5-ee.0 · sucrose is also metabolized via the oxidative pentose phos-phate pathway (29) to produce...

Plant Physiol. (1992) 100, 1-60032-0889/92/1 00/0001/06/$01 .00/0

_5-EE.0.IReceived for publication August 5, 1991

Accepted January 15, 1992

Anaerobic Metabolism in Plants'

Robert A. Kennedy*, Mary E. Rumpho, and Theodore C. Fox

Department of Botany and the Maryland Agricultural Experiment Station, University of Maryland,College Park, Maryland 20742

ABSTRACT

Exposure to oxygen deficits is more widespread in biologicalsystems than is commonly believed. Until recently, the generalperception of anaerobic metabolism was often limited to the in-duction of alcoholic or lactic acid fermentation as the sole bio-chemical response to hypoxia/anoxia. Developments in the physi-ology, biochemistry, and molecular biology of anaerobic responsesin invertebrates, lower plants, and higher plants have demonstratedthat, depending upon the species, anaerobic metabolism may en-compass much more than simple glycolytic metabolism. Here,recent progress in elucidating the mechanism(s) determining tol-erance versus intolerance to anaerobic environments in higherplants is discussed, drawing most heavily on experimental systemsusing seeds or seedlings.

Anaerobiosis is a universal biological phenomenon thatoccurs to some extent in all organisms. At one extreme,obligate anaerobes exist in a wide range of habitats, fromorganisms living in thermal vents to symbiotic nitrogen-fixingbacteria. Many organisms, including microbes, invertebrates,and vertebrates experience anoxia, with some species beinghighly adapted for prolonged survival under these conditions(9, 11).

Anaerobiosis in plants is even more common. The occur-

rence and consequences of oxygen deficits caused by waterare varied, ranging from the obvious example of crop lossesdue to seasonal flooding to the environmental impact causedby algal blooms, hypoxia, and drastic changes in the qualityof the nation's marine and fresh waters. Higher plants havean absolute requirement for oxygen to sustain metabolismand growth; most plant tissues will tolerate anoxia for onlyshort periods of time before irreversible damage occurs (10).At the same time, 72% of the earth's surface is covered withwater or has submerged soils or sediments that restrict oxygendiffusion (25). To combat this, aquatic species (plant andanimal) have evolved mechanisms to overcome the funda-mental problem during anoxia, the requirement for oxygenas a terminal electron acceptor during respiration. As a group,

1Supported by U.S. Department of Agriculture Competitive Re-search Grant No. 87-CRCR-1-2595 to R.A.K. and M.E.R., a HermanFrasch Foundation grant in Agricultural Chemistry to R.A.K., andthe University of Maryland Agricultural Experiment Station, Contri-bution No. 8387.

however, these organisms are among the most varied andleast understood, especially in their adaptive physiology.Although there has long been research interest in the area

of environmental stress, a marked increase in plant stressresearch has occurred in the last 5 to 10 years, both as aresult of improvements in our understanding of the under-lying biochemistry and especially because of new insight intoenvironmental effects on gene expression (see ref. 33 forreview). Numerous studies have been carried out using bothtolerant and intolerant seeds and seedlings in varying exper-imental conditions. The main objective of this review is topresent an integrated picture of recent developments in an-aerobiosis research with emphasis on metabolic adaptations.

ANAEROBIOSIS AND SEED GERMINATION

It is generally believed that anaerobiosis occurs to varyingdegrees in the seeds of many higher plants during imbibition,before rupture of the testa (2). During the early hours ofgermination, the seed coat is impermeable to oxygen; seedsrapidly generate high respiratory quotients and exhibit in-creased ADH2 activities and active alcoholic fermentation.Soon after germination, protrusion of the radicle and/or shootinduces aerobic respiration, and ADH activity declines tonegligible levels. In flooded or even waterlogged soils, theseeds of most plants do not germinate and, therefore, quicklylose viability (23). Most agricultural plants, including barley,maize, oats, sorghum, and wheat, tolerate even the mosttransient waterlogging poorly. Only six species of higherplants are known to germinate and grow under strict anoxia:the four Echinochloa species studied in our laboratory, rice(Oryza sativa), and the African legume, Erythina caffra (36).The most extensively studied of these species are rice andEchinochloa phyllopogon.

