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R E V I E W

Vera Go hre Uta PaszkowskiContribution of the arbuscular mycorrhizal symbiosis to heavy metal

phytoremediation

Received: 14 October 2005 / Accepted: 3 January 2006 / Published online: 23 March 2006 Springer-Verlag 2006

Abstract High concentrations of heavy metals (HM) inthe soil have detrimental effects on ecosystems and are arisk to human health as they can enter the food chain viaagricultural products or contaminated drinking water.

Phytoremediation, a sustainable and inexpensive tech-nology based on the removal of pollutants from theenvironment by plants, is becoming an increasinglyimportant objective in plant research. However, asphytoremediation is a slow process, improvement ofefficiency and thus increased stabilization or removal ofHMs from soils is an important goal. Arbuscularmycorrhizal (AM) fungi provide an attractive system toadvance plant-based environmental clean-up. Duringsymbiotic interaction the hyphal network functionallyextends the root system of their hosts. Thus, plants insymbiosis with AM fungi have the potential to take upHM from an enlarged soil volume. In this review, we

summarize current knowledge about the contribution ofthe AM symbiosis to phytoremediation of heavy metals.

Keywords Arbuscular mycorrhizal symbiosis Glomus Heavy metal Phytoremediation

Abbreviations AM: Arbuscular mycorrhizal HM: Heavy metals EC50: Effective concentrationreducing germination or hyphal growth to 50%

Introduction

Heavy metals are grouped into one category of 53 ele-ments with specific weight higher than 5 g/cm3 (Holl-eman and Wiberg 1985; Weast 1984). Trace elementssuch as Cu, Fe, Mn, Ni and Zn are essential for normalgrowth and development of plants. They are required innumerous enzyme catalyzed or redox reactions, in elec-tron transfer and have structural function in nucleic acidmetabolism (Zenk 1996). Conversely, metals like Cd, Pb,Hg, and As are not essential (Mertz 1981). HM occurmainly in terrestrial or aquatic ecosystems although theycan be also emitted into the atmosphere. For land plantsthe root is typically the organ, which is in immediatecontact with metal ions. Acquisition at the rootsoilinterface, partitioning throughout the plant organismand cellular homeostasis of essential HM must be wellcontrolled by the plant to avoid deficiency as well asexcess. Essential HM are taken up by specific uptakesystems, but at high concentrations they also can enterthe cell via non-specific transporters. Non-essential HMcan enter the root via passive diffusion, but also usinglow-affinity metal transporter with broad specificity(Hall and Williams 2003).

At high concentrations, HM interfere with essentialenzymatic activities by modifying protein structure or byreplacing a vital element resulting in deficiency symp-toms. The plasma membrane is particularly vulnerableto HM toxicity since membrane permeability and thusfunctionality can be affected by alterations of important

membrane intrinsic proteins such as H+-ATPases (Hall2002 and citations therein). Also the production ofreactive oxygen species leading to oxidative damage ofplant tissue occurs in response to elevated HM levels(Schu tzendu bel and Polle 2002). As a consequence tox-icity symptoms such as chlorosis, growth retardation,browning of roots, effects on both photosystems, cellcycle arrest and others can be observed. Plants haveevolved several mechanisms to maintain ion homeostasisunder elevated HM concentrations (Clemens 2001; Hall

V. Go hreDepartment of Molecular Biology and Plant Biology,University of Geneva, 30 Quai Ernest Ansermet,CH-1211 Geneva 4, Switzerland

U. Paszkowski (&)Department of Plant Biology, University of Geneva,30 Quai Ernest Ansermet, CH-1211 Geneva 4, SwitzerlandE-mail: uta.paszkowski@bioveg.unige.chTel.: +41-22-3793022Fax: +41-22-3793107

Planta (2006) 223: 11151122DOI 10.1007/s00425-006-0225-0

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2002, Fig. 1). These rely on circumventing the genera-tion of physiologically intolerable concentrations of HMat susceptible locations within the cell by regulatingacquisition, enrichment, trafficking and detoxification ofHM (Clemens 2001). Over the past years progress hasbeen made in identifying components of such mecha-nisms. Several genes encoding metal uptake transporterproteins such as Nramp3 (Thomine et al. 2000), proteinsinvolved in metal trafficking like RAN1 (Hirayama et al.

