration. NewPhytol 172:261–271 (2006).

32 Dakora FD and Phillips DA, Root exudates as mediators of mineralacquisition in low-nutrient environments. Plant Soil 245:35–47(2002).

33 Hinsinger P, Bioavailability of trace elements as related to root-induced chemical changes in the rhizosphere. Proc. Ext. Abstr.of 5th Int. Conf. on Biogeochemistry of Trace Elements, Vienna,pp. 152–153 (1999).

34 Mench M, Vangronsveld J, Lepp N, Ruttens A, Bleeker P andGebelen W, Use of soil amendments to attenuate trace elementexposure: sustainability, side effects, and failures, in NaturalAttenuation of Trace Element Availability in Soils, ed. byHamon R, McLaughlin M and Lombi E. SETAC Press, Pensacola,FL, pp. 197–228 (2007).

35 Kumpiene J, Lagerkvist A and Maurice C, Stabilization of As, Cr, Cu,Pb and Zn in soil using amendments – a review. Waste Manag28:215–225 (2008).

36 Hartley W and Lepp NW, Effect of in situ soil amendments onarsenic uptake in successive harvests of ryegrass (Lolium perennecv Elka) grown in amended As-polluted soils. Environ Pollut156:1030–1040 (2008).

37 Gabriel PF, Innovative technologies for contaminated siteremediation: focus on bioremediation. J Air Waste Manag Assoc41:1657–1660 (1991).

38 Gupta SK, Herren T, Wenger K, Krebs R and Hari T, In situ gentleremediation measures for heavy metal polluted soils, inPhytoremediation of Contaminated Soil and Water, ed. by Terry Nand Banuelos GS. CRC Press, New York, NY, pp. 303–322 (1999).

39 Mench M, Vangronsveld J, Lepp N, Bleeker P, Ruttens A andGeebelen W, Phytostabilisation of metal-contaminated sites, inPhytoremediation of Metal-contaminated Soils (Nato Science SeriesIV: Earth and Environmental Sciences 68), ed. by Echevarria G,Morel JL and Goncharova N. Springer, Dordrecht, pp. 109–190(2006).

40 Ascher J, Ceccherini MT, Landi L, Mench M, Pietramellara G,Nannipieri P, et al, Composition, biomass and activity of microflora,and leaf yields and foliar elemental concentrations of lettuce, afterin situ stabilization of an arsenic-contaminated soil. Appl Soil Ecol41:351–359 (2009).

41 Alloway BJ (ed.), Heavy Metals in Soils. Blackie Academic andProfessional, London (1995).

42 Vangronsveld J, Mench M, Lepp NW, Boisson J, Ruttens A, Edwards R,et al, In situ inactivation and phytoremediation of metal- andmetalloid-contaminated soils: field experiments, in Bioremediationof Contaminated Soils, ed. by Wise J, Trantolo D, Cichon E, Inyang Hand Stotmeister U. Marcel Dekker, New York, NY, pp. 859–884(2000).

43 Ruttens A, Colpaert JV, Mench M, Boisson J, Carleer R andVangronsveld J, Phytostabilization of a metal contaminated sandysoil. II: Influence of compost and/or inorganic metal immobilizingsoil amendments on metal leaching. Environ Pollut 144:533–539(2006).

44 McLaughlin MJ, Parker DR and Clarke JM, Metals andmicronutrients – food safety issues. Field Crops Res 60:142–163(1999).

45 Bes C and Mench M, Remediation of copper-contaminated topsoilsfrom a wood treatment facility using in situ stabilisation. EnvironPollut 156:1128–1138 (2008).


13

62


46 Krebs R, Gupta SK, Furrer G and Schulin R, Solubility and plant uptakeof metals with and without liming of sludge amended soils.J Environ Qual 27:18–23 (1998).

47 Singh BR, Narwal RP, Jeng AS and Almås Å, Crop uptake andextractability of cadmium in soils naturally high in metals atdifferent pH levels. Commun Soil Sci Plant Anal 26:2123–2142(1995).

48 Maier NA, McLaughlin MJ, Heap M, Butt M, Smart MK andWilliams CMJ, Effect of current season applications of calciticlime on pH, yield and cadmium concentration of potato (Solanumtuberosum L.) tubers. Nutr Cycl Agroecosyst 47:1–12 (1997).

49 Singh BR and Myhr K, Cadmium uptake by barley as affected by Cdsources and pH levels. Geoderma 84:185–194 (1998).

50 Boekhold AE, Temminghoff EJM and van der Zee SEATM, Influenceof electrolyte composition and pH on cadmium sorption by anacid soil. J Soil Sci 44:85–96 (1993).

51 Mench MJ, Didier V, Loffler M, Gomez A and Masson P, A mimicked in-situ remediation study of metal contaminated soils with emphasison Cd and Pb. J Environ Qual 23:58–63 (1994).

52 Czupyrna G, Levy RD, Maclean AI and Gold H, In situ immobilizationof heavy metal-contaminated soils. Pollution Technology Rev. No.173, Noyes Data Corp., Park Ridge, NJ (1989).

53 Shende A, Juwarker AS and Dara SS, Use of fly ash in reducing heavymetal toxicity to plants. Resour Conserv Recycl 12:221–228 (1994).

54 Vangronsveld J, Sterckx J, Van Assche F and Clijsters H, Rehabilitationstudies on an old non-ferrous waste dumping ground. Effect ofrevegetation and metal immobilization by beringite. J GeochemExplor 52:221–229 (1995).

55 Ruttens A, Mench M, Colpaert JV, Boisson J, Carleer R andVangronsveld J, Phytostabilization of a metal contaminated sandysoil. I: Influence of compost and/or inorganic metal immobilizingsoil amendments on phytotoxicity and plant availability of metals.Environ Pollut 144:524–532 (2006).