E. phyllopogon is a highly adapted rice field weed. InCalifornia rice culture, the seed bed (containing E. phyllopogonseeds) is leveled and flooded with 20 to 25 cm of water 2weeks before aerial seeding of the rice crop. Within a fewhours of flooding, the seed bed (soil/water interface and thefirst few millimeters of the paddy soil) becomes anaerobicdue to microbial activity (6). Because E. phyllopogon germi-nates below the soil/water interface, seed germination andthe first few days of seedling growth are restricted to an

2Abbreviations: ADH, alcohol dehydrogenase; LDH, lactate de-hydrogenase; PDC, pyruvate decarboxylase; TCA, tricarboxylic acid;ASPs, anaerobic stress proteins; HSPs, heat shock proteins.

1

Dow

nloaded from https://academ

ic.oup.com/plphys/article/100/1/1/6086899 by guest on 04 August 2021

Plant Physiol. Vol. 100, 1992

anaerobic environment. Our previous research (see refs. 15and 17 for review) has shown that this species is highlyadapted to this environment metabolically and that it pos-sesses a comprehensive and coordinated metabolism duringanaerobic growth (general scheme shown in Fig. 1). Theseadaptations appear to consist of most of the major aerobicrespiratory pathways in addition to unique anaerobic bio-chemical adaptations that may impart tolerance.

METABOLIC ADAPTATIONS TO FLOODING

Because of the importance and easily recognized symptomsof flooding stress in many important crop plants such aswheat, maize, and soybean, flooding tolerance has beenstudied scientifically for many years and has lead to a numberof theories or hypotheses to explain it. Two theories havebeen most important.

Crawford's Metabolic Theory for Flooding Tolerance

Flood tolerance in plants has classically been related to theamount of ethanol produced by a particular plant species andits presumed toxic effects on metabolism and cell structure.The first attempt to explain flood tolerance in comprehensivemetabolic terms was proposed by McManmon and Crawford(19) in their 'metabolic theory of flooding tolerance' in 1971.Simplified, this theory held that flood tolerance in plantsdepended upon decreased ethanol production due to lowADH activity, thereby reducing the presumed toxic effects ofethanol. Implicit in this theory was the ability of some tolerantplants to reroute glycolytic intermediates to alternate endproducts such as malate, lactate, or other organic acids.Although the major tenets of this hypothesis have since beenshown to be incorrect for the most part (5, 12, 28, 30, 37),this theory did include most of the factors (ethanol, ADHactivity, and shifts to alternate fermentative end products)that continue to be widely studied as factors important inflood tolerance. Regardless of the validity of the initial hy-potheses, the research Crawford and his colleagues con-ducted (see ref. 10 for summary) was a stimulus to researchin the general area of flood tolerance and especially in pro-moting the concept of unifying metabolic theories for partic-ular plant stresses.

Plants apparently have several mechanisms for dealingwith potentially toxic levels of ethanol. These include ventingof ethanol to the external (usually aquatic) medium and thetransport of ethanol from tissues in poorly aerated regions towell-aerated regions where it is metabolized (10, 30). Sometolerant species also delay or avoid accumulation of ethanolby diverting glycolytic intermediates to alternate end productssuch as lactate, malate, succinate, y-aminobutyrate, and ala-nine (10). The question of whether ethanol is toxic to plantsremains, and, regardless of whether venting or transport ofethanol occurs, many plants can accommodate ethanol con-centrations much higher than those found in nature (12, 30).

