AdaptedfromWilson,R.andWhitney,S(2015).Photosynthesis:GettingittogetherforCO2fixation.NaturePlants1:15130;TomDonaldCarbon-fixingreactionsofphotosynthesisCO2+H2O

O2+sugarsLifeFoodRubiscoOverviewRibulose-bisphosphatecarboxylase/oxygenase(akaRubisco)Calvin-BensoncyclePhotorespirationCarbon-concentratingmechanismsC4photosynthesisCrassulaceanAcidMetabolism(CAM),EffectsofincreasingatmosphericCO2levelsonplantsEngineeringmoreefficientC3plantsEngineeringphotosynthesisforafuturewithelevatedCO2LessonoutlineOverview:Photosynthesiscaptureslightenergyasreducedcarbon“Lowenergy”,oxidizedcarbonincarbondioxideEnergyinputfromsunlight“Highenergy”,reducedcarbonOxygenisreleasedasabyproduct6CO2+6H2OC6H12O6+6O2ThefirststepistheconversionoflightenergyintoATPandreducingpower,NADPHInthesecondstep,thepowerfromATPandNADPHisusedtomakehigh-energysugarsfromCO2ATPNADPHLight-dependentreactionsCarbon-fixingreactionsPhotosynthesisistwosetsofconnectedreactionsAdaptedfromKramer,D.M.,andEvans,J.R.(2010).Theimportanceofenergybalanceinimprovingphotosyntheticproductivity.PlantPhysiol.155:70–78.Chloroplast2H2OO2+2H++2ADPH+ATPTheLIGHTreactionstakeplaceinthethylakoidmembranesTheCARBON-FIXINGreactionstakeplaceinthechloroplaststroma2NADP+2NADPH2H+e?e?LightreactionsproduceO2,ATPandNADPHAdaptedfromKramer,D.M.,andEvans,J.R.(2010).Theimportanceofenergybalanceinimprovingphotosyntheticproductivity.PlantPhysiol.155:70–78.2H2OO2+2H++2ADPH+ATPATPsynthasePhotosystemII(PSII)PhotosystemI(PSI)Cytochromeb6fcomplexThereactionsrequireseverallargemulti-proteincomplexes:twolightharvestingphotosystems(PSIandPSII),thecytochromeb6f

complex,andATPsynthase2NADP+2NADPH2H+e?e?ThroughtheCalvin-Bensoncycle,ATPandNADPHareusedtofixCO2Adaptedfrom:Buchanan,B.B.,Gruissem,W.andJones,R.L.(2000)BiochemistryandMolecularBiologyofPlants.AmericanSocietyofPlantPhysiologists.Rubisco3xCO23xRibulose-1,5-bisphosphate6ATP6ADP+6Pi6NADP++6H+6NADPHEnergyinputReducingpowerinputForevery3CO2fixed,oneGAPisproducedforbiosynthesisandenergy5xGAP1xGAPEachCO2fixedrequires3ATPand2NADPHCarboxylationRegeneration3ATP3ADP+3Pi6x3-phosphogylcerate(3-PGA)Reduction6xglyceraldehyde3-phosphate(GAP)Someplantshaveadditional,

CO2-concentratingstepsBecauseRubiscohasanoxygenaseaswellasacarboxylaseactivity,someplantsconcentrateCO2topromotethecarboxylationreactionPEPcarboxylaseRubiscoCCCHCO3-CCCCCCCCCCCCCalvin-BensoncycleC4cycleEnergyinputRegenerationCO2Ribulose-1,5-bisphosphatePhosphoenolpyruvate(PEP)Thiscycleproducesafour-carboncompound,soitiscalledtheC4cycleTheresultoftheC4cycleistoelevatetheconcentrationofCO2atthesiteofRubiscoactionRubiscocarboxylationusesCO2,areactionthatiscompetitivelyinhibitedbyO2PEPCusesHCO3-(bicarbonate),socompetitionwithO2isnotaproblemRubiscocatalyzesbothcarboxylationandoxygenationreactionsTabita,F.R.,Satagopan,S.,Hanson,T.E.,Kreel,N.E.andScott,S.S.(2008).DistinctformI,II,III,andIVRubiscoproteinsfromthethreekingdomsoflifeprovidecluesaboutRubiscoevolutionandstructure/functionrelationships.J.Exp.Bot.59:1515-1524

