Photorespiration

内容需要下载文档才能查看 内容需要下载文档才能查看

Available online at http://wendang.chazidian.com

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

http://wendang.chazidian.com

250Physiologyandmetabolism

Figure

1

Thephotorespiratorycarbonandnitrogencyclecomprisesenzymesandtransportersdistributedbetweenchloroplasts,peroxisomes,mitochondriaandcytosol.DiT1andDiT2:dicarboxylatetransporter1and2,aretheonlytransportersidentifiedatthegenomiclevel[33,34].CAT:catalase;GDC:glycinedecarboxylase;GGAT:glutamate:glyoxylateaminotransferase;GLYK:glyceratekinase;GO:glycolateoxidase;GOGAT:

glutamate:oxoglutarateaminotransferase;GS:glutaminesynthetase;HPR1:peroxisomalhydroxypyruvatereductase;HPR2:cytosolic

hydroxypyruvatereductase;PGP:phosphoglycolatephosphatase;RubisCO:ribulose-1,5-bisphosphatecarboxylase/oxygenase;RubP:ribulose-1,5-bisphosphate;SGAT:serine-glutamateaminotransferase;SHMT:serinehydroxymethyltransferase;THF:tetrahydrofolate;5,10-CH2-THF:5,10-methylene-THF;3-PGA:3-phosphoglycerate.Thedashedlinerepresentsmanyenzymaticsteps.

Mitochondrialconversionofglycineintoserineandtheneedforammoniare-assimilation

Inthemitochondria,glycineisdecarboxylatedanddea-minatedbytheglycinedecarboxylasecomplex(GDC)yieldingCO2,NH4+,NADHand5,10-methylenetetra-hydrofolate.Thelatterisusedbyserinehydroxymethyl-transferase(SHMT)tosynthesizeserinebytransferringtheactivatedC1unitontoanothermoleculeofglycine(Figure1).

GDCisahetero-tetramerandArabidopsispossessestwogeneseachencodingforP(At4g33010andAt2g26080)

CurrentOpinioninPlantBiology2010,13:249–256

andL(At3g17240andAt1g48030)proteins,threegenesforH(At2g35370,At2g35120andAt1g32470)proteinsandonegenefortheT(At1g11860)protein[5].OnlyfewT-DNA-taggedmutantsinthesegeneswereanalyseduntilpresent.Recentstudiesshowedthatindividualknock-outsoftheP-proteingenesgrownormally[6].Incontrast,thecombinedknock-outofbothgenesislethalevenatnon-photorespiratoryconditionsprovid-ingevidencethattheGDCreactioncannotbebypassed[6].GDCseemstobeindispensableatleastforone-carbonmetabolism(Figure2),byrecyclingglycineorigi-natedfromextramitochondrialSHMT.InArabidopsis,

http://wendang.chazidian.com

PhotorespirationMaurinoandPeterhansel251

Figure

2

SimplifiedschemeillustratingtheconnectionofC1-metabolismtothephotorespiratorycarboncycle.

At4g37930encodesthephotorespiratorySHMT1asknock-outmutantsinthisSHMTgenearelethalatambientCO2levelsbutgrownormallyathighCO2[7].InthecourseoftheGDCreaction,one-quarteroftheboundcarbonislostasphotorespiratoryCO2andanequimolaramountofNH4+isreleased,whichneedstobere-assimilatedthroughtheglutaminesynthetase(GS)/ferredoxin-dependentglutamate:oxoglutarateamino-transferase(Fd-GOGAT)cycle[8].Therelevanceofthisnitrogenre-assimilationpathwaywasrecentlycon?rmedasmutantsinFd-GOGAT(At5g04140)displayedatypicalphotorespiratoryphenotype[9 ].

Peroxisomalproductionofglycerateanditschloroplasticphosphorylationinto3-PGA

inArabidopsis(At2g13360)anditsdisruptionresultsinplantsthatareunviableundernormalatmosphericcon-ditions[10].Onthecontrary,disruptionofHPR1(At1g68010)causednegligibleeffectsongrowthinnor-malairindicatingtheexistenceofanalternativereaction[11 ](seebelow).

Thetransportofglyceratetothechloroplastsanditsphosphorylationto3-PGAbyD-glycerate3-kinase(GLYK)completesthephotorespiratorypathway(Figure1).InArabidopsis,GLYKisencodedbyasingle-copygene(At1g80380).Lossofitsfunctionresultsinplantsthatarenotviableinnormalair[12].PlantGLYKsrepresentanovelproteinfamilyastheyarestructurallyandphylogeneticallydifferentfromknownGLYKsfromnon-photosyntheticorganisms[12].