Davies-Roberts pH Stat Hypothesis

Another hypothesis to explain short-term flood tolerancein some plants was first proposed by Davies (5) and involves

v toOPPP

GLYCOLYSIS F-6-P a Nucleic

fr R5i , Acids

0-6-.P ~R5

cutellum/) tDNADP H OPPPLI '_--,- to FA

rYsucrose / Synthesis

ANAEROBIC

Figure 1. Major flow of carbon in seeds of E. phyllopogon germi-nated under nitrogen. The proposed integration of metabolic path-ways that operate in E. phyllopogon seedlings under anaerobicconditions is detailed. We have shown that all of the major aerobicrespiratory pathways function in this species during anoxia, inaddition to some apparently adaptive anaerobic biochemical reac-tions. Under anaerobic conditions, sucrose is presumed to be themajor form of carbohydrate transported to the shoots (coleoptileand primary leaf) (14). The sucrose is metabolized via glycolysis toproduce ethanol and much lesser amounts of lactate and malate(28). Sucrose is also metabolized via the oxidative pentose phos-phate pathway (29) to produce intermediates for nucleic acidsynthesis and reducing equivalents (NADPH). We have shown thatlipids are actively synthesized and accumulate under anaerobicconditions in E. phyllopogon (15). This activity could serve as amechanism for the reoxidation of NADPH produced by the oxida-tive pentose phosphate pathway, thus permitting cyclic metabo-lism. In this sense, lipids would serve as "alternate electron accep-tors" during anaerobic germination. Although our data indicate thatacetyl CoA is used for lipid synthesis, we also have evidence that itenters the TCA cycle. Mitochondrial activity, including both theTCA cycle (7) and electron transport (16), is present in the absenceof molecular oxygen. (Figure reprinted with permission of Chapman& Hall, publishers, London, England, Ecology and Management ofWetlands, TC Fox, RA Kennedy, AA Alani.) OPPP, Oxidative pen-tose phosphate pathway; OAA, oxaloacetate; AA, amino acid;FADH, reduced flavin adenine dinucleotide; Kg, a-ketoglutarate;EtOH, ethyl alcohol; F-6-P, fructose-6-phosphate; G-6-P, glucose-6-phosphate; X5P, xylulose-5-phosphate; R5P, ribose-5-phosphate;Ru5P, ribulose-5-phosphate; FA, fatty acid.

2 KENNEDY ET AL.

Dow



ANAEROBIC METABOLISM IN PLANTS

the tight regulation of the pH stat to prevent cytoplasmicacidosis. Under this hypothesis, the relative rate of synthesisof lactate versus ethanol depends upon the cytoplasmic pH.Under anaerobic conditions, pyruvate is initially convertedto lactate, but as the cytosolic pH decreases, LDH activity isinhibited, PDC activity is stimulated, and ethanol synthesispredominates. Recently, the plausibility of this scenario op-erating in some instances was convincingly demonstrated byRoberts (26) using noninvasive 31p and `3C NMR techniques.During the first 20 min of anoxia, the cytoplasmic pH inmaize root-tip cells was shown to decrease from 7.3 to 6.8before leveling off. Three factors may have prevented the pHfrom declining even further: transport of lactate into thevacuole, enhanced activity of PDC and ADH, and reducedactivity of LDH at lower pH values. At least in maize roots,the cytoplasmic pH has been shown (27) to be stable for upto 10 h after undergoing this type of metabolic shift. Duringlong periods of hypoxia, however, protons may leak fromthe vacuole, and the vacuolar pH increases as the cytoplasmicpH decreases (27). In maize and pea seeds, both relativelyflood-intolerant plants, death is correlated with the collapseof the vacuolar-cytoplasmic proton gradient (27). However,leakage of other ions such as Pi may also be important indetermining flood tolerance.Although shifts in anaerobic metabolism involving pH

changes have been shown to occur in several plant species(20, 27), this mechanism does not operate in all organs of thesame species (maize roots versus seeds, for example) andaffords resistance for only limited periods of time. Mostimportant, this hypothesis cannot serve as a unifying theoryfor flood tolerance because it does not appear to operate inthe same manner in plants that are truly flood tolerant, suchas rice and E. phyllopogon. In preliminary experiments, wefound that E. phyllopogon did not exhibit the kind of pHchanges that maize does (unpublished results, T.C. Fox,J.K.M. Roberts, and R.A. Kennedy). Whereas maize rootsexhibit a marked decrease in pH upon transfer to an hypoxicenvironment, the pH declined transiently in E. phyllopogonand then recovered. Using 31P NMR, Menegus et al. (20, 21)recently confirmed those results in a survey of several 're-sistant' and 'sensitive' plants. They concluded that not onlydid anoxia cause an overall acidification of the cell sap insensitive plants such as wheat and barley but an 'alkaliniza-tion' of the cytoplasm in resistant plants such as rice andEchinochloa occurred also. Furthermore, these workers con-cluded that resistance to anoxia Awas inversely correlatedwith the extent of cytoplasmic acidification.' Thus, it seemsclear that pronounced acidification of the cytoplasm doesoccur and that it is an early indicator of anaerobic stress,particularly in flood-sensitive plants. On the other hand,flood-tolerant plants seem to possess alternative mecha-nisms/responses beyond the initial transient changes in pHobserved.