1999) or in transport to the vacuole e.g. AtHMA3(Gravot et al. 2004) have been described from plants(reviewed in Clemens 2001; Guerinot 2000; Hall andWilliams 2003; Himelblau and Amasino 2000). The ba-sic principles of detoxification mechanisms include theextracellular HM-chelation by root exudates and/orbinding of HM to the rhizodermal cell walls uptake ofHM avoiding. Active plant efflux systems control cyto-solic concentrations of HM. Intracellularly the plant cellproduces chelating agents such as phytochelatins andmetallothioneins, which have high-affinity HM bindingproperties. The resulting complex can finally be exported

from the cytoplasm across the tonoplast and becomesequestered inside the vacuole (Hall 2002, Fig. 1). Alsoother organelles are involved in storage. In plants Fe isstored bound to ferritin inside chloroplasts (Briat andLobreaux 1997). In Schizosaccharomyces pombe amember of the cation diffusion facilator (CDF) familyhas been identified that has a role in accumulation ofHM in the ER (Clemens et al. 2002). Proteins withsimilar function can be expected to exist in the ER ofplants.

Man-made soil contamination resulting from mining,industry, agriculture and military activities resulted inhigh local concentrations of HM. In uncontaminatedsoil concentrations vary in orders of magnitudes, but onaverage the concentrations are, e.g., Zn: 80 ppm, Cd:0.10.5 ppm and Pb: 15 ppm. However, in polluted soildramatically higher concentrations were found, e.g., Zn:>20,000 ppm, Cd: >14,000 ppm and Pb: >7,000 ppm(http://www.speclab.com/elements/). Since HM are notbiodegradable and may enter the food chain, they are along-term threat to both the environment and human

mechanisms present in the plant cell

me

cellwal

plant cell

Rhizodermis Cortex

Rhizodermis Cortex

eh

app

arb

Fig. 1 HM detoxificationmechanisms of plants and fungiin arbuscular mycorrhizasymbioses. 1 Chelating agentsare secreted that bind metals inthe soil, e.g. histidine andorganic acids from the plant,glomalin from the fungus. 2Binding of HM to cell wallcomponents in plants and fungi.3 The plasma membrane as aliving, selective barrier in plantsand fungi. 4 Specific and non-specific metal transporters andpores in the plasma membrane

of plants and fungi (active andpassive import). 5 Chelates inthe cytosol, e.g.,metallothioneins (plants andfungi), organic acids, aminoacids, and metal-specificchaperons (shown for plants,assumed for AM fungi). 6Export via specific or non-specific active or passivetransport from plant or fungalcells. 7 Sequestration of HM inthe vacuole of plant and fungalcells. 8 Transport of HM in thehyphae of the fungus. 9 Inarbuscules, metal export from

the fungus and import intoplant cells via active or passivetransport

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health (Jarup 2003). Conventional soil remediationpractices in the past have relied mainly on the excavationof the contaminated soil. However, physical displace-ment, transport and storage or alternatively soil washingare expensive procedures and leave a site behind devoidof any soil microflora. Furthermore translocation ofHM polluted soil instead of solving the problem rathershifts it to upcoming generations (Vidali 2001). In con-trast plants offer an inexpensive and sustainable on-siteapproach (Kra mer 2005; Peuke and Rennenberg 2005;Salt et al. 1998), which relates to the above-describedmechanisms for HM detoxification (Hall 2002 Fig. 1).There are two main strategies that use plants either tobind HM in the soil (phytostabilization) or to importand store HM in the plants above-ground tissues(phytoextraction).

Arbuscular mycorrhizal (AM) fungi occur in the soilof most ecosystems, including polluted soils. Byacquiring phosphate, micronutrients and water anddelivering a proportion to their hosts they enhance thenutritional state of their hosts. Similarly, HM are takenup via the fungal hyphae and can be transported to the

plant. Thus, in some cases mycorrhizal plants can showenhanced HM uptake and root-to-shoot transport(phytoextraction) while in other cases AM fungi con-tribute to HM immobilization within the soil (phyto-stabilization). The result of mycorrhizal colonization onclean-up of contaminated soils depends on the plantfungusHM combination and is influenced by soil con-ditions. The significance of AM fungi in soil remediationhas lately been recognized (reviewed in Gaur andAdholeya 2004; Khan 2005). In this present review, wesummarize current knowledge of the suitability of AMfungi for soil remediation, focusing on recent develop-ments and novel molecular aspects. Other important

factors affecting the rhizosphere and thus phytoremedi-ation e.g. pH and soil microbes such as bacteria andectomycorrhiza are not considered here.