56 Mench M, Vangronsveld J, Lepp NW and Edwards R, Physico-chemical aspects and efficiency of trace element immobilizationby soil amendments, in Metal Contaminated Soils: In-situ Inac-tivation and Phytoremediation, ed. by Vangronsveld JD andCunningham SD. Springer, Dordrecht, pp. 151–182 (1998).

57 Loland JØ and Singh BR, Extractability and plant uptake of copperin contaminated coffee orchard soils as affected by differentamendments. Acta Agric Scand B 54:121–128 (2004).

58 Krebs R, Gupta SK, Furrer G and Schulin R, Gravel sludge as animmobilizing agent in soils contaminated by heavy metals. WaterAir Soil Pollut 115:465–479 (1999).

59 Rebeda I and Lepp NW, The use of synthetic zeolites to reduce plantuptake and phytotoxicity in two polluted soils, in Biogeochemistryof TraceEelements, ed. by Davis BE. Science and Technology Letters,Northwood, pp. 81–87 (1994).

60 Mench M, Bussiere S, Boisson J, Castaing E, Vangronsveld J, Ruttens A,et al, Progress in remediation and revegetation of the barren Jalesgold mine soil after in situ treatments. Plant Soil 249:187–202(2003).

61 McGrath SP, Chaudhry AM and Giller KE, Long-term effects of metalsin sludge on soils, microorganisms and plants. J Ind Microbiol14:94–104 (1995).

62 McBride MB, Environmental Chemistry of Soils. Oxford University Press,New York, NY (1994).

63 Cundy AB, Hopkinson L and Whitby RL, Use of iron-based tech-nologies in contaminated land and groundwater remediation:a review. Sci Total Environ 400:42–51 (2008).

64 Lombi E, Zhao F, Zhang G, Sun B, Fitz W, Zhang H, et al, In situ fixationof metals in soils using bauxite residue: chemical assessment.Environ Pollut 118:435–443 (2002).

65 Scheckel KG and Ryan JA, Spectroscopic speciation and quan-tification of lead in phosphate-amended soils. J Environ Qual33:1288–1295 (2004).

66 Mench M, Vangronsveld J, Didier V and Clijsters H, Evaluation ofmetal mobility, plant availability and immobilization by chemicalagents in a limed silty soil. Environ Pollut 86:279–286 (1994).

67 Berti WR and Cunningham SD, In-place inactivation of Pb in Pbcontaminated soils. Environ Sci Technol 31:1359–1364 (1997).

68 Chlopecka A and Adriano DC, Mimicked in situ stabilization of metalin a cropped soil. Bioavailability and chemical forms of Zn. EnvironSci Technol 30:3294–3303 (1996).

69 Mench M, Vangronsveld J, Beckx C and Ruttens A, Progress in assistednatural remediation of an arsenic contaminated agricultural soil.Environ Pollut 144:51–61 (2006).

70 Renella G, Landi L, Ascher J, Ceccherini MT, Pietramellara G andMench M, Long-term effects of aided phytostabilisation of traceelements on microbial biomass and activity, enzyme activities, andcomposition of microbial community in the Jales contaminatedmine soils. Environ Pollut 152:702–712 (2008).

71 McKenzie RM, The surface charge on manganese dioxides. Aust J SoilRes 19:41–50 (1981).

72 Wenger K, Gupta SK, Furrer G and Schulin R, The role of ni-trilotriacetate in copper uptake by tobacco. J Environ Qual32:1669–1676 (2003).

73 Kayser A, Wenger K, Keller A, Gupta SK and Schulin R, Enhancementof phytoextraction of Zn, Cd, and Cu from calcareous soil: the useof NTA and sulfur amendments. Environ Sci Technol 34:1778–1783(2000).

74 Blaylock MJ, Salt DE, Dushenkov S, Zakharova O, Gussman C,Kapulnik Y, et al, Enhanced accumulation of Pb in Indian mustardby soil-applied chelating agents. Environ Sci Technol 31:860–865(1997).

75 Athalye VV, Ramachandran V and D’Souza TJ, Influence of chelatingagents on plant uptake of 51Cr, 210Pb and 210Po. Environ Pollut89:47–53 (1995).

76 Mench M, Bert V, Gawronski S, Schwitzguebel JP, Schroder P andVangronsveld J, Assessment of successful experiments andlimitations of phytotechnologies: II. Outcomes at field scale andoutlook from COST Action 859. Environ Sci Pollut Res 16:876–900(2011).

77 Cunningham SD and Ow DW, Promises and prospects of phyto-remediation. Plant Physiol 110:715–719 (1996).

78 Verbruggen N, Hermans C and Schat H, Molecular mechanisms ofmetal hyperaccumulation in plants. New Phytol 181:759–776(2009).

79 Nehnevajova E, Herzig R, Bourigault C, Bangerter S andSchwitzguebel JP, Stability of enhanced yield and metal uptakeby sunflower mutants for improved phytoremediation. IntJ Phytoremediation 11:329–346 (2009).

80 Greger M and Landberg T, Influence of Salix cultivation on Cd inwheat grains. COST 859 WG4 Workshop, Verneuil-en-Halatte, p.76(2008).

81 Sauerbeck DR, Plant element and soil properties governing uptakeand availability of heavy metals derived from sewage sludge.Water Air Soil Pollut 58:227–237 (1991).

82 Smolders E and McLaughlin MJ, Chloride increases Cd uptake inSwiss chard in a resin-buffered nutrient solution. Soil Sci Soc Am J60:1443–1447 (1996).

83 Ebbs SD, Lasat MM, Brady DJ, Cornish J, Gordon R and Kochian LV,Phytoextraction of cadmium and zinc from a contaminated soil.J Environ Qual 26:1424–1430 (1997).

84 Mortvedt JJ, Cadmium levels in soils and plants from some long–termfertility experiments in the USA. J Environ Qual 16:137–142 (1987).

85 Jones KC and Johnston AE, Cadmium in cereal grain and herbagefrom long-term experimental plots at Rothamsted. Environ Pollut57:199–216 (1989).