bypermissionofOxfordUniversityPress.ThereforeitfunctionsaseitheracarboxylaseoranoxygenaseRecyclingthewastefuloxygenationproduct2-PGcostsenergyandreleasesfixedCO2Ribulose1,5-bisphosphatecarboxylase/oxygenase(Rubisco)catalyzesthereactionbetweenRibulose1,5-bisphosphate(RuBP)andCO2orO2RubiscoactivesiteCarboxylationOxygenationCarboxylationproducestwomoleculesof3-phosphoglycerate(3-PGA)Oxygenationproducesone3-PGAandone2-phosphogylcolate(2-PG)Andersson,I.(2008).CatalysisandregulationinRubisco.J.Exp.Bot.59:1555–1568bypermissionofOxfordUniversityPress.RuBPRibulose1,5-bisphosphate2x3-PGA3-phosphoglycerateCalvin-BensonCycleEnergyRubiscocarboxylatesRuBP,producingtwomoleculesof3-PGARubiscooxygenatesRuBP,producingone3-PGAandone2-PGRuBPRibulose1,5-bisphosphate2-PG2-phosphoglycolate+3-PGA3-phosphoglycerateCO2PhotorespirationCalvin-BensonCycleEnergyPhotorespirationrecoverscarbonfrom2-PGinanenergy-requiringprocessAndersson,I.(2008).CatalysisandregulationinRubisco.J.Exp.Bot.59:1555–1568bypermissionofOxfordUniversityPress.RubiscodoesnotdiscriminatewellbetweenO2andCO2ReprintedwithpermissionfromStec,B.(2012).StructuralmechanismofRuBisCOactivationbycarbamylationoftheactivesitelysine.Proc.Natl.Acad.Sci.USA.102:18785–18790.ManyothercarboxylasesincludingPEPCbindtoandusebicarbonate(HCO3-)asasubstrate,thusavoidingcompetitionbyO2