AM fungi in heavy metal contamination

Arbuscular mycorrhizal fungi of the phylum of theGlomeromycota (Schu ssler et al. 2001) are a naturalconstituent of the soil of most ecosystems. They interactwith the roots of more than 80% of terrestrial plants andcan be considered functional extensions of plant rootsconsiderably enlarging the soil volume for nutrient up-

take (Harrison 1999). To use the AM symbiosis forphytoremediation, it is important to understand how thefungus itself and establishment of the symbiosis are af-fected by contaminated soils. Spores and pre-symbiotichyphae are generally sensitive to HM in the absence ofplants. EC50 values (effective concentration reducinggermination or hyphal growth to 50%) vary with thestrain, but overall negative effects at high HM concen-trations are observed (Shalaby 2003): spores from HM-polluted and unpolluted soils were isolated and theirgermination and subsequent hyphal growth were

assessed in vitro (monoxenical cultures) in the presenceof Zn, Pb and Cd. Germination and hyphal growth wereinhibited by HM in all cases. However, spores frompolluted soils were more tolerant to elevated concen-trations of each of the three HM than spores fromuncontaminated soils. This naturally occurring resis-tance is likely due to phenotypic plasticity rather thangenetic changes in the spores, because tolerance is lostafter one generation in the absence of HM (Shalaby2003). Enhanced tolerance to specific HM of fungi iso-lated from soils contaminated with Zn, Pb, Cd or Cu hasbeen observed frequently (reviewed in Gaur and Adho-leya 2004). Furthermore, independent studies examinedspore count and colonization efficiency of sewagesludge-treated sites and revealed that spores tolerant toincreased HM application readily colonize host rootsdespite low spore counts (Del Val et al. 1999; Jacquot-Plumey et al. 2001). It remains to be studied, if the sporecount decreases due to spore formation efficiency or dueto spore death in the soil. Examination of the effect ofZn, Pb and Cd on pre-symbiotic (e.g., spore germinationand hyphal extension), and symbiotic (e.g., extraradical

mycelial growth and sporulation) fungal life stages oftwo Glomus species showed Glomus intraradices to bemore tolerant to each of the metals than Glomus etu-nicatum (Pawlowska and Charvat 2004). Thus, tolerancevaries depending on the fungal genotype. Mixtures ofHM lead to synergistic or antagonistic interactions,increasing or decreasing toxicity of one metal alone. Forexample, addition of Zn was antagonistic to the toxicityof Pb and/or Cd on pre-symbiotic hyphal growth, whilePb and Cd acted synergistically (Shalaby 2003). In cur-rent phytoremediation practices, plants are generallyintroduced into soil without established symbioses. Thefact that AM fungi occur in contaminated soil makes

adapted, indigenous inocula a promising tool for soilremediation efforts. Remediation could be carried outon-site without moving large amounts of soil to decon-tamination facilities. If the soil has already been treatedand thus the mycorrhizal population is decreased ordestroyed, inoculation with HM-resistant fungi fromsimilar soils could speed up establishment of the sym-biosis and improve soil remediation.

Phytostabilization

Phytostabilization prevents spreading of HM into the

soil environment as well as their leakage from the soildue to erosion. Metal-tolerant plant species with exten-sive root systems and good soil cover prevent wind and/or water erosion of HM and therefore serve well forphytostabilization strategies. Immobilization of HMwithin the rhizosphere is accomplished by precipitationof HM within the soil, adsorption onto the root surfaceor uptake and accumulation within roots. Commerciallyavailable plant species include those with tolerance tosingle or multiple HM (reviewed in Wong 2003). AMfungi contribute to the immobilization of HM in the soil

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beyond the plant rhizosphere and thereby improvephytostabilization. The fungus employs strategies simi-lar to those of its host. Among these are immobilizationof metals by compounds secreted by the fungus, pre-cipitation in polyphosphate granules in the soil,adsorption to fungal cell walls, and chelation of metalsinside the fungus (Fig. 1, Gaur and Adholeya 2004).