GLYCOLYSISUntil the last 10 years, the prevailing view of anaerobic

metabolism was basically synonymous with glycolysis. Sincethen, because of our better understanding of the extensivemetabolism conducted by anaerobic-tolerant species such asrice and Echinochloa (15, 17), we now realize that anaerobic

metabolism involves far more than just straight-chain fer-mentative reactions. Nonetheless, research on glycolysisdominated early studies on flooding stress and continues tobe among the most studied aspect of flood-related research.

Glycolysis is a ubiquitous pathway that operates underboth aerobic and anaerobic conditions. If deprived of 02 foreven short periods of time, many plants accelerate glycolysisby exhibiting a Pasteur effect, but the adenylate energycharge is low (22). Most anaerobic-intolerant plants exhibit apronounced Pasteur effect (2). E. caffra, a tolerant species,also displays a Pasteur effect (36), whereas E. phyllopogonand rice do not (14). Thus, the occurrence of a Pasteur effectper se does not correlate with tolerance or intolerance toanoxia. The following glycolytic enzyme systems are amongthe most important and often studied.

1. PDC is perhaps at the most important branch point inanaerobic metabolism and catalyzes the decarboxylation ofpyruvate, yielding CO2 and acetaldehyde (precursor ofethanol). PDC is constitutive in many species and varioustissues. In maize, PDC activity increases 5- to 9-fold duringanoxia, whereas PDC mRNA is induced 20-fold (13). Themajor difference between maize PDC and yeast and bacteriaPDC is the addition of 44 predominately hydrophobic aminoacids to the amino terminus of the maize sequence (13). Thissequence is rich in alanine, an alternate product of PDCmetabolism, during anaerobiosis. Cellular levels of this aminoacid may specifically influence PDC translation in maize.

2. LDH catalyzes the formation of lactate from pyruvate,is present as multiple isozymes, and is anaerobically inducedin several species (10). Lactate fermentation frequently occursin anaerobic roots and germinating seeds, but only transientlyafter transfer from aerobic to anaerobic conditions (as dis-cussed above). Induced LDH activity supports continuedlactate fermentation, maintaining the redox balance withoutthe carbon loss associated with ethanol fermentation.

3. Without question, ADH is the most studied enzymerelating to anaerobiosis. In response to anaerobic stress, ADHactivity increases in most plants during anoxia, althoughexceptions do exist. For example, E. caffra (tolerant) (36) andGlycine max cv Prize (intolerant) (3) are both examples ofplants that do not increase ADH activity under low oxygenconcentrations. The G. max cv Williams, on the other hand,exhibits an increase in ADH activity in response to anoxia(8). Although ADH activity is apparently not required forgrowth of maize in air, ADH null mutants demonstrate thatADH activity is essential for extended survival of this speciesduring flooding (18, 34). Maize ADH- mutants survive onlya few hours, whereas normal seedlings survive 3 d of anoxia.Although the research with maize ADH- mutants indicates

that a minimum ADH activity is required to survive evenshort periods of flooding, our research on Echinochloa spp.indicates that tolerance does not correlate with the level ofactivity or the number ofADH isozymes (17). However, theremay be a relationship between ADH activity and its locationwithin the plant. We have found that flood-tolerant speciessuch as rice and E. phyllopogon have two-thirds of their ADHactivity in the shoot, whereas flood-intolerant species suchas maize and pea have two-thirds of their ADH activity inthe root (4). Under anaerobic conditions, allocation of seedreserves and fermentative energy production to shoots, which

3

Dow




elongate into an aerobic zone, would be energetically advan-tageous and may represent a flood tolerance 'strategy' inthose species. In contrast, mature maize leaves do not exhibitincreased ADH levels and cannot survive even short periodsof anoxia (6).