86 Jeng AS and Singh BR, Cadmium status of soils and plants fromlong-term fertility experiments in Southern Norway. Plant Soil175:67–74 (1995).

87 Grant CA, Bailey LD, McLaughlin MJ and Singh BR, Managementfactors which influence cadmium concentration in crops, inCadmium in Soils and Plants, ed. by McLaughlin MJ and Singh BR.Kluwer Academic, Dordrecht, pp. 151–198 (1999).

88 Tiller KG, Oliver DP, McLaughlin MJ, Conyers MK, Merry RH andNaidu R, Managing cadmium contamination of agricultural land,in Remediation of Soils Contaminated by Metals, ed. by Iskandar IK.Science and Technology Letters, Northwood, pp. 225–255 (1997).

89 Shao G, Chen M, Wang W, Mou R and Zhang G, Iron nutrition affectscadmium accumulation and toxicity in rice plants. Plant GrowthRegul 53:33–42 (2007).

90 Oliver DO, Schulz JE, Tiller KG and Merry RH, The effect of croprotations and tillage practices on cadmium concentration in wheatgrain. Aust J Soil Res 44:1221–1234 (1993).

91 Jonsson JO and Eriksson J, The effect of fertilization for higherprotein content on Cd-level in winter wheat grain, in ConferenceProceedings 7th ICOBTE, Vol. 3, ed. by Gobran GR. SwedishUniversity of Agricultural Sciences, Uppsala, pp. 242–243 (2003).


13

63


92 Wångstrand H, Eriksson J and Oborn I, Cadmium concentration inwinter wheat as affected by nitrogen fertilization. Eur J Agron26:209–214 (2007).

93 He QB and Singh BR, Crop uptake of cadmium from phosphorusfertilizers: I. Yield and cadmium content. Water Air Soil Pollut74:251–265 (1994).

94 Loganathan P and Hedley MJ, Downward movement of cadmiumand phosphorus from phosphatic fertilizer in a pasture in NewZealand. Environ Pollut 95:319–324 (1997).

95 Gray CW, McLaren RG, Roberts AHC and Condron LM, The effect oflong-term phosphatic fertilizer application on the amount andforms of cadmium in soils under pasture in New Zealand. Nutr CyclAgroecosyst 54:267–277 (1999).

96 He QB and Singh BR, Plant availability of Cd in soils: I. Extractabilityof Cd in newly and long-term cultivated soils. Acta Agric Scand B43:134–141 (1993).

97 Singh BR, Cadmium and fluoride uptake by oats and rape fromphosphate fertilizer in two different soils. Norweg J Agric Sci4:239–249 (1990).

98 McLaughlin MJ, Tiller KG, Naidu R and Stevens DG, The behaviourand environmental impact of contaminants in fertilizers. Aust J SoilRes 34:1–54 (1996).

99 Grant CA, Bailey LD and Therrien MC, The effect of N, P and KClfertilizers on grain yield and Cd concentration of malting barley.Fertilizer Res 45:153–161 (1996).

100 McLaughlin MJ, Tiller KG and Smart MK, Speciation of cadmium insoil solutions of saline/sodic soils and relationship with cadmiumconcentrations in potato tubers. Aust J Soil Res 35:1–16 (1997).

101 Oliver DO, Hannam R, Tiller KG, Wilhelm NS, Merry RH and Cozens GD,The effects of zinc fertilization on cadmium concentration in wheatgrain. J Environ Qual 23:705–711 (1994).

102 McLaughlin MJ, Hamon RE, Maier NA, Corell R, Smart MKand Grant CD, Screening of phytoremediation and in-situimmobilization techniques to remediate cadmium-contaminatedagricultural soils, in Proceedings National Soils Conference –Environmental Benefits of Soil Management, Brisbane, Australia, ed.by Muluey P. Australian Soil Science Society, Sydney, pp. 229–236(1998).

103 Grant CA and Bailey LD, Effect of phosphorus and Zn fertilizersmanagement on Cd accumulation in flax seed. J Sci Food Agric73:307–314 (1997).

104 Jiao Y, Grant CA and Bailey LD, Effect of P and Zn fertilizer on Cduptake and distribution in flax and durum wheat. J Sci Food Agric84:777–785 (2004).

105 Koleli N, Eker S and Cakmak I, Effect of Zn fertilization on Cd toxicityin durum and bread wheat grown in Zn deficient soil. EnvironPollut 131:453–459 (2004).

106 Khoshgoftar AH, Shariatmadari H, Karimian N, Kalbasi M, van derZee SEATM and Parker DR, Salinity and zinc application effectson phytoavailability of cadmium and zinc. Soil Sci Soc Am J68:1885–1889 (2004).

107 Lux A, Vaculık M, Luxova M and Kodama S, Alleviating effect of siliconon cadmium toxicity in hydroponically cultivated maize, in -OmicsApproaches and Agricultural Management: Driving Forces to ImproveFood Quality and Safety?, ed. by Bouchardon JL, Faure O andLeclerc JC. Ecole Nationale Superieure des Mines, Saint-Etienne,France, p. 86 (2006).

108 Nwugo CC and Huerta AJ, Silicon-induced cadmium resistance in rice(Oryza sativa). J Plant Nutr Soil Sci 171:841–848 (2008).

109 Greger M and Landberg T, Influence of Si on Cd in wheat, inContaminants and Nutrients Availability, Accumulation/Exclusionand Plant –Microbia–Soil Interactions, ed. by Liskova D, Lux A andMartinka M. Copycentrum PACI, Bratislava, p. 49 (2008).

110 McLaughlin MJ, Palmer LT, Tiller KG, Beech TA and Smart MK,Increased soil salinity causes elevated cadmium concentrations infield-grown potato tubers. J Environ Qual 23:1013–1018 (1994).