Glomalin is an example of an insoluble glycoproteinthat is produced and released by AM fungi and bindsHM in the soil (Gonzalez-Chavez et al. 2004; Wrightand Upadhyaya 1996, 1998). Glomalin can be extractedfrom the soil together with significant amounts of boundHM. Gonzalez-Chavez et al. (2004) found that up to4.3 mg Cu, 0.08 mg Cd and 1.12 mg Pb per gram glo-malin could be extracted from polluted soils that hadbeen inoculated with laboratory cultures of AM fungi.In an in vitro experiment, Gigaspora rosea sequestered28 mg Cu per gram glomalin. Since there is a correlationbetween the amount of glomalin in the soil and theamount of HM bound, fungal strains with significantsecretion of glomalin should be more suitable for bio-stabilization efforts.

Binding of HM to chitin in the fungal cell wall re-duces its local concentration in the soil. Passiveadsorption to the hyphae leads to binding of up to0.5 mg Cd per mg dry biomass (Joner et al. 2000). Dueto the large surface area presented by fungi in the soilhyphal binding is an important sink for HM. Further-more, HM-tolerant fungi have a higher affinity for HM(24 times more than roots) and are therefore particu-larly suitable for fixing HM in the soil (Joner et al. 2000).

The uptake of Pb and its immobilization were foundto be higher in roots of mycorrhizal than non-mycor-rhizal plants (Chen et al. 2005). Enhanced influx of Pbinto plant roots is generally observed upon mycorrhizal

colonization. Furthermore, sequestration of Pb in theroots was found to be correlated with an increase in thenumber of fungal vesicles in highly colonized species.Similar to plant and fungal vacuoles, fungal vesicles maybe involved in storing toxic compounds and, thereby,could provide an additional detoxification mechanism.Enhanced metal tolerance of mycorrhizal plants hasbeen frequently observed (Gaur and Adholeya 2004).For example, the Glomus isolate Br1 obtained fromroots of Viola calaminaria grown on HM contaminatedsoil colonized maize, alfalfa, barley and V. calaminariaand allowed each plant species to complete their lifecycle on highly HM polluted soil. A common, not HM-

adapted, isolate ofG. intraradices also permitted growth,but to a lower extent, whereas non-colonized plants diedon the same soil (Hildebrandt et al. 1999). An explana-tion for this observation is offered by another study inmaize where it has been found that HM are selectivelyretained in the inner parenchyma cells coinciding withfungal structures (Kaldorf et al. 1999). Since hyphae ofHM tolerant AM fungi display a higher affinity to HMthan plant cells (Joner et al. 2000) HM possibly becomeimmobilized at or within the fungus. Specific accumu-lation of HM in colonized tissue may represent the

predominant mechanism of AM fungi-mediated intra-radical detoxification. This fungus-associated loss ofHM from plant tissue could also account for the reducedgene expression of HM-inducible plant genes uponmycorrhizal colonization (dilution effect Burleighet al. 2003; Ouziad et al. 2005). Similarly, in pea (Rivera-Becerril et al. 2002), clover (Medina et al. 2005) andribwort (Hutchinson et al. 2004) Cd is stabilized in theroot system of AM plants. Rivera-Becerril et al. (2002)suggested a mycorrhiza-buffering of Cd-stress, whichthe authors attributed to detoxification mechanismsdescribed earlier in this review. Para di et al. (2003) laterassigned this effect to changes in polyamine metabolism.These authors hypothesized that alterations in poly-amine content and ratio in AM plants lead to Cd tol-erance. Thus, AM symbioses seem to create a morebalanced environment that ultimately allows roots tocope with higher HM concentrations possibly byenriching HM at or in fungal structures. On the fungalside induction of oxidative stress-related genes is ob-served in extraradical mycelium upon HM stress (Ouz-iad et al. 2005). It remains to be determined whether or

not the fungus mounts a protection to oxidative stressalso within the intraradical structures.

In clover and maize, increased retention of Zn in theroots of AM plants has been reported in several studies(Chen et al. 2001, 2003; Zhu et al. 2001). However, un-der Zn limitation, mobilization of Zn and transfer to theshoot is improved by the AM symbiosis (Chen et al.2003), reflecting the role of Zn as a micronutrient andthe beneficial role of the symbiosis on nutrient supply.