Alternativephotorespiratoryreactions

Glycinegeneratedinthemitochondriaistransportedtotheperoxisomeswhereitisconvertedtoglyceratethroughthesequentialactionofserine:glyoxylateamino-transferase(SGAT)andhydroxypyruvatereductase(HPR1;Figure1).SGATisencodedbyasinglegene

http://wendang.chazidian.com

Inarecentpublication,Timmetal.[11 ]presentedbiochemicalandgeneticevidencethatHPR2(At1g79870)isthecytosolicenzymethatprovidesa

CurrentOpinioninPlantBiology2010,13:249–256

252Physiologyandmetabolism

bypasstotheperoxisomalconversionofglycineintoglycerate(Figure1).Thisreactionservesasamechanismfortheutilizationofhydroxypyruvateleakingfromtheperoxisomesandextendsthephotorespiratorypathwaytothecytosoliccompartment.Inlinewiththis,combineddeletionoftheperoxisomalandcytosolicreactionsisdetrimentaltoair-grownmutants[11 ].

Mostgreenalgaeconvertglycolatetoglycerateinthemitochondrium[13].Knock-outofglycolatedehydrogen-ase(GlyDH),the?rstenzymeinthispathway,inChla-mydomonasresultsinahigh-CO2-requiringphenotype[14].Ithasbeensuggestedthatthismitochondrialpath-wayisconservedinhigherplants,becauseanArabidopsisproteinwithhomologytoChlamydomonasGlyDHistargetedtomitochondria[15]andinsertionmutantsshowedalteredphotorespiratorypropertiesalbeitnotbeingaffectedingrowth[16].ThisviewhasbeenrecentlychallengedbycharacterizationoftheArabidopsishomologenzyme[17 ],whichshowsamuchhigherpre-ferenceforD-lactatecomparedtoglycolatebecauseofanextremelylowcatalyticratewiththelattersubstrate.Moreover,thisenzymewasshowntoparticipateinplantainthemethylglyoxalpathway[17 ],suggestingthattheancientphotorespiratorypathwayhasbeenre-directedtoanewfunctioninhigherplants.

Complexregulationofthemitochondrialreactionsofphotorespiration

occursunderhighlight,droughtandsalt-stressandCO2-freeair[19,20].Thistransferofreducingequivalentsmightalsobeimportantfornitrateassimilation[21].Moreover,photorespirationwouldbeimportantforavoid-ingthesuppressionoftherepairofphotodamagedphoto-systemII,asdifferentphotorespiratorymutantsofArabidopsisshowedinhibitionofthesynthesisoftheD1proteinattheleveloftranslation[22].Finally,throughH2O2productionandpyridinenucleotideinteractions,photorespirationmakesakeycontributiontocellularredoxhomeostasis[23].

Photorespirationisrequiredinallphotosyntheticorganisms

ThedualfunctionofRubisCOiscommontoallphoto-syntheticorganisms[24].Itisthereforeplausibletoassumethatallphotosyntheticorganismsalsorequireapathwayforphosphoglycolateconversion.Accordingtothis,genesencodingphotorespiratoryenzymesarefoundthroughoutthegreenlineage[13,25].However,cyano-bacteria,algaeandhigherplantsdevelopedmechanismsfortheconcentrationofCO2aroundRubisCOresultinginanef?cientsuppressionofphosphoglycolateproduction(Figure3).Itwasassumedthattheseorganismsdonotstrictlyrequirephotorespiration.Tworecentpapershavenowfalsi?edthishypothesis.Inthe?rststudy,Zelitchetal.[26 ]isolatedamaizemutantwithatransposonintegrationintheGO1genethatencodesthemajorglycolateoxidaseenzymeinmaize.MaizeisaC4plantand,thus,largelysuppressesphotorespirationbypump-ingCO2intothevicinityofRubisCO([Figure3]).Unex-pectedly,homozygousmutantplantswerenotcapableofsurvivinginnormalairandrequiredCO2enrichmentfornormalgrowth.Furthermore,glycolateaccumulatedintheseplantsandphotosynthesiswassuppressedbyhighoxygenconcentrations.AllthesefeaturesarereminiscentofphotorespiratorymutantsinC3plants[27].ThelowratesofRubisCOoxygenaseactivityinC4plantssee-minglyproduceenoughphosphoglycolatetoinhibitphotosynthesiswhenaccumulating.Thisoftenmen-tioned,butpoorlystudiedtoxiceffectofphosphoglyco-lateandotherphotorespiratorymetabolitesonphotosynthesisseemstobeequallyimportantforgrowthinhibitionunderconditionsofhighoxygenaseactivityasthelossofCO2orNH4+duringlaterstepsofthephoto-respiratorypathway.

Inthesecondstudy,photorespiratorymutantsofthecyanobacteriumSynechocystiswereanalysed[28 ].Analo-goustothesituationinmaize,oxygen?xationbyRubisCOisalsolowincyanobacteriabecauseofanef?cientCO2-concentratingmechanism.Moreover,atleastsomecyanobacteriaarecapableofexcretingexcessglycolate[29].Despiteofthelowratesofphosphoglyco-latesynthesis,Synechocystisestablishedthreedifferentroutesforthemetabolismofthiscompound(Figure4):onepathwayremindsofthemetabolismthatbacteriause

http://wendang.chazidian.com

Recently,Jamaietal.[9 ]reportedthatFd-GOGAT,acomponentofthephotorespiratorynitrogenassimilationcycleinthechloroplast,isdualtargetedtothemitochon-dria.Here,theproteinseeminglydoesnotexertanenzymaticfunction,butratherassociateswithSHMT1.Furthermore,two10-formylTHFdeformylases(10-FDF;At4g17360andAt5g47435),componentsofthemitochondrialTHFcycle,wereshowntobeessentialforphotorespirationastheyopposetheaccumulationof5-formylTHF([Figure2])[18 ].ThiscompoundissynthesizedbySHMTinasidereactionusing5,10-methenyl-THFassubstrateandisastronginhibitoroftheSHMTactivity.ItwasspeculatedthattheroleofFd-GOGATboundtoSHMT1mightbetoreducethesensitivityofSHMT1to5-formylTHFortoinhibititsaccumulation[9 ].

Whyphotorespiration?

Photorespirationlowersphotosyntheticef?ciencyinthatCO2andammoniashouldbere-assimilatedwiththeconcomitantconsumptionofbothATPandreducingpower[19].Sowhyphotorespiration?AlthoughthemainfunctionofphotorespirationwouldbetherecoveryofcarbondivertedbytheoxygenaseactivityofRubisCOsomeotherfunctionshavealsobeenproposed.Bytrans-portingexcessreducingequivalentsfromthechloroplastphotorespirationmaybeamechanismforpreventingtheoverreductionofthestroma,andthusphotoinhibition,as

CurrentOpinioninPlantBiology2010,13:249–256

PhotorespirationMaurinoandPeterhansel253

Figure

3

CCMincyanobacteria,algaeandhigherplants.

togrowonglycolateasacarbonsource,thesecondresemblesthephotorespiratorypathwayfoundinhigherplants,andthethirdinvolvesthecompleteoxidationofglycolatetoCO2.Whereasmutantsinsinglepathwayswereimpairedingrowth,onlyknock-outofallthreepath-waysforglycolateconversionresultedinaconditionallethalphenotype.Thesedataareimportantfortworeasons:First,theysuggestthatthephotorespiratorypath-waymighthaveevolvedbeforetheendosymbiosisofcyanobacteriaformingthe?rstphotosyntheticeukaryotes.Second,acompleteinterruptionofphosphoglycolatemetabolismisprobablylethalforalltestedphotosyntheticorganismsundernormalgrowthconditionsindependentoftheamountsofphosphoglycolateformed.

Manipulationofphotorespirationandgrowth

Intheory,anyreductioninphotorespirationshouldenhanceCO2?xationandthereforegrowth.However,

http://wendang.chazidian.com

asdiscussedabove,disruptionofphotorespirationcausesstronglyretardedgrowthofmutantplants.AnalternativeisthedeviationofsomeofthephosphoglycolateformedbyRubisCOintoalternativepathways.Kebeishetal.[30]describedtheinstallationofthebacterialglycolateoxi-dationpathwayinArabidopsischloroplaststhatconvertsglycolateinthreestepstoglycerateandthusestablishesaphotorespiratorybypass.ThispathwayincludesaCO2releasestepanalogoustotheC2pathway,buttheauthorsprovidedevidencethatshiftingCO2releasefromthemitochondriumtothechloroplastincreasedtheCO2concentrationaroundRubisCOandasaresultreducedRubisCO’soxygenaseactivityinvivo.Moreover,energyandreducingequivalentsmaybesavedinthebypass:itdoesnotincludereleaseandre?xationofammoniaandtheenergyfromglycolateoxidationissavedinreducingequivalentsandnotburnedbytheformationofH2O2.Consequently,transgeniclinesshowedenhancedshoot

CurrentOpinioninPlantBiology2010,13:249–256