RubiscocatalyticsitewithO2(left)andCO2(right)RubiscoisfoundinavarietyofformsWhitney,S.M.,Houtz,R.L.andAlonso,H.(2011).AdvancingourunderstandingandcapacitytoengineerNature’sCO2-sequesteringenzyme,Rubisco.PlantPhysiol.155:27-35;SeealsoTabita,F.R.,Satagopan,S.,Hanson,T.E.,Kreel,N.E.andScott,S.S.(2008).DistinctformI,II,III,andIVRubiscoproteinsfromthethreekingdomsoflifeprovidecluesaboutRubiscoevolutionandstructure/functionrelationships.J.Exp.Bot.59:1515-1524.FormI:Plantsandmostcyanobacteriahaveahexadecamericformconsistingof8Largesubunitsand8Smallsubunits(L8S8)OnlyFormIhassmallsubunitsFormII:SomebacteriaandsomedinoflagellatealgaehaveformII,madeofdimersofL,rangingfromL2toL8FormIII:Somearchaea.FormIIIisfoundasL2,(L2)4or(L2)5.FormIIIalsoservesanon-photosyntheticfunctionFormIV:Rubisco-LikeProteinswithnocatalyticactivity.Somebacteria.FoundasL2LSUSSUEvolutionarytimelineandphylogeneticdistributionofRubiscoWhitney,S.M.,Houtz,R.L.andAlonso,H.(2011).AdvancingourunderstandingandcapacitytoengineerNature’sCO2-sequesteringenzyme,Rubisco.PlantPhysiol.155:27-35.CO2(hypothetical)CO2O2(hypothetical)O2DifferentformsofRubiscohavedifferentcatalyticpropertiesShihP.M.,OcchialiniA.,CameronJ.C.,AndralojcP.J.,ParryM.A.J.andKerfeldC.A.(2016).BiochemicalcharacterizationofpredictedPrecambrianRuBisCO.NatCommun7:10382.SeealsoWhitney,S.M.,Houtz,R.L.andAlonso,H.(2011).AdvancingourunderstandingandcapacitytoengineerNature’sCO2-sequesteringenzyme,Rubisco.PlantPhysiol.155:27-35.SpecificityfactorindicatesthespecificityoftheenzymeforCO2relativetoO2.HighermeansmorespecificforCO2.RubiscoinredalgaeismorespecificbutslowerthanthatinplantsRubiscoincyanobacteriaalgaeisfasterbutlessspecificthanthatinplantsInmanyorganisms,theenzymecannotbebothfastandspecificRubisco’scatalyticinefficiencymeansthatplantsinvesthugelyinRubiscoRUBISOfixes1015gCperyear,about1/7thofallatmosphericCO2MaleneAlgaeandC4plantshavecarbon-concentratingmechanismssocangetbywithmuchlessRubiscoprotein.RUBISOisslow,catalyzing3–10reactionspersecond(comparetocarbonicanhydrase,at500,000rxns/sec)RubiscoCATomakeupforRubisco’scatalyticdeficiencies,plantsmakeLOTSofit.InC3plants,50%ofsolubleleafproteincanbeRubisco.C3C4GreenalgaRubiscoactivityisregulatedatmanylevelsincludingbylightandmetabolitesTranscriptionofsmallsubunitgenesinnucleargenomeTranscriptionoflargesubunitgeneinplastidgenomeAssemblyThe16-subunitholoenzymeisassembledsequentiallyandinvolvesthecontributionsofseveraldedicatedchaperoneproteinsTranscriptionReversibleinhibitionSeveralinhibitorysugarphosphatesbindtoRubisco.InactiveActiveInhibitorremovalandenzymeactivationtunesRubiscoactivitytoenvironmentalconditionsL2L8L8S8SeeHauser,T.,Popilka,L.,Hartl,F.U.,andHayer-Hartl,M.(2015).RoleofauxiliaryproteinsinRubiscobiogenesisandfunction.Nat.Plants1:15065;Wilson,R.,andWhitney,S.(2015).Photosynthesis:GettingittogetherforCO2fixation.Nat.Plants1:15130.InplantsRubiscoassemblesfromlargeandsmallsubunitsinthechloroplastReprintedbypermissionfromMacmillanPublishersLtd:Wilson,R.andWhitney,S(2015).Photosynthesis:GettingittogetherforCO2fixation.NaturePlants1:15130;SeealsoWhitney,S.M.,Houtz,R.L.andAlonso,H.(2011).AdvancingourunderstandingandcapacitytoengineerNature’sCO2-sequesteringenzyme,Rubisco.PlantPhysiol.155:27-35.1.LSUisencodedbyasinglegeneinthemulticopychloroplastgenome2.SSUisencodedbyseveralgenesinthenucleargenome3.ChaperonesincludingDNAJ,BSDIIandRbcXareneededforholoenzymeassembly4.RubiscoactivaseisneededtoactivateRubiscoL8S8hexadecamericformBlue=SmallsubunitsGreen=LargesubunitsAlysineintheactivesitemustbecarbamylatedforactivityWhitney,S.M.,Houtz,R.L.andAlonso,H.(2011).AdvancingourunderstandingandcapacitytoengineerNature’sCO2-sequesteringenzyme,Rubisco.PlantPhysiol.155:27-35.

Lorimer,G.H.andMiziorko,H.M.(1980).Carbamateformationonthe.epsilon.-aminogroupofalysylresidueasthebasisfortheactivationofribulosebisphosphatecarboxylasebycarbondioxideandmagnesium2+.