111 Weggler-Beaton K, McLaughlin MJ and Graham RD, Salinity increasescadmium uptake by wheat and Swiss chard from soil amendedwith biosolids. Aust J Soil Res 38:37–45 (2000).

112 McLaughlin MJ, Lembrechts RM, Smolders E and Smart MK, Effect ofsulphate on cadmium uptake by Swiss chard: II. Effect due tosulphate addition to soil. Plant Soil 202:217–222 (1998).

113 Edwards JH, Wood CW, Thuelow DL and Ruf ME, Tillage and croprotation effect on fertility status of a Hapludult soil. Soil Sci Soc AmJ 56:1577–1582 (1992).

114 Zuo YM, Zhang FS, Li X and Cao YP, Studies on the improvementin iron nutrition of peanut by intercropping with maize on acalcareous soil. Plant Soil 220:13–25 (2000).

115 Zhang FS, Romheld V and Marschner H, Diurnal rhythm of release ofphytosiderophores and uptake rate of zinc in iron-deficient wheat.Soil Sci Plant Nutr 37:671–678 (1991).

116 Li YM, Chaney RL, Schneiter AA, Elias E and Green CE, Genetic andsoil factors related to Cd levels in sunflower kernels and durumwheat. Agron Abstr p. 277 (1993).

117 Arao T and Ae N, Genotypic variations in cadmium levels of rice grain.Soil Sci Plant Nutr 49:473–479 (2003).

118 Liu J, Qian M, Cai G, Yang J and Zhu Q, Uptake and translocation ofCd in different rice cultivars and the relation with Cd accumulationin rice grain. J Hazard Mater 143:443–447 (2007).

119 Oliver DP, Gartrell JW, Tiller KG, Correll R, Cozens GD and Youngberg B,Differential responses of Australian wheat cultivars to cadmiumconcentration in wheat grain. Aust J Agric Res 46:873–886 (1995).

120 Liu J, Qian M, Cai G, Zhu Q and Wong MH, Variations between ricecultivars in root secretion of organic acids and the relationshipwith plant cadmium uptake. Environ Geochem Health 29:189–195(2007).

121 Harris NS and Taylor GJ, Cadmium uptake and translocation inseedlings of near isogenic lines of durum wheat that differ ingrain cadmium accumulation. BMC Plant Biol 4:(2004). DOI: 10-1186/1471-2229-4-4.

122 Arao T and Ishikawa S, Genotypic differences in cadmium concen-tration and distribution of soybeans and rice. Jpn Agric Res Q40:21–30 (2006).

123 Ishikawa S, Ae N, Sugiyama M, Murakami M and Arao T, Genotypicvariation in shoot cadmium concentration in rice and soybean insoils with different levels of cadmium contamination. Soil Sci PlantNutr 51:101–108 (2005).

124 Greger M and Lofsted M, Comparison of uptake and distribution ofcadmium in different cultivars of bread and durum wheat. Crop Sci44:501–507 (2004).

125 Clarke JM, Norvell WA, Clarke FR and Buckley WT, Concentration ofcadmium and other elements in the grain of near-isogenic durumlines. Can J Plant Sci 82:27–33 (2002).

126 Clarke JM, McCaig TN, DePauw RM, Knox RE, Clarke FR andFernandez MR, Strong field durum wheat. Can J Plant Sci85:651–654 (2005).

127 Zhang L, Zhang L and Song FB, Cadmium uptake and distributionby different maize genotypes in maturing stage. Commun Soil SciPlant Anal 39:1517–1531 (2008).

128 Grispen VMJ, Nelissen HJM and Verkleij JAC, Phytoextraction withBrassica napus L.: a tool for sustainable management of heavymetal contaminated soils. Environ Pollut 144:77–83 (2006).

129 Zorrig W, Sarrobert C, Rouached A, Maisonneuve B, Davidian J-C,Abdelly C, et al, Genetic and physiological determinants con-trolling cadmium accumulation in lettuce (Lactuca sativa), inUptake, Sequestration and Detoxification – an Integrated Approach,ed. by Erdei L. Cost action 859 conference univ.of Szeged, Hungary,p. 75 (2009).

130 Clarke JM, Leisle D, DePauw RM and Thiessen LL, Registration of fivepairs of durum wheat genetic stocks near-isogenic for cadmiumconcentration. Crop Sci 37:1–297 (1997).

131 Hart JJ, Welch RM, Norvell WA, Clarke JM and Kochian LV, Zinc effectson cadmium accumulation and partitioning in near-isogenic linesof durum wheat that differ in grain cadmium concentration. NewPhytol 167:391–401 (2005).

132 Mench M, Baize D and Mocquot B, Cadmium availability to wheat infive soil series from the Yonne district, Burgundy, France. EnvironPollut 95:93–103 (1997).

133 Mench M, Winkel B, Baize D and Bodet JM, French bread wheatcultivars differ in grain Zn concentrations, in Challenges ofImproving Quality and Safety of Food Crops, Cost 854 meetingLillehammer, Norway, p. 31 (2008).

134 Clemens S, Palmgren MG and Kramer UA, Long way ahead:understanding and engineering plant metal accumulation. TrendsPlant Sci 7:309–315 (2002).

135 Clemens S, Toxic metal accumulation, responses to exposure andmechanisms of tolerance in plants. Biochimie 88:1707–1719(2006).

136 Ramesh SA, Shin R, Eide DJ and Schachtman DP, Differential metalselectivity and gene expression of two zinc transporters from rice.Plant Physiol 133:126–134 (2003).


13

64


137 Guerinot ML, The ZIP family of metal transporters. Biochim BiophysActa 1465:190–198 (2000).

138 Eide D, Broderius M, Fett J and Guerinot ML, A novel iron-regulatedmetal transporter from plants identified by functional expressionin yeast. Proc Natl Acad Sci USA 93:5624–5628 (1996).