MITOCHONDRIA: TCA CYCLE

Mitochondria have always generally been assumed to beinoperative during anaerobiosis because the requirement for02 as a terminal electron acceptor in higher plants is absolute.Without 02, all pyridine nucleotides are presumed to bereduced, and normal cyclic metabolism stops. In support ofthis view, mitochondria have been reported to develop ab-normally under low or no 02 in a number of plants (38). Incontrast, mitochondria in E. phyllopogon grown in nitrogenare virtually indistinguishable from those grown in air (14),suggesting an unusual degree of stability and/or functioningduring anoxia.During anoxia, invertebrates commonly utilize a partial

TCA cycle (incomplete 'cycle') that is catalyzed by citratesynthase, aconitase, isocitrate dehydrogenase, 2-oxoglutaratedehydrogenase, and succinyl CoA synthase, and this anaer-obic sequence replaces the usual aerobic pathways in theseorganisms (9). It has long been recognized that succinateaccumulates in plants during anoxia (10, 20), as well as inyeast and various animals (9). Accumulation of malate orsuccinate (so-called 'alternate end products' [10]) permitsreoxidation of pyridine nucleotides. If malate synthesis occursby carboxylation of phosphoenolpyruvate via phosphoenol-pyruvate carboxykinase, additional ATP is formed. In hel-minths, these reactions yield ATP and oxaloacetate (9). NADis oxidized as oxaloacetate is reduced to malate. In turn,malate is metabolized to succinate by two routes. In thefavored pathway, malate is converted to fumarate and thensuccinate by fumarase and succinate dehydrogenase, respec-tively, resulting in the oxidation of flavin adenine dinucleo-tide. In these facultative anaerobes, metabolism of 1 mol ofglucose and glutamate yields a net gain of at least 3 mol ofATP and GTP. Coupling these substrate level phosphoryla-tions to glycolysis increases the level of high-energy phos-phate compounds. Although similar reactions also have beensuggested to occur in higher plants under anoxia (9), the roleof mitochondria to anaerobic metabolism in plants has beenexamined very little. In E. phyllopogon, TCA cycle interme-diates are labeled when exogenously applied ["4C]sucrose,['4C]glucose, and [14C]acetate are metabolized (30). In addi-tion, all TCA cycle enzymes are present in anaerobicallygrown E. phyllopogon seedlings, although the activity of someenzymes is lower than that in aerobically grown seedlings(7).

MITOCHONDRIA: ELECTRON TRANSPORT

In coleoptiles of anaerobically grown rice, mitochondriacontain the usual set of plant cytochromes, although theamount is two to three times less than in coleoptiles grownin air (35). Mitochondria from these 'anaerobic coleoptiles'oxidize TCA cycle substrates and NADH in vitro. In E.phyllopogon, mitochondria isolated from anaerobically grown

seedlings rapidly develop the capacity to reduce oxygen upontransfer to air (16). The mitochondria have the same comple-ment of cytochromes in both air- and nitrogen-grown seed-lings, including the unusual absorption maximum at A629.The Cyt absorption characteristics, the depletion of endoge-nous nitrate, TCA cycle activity, and the integrity of mito-chondria in anaerobically grown E. phyllopogon seedlingshave led us to assume that the mitochondria may play a veryactive and crucial role in the unusual ability of this organismto grow during anoxia.

PROTEIN METABOLISM

Polyribosomes are known to dissociate upon exposure toseveral different stresses including desiccation, heat, wound-ing, and anaerobiosis (1, 24). Accompanying the dissociationof polyribosomes under anoxia is a rapid decrease in proteinsynthesis and altered polypeptide profiles, as observed inmaize, pea, rice, and soybean (8). In the latter species, poly-somes rapidly reform when returned to air. Polyribosomedissociation is not due to degradation of mRNA, however,because mRNA encoding preexisting proteins are translatablein vitro after 5 h of hypoxia. Rather, changes in cytoplasmicpH associated with hypoxia in maize root tips alter theelongation efficiency of specific peptides (39). At pH 6.5, oneset of mRNAs is preferentially translated, which is distinctlydifferent from 'normal' protein synthesis at pH 7.5. Thestalled ribosome-mRNA complex is thought to retard degra-dation of the mRNA under conditions that are not conduciveto efficient translation. Moreover, hypoxia altered the quan-tity and electrophoretic mobility of ribosomal proteins (1),whereas a 31-kD protein was underphosphorylated relativeto aerobic controls. These changes in ribosome structure werereversed upon return to aerobic conditions. Apparently, thehypoxic response in maize entails some rearrangements inribosome structure that affect the selectivity and efficiency oftranslation. Although polyribosomes completely dissociate ina number of plant species during hypoxia, this does notappear to occur in the only two flood-tolerant species exam-ined, E. phyllopogon (our unpublished data) and rice (22),even under anoxic conditions.The gene response of flood-tolerant and -intolerant plants

to anoxia is distinctly different. In flood-sensitive plants suchas maize, anaerobiosis induces a general repression of genesexpressed under aerobic conditions and the synthesis of afew ASPs (the acronym ANP [anaerobic proteins] was origi-nally used by Sachs et al. [32]. However, for consistency withthe more well-known HSP system, we have adapted Nover's[24] convention of ASPs). The plants do not overcome thisrepression because normal protein synthesis never resumesand the plants die within 3 d unless aerobic conditions arerestored (32). Several ASPs have been identified in the maizesystem: ADH1 and ADH2, glucose-6-P isomerase, aldolase,sucrose synthase, cytosolic glyceraldehyde-3-phosphate de-hydrogenase, LDH, and PDC (see ref. 31 for discussion). Ofthe ASPs that have been identified, all but one (sucrosesynthase) are glycolytic enzymes, and all are involved in theearly carbohydrate catabolism that occurs in germinatingseeds.The response to anoxia in tolerant Echinochloa species,

4 KENNEDY ET AL.

Dow



ANAEROBIC METABOLISM IN PLANTS

however, is very different. After an initial period duringwhich ASPs are synthesized, aerobic protein synthesis re-sumes within 24 h after the onset of anoxic conditions (ourunpublished results). In E. phyllopogon, anaerobic polypeptidepatterns are surprisingly similar to those obtained underaerobic conditions. Several polypeptides are also labeledwhen rice seedlings are transferred to anaerobic conditions(22), but incorporation of radioactive amino acids into proteindecreases with time. During anoxia and during air/nitrogentransitions, proteins also turn over, but they eventually reachsteady-state levels. Despite the common native habitat of riceand E. phyllopogon and their extreme resistance to flooding,the overall polypeptide profiles of rice and Echinochloa differsignificantly when grown anaerobically, although several ofthe enhanced or induced proteins are similar.Although altered gene expression is induced by a number

of environmental stresses, most notably by heat stress withthe induction of specific HSPs, the ASPs expressed duringanaerobiosis are different from that produced by otherstresses (24). The only exception to this may be a 27- to 28-kD HSP induced in soybean during heat stress that alsoappears to be produced during anoxia.

SUMMARY AND FUTURE RESEARCHCONSIDERATIONS

In this review, we have considered the biochemical adap-tations of plants to anoxia. As discussed, low-oxygen stressis common among biological organisms, and in plants it hasbeen the subject of experimental studies since the early 1900s.However, stereotypical views of 'flooding stress' have per-sisted in the literature and classroom until the last few years,namely, that the metabolic response of plants to low/nooxygen only involves glycolysis, resulting in a toxic accumu-lation of ethanol.

In the past 5 to 10 years, there has been a confluence ofresearch in applying new molecular techniques to appliedproblems and a greatly increased scientific awareness ofenvironmental and stress physiology. Because of this, a tre-mendous amount of information concerning the responses)of plants to various stresses, including anoxia, has beengenerated. This information runs the gamut from descriptiveresponses of enzymes to anoxia to a sophisticated molecularunderstanding of gene induction and coordinated regulation.Yet, the majority of these studies have added only to ourunderstanding of a small segment of anaerobic metabolism,the classical reactions associated with glycolysis. Further-more, the most common experimental systems used to studyflood tolerance, such as maize, soybean, and pea, have anextremely limited tolerance to anoxia compared to manynaturally occurring flood-tolerant plant species. Thus, weknow more about the short-term response of certain agro-nomic plant species to flooding, but we are only slightlycloser to understanding the real mechanisms) for anaerobictolerance in nature.

LITERATURE CITED

1. Bailey-Serres J, Freeling M (1990) Hypoxic stress-inducedchanges in ribosomes of maize seedling roots. Plant Physiol94: 1237-1243

2. Bewley JD, Black M (1978) Physiology and Biochemistry ofSeeds, Vol 1. Springer-Verlag, New York

3. Brzrezinski R, Talbot BG, Brown D, Klimceszko D, BlakeleySD, Thirion JP (1986) Characterization of alcohol dehydro-genase in young soybean seedlings. Biochem Genet 24:643-656

4. Cobb BG, Kennedy RA (1987) Distribution of alcohol dehydro-genase in roots and shoots of rice (Oryza sativa) and Echinoch-loa seedlings. Plant Cell Environ 10: 633-638

5. Davies DD (1980) Anaerobic metabolism and production oforganic acids. In PK Stumpf, EE Conn, eds, The Biochemistryof Plants: A Comprehensive Treatise, Vol 2. Academic Press,New York, pp 581-611

6. Drew MC (1990) Sensing soil oxygen. Plant Cell Environ 13:681-693

7. Fox TC, Kennedy RA (1991) Mitochondrial enzymes in aerobi-cally and anaerobically germinated seedlings of Echinochloaand rice. Planta 184: 510-514

8. Ho T-HD, Sachs MM (1989) Stress-induced proteins: character-ization and the regulation of their synthesis. In A Marcus, ed,The Biochemistry of Plants, Vol 15. Academic Press, NewYork

9. Hochachka PW (1986) Defense strategies against hypoxia andhypothermia. Science 231: 234-241

10. Hook DD, Crawford RMM (1978) Plant Life in AnaerobicEnvironments. Ann Arbor Science, Ann Arbor, MI

11. Jackson MB, Davies DD, Lambers H (1991) Plant Life underOxygen Deprivation: Ecology, Physiology and Biochemistry.SPB Ac Pub, The Hague, The Netherlands

12. Jackson MB, Herman B, Goodenough A (1982) An examinationof the importance of ethanol in causing injury to floodedplants. Plant Cell Environ 8: 163-172

13. Kelley PM (1989) Maize pyruvate decarboxylase mRNA is in-duced anaerobically. Plant Mol Biol 13: 213-222

14. Kennedy RA, Barrett SCH, VanderZee D, Rumpho ME (1980)Germination and seedling growth under anaerobic conditionsin Echinochloa crus-galli (barnyard grass). Plant Cell Environ3: 243-248

15. Kennedy RA, Fox TC, Everard JD, Rumpho ME (1991) Bio-chemical adaptations to anoxia: potential role of mitochondrialmetabolism to flood tolerance in Echinochloa (barnyard grass).In MB Jackson, DD Davies, H Lambers, eds, Plant Life underOxygen Deprivation: Ecology, Physiology and Biochemistry.SPB Academic Publishing, The Hague, The Netherlands, pp217-227

16. Kennedy RA, Fox TC, Siedow JN (1987) Activities of isolatedmitochondria and mitochondrial enzymes from aerobicallyand anaerobically germinated barnyard grass (Echinochloa)seedlings. Plant Physiol 85: 474-480

17. Kennedy RA, Rumpho ME, Fox TC (1987) Germination phys-iology of rice and rice weeds: metabolic adaptation to anoxia.In RMM Crawford, ed, Plant Life in Aquatic and AmphibiousHabitats. Blackwell Press, Oxford, UK, pp 193-203

18. Lemke-Keyes CA, Sachs MM (1989) Anaerobic tolerant null: amutant that allows AdhI nulls to survive anaerobic treatment.J Hered 80: 316-319

19. McManmon M, Crawford RMM (1971) A metabolic theory offlooding tolerance: the significance of enzyme distribution andbehavior. New Phytol 70: 299-306

20. Menegus F, Cattaruzza L, Chersi A, Fronza G (1989) Differ-ences in the anaerobic lactate-succinate production and in thechanges of cell sap pH for plants with high and low resistanceto anoxia. Plant Physiol 90: 29-32

21. Menegus F, Cattaruzza L, Mattana M, Beffagna N, Ragg E(1991) Response to anoxia in rice and wheat seedlings. PlantPhysiol 95: 760-767

22. Mocquot B, Prat C, Mouches C, Pradet P (1981) Effect of anoxiaon energy charge and protein synthesis in rice embryo. PlantPhysiol 68: 636-640

23. Morinaga T (1926) Germination of seeds under water. Am J Bot13: 126-140

24. Nover L (1989) Anaerobic stress. In L Nover, D Neumann, K-D

5

Dow




Scharf, eds, Heat Shock and Other Stress Response Systemsof Plants. Springer-Verlag, New York, pp 89-90

25. Ponnamperuma FN (1972) The chemistry of submerged soils.Adv Agron 24: 29-95

26. Roberts JKM (1989) Cytoplasmic acidosis and flooding tolerancein crop plants. In DD Hook, ed, The Ecology and Manage-ment of Wetlands, Vol 1. Croom-Helm Press, London, UK,pp 392-397

27. Roberts JKM, Callis J, Jardetsky D, Walbot V, Freeling M(1984) Cytoplasmic acidosis as a determinant of floodingintolerance in plants. Proc Natl Acad Sci USA 81: 6029-6033

28. Rumpho ME, Kennedy RA (1981) Anaerobic metabolism ingerminating seeds of Echinochloa crus-galli (barnyard grass).Metabolite and enzyme studies. Plant Physiol 68: 165-168

29. Rumpho ME, Kennedy RA (1983) Activity of the pentosephosphate and glycolytic pathways during anaerobic germi-nation of Echinochloa crus-galli (barnyard grass) seeds. J ExpBot 34: 893-902

30. Rumpho ME, Kennedy RA (1983) Anaerobiosis in Echinochloacrus-galli (barnyard grass) seedlings. Intermediary metabolismand ethanol tolerance. Plant Physiol 72: 44-49

31. Sachs MM (1991) Molecular response to anoxic stress in maize.In MB Jackson, DD Davies, H Lambers, eds, Plant Life underOxygen Deprivation: Ecology, Physiology and Biochemistry.SPB Academic Publishing, The Hague, The Netherlands, pp129-139

32. Sachs MM, Freeling M, Okimoto R (1980) The anaerobicproteins of maize. Cell 20: 761-767

33. Sachs MM, Ho THD (1986) Alteration of gene expression duringenvironmental stress in plants. Annu Rev Plant Physiol 37:363-376

34. Schwartz D (1969) An example of gene fixation resulting fromselective advantage in suboptimal conditions. Am Nat 103:479-481

35. Shibaska M, Tsuji H (1991) Changes in the respiratory systemof submerged rice seedlings after exposure to air. In MBJackson, DD Davies, H Lambers, eds, Plant Life under OxygenDeprivation: Ecology, Physiology and Biochemistry. SPB Ac-ademic Publishing, The Hague, The Netherlands, pp 169-186

36. Small JGC, Potgieter GP, Botha C (1989) Anoxic seed germi-nation of Erythinia caffra: ethanol fermentation and responseto metabolic inhibitors. J Exp Bot 40: 375-381

37. Smith AM, ap Rees, T (1979) Pathways of carbohydrate fer-mentation in the roots of marsh plants. Planta 146: 327-334

38. Vartapetian BB, Andreeva IN, Kozlova GI (1976) The resist-ance to anoxia and the mitochondrial fine structure of riceseedlings. Protoplasma 88: 215-224

39. Webster C, Kim C-Y, Roberts JKM 1991 Elongation and ter-mination reactions of protein synthesis on maize root tippolyribosomes studied in a homologous cell-free system. PlantPhysiol 96: 418-425

KENNEDY ET AL.6

Dow



5-ee.0 · sucrose is also metabolized via the oxidative pentose phos-phate pathway (29) to produce...

Documents