Biochemistry.19:5321-5328.Asingleactivesiteshowingcarbamylatedlysine(K201X),Mg,andCABP,ananalogueofthesix-carbonreactionintermediateEnz–Lys-NH2(inactive)Enz–Lys-NH-COO-

Enz COO-

Lys-NHMg2+(active)ACTIVATIONRubiscoissubjecttoinactivationwhichcanbereversedbyRubiscoactivaseReprintedbypermissionfromMacmillanPublishersLtd:Wilson,R.andWhitney,S(2015).Photosynthesis:GettingittogetherforCO2fixation.NaturePlants1:15130;AdaptedfromHauser,T.,Popilka,L.,Hartl,F.U.andHayer-Hartl,M.(2015).RoleofauxiliaryproteinsinRubiscobiogenesisandfunction.NaturePlants.1:15065.SeeMueller-Cajar,O.,Stotz,M.,Wendler,P.,Hartl,F.U.,Bracher,A.andHayer-Hartl,M.(2011).StructureandfunctionoftheAAA+proteinCbbX,ared-typeRubiscoactivase.Nature.479:194-199.XuBP(anisomerofRuBP)andCA1ParecompetitiveinhibitorsthatbindtightlytoRubisco,inactivatingitRubiscoactivase(RAorRCA)usestheenergyofATPtoremoveinhibitors,therebyactivatingRubiscoRubisco(inactive)(EI)Rubisco(inactive)(ECMI)Rubisco(E)Rubisco(active)(ECM)CarbonfixationSugarphosphateinhibitorsActivatingCO2+Mg2+SugarphosphateinhibitorsRubiscoactivaseisheatlabile.Heat-stabilizedRAcanconferthermotoleranceReprintedwithpermissionfromCrafts-Brandner,S.J.andSalvucci,M.E.(2000).RubiscoactivaseconstrainsthephotosyntheticpotentialofleavesathightemperatureandCO2.Proc.Natl.Acad.Sci.97:13430-13435;Kurek,I.,Chang,T.K.,Bertain,S.M.,Madrigal,A.,Liu,L.,Lassner,M.W.andZhu,G.(2007).EnhancedthermostabilityofArabidopsisRubiscoactivaseimprovesphotosynthesisandgrowthratesundermoderateheatstress.PlantCell.19:3230-3241.

See

Carmo-Silva,E.,Scales,J.C.,Madgwick,P.J.andParry,M.A.J.(2015).OptimizingRubiscoanditsregulationforgreaterresourceuseefficiency.PlantCellEnviron.38:1817-1832.Invivoandinvitro,RubiscoislimitedathighertemperaturesbyRubiscoactivase’sheatsensitivitySeeddevelopmentislimitedat26°inwildtypeplants,butrestoredinplantswithheat-stabilizedRubiscoactivaseCarbonfixationisregulatedandcontrolledbymanyotherfactorsKaiserE.,MoralesA.,HarbinsonJ.,KromdijkJ.,HeuvelinkE.andMarcelisL.F.M.(2015).Dynamicphotosynthesisindifferentenvironmentalconditions.J.Exp.Bot.66:

2415-2426

bypermissionofOxfordUniversityPressIrradiance(I)Temperature(T)Leaf-to-airVaporPressureDeficit(VPD)Stomatalconductance(gs)Ca,Ci,Cc=ambient,substomatalandchloroplast[CO2]Mesophyllconductance(gm)Rubiscoactivase(Rca)CellwallAdaptedfrom:Buchanan,B.B.,Gruissem,W.andJones,R.L.(2000)BiochemistryandMolecularBiologyofPlants.AmericanSocietyofPlantPhysiologists.CarboxylationReductionRubisco3xCO23xRibulose-1,5-bisphosphate6x3-phosphogylcerate6ATP6ADP+6Pi6NADP++6H+6NADPHEnergyinputReducingpowerinput6xglyceraldehyde3-phosphate(GAP)RegenerationForevery3CO2fixed,oneGAPisproducedforbiosynthesisandenergy5xGAP1xGAP3ATP3ADP+3PiEachCO2fixedrequires3ATPand2NADPHTheCalvin-Bensoncycleregeneratesribulose1,5-bisphosphate6xNADP+3xCO23xRuBP6x3-PGAPhosphoglycerateCarboxylationphaseReductionphaseRegenerationphase3xH2O6xATP6xADP6xNADPH6xPi6xGAPGlyceraldehyde3-phosphateEnergy1GAP5GAP3xADP3xATPTheCalvin-BensoncycleprovidescarbonskeletonsforgrowthtooSeeRaines,C.A.(2011).IncreasingphotosyntheticcarbonassimilationinC3plantstoimprovecropyield:Currentandfuturestrategies.PlantPhysiol.155:36-42.Steadystate:Threecarbonsoutforthreecarbonsin.[RuBP]staysconstantCapacitybuilding:NewlyfixedcarbonusedtoproducemoreRuBP.[RuBP]increasesCapacitydeclining:Carbonskeletonsremovedfromcyclefasterthantheyarereplenished.[RuBP]decreasesTheCalvin-BensoncycleprovidescarbonskeletonsforgrowthtooRaines,C.A.(2011).IncreasingphotosyntheticcarbonassimilationinC3plantstoimprovecropyield:Currentandfuturestrategies.PlantPhysiol.155:36-42.Theactivitiesofmanyenzymes,particularlySBPaseandFBPase,determinethefateofthenewlyfixedcarbon.TransgenictobaccooverexpressingSBPaseshowenhancedcapacityforphotosynthesisDifferentfactorslimittherateofphotosynthesisundervariousconditionsReprintedwithpermissionfromWileyfromSage,R.F.,andKubien,D.S.(2007).ThetemperatureresponsesofC3andC4photosynthesis.PlantCellEnviron.30:1086–1106.CO2solubilityrelativetoO2andRubiscoselectivitybothdecreasewithincreasingtemperature.Therefore,as[CO2]increasesphotosynthesisbecomeslesssensitivetoinhibitionathightemperatures.Athigh[CO2],theregenerationofRuBPislimitingexceptathightemperaturesAtlowtomoderate[CO2],therateofRubiscocarboxylationlimitsphotosynthesisPhotorespirationrecyclesproductsofRubisco’soxygenationreactionAdaptedfromPeterhanselC.,HorstI.,NiessenM.,BlumeC.,KebeishR.,KürkcüogluS.,andKreuzalerF.(2010)Photorespiration.TheArabidopsisBook8:e0130.doi:10.1199/tab.0130RubiscoCO2O23-PGA3-phosphoglycerateSugars3-PGA