These examples illustrate mechanisms by which AMfungi immobilize HM within the soil or within roots andreflect the suitability of AM fungi for phytostabilizationapplications (Fig. 2). However, it must be emphasized

that the system is complex and general predictions aredifficult to be made. For example, each contaminatedsite has a specific profile of pollutants for which anappropriate combination of fungal and plant genotypesmust be established. Second, other interactions takeplace in the soil that could positively or negatively affectthe expected efficiency of HM stabilization. However, insummary it can be concluded that indigenous fungi fromcontaminated soils are most suitable for phytostabili-zation.

Phytoextraction

Phytoextraction is a rather recent technology and rep-resents the most effective and hence attractive strategy toclean up contaminated soils (Kra mer, 2005). It relies onplants with high root-to-shoot transfer accumulatinghigh amounts of HM in their above-ground parts.Alternatively it employs plants producing high biomasswith normal concentrations of HM (Fig. 2, Salt et al.1998). Consequently, HM become removed togetherwith the plant upon harvest and can be either recaptured(phytomining), used to produce energy by combustion

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or stored as low-volume dried material (Kra mer 2005;

Peuke and Rennenberg 2005). However, the soil clean-up is slow requiring many years to decrease soil con-tamination by half (McGrath and Zhao 2003). Bio-availability of HM in the soil represents one of the majorconstraints for rapid phytoremediation. The efficiency ofphytoextraction depends on biomass production by theplants and their metal tolerance. Naturally occurringhyperaccumulators can enrich HM by 100- to 1,000-foldrelative to non-accumulators without displaying toxicitysymptoms (Peuke and Rennenberg 2005 and citationstherein). However, the majority of these species producemodest amounts of biomass. Brake fern (Pteris vittata)is one plant species suitable for phytoextraction and one

variety has been used commercially for As removal fromcontaminated sites (edenfern; http://www.eden-space.com/index.html). Possible ways to accelerate theclean-up process even in non-hyperaccumulators includethe addition of chelates resulting in induced HMaccumulation by plants (Salt et al. 1998). Differentsynthetic chelates show selective high affinity to certainHM, e.g. EDTA (ethylenediamine tetraacetate) to Pb,enabling chelate-assisted phytoremediation to soilscontaminated by different HM. Improved soil remedia-tion can also be achieved by increasing HM acquisition

by root cells coupled to boosting transport to the shootand detoxification within plant cells (Kra mer 2005 andcitations therein).

It was discovered recently that addition of AM fungifurther enhances the uptake and accumulation of As inP. vittata (Leung et al. 2006). At the highest As con-centration tested (100 mg per kg soil), non-colonizedplants accumulated 60.4 mg As per kg while plantscolonized by AM fungi isolated from an As mineaccumulated 88.1 g As per kg. This was accompanied byenhanced growth, possibly due to improved phosphate(Pi) nutrition reaching 36.3 mg per pot in non-colonizedand 257 mg per pot in colonized plants. Both effectscombined allow for higher recovery of HM.

Berkheya coddii belongs to the Asteracea family andlike P. vittata is also recognized as a phytoextractioncrop. It is used for phytomining, i.e. the recovery ofmetals from plant tissues (Salt et al. 1998). The biomassof this Ni-hyperaccumulator was found to be at leasttwice as high in plants colonized by adapted AMfungi than in non-mycorrhizal controls. Furthermore,mycorrhizal plants accumulated 30% more Ni (Turnau

and Mesjasz- Przybylowicz 2003). Commercial effortshave already been made towards phytomining of Ni,since this technology is less expensive than classicalextraction methods. It would be worthwhile to combineclean-up and recovery assays to minimize costs andmaximize recyling of HM. Non-hyperaccumulators canalso be used for phytoextraction if they are sufficientlytolerant to HM and produce high biomass. Mycorrhizaltomato plants, for example, had at least 30% higher rootand shoot biomass than non-colonized controls at Asconcentrations up to 75 mg As per kg soil, which coin-cided with higher Pi uptake (Liu et al., 2005). No effectwas observed at higher concentrations. Elevated shoot

As concentrations paralleled the increase in biomass. Amaximum of 39% (As in shoot/total As) was reached at75 mg As per kg soil in colonized plants. However, itshould be noted that, in general, As was accumulatedpreferentially in the roots.

In thyme (Thymus polytrichus), enhanced biomasswas linked to elevated Pi acquisition and Zn concen-tration within the shoots. This effect was more pro-nounced in plant cuttings exhibiting low Zn tolerance:15 mg as against 210.6 mg dry weight (dw) of controland colonized plants, respectively, which correspondedto 140 and 210 mg Zn per kg dw (Whitfield et al. 2004).

As mentioned previously, addition of chelating

agents enhances the bioavailability of HM and thus theefficiency of phytoextraction in the absence of the AMsymbiosis (Salt et al. 1998). Application of EDTA (orEDDS, ethylene-diaminedisuccinate) had no negativeeffect on the infectivity of AM fungi (Grcman et al.2003). Using maize as a model plant Chen et al. (2004)found that addition of EDTA led to phytotoxic con-centrations of Zn in the plant reflected by growthretardation. Colonization by AM fungi diminished thephytotoxic effect of higher Zn levels and thereby con-tributed overall to increased mobilization of Zn from the

phytoextraction

enhancedbiomass

immobilizationin rhizosphere

accumulationin shoot

addition of AMF

phytostabilization

Fig. 2 Contribution of AM fungi to phytoremediation of HM.Top: Non-mycorrhizal plant in HM-polluted soil. Left panelImproved stabilization of HM in soil upon mycorrhizal coloniza-tion; favored for phytostabilization. Right panel Enhanced uptakeand transfer of HM to the shoot (left plant) and increased biomassof plants resulting from AM-fungi enhanced nutrition leading toincreased removal of HM from soil (right plant); beneficial forphytoextraction. (red dots HM in the soil; orange or red plant orfungal parts: low and high concentrations of HM, respectively)

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induction of HM transporters during the symbiosiscould improve translocation to the plant. Powerfulcandidates are promoters of the symbiotic Pi trans-porters. These are available from solanaceous (Rauschet al. 2001), leguminous (Harrison et al. 2002) andpoaceous (Paszkowski et al. 2002) plant species. Thecombination of enhanced uptake with enhanced root-to-shoot transport is particularly promising for phytoex-traction strategies (as previously already mentioned forAs) and could involve homologs of genes such as AtH-MA4 (Verret et al. 2004). Also an appropriate choice ofthe plant system is critical. Despite important informa-tion gained from the model plant Arabidopsis thaliana, itis not a host of AM fungi and hence is not suitable inthis context. Tomato, bean and maize, for example, arepromising non-hyperaccumulators previously used inHM stress experiments (Guo et al. 1996; Liu et al. 2005)that can be transformed and are hosts of AM fungi. Thegenomes of rice and poplar (both hosts of AM fungi)have been completely sequenced and both offer micro-array platforms to identify candidate genes, for example,for HM uptake in the presence or the absence of AM

fungi. Furthermore, poplar is already being used inphytoremediation practices because it (a) producesextensive root systems and above-ground biomass, (b)grows rapidly and has an efficient rootshoot transportdue to intense water uptake and transpiration rate, and(c) is transformable (Kra mer 2005; Peuke and Rennen-berg 2005). Since AM fungi can compensate for ineffi-cient plant nutrient and HM uptake, they should beintegrated into the design of future soil clean-up strate-gies with, for example, poplar. Studies of contaminatedsites provide fungal isolates highly suitable for phyto-remediation. Also, mechanistic data are being obtainedin artificial laboratory experiments that might help the

design of new strategies. However, such data must bevalidated in field experiments.

Conclusions

A vast amount of literature is available on the effects ofmycorrhizal colonization on plants under HM stress.Until recently, contradictory observations and widevariation in results were reported and are reflected intwo recent reviews (Gaur and Adholeya 2004; Khan2005). It was, therefore, challenging to try to draw

general conclusions about the usefulness of AM fungifor soil remediation. However, enhanced understandingof mycorrhizal biology and of the HM tolerance ofplants and fungi has defined valuable parameters forimproving phytoremediation. One example is the use ofadapted indigenous fungal strains that are more suitablefor phytostabilization and extraction purposes thanlaboratory strains. As a result, the number of mycor-rhizal publications reporting the successful employmentof AM fungi for these two phytoremediation strategieshas multiplied over the last 5 years and can be expected

to increase further. It was our intention with this reviewto draw attention to research illustrating the utility ofAM fungi in soil remediation (schematically summarizedin Fig. 2). It will be important in future to include theAM symbiosis in the design of both research plans andapplications, with the ultimate goal of increasing theefficiency of phytoremediation.

Acknowledgements We apologize to all those researchers whose

work we overlooked or could not include because of page limita-tions. We are grateful to Patrick King for critical reading of themanuscript.

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