139 Vert G, Grotz N, Dedaldechamp F, Gaymard F, Guerinot ML, Briat JF,et al, IRT1, an Arabidopsis transporter essential for iron uptake fromthe soil and for plant growth. Plant Cell 14:1223–1233 (2002).

140 Henriques R, Jasik J, Klein M, Martinoia E, Feller U, Schell J, et al,Knock-out of Arabidopsis metal transporter gene IRT1 results iniron deficiency accompanied by cell differentiation defects. PlantMol Biol 50:587–597 (2002).

141 Bughio N, Yamaguchi H, Nishizawa NK, Nakanishi H and Mori S,Cloning an iron-regulated metal transporter from rice. J Exp Bot53:1677–1682 (2002).

142 Ishimaru Y, Suzuki M, Tsukamoto T, Suzuki K, Nakazono M,Kobayashi T, et al, Rice plants take up iron as an Fe3+-phyto-siderophore and as Fe2+. Plant J 45:335–346 (2006).

143 Korshunova YO, Eide D, Clark WG, Guerinot ML and Pakrasi HB, TheIRT1 protein from Arabidopsis thaliana is a metal transporter witha broad substrate range. Plant Mol Biol 40:37–44 (1999).

144 Fox TC and Guerinot ML, Molecular biology of cation transport inplants. Annu Rev Plant Physiol Plant Mol Biol 49:669–696 (1998).

145 Moog PR, Vanderkooij TAW, Bruggemann W, Schiefelbein JW andKuiper PJC, Responses to iron-deficiency in Arabidopsisthaliana – the turbo iron reductase does not depend on theformation of root hairs and transfer cells. Planta 195:505–513(1995).

146 Robinson NJ, Procter CM, Connolly EL and Guerinot ML, A ferric-chelate reductase for iron uptake from soils. Nature 397:694–697(1999).

147 Cohen CK, Fox TC, Garvin DF and Kochian LV, The role of iron-deficiency stress responses in stimulating heavy-metal transportin plants. Plant Physiol 116:1063–1072 (1998).

148 Connolly EL, Fett JP and Guerinot ML, Expression of the IRT1 metaltransporter is controlled by metals at the levels of transcript andprotein accumulation. Plant Cell 14:1347–1357 (2002).

149 Connolly EL, Campbell NH, Grotz N, Prichard CL and Guerinot ML,Overexpression of the FRO2 ferric chelate reductase conferstolerance to growth on low iron and uncovers post-transcriptionalcontrol. Plant Physiol 133:1102–1110 (2003).

150 Grotz N, Fox T, Connolly E, Park W, Guerinot ML and Eide D,Identification of a family of zinc transporter genes from Ara-bidopsis that respond to zinc deficiency. Proc Natl Acad Sci USA95:7220–7224 (1998).

151 Pence NS, Larsen PB, Ebbs SD, Letham DLD, Lasat MM, Garvin DF,et al, The molecular physiology of heavy metal transport in theZn/Cd hyperaccumulator Thlaspi caerulescens. Proc Natl Acad SciUSA 97:4956–4960 (2000).

152 Briat JF, Curie C and Gaymard F, Iron utilization and metabolism inplants. Curr Opin Plant Biol 10:276–282 (2007).

153 Briat JF and Gaymard F, Iron nutrition and interactions inplants – preface. Plant Physiol Biochem 45:259 (2007).

154 Rogers EE, Eide DJ and Guerinot ML, Altered selectivity in anArabidopsis metal transporter. Proc Natl Acad Sci USA97:12356–12360 (2000).

155 Plaza S, Tearall KL, Zhao FJ, Buchner P, McGrath SP andHawkesford MJ, Expression and functional analysis of metaltransporter genes in two contrasting ecotypes of the hyper-accumulator Thlaspi calerulescens. J Exp Bot 58:1717–1728 (2007).

156 Sappin-Didier V, Vansuyt G, Mench M and Briat J-F, Cadmium avail-ability at different soil pH to transgenic tobacco overex-pressing ferritin. Plant Soil 270:189–197 (2005).

157 Higgins CF, ABC transporters – from microorganisms to man. AnnuRev Cell Biol 8:67–113 (1992).

158 Hyde SC, Emsley P, Hartshorn MJ, Mimmack MM, Gileadi U, Pearce SR,et al, Structural model of ATP-binding proteins associated withcystic-fibrosis, multidrug resistance and bacterial transport. Nature346:362–365 (1990).

159 Sanchez-Fernandez R, Davies TGE, Coleman JOD and Rea PA, TheArabidopsis thaliana ABC protein superfamily, a completeinventory. J Biol Chem 276:30231–30244 (2001).

160 Linton KJ and Higgins CF, The Escherichia coli ATP-binding cassette(ABC) proteins. Mol Microbiol 28:5–13 (1998).

161 Martinoia E, Klein M, Geisler M, Bovet L, Forestier C, Kolukisaoglu U,et al, Multifunctionality of plant ABC transporters – more than justdetoxifiers. Planta 214:345–355 (2002).

162 Rea PA, Plant ATP-binding cassette transporters. Annu Rev Plant Biol58:347–375 (2007).

163 Jungwirth H and Kuchler K, Yeast ABC transporters – a tale of sex,stress, drugs and aging. FEBS Lett 580:1131–1138 (2006).

164 Li ZS, Lu YP, Zhen RG, Szczypka M, Thiele DJ and ReaPA, A new pathway for vacuolar cadmium sequestrationin Saccharomyces cerevisiae: YCF1-catalyzed transport ofbis(glutathionato)cadmium. Proc Natl Acad Sci USA 94:42–47(1997).

165 Ortiz DF, Ruscitti T, Mccue KF and Ow DW, Transport of metal-bindingpeptides by HMT1, a fission yeast ABC-type vacuolar membrane-protein. J Biol Chem 270:4721–4728 (1995).