+2-PG2-phosphoglycolate2x2-PG2-phosphoglycolatePhotorespiration:Asetofreactionsthatusesenergytoconvert2x2-PGto3-PGA,CO2andNH3ATPNAD(P)HADP+PiNAD(P)+CO2andNH3GLUα-ketoglutarate2x3-PGA3-phosphoglycerateAlternativepathwaysforglyoxylateconverstionincyanobacteriaThethreepathwaysforglyoxylateconversioninSynechocystisPeterhanselC.,HorstI.,NiessenM.,BlumeC.,KebeishR.,KürkcüogluS.,andKreuzalerF.(2010)Photorespiration.TheArabidopsisBook8:e0130.doi:10.1199/tab.0130Photorespirationisamulti-organellarprocess2-PGisdephosphorylatedtoglycolateandtransportedtoperoxisomeGlycolateisoxidizedtoglyoxylate,producingH2O2Glyoxylateistransaminatedtoglycine,whichistransportedtothemitochondrionHagemann,M.,Kern,R.,Maurino,V.G.,Hanson,D.T.,Weber,A.P.M.,Sage,R.F.andBauwe,H.(2016).Evolutionofphotorespirationfromcyanobacteriatolandplants,consideringproteinphylogeniesandacquisitionofcarbonconcentratingmechanisms.J.Exp.Bot.67:2963-2976.PhotorespirationreleasesCO2andNH3fromorganicform2GlycineinSerineoutInthemitochondrion,oneglycineisdecarboxylated(releasingCO2)anddeaminated(releasingNH3)bytheGlycineDecarboxylaseComplex(GDC).ThecentralCofglycineistransferredtoaonecarboncarrier(tetrahydrofolate)andthentransferredtoanotherglycinetoproduceserine,byserinehydroxymethyltransferase(SHMT)Hagemann,M.,Kern,R.,Maurino,V.G.,Hanson,D.T.,Weber,A.P.M.,Sage,R.F.andBauwe,H.(2016).Evolutionofphotorespirationfromcyanobacteriatolandplants,consideringproteinphylogeniesandacquisitionofcarbonconcentratingmechanisms.J.Exp.Bot.67:2963-2976.Photorespiration:GlycinedecarboxylasecomplexdisassemblesglycineYikrazuulGlycineCO2releasedNH3releasedCH2transferredtocarboncarrier,thentoGlytomakeSerGDCmakesup10%ofleafmitochondrialproteinsinthelightGDCcomplexreactioncycleInthechloroplast,glycerateisphosphorylatedto3-PGAwhichre-enterstheCalvin-BensoncycleTheresultingserineistransportedbackintotheperoxisomewhereitisreducedtoglycerateHagemann,M.,Kern,R.,Maurino,V.G.,Hanson,D.T.,Weber,A.P.M.,Sage,R.F.andBauwe,H.(2016).Evolutionofphotorespirationfromcyanobacteriatolandplants,consideringproteinphylogeniesandacquisitionofcarbonconcentratingmechanisms.J.Exp.Bot.67:2963-2976.PhotorespiratorybypassescanminimizeenergeticcostsofphotorespirationReprintedbypermissionfromMacmillanPublishersLtd:Leegood,R.C.(2007).Awelcomediversionfromphotorespiration.NatBiotech.25:539-540.SeealsoKebeish,R.,Niessen,M.,Thiruveedhi,K.,Bari,R.,Hirsch,H.-J.,Rosenkranz,R.,Stabler,N.,Schonfeld,B.,Kreuzaler,F.andPeterhansel,C.(2007).ChloroplasticphotorespiratorybypassincreasesphotosynthesisandbiomassproductioninArabidopsisthaliana.NatBiotech.25:593-599.Severalstrategiestolimittheenergeticcostofphotorespirationhavebeenexamined.Herefivebacterialenzymesaretargetedtothechloroplastwheretheyrecycleglycolatetoglycerate.ThisbypassallowstheCO2tobeeffectivelyrefixed,andeliminatestheneedtorefixNH3.Blue=photorespiration,Red=engineeredbypassSummary:Rubisco,Calvin-BensoncycleandphotorespirationAlmostallorganiccarbonisfixedintoorganicformbyRubisco;1/7thofatmosphericcarbonisfixedeachyearRubiscocarboxylatesRuBP,toproducetwomoleculesof3-PGA3-PGAenterstheCalvin-Bensoncycle,regeneratingRuBPRubiscoalsooxygenatesRuBP,producing3-PGAand2-PG2-PGisrecycledintoRuBPthroughenergeticallycostlyphotorespirationRuBPCalvin-BensoncycleCO2orO2RubiscoCarbon-concentratingmechanismsinbacteria,algaeandplantsSun,Y.,Casella,S.,Fang,Y.,Huang,F.,Faulkner,M.,Barrett,S.andLiu,L.