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Photorespiration

Photorespiration

Photorespiration

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Photorespiration:currentstatusandapproachesformetabolicengineering

VeronicaGMaurino1andChristophPeterhansel2

Photorespirationresultsfromtheoxygenasereactioncatalysedbyribulose-1,5-bisphosphatecarboxylase/oxygenaseandservesasacarbonrecoverysystem.Itcomprisesenzymaticreactionsdistributedinchloroplasts,peroxisomesand

mitochondria.Therecentdiscoveryofacytosolicbypassandtherequirementofcomplexformationbetweensome

photorespiratoryproteinsaddedadditionallevelsofcomplexitytotheknownpathway.Photorespirationmayhaveevolvedinboth,C3andC4plants,topreventanaccumulationoftoxiclevelsofglycolate.Moreover,itissuggestedthat

photorespirationevolvedincyanobacteriabeforetheoriginofchloroplasts.Syntheticdetours,reminiscentofsecondaryphotorespiratorypathwaysnaturallyoccurringin

cyanobacteria,wereinstalledinArabidopsisthalianatobypassphotorespiration.AnenrichmentofCO2inthechloroplastandpositiveeffectsonplantgrowthraisedthequestionwhythesepathwayshavebeenlostfromhigherplants.

Addresses1

¨tzuKo¨ln,Zu¨lpicherStr.47b,50674BotanischesInstitut,Universita

Cologne,Germany2

¨tHannover,Herrenha¨userstr.2,¨rBotanik,Leibniz-UniversitaInstitutfu

30419Hannover,Germany

Correspondingauthor:Maurino,VeronicaG(v.maurino@uni-koeln.de)andPeterhansel,Christoph(cp@botanik.uni-hannover.de)

CO2/O2ratiointhechloroplastandtheCO2/O2speci cityfactorofRubisCO(V=VC/VO=VC/KCÂKO/VOÂ[CO2]/[O2],whereKCandKOrepresenttheMichaelisMentenconstantsforCO2andO2,andVCandVOthemaximalvelocitiesforcarboxylationandoxygenation),whichindicatesthepreferenceoftheenzymeforCO2overO2andvariesbetweenspecies.ThephotorespiratoryC2cycleservesasacarbonrecoverysystemconverting2-PGto3-PGAthatcanre-enterthereductivecycle.Thispathwayiscoordinatedbetweenfourcellularcompartmentsandusesamultitudeofenzymesandtransportprocesses(Figure1).

CurrentstatusofthephotorespiratorypathwayinthemodelplantArabidopsisthaliana

Chloroplasticproductionofglycolateanditsperoxisomalconversionintoglycine

CurrentOpinioninPlantBiology2010,13:249–256ThisreviewcomesfromathemedissueonPhysiologyandmetabolism

EditedbyUweSonnewaldandWolfB.FrommerAvailableonline23rdFebruary20101369-5266/$–seefrontmatter

#2010ElsevierLtd.Allrightsreserved.DOI10.1016/j.pbi.2010.01.006

First,2-PGisdephosphorylatedinthechloroplaststhrough2-phosphoglycolatephosphatase(PGLP)andtheglycolateproducedistransportedtoperoxisomesforfurthermetabolization(Figure1).A.thaliana(Arabi-dopsis)possessestwogenesencodingactivePGLPs,butonlyPGLP1(At5g36700)participatesinphotorespiraton[1].Knock-outmutantsofthisgenehaveverylowleafPGLPactivityandcannotsurviveinnormalairbutgrowwellinaCO2-enrichedenvironmentwherephotorespira-tionislow[1].

Intheperoxisomes,glycolateoxidase(GO)catalysestheoxidationofglycolateintoequimolaramountsofglyox-ylateandH2O2.Catalase(CAT)degradesH2O2andglutamate:glyoxylateaminotransferase(GGAT)transa-minatesglyoxylateintoglycine,whichisfurthertrans-portedtothemitochondria(Figure1).GO-suppressedriceshowedthetypicalconditionallethalhigh-CO2-requiringphenotype[2].Moreover,GOwasfoundtoexertastrongregulationoverphotosynthesis,possiblethroughafeedbackinhibitiononRubisCOactivase[2].Arabidopsishastwogenesencodingperoxisomalgluta-mate:glyoxylateaminotransferase(GGAT1,At1g23310andGGAT2,At1g70580)showntoparticipateinphoto-respiration[3,4].Knock-outmutantsofGGAT1showedaweakresidualGGATactivity,repressedgrowthinnormalairandhighlightintensity,butnormalgrowthatelevatedCO2conditions[4].Thispartialphotorespiratorypheno-typesuggeststhatboth,GGAT1andGGAT2,maycontributetophotorespiration.However,GGAT1ishighlyexpressedinbothleavesandroots,indicatingthatitsfunctionisnotrestrictedtophotorespiration.

CurrentOpinioninPlantBiology2010,13:249–256

Introduction

Plantphotosyntheticcarbonmetabolismiscomposedoftwoconnectedpathways:thereductivephotosyntheticcarbonmetabolism,alsoknownastheC3orCalvincycle,andtheoxidativephotosyntheticcarbonmetab-olism,alsoknownastheC2cycleorphotorespiratorypathway(Figure1).Bothcyclesareinitiatedbytheactionofribulose-1,5-bisphosphatecarboxylase/oxygenase(RubisCO)onribulose-1,5-bisphosphate(RubP).Car-boxylationofRubPyields3-phosphoglycerate(3-PGA),whileitsoxygenationleadstothesynthesisofonemol-eculeof3-PGAandonemoleculeof2-phosphoglycolate(2-PG).The3-PGA/2-PGratioisdeterminedbythe

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