166 Song WY, Sohn EJ, Martinoia E, Lee YJ, Yang YY, Jasinski M, et al,Engineering tolerance and accumulation of lead and cadmiumin transgenic plants. Nature Biotechnol 21:914–919 (2003).

167 Bovet L, Eggmann T, Meylan-Bettex M, Polier J, Kammer P, Marin E,et al, Transcript levels of AtMRPs after cadmium treatment:induction of AtMRP3. Plant Cell Environ 26:371–381 (2003).

168 Kim DY, Bovet L, Kushnir S, Noh EW, Martinoia E and Lee Y, AtATM3is involved in heavy metal resistance in Arabidopsis. Plant Physiol140:922–932 (2006).

169 Tommasini R, Vogt E, Fromenteau M, Hortensteiner S, Matile P,Amrhein N, et al, An ABC-transporter of Arabidopsis thaliana hasboth glutathione-conjugate and chlorophyll catabolite transportactivity. Plant J 13:773–780 (1998).

170 Chen S, Sanchez-Fernandez R, Lyver ER, Dancis A and Rea PA,Functional characterization of AtATM1, AtATM2, and AtATM3,a subfamily of Arabidopsis half-molecule ATP-binding cassettetransporters implicated in iron homeostasis. J Biol Chem282:21561–21571 (2007).

171 Gaillard S, Jacquet H, Vavasseur A, Leonhardt N and ForestierC, AtMRP6/AtABCC6, an ATP-binding cassette transporter geneexpressed during early steps of seedling development and up-regulated by cadmium in Arabidopsis thaliana. BMC Plant Biol 8:22(2008). DOI:10.1186/1471-2229-8-22.

172 Crouzet J, Trombik T, Fraysse AS and Boutry M, Organization andfunction of the plant pleiotropic drug resistance ABC transporterfamily. FEBS Lett 580:1123–1130 (2006).

173 Smart CC and Fleming AJ, Hormonal and environmental regulationof a plant PDR5-like ABC transporter. J Biol Chem 271:19351–19357(1996).

174 Ducos E, Fraysse AS and Boutry M, NtPDR3, an iron-deficiencyinducible ABC transporter in Nicotiana tabacum. FEBS Lett579:6791–6795 (2005).

175 Moons A, Ospdr9, which encodes a PDR-type ABC transporter, isinduced by heavy metals, hypoxic stress and redox perturbationsin rice roots. FEBS Lett 553:370–376 (2003).

176 Lee M, Lee K, Lee J, Noh EW and Lee Y, AtPDR12 contributes to leadresistance in Arabidopsis. Plant Physiol 138:827–836 (2005).

177 Kobae Y, Sekino T, Yoshioka H, Nakagawa T, Martinoia E andMaeshima M, ABC transporter, AtPDR8 is implicated in pathogenresistance. Plant Cell Physiol 47:309–318 (2006).

178 Stein M, Dittgen J, Sanchez-Rodriguez C, Hou BH, Molina A, Schulze-Lefert P, et al, Arabidopsis PEN3/PDR8, an ATP binding cassettetransporter, contributes to nonhost resistance to inappropriatepathogens that enter by direct penetration. Plant Cell 18:731–746(2006).

179 Kim DY, Bovet L, Maeshima M, Martinoia E and Lee Y, The ABCtransporter AtPDR8 is a cadmium extrusion pump conferringheavy metal resistance. Plant J 50:207–218 (2007).

180 Lee J, Bae H, Jeong J, Lee JY, Yang YY, Hwang I, et al, Functionalexpression of a bacterial heavy metal transporter in Arabidopsisenhances resistance to and decreases uptake of heavy metals.Plant Physiol 133:589–596 (2003).

181 Zhou J and Goldsbrough PB, Functional homologs of fungalmetallothionein genes from Arabidopsis. Plant Cell 6:875–884(1994).

182 Kramer U, Cotter-Howells JD, Charnock JM, Baker AJM and Smith JAC,Free histidine as a metal chelator in plants that accumulate nickel.Nature 379:635–638 (1996).

183 Salt DE, Prince RC, Baker AJM, Raskin I and Pickering IJ, Zincligands in the metal hyperaccumulator Thlaspi caerulescens as


13

65


determined using X-ray absorption spectroscopy. Environ SciTechnol 33:712–717 (1999).

184 Callahan DL, Baker AJ, Kolev SD and Wedd AG, Metal ion ligands inhyperaccumulating plants. J Biol Inorg Chem 11:2–12 (2006).

185 Roth U, von Roepenack-Lahaye E and Clemens S, Proteome changesin Arabidopsis thaliana roots upon exposure to Cd2+ . J Exp Bot57:4003–4013 (2006).

186 Vatamaniuk O, Mari S, Lu Y and Rea P, AtPCS1, a phytochelatinsynthase from Arabidopsis: isolation and in vitro reconstitution.Proc Natl Acad Sci USA 96:7110–7115 (1999).

187 Vogeli-Lange R and Wagner GJ, Subcellular-localization of cadmiumand cadmium-binding peptides in tobacco-leaves – implication ofa transport function for cadmium-binding peptides. Plant Physiol92:1086–1093 (1990).

188 Ortiz DF, Kreppel L, Speiser DM, Scheel G, Mcdonald G and Ow DW,Heavy-metal tolerance in the fission yeast requires an ATP-binding cassette-type vacuolar membrane transporter. EMBO J11:3491–3499 (1992).

189 Gaxiola RA, Fink GR and Hirschi KD, Genetic manipulation of vacuolarproton pumps and transporters. Plant Physiol 129:967–973 (2002).

190 Salt DE and Wagner GJ, Cadmium transport across tonoplast ofvesicles from oat roots – evidence for a Cd2+/H+ antiport activity.J Biol Chem 268:12297–12930 (1993).

191 Shigaki T and Hirschi K, Characterization of CAX-like genes in plants:implications for functional diversity. Gene 257:291–298 (2000).

192 Shigaki T, Barkla BJ, Miranda-Vergara MC, Zhao J, Pantoja O andHirschi KD, Identification of a crucial histidine involved in metaltransport activity in the Arabidopsis cation/H+ exchanger CAX1.J Biol Chem 280:30136–30142 (2005).

193 Korenkov V, Hirschi K, Crutchfield JD and Wagner GJ, Enhancingtonoplast Cd/H antiport activity increases Cd, Zn, and Mntolerance, and impacts root/shoot Cd partitioning in Nicotianatabacum L. Planta 226:1379–1387 (2007).

194 Korenkov V, King B, Hirschi K and Wagner GJ, Root-selectiveexpression of AtCAX4 and AtCAX2 results in reduced laminacadmium in field-grown Nicotiana tabacum L. Plant Biotechnol J7:219–226 (2009).

195 Cellier M, Prive G, Belouchi A, Kwan T, Rodrigues V, Chia W, et al,NRAMP defines a family of membrane-proteins. Proc Natl AcadSci USA 92:10089–10093 (1995).

196 Alonso JM, Hirayama T, Roman G, Nourizadeh S and Ecker JR, EIN2,a bifunctional transducer of ethylene and stress responses inArabidopsis. Science 284:2148–2152 (1999).

197 Maser P, Thomine S, Schroeder JI, Ward JM, Hirschi K, Sze H, et al,Phylogenetic relationships within cation transporter families ofArabidopsis. Plant Physiol 126:1646–1667 (2001).

198 Curie C, Alonso JM, Le Jean M, Ecker JR and Briat JF, Involvement ofNRAMP1 from Arabidopsis thaliana in iron transport. Biochem J347:749–755 (2000).

199 Thomine S, Wang RC, Ward JM, Crawford NM and Schroeder JI,Cadmium and iron transport by members of a plant metaltransporter family in Arabidopsis with homology to NRAMP genes.Proc Natl Acad Sci USA 97:4991–4996 (2000).

200 Lanquar V, Lelievre F, Bolte S, Hames C, Alcon C, Neumann D, et al,Mobilization of vacuolar iron by AtNRAMP3 and AtNRAMP4 isessential for seed germination on low iron. EMBO J 24:4041–4051(2005).

201 Arguello JM, Identification of ion-selectivity determinants in heavy-metal transport P-1B-type ATPases. J Membr Biol 195:93–108(2003).

202 Axelsen KB and Palmgren MG, Evolution of substrate specificities inthe P-type ATPase superfamily. J Mol Evol 46:84–101 (1998).

203 Mills RF, Krijger GC, Baccarini PJ, Hall JL and Williams LE, Functionalexpression of AtHMA4, a P-1B-type ATPase of the Zn/Co/Cd/Pbsubclass. Plant J 35:164–176 (2003).

204 Gravot A, Lieutaud A, Verret F, Auroy P, Vavasseur A and Richaud P,AtHMA3, a plant P-1B-ATPase, functions as a Cd/Pb transporter inyeast. FEBS Lett 561:22–28 (2004).

205 Morel M, Crouzet J, Gravot A, Auroy P, Leonhardt N, Vavasseur A,et al, AtHMA3, a P-1B-ATPase allowing Cd/Zn/Co/Pb vacuolarstorage in Arabidopsis. Plant Physiol 149:894–904 (2009).

206 Hussain D, Haydon MJ, Wang Y, Wong E, Sherson SM, Young J, et al,P-type ATPase heavy metal transporters with roles in essential zinchomeostasis in Arabidopsis. Plant Cell 16:1327–1339 (2004).

207 Verret F, Gravot A, Auroy P, Leonhardt N, David P, Nussaume L, et al,Overexpression of AtHMA4 enhances root-to-shoot translocation

of zinc and cadmium and plant metal tolerance. FEBS Lett576:306–312 (2004).

208 Lloyd JR, Microbial reduction of metals and radionuclides. FEMSMicrobiol Rev 27:411–425 (2003).

209 Islam FS, Gault AG, Boothman C, Polya DA, Charnock JM, Chatterjee D,et al, Role of metal-reducing bacteria in arsenic release from Bengaldelta sediments. Nature 430:68–71 (2004).

210 Caccavo Jr F, Lonergan DJ, Lovley DR, Davis M, Stolz JF andMcInerney MJ, Geobacter sulfurreducens sp. Nov., a hydrogen- andacetateoxidizing dissimilatory metal-reducing microorganism.Appl Environ Microbiol 60:3752–3759 (1994).

211 Methe BA and Nelson KE, Genome of Geobacter sulfurreducens: metalreduction in subsurface environments. Science 302:1967–1969(2003).

212 Islam FS, Pederick RL, Gault AG, Adams LK, Polya DA, Charnock JM,et al, Interactions between the Fe(III)-reducing bacterium Geo-bacter sulfurreducens and arsenate, and capture of the metalloidby biogenic Fe(II). Appl Environ Microbiol 71:8642–8648 (2005).

213 Lynch JM and Whipps JM, Substrate flow in the rhizosphere. PlantSoil 129:1–10 (1990).

214 Courchesne F, Cloutier-Hurteau B and Turmel MC, Relevance ofrhizosphere research to the ecological risk assessment of tracemetals in soils. Hum Ecol Risk Assess 14:54–72 (2008).

215 Dong J, Mao WH, Zhang GP, Wu FB and Eai Y, Root excretion andplant tolerance to cadmium toxicity – a review. Plant Soil Environ53:193–200 (2007).

216 Li TQ, Yang XE and Jin XF, Root responses and metal accumulation intwo contrasting ecotypes of Sedum alfredii Hance under lead andzinc toxic stress. J Environ Sci Health A 40:1081–1096 (2005).

217 Kamaludeen SPB and Ramasamy K, Rhizoremediation of metals:harnessing microbial communities. Indian J Microbiol 48:80–88(2008).

218 Cieslinski G, Van Rees KCJ, Szmigielska AM, Krishnamurti GSR andHuang PM. Low-molecular-weight organic acids in rhizospheresoils of durum wheat and their effect on cadmium bio-accumulation. Plant Soil 203:109–117 (1998).

219 Belimov AA, Safronova VI, Sergeyeva TA, Egorova TN, Matveyeva VA,Tsyganov VE, et al, Characterization of plant growth-promotingrhizobacteria isolated from polluted soils and containing 1-aminocyclopropane-1-carboxylate deaminase. Can J Microbiol47:642–652 (2001).

220 Belimov AA, Kunakova AM, Safronova VI, Stepanok VV, Yudkin LYu,Alekseev YuV, et al, Employment of rhizobacteria for the ino-culation of barley plants cultivated in soil contaminated with leadand cadmium. Microbiology (Moscow) 73:99–106 (2004).

221 Rabie GH and Almadini AM, Role of bioinoculants in developmentof salt-tolerance of Vicia faba plants under salinity stress. AfrJ Biotechnol 4:210–222 (2005).

222 Mayak S, Tirosh T and Glick BR, Plant growth-promoting bacteriaconfer resistance in tomato plants to salt stress. Plant PhysiolBiochem 42:565–572 (2004).

223 Ashraf M, Ghorbanli M and Ebrahimzadeh H, Improved growth ofsalinity-stressed soybean after inoculation with salt pre-treatedmycorrhizal fungi. J Plant Physiol 164:1144–1151 (2007).

224 Kloepper JW, Lifshitz R and Zablotowicz RM, Free living bacterialinocula for enhancing crop productivity. TrendsBiotechnol 7:39–44(1989).

225 Burd GI, Dixon DG and Glick BR, Plant growth-promoting bacteriathat decrease heavy metal toxicity in plants. Can J Microbiol46:237–245 (2000).

226 Salt DE, Blaylock M, Kumar NP, Dushenkov BA, Ensley BD, Chet I, et al,Phytoremediation: a novel strategy for the removal of toxic metalsfrom the environment using plants. Biol Technol 13:468–474(1995).

227 Shilev S, Ruso J, Puig A, Benlloch M, Jorrin J and Sancho ED,Rhizospheric bacteria promote sunflower (Helianthus annuus L.)plant growth and tolerance to heavy metals. Minerva Biotecnol13:37–39 (2001).

228 Shilev S, Benlloch M, Dios-Palomares R, Enrique D and Sancho ED,Phytoremediation of metal-contaminated soil for improving foodsafety, in Predictive Modelling and Risk Assesment, ed. by Costa Rand Kristbergsson K. Springer, Dordrecht, pp. 225–242 (2008).

229 Lambrecht M, Okon Y, Vande Broek A and Vanderleyden J, Indole-3-acetic acid: a reciprocal signalling molecule in bacteria–plantinteractions. Trends Microbiol 8:298–300 (2000).


13

66


230 Sharma A and Johri BN, Combat of iron-deprivation through a plantgrowth promoting fluorescent Pseudomonas strain GRP3A inmung bean. Microbiol Res 158:77–81 (2003).

231 Schalk IJ, Hennard C, Durgave L, Poole K, Abdallah MH and Pattus F,Free pyoverdine binds to its outer membrane receptor FpvA inPseudomonas aeruginosa: a new mechanism for membrane irontransport. Mol Microbiol 39:351–360 (2001).

232 Glick BR, The enhancement of plant growth by free-living bacteria.Can J Microbiol 41:109–117 (1995).

233 Glick BR, Penrose M and Li J, A model for the lowering of plantethylene concentrations by plant growth-promoting bacteria.J Theor Biol 190:63–65 (1998).

234 Penrose DM and Glick BR, Levels of 1-aminocyclopropane-1-carboxylic acid (ACC) in exudates and extracts of canola seedstreated with plant growth-promoting bacteria. Can J Microbiol47:368–372 (2001).

235 Duffy BK and Defago G, Environmental factors modulating antibioticand siderophore biosynthesis by Pseudomonas fluorescensbiocontrol strains. Appl Environ Microbiol 65:2429–2438 (1999).

236 International Organization for Standardization (ISO), Soil Quality,Guidance on the Determination of Background Values. ISO/DIS19258 (2005).

237 Gawlik BM and Bidoglio G, Background Values in European Soilsand Sewage Sludge. Part III. Conclusions, Comments and Re-commendations (European Commission, EUR 22265 EN). Office for

Official Publications of the European Communities, Luxembourg(2006).

238 Mathieu A, Baize D, Raoul C and Daniau C, Regional guidelines fortrace metal concentrations in soils: their use in health riskassessments. Environ Risques Sante 7:112–122 (2008).

239 Baize D and Sterckeman T, Of the necessity of knowledge of thenatural pedo-geochemical background content in the evaluationof the contamination of soils by trace elements. Sci Total Environ264:127–139 (2001).

240 Jensen J and Mesman M (eds), Ecological risk assessment ofcontaminated land. Decision support for site specific inves-tigations. Report Number 711701047, National Institute of PublicHealth and the Environment (RIVM), Bilthoven (2006).

241 Dıez M, Simon M, Martın F, Dorronsoro C, Garcıa I and Van Gestel CAM,Ambient trace element background concentrations in soils andtheir use in risk assessment. Sci Total Environ 407:4622–4632(2009).

242 EFSA Panel on Contaminants in the Food Chain, Scientific opinion:cadmium in food. EFSA J 980:1–139 (2009). [Online]. Available:http://www.nutfruit.org/UserFiles/Image/pdf/reg5 apr09.pdf [20April 2010].

243 Food Standards Agency, Measurement of the concentrationsof metals and other elements from the 2006 UK total dietstudy. [Online]. Available: http://www.food.gov.uk/multimedia/pdfs/fsis0109metals.pdf [21 April 2010].


safety of food crops on land contaminated with trace elements

Documents