(2016).Lightmodulatesthebiosynthesisandorganizationofcyanobacterialcarbonfixationmachinerythroughphotosyntheticelectronflow.PlantPhysiol.171:530-541.Engel,B.D.,Schaffer,M.,KuhnCuellar,L.,Villa,E.,Plitzko,J.M.andBaumeister,W.(2015).NativearchitectureoftheChlamydomonaschloroplastrevealedbyinsitucryo-electrontomography.eLife.4:e04889.CyanobacteriaconcentrateCO2inproteinstructurescalledcarboxysomes(herelabeledwithGFP)flagellachloroplastnucleuspypyrenoidManyalgaeconcentrateCO2inpyrenoidswithintheirchloroplastsC4andCAMplantsusephosphoenolpyruvatecarboxylase(PEPC)fortheprimarycarboxylationreaction,concentratingorstoringCO2upstreamofRubiscoHCO2-CO2C4acidsCO2CBcyclePEPCRubiscoCarbonicanhydrase(CA)Pyrenoids,possiblycarboxysomes,evolvedlongafterplastidendosymbiosisAdaptedfromPrice,G.D.,Badger,M.R.andvonCaemmerer,S.(2011).TheprospectofusingcyanobacterialbicarbonatetransporterstoimproveleafphotosynthesisinC3cropplants.PlantPhysiol.155:20-26;EndosymbioticoriginofchloroplastsCAMandC4plantsevolvedThebacterialcarboxysomesequestersRubiscoandconcentratesCO2AdaptedfromPrice,G.D.,Badger,M.R.andvonCaemmerer,S.(2011).TheprospectofusingcyanobacterialbicarbonatetransporterstoimproveleafphotosynthesisinC3cropplants.PlantPhysiol.155:20-26;Badger,M.R.,andPrice,G.D.(2003).CO2concentratingmechanismsincyanobacteria:molecularcomponents,theirdiversityandevolution.J.Exp.Bot.54:609–622.ToyeatesHCO3-CO2HCO3-HCO3-CO2CARubiscoBacteriaconcentrateCO2andRubiscoincarboxysomes,atypeofbacterialmicrocompartment.CarboxysomescanconcentrationCO21000xaboveambient.InnermembranetransportersforHCO3-andthylakoidtransportersforCO2bringinorganiccarbonintothecellCarboxysomeshaveanicosahedral(20sided)proteinshell(blue)thatresistsCO2efflux,intowhichRubisco(green)ispackedCarbonicanhydrase(CA)catalyzesthereversiblereaction:CO2+H2O HCO3-+H+PyrenoidsconcentratecarboninsomephotosyntheticeukaryotesAngiospermsGymnospermsFernsMossesGreenalgaeRedalgaeanddiatomsNoNoNoNoNoSomeSomeSomeLiverwortsHornwortsBryophytesVascularplantsPyrenoiddistributionPyrenoidsarefoundinsomebutnotallgreenalgae,redalgaeanddiatoms.TheyareofteninducedinresponsetolowCO2.Mostlandplantslackpyrenoids,butsomehornworts(bryophytes)havethem.AnthocerosagrestisAdaptedfromVaughn,K.C.,Campbell,E.O.,Hasegawa,J.,Owen,H.A.andRenzaglia,K.S.Thepyrenoidisthesiteofribulose1,5-bisphosphatecarboxylase/oxygenaseaccumulationinthehornwort(Bryophyta:Anthocerotae)chloroplast.Protoplasma.156:117-129.BerndHflagellachloroplastnucleuspyPyrenoidsarehighlypackagedRubisco,interspersedwiththylakoidmembranesEngel,B.D.,Schaffer,M.,KuhnCuellar,L.,Villa,E.,Plitzko,J.M.andBaumeister,W.(2015).NativearchitectureoftheChlamydomonaschloroplastrevealedbyinsitucryo-electrontomography.eLife.4:e04889.ThylakoidsStarchRubiscoRubiscoAnabundantrepeatprotein,EPYC1,linksRubiscotoformpyrenoidsReprintedwithpermissionfromMackinder,L.C.M.,etal.andJonikas,M.C.(2016).ArepeatproteinlinksRubiscotoformtheeukaryoticcarbon-concentratingorganelle.Proc.Natl.Acad.Sci.USA113:

5958-5963.Loss-of-functionepyc1Chlamydomonascellsformsmallerpyrenoids(yellowarrow)thanwild-typecellsTheepyc1pyrenoidscontainlessRubisco,asshownbyfluorescently-taggedRBCS1(top)andimmunogoldTEM(bottom)EPYC1hasfourconservedrepeatsthatmayservetolinkRubiscoholoenzymestogethertoformthepyrenoidCO2isconcentratedinpyrenoidsviatransportersandcarbonicanhydraseAdaptedfromMeyer,M.andGriffiths,H.(2013).OriginsanddiversityofeukaryoticCO2-concentratingmechanisms:lessonsforthefuture.J.Exp.Bot.64:769-786.HCO3-ispoorlymembranepermeable,butspecifictransportersmoveitintochloroplastsandthylakoidsCarbonicanhydrase(CA)convertsHCO3-toCO2CO2

ishighlymembranepermeablebutdiffusionisslowedbythepyrenoid’sstarchlayerPlasmamembraneChloroplastenvelopeChloroplastPyrenoid(Rubisco)Starchlayer:CO2diffusionbarrierThylakoidsHCO3-HCO3-HCO3-BicarbonatetransporterHCO3-CO2CAPEPC-basedcarboxylationreactionsinplants:C4andCAMChristin,P.-A.,Arakaki,M.,Osborne,C.P.andEdwards,E.J.(2015).GeneticenablersunderlyingtheclusteredevolutionaryoriginsofC4photosynthesisinangiosperms.Mol.Biol.Evol.32:846-858,bypermissionofOxfordUniversityPress.CO2fixationCO2fixationStorageinvacuolesC4aciddecarboxylationC4aciddecarboxylationtransporttransportRubiscoRubiscoPEPCPEPCCarboxylasesspatiallyseparatedCarboxylasestemporallyseparatedTheadvantageoftheC4pathwayisthatitavoidsphotorespirationCAMconserveswaterbyopeningstomataatnight,andalsoavoidsphotorespirationHCO3-HCO3-C4photosynthesishasevolvedmorethan60times,mainlyinhot,dryregionsSageR.F.,ChristinP.-A.,andEdwardsE.J.(2011).TheC4plantlineagesofplanetEarth.J.Exp.Bot.62:3155-3169bypermissionofOxfordUniversityPressCentersoforiginforsomeC4plantsC4photosynthesissignificantlyincreasesbiomassaccumulationinwarm,sunny,dryconditions.Thisstrongadvantagehasbeenselectedforrepeatedly.C4photosynthesisisadvantageousindry,hot,sunnyregions(1)PEPcarboxylaseRubiscoCCCCCCalvin-BensoncycleCO2Ribulose-1,5-bisphosphateCCCHCO3-C4cycleCO2CO2H2OH2OPEPCfixesCO2effectivelyevenwheninternal[CO2]islow,meaningthatC4plantsoftenhaveanadvantageoverC3whenwaterislimitingPlantsclosetheirstomatatoconservewater,butthisalsopreventsCO2fromenteringtheleafC4photosynthesisisadvantageousindry,hot,sunnyregions(2)C3plantC4plant35°CIrradianceFullsunlightPhotosynthesis10°CC3plantC4plantIrradianceFullsunlightPhotosynthesisC3plantshaveaslightadvantageatcooltemperature(theadditionalcarboxylationstepsofC4requireenergy)Becausephotorespirationincreaseswithtemperature,C4plantshaveanadvantagehighertemperaturesBecausecarbon-fixationinC4plantsisnotcarbon-limited,theyareabletotakeadvantageofhighlightintensities;notethathereC4photosynthesisisnotsaturatedinfullsunlightAdaptedfromC4photosynthesis,PlantsinActionPhotoinhibitiondramaticallylimitsC3photosynthesisathighertemperatures50Netphotosynthesis(μmol/m2-sec)01020403050600102040305060Leaftemperature(oC)20304015352545330ppmCO21000ppmCO2C4C3C4C3Photorespirationincreasesathightemperaturesbecause(a)Rubiscoselectivitydecreases,and(b)andtherelativesolubilityofCO2toO2decreases.Bysuppressingphotorespiration,C4photosynthesiscanincreasewithincreasingT(toacertainlevel).RedrawnfromPearcy,R.W.andEhleringer,J.(1984).ComparativeecophysiologyofC3andC4plants.PlantCellEnviron.7:1-13.Whenphotorespirationissuppressedbyincreasing[CO2],C3photosynthesisrespondstoincreasingTlikeC4photosynthesisPulse-chasestudy:14CO2labelaccumulatesfirstinmalateandotherC4acids,then3-PGAHistory: