Quantifying the roles of algal photosynthetic electron pathways.pdf

Tansley insightQuantifying the roles of algal photosyntheticelectron pathways:a milestone towardsphotosynthetic robustnessAuthor for correspondence:Adrien BurlacotEmail:aburlacotcarnegiescience.eduReceived:31 March 2023Accepted:1 September 2023Adrien Burlacot1,21Department of Plant Biology,Carnegie Institution for Science,Stanford,CA 94305,USA;2Department of Biology,StanfordUniversity,Stanford,CA 94305,USAContentsSummary2197I.Introduction2197II.Molecular mechanisms2199III.Physiological role of alternative electron transport,a quantitative point of view2199IV.Potential of AEFs and conclusions2202Acknowledgements2202References2202New Phytologist(2023)240:21972203doi:10.1111/nph.19328Key words:ecophysiology,microalgae,photoprotection,photosynthesis,photosynthetic electron flows,protongradient.SummaryDuringphotosynthesis,electrontransportreactionsgenerateandshuttlereductanttoallowCO2reduction by the CalvinBensonBassham cycle and the formation of biomass building block intheso-calledlinearelectronflow(LEF).However,innature,environmentalparameterslikelightintensity or CO2availability can vary and quickly change photosynthesis rates,creating animbalance between photosynthetic energy production and metabolic needs.In addition to LEF,alternative photosynthetic electron flows are central to allow photosynthetic energy to matchmetabolicdemandinresponsetoenvironmentalvariations.Microalgaearguablyharbouroneofthe most diverse set of alternative electron flows(AEFs),including cyclic(CEF),pseudocyclic(PCEF)and chloroplast-to-mitochondria(CMEF)electron flow.While CEF,PCEF and CMEFhave large functional overlaps,they differ in the conditions they are active and in their role forphotosynthetic energy balance.Here,I review the molecular mechanisms of CEF,PCEF andCMEF in microalgae.I further propose a quantitative framework to compare their keyphysiological roles and quantify how the photosynthetic energy is partitioned to maintain abalanced energetic status of the cell.Key differences in AEF within the green lineage and thepotential of rewiring photosynthetic electrons to enhance plant robustness will be discussed.I.IntroductionOn Earth,photosynthesis constitutes the main biological actor oforganic and inorganic carbon cycling on our planet,with thenatural carbon circulation initiated by photosynthetic fixation ofAdrien Burlacot is a finalist of the 2023 New Phytologist Tansley Medalcompetition for excellence in plant science.See Slater&Dolan(2023,240:21712172)for more details.?2023 The AuthorsNew Phytologist?2023 New Phytologist FoundationNew Phytologist(2023)240:ReviewCO2being 10 times larger than the amount of anthropogenicCO2emissions(IPCC,2021),andmicroalgaecontributetohalfofthis circulation(Field et al.,1998).During photosynthesis,CO2fixation is performed thanks to the combined activity ofphotosystem(PS)IIandI,whichusesunlighttogenerateNADPH(in a so-called linear electron flow LEF)and a trans-thylakoidalproton motive force(pmf)composed of a proton gradient and anelectrical gradient which is used by the ATP synthase to generateATP(Fig.1).Both ATP and NADPH are used by the CalvinBensonBassham(CBB)cycle and the downstream metabolism totransform CO2into biomass building blocks.However,the LEFdoes not generate enough ATP for the requirements of the CBBcycle(Allen,2002)nor for downstream metabolic reactions,andalternative electron flows(AEFs)compensate for this deficiency bygenerating an extra pmf without the net generation of NADPH(Allen,2003).Innaturalhabitats,lightintensity,CO2andnutrientavailability,ortemperaturevary,affectingphotosyntheticrates,themetabolic demand for energetic vectors or both.An imbalancebetweenmetabolicdemandandphotosyntheticenergyproductioncan cause the accumulation of harmful chemicals like reactiveoxygen species that lead to photo-oxidative damage.In particular,any damage to the PSI is dangerous since PSI recovery requires thecompletesynthesisofthePSIcomplex,takingdaysforfullrecovery(Bernhard Teicher et al.,2000).The electron transport capacity ofvarious AEF is hence dynamically tuned in response to environ-mental variations,allowing to match metabolic demand for ATPand NADPH without overproduction of energetic vectors,thusprotecting the cell from photodamage.Despite their central role,amajorquestionremains:HowmanyelectronsflowingthrougheachAEFareneededtobalancetheenergeticstatusofthecellinresponseto environmental conditions?Here,I review the main alternativeelectron transport pathways in microalgae,their roles and theH+LumenCytosolChloroplast envelopeATPaseThylakoidPSII H2O O2+1H+2H+2H+H2O1H+O2PCEFCyt b6/fCyt b6/fAFLVsBPSICEF2H+2H+Cyt b6/fPSIPSII2H+2H+PSI*CBBCO2ATPLEFStroma*CMEFvia III/IVMitochondrionAOXATP2H+IIIIIVCMEFvia AOX H2O O2+1H+2H+2H+2H+1H+1H+H2O O2+1H+O2+1H+H2O2.5 H+per e2 H+per e5.5 H+per e3 H+per eCMEF2.5 H+per e in the chloroplastPGR5H+PGRL1Fig.1 Mainalternativeelectronpathwayspresentingreenmicroalgae.Schematicofthephotosyntheticelectrontransportchainwithlinearelectronflow(LEF,brown arrow)generating NADPH(brown stars)and trans-thylakoidal proton(H+)gradient the latter being a component of the pmf that is used for ATPsynthesis.Both NADPH and ATP are used by the CalvinBensonBassham cycle(CBB)to fix CO2.Cyclic electron flow(CEF,right pink rectangle)andpseudocyclicelectronflow(PCEF,leftpurplerectangle)contributeinthechloroplasttothepmf.Thechloroplast-to-mitochondriaelectronflow(CMEF,topredrectangle)shuttlesreductantusingmultiplemetabolicpathwaysandconvertsitintoATPviaeitherthealternativeoxidase(AOX)pathway(CMEFviaAOX)orthe complex III/IV pathway(CMEF via III/IV).For each electron pathway,the details of theoretical H+translocation reactions occurring per electron(e?)areshownaswellastheoverallH+translocatedpere?.NoteherethatCMEFtranslocatesH+acrossthethylakoidmembrane(2.5H+pere?,sameforbothCMEFpathways)as well as across the mitochondrial membrane.ATP,adenosine triphosphate;FLVs,flavodiiron proteins A and B;NADPH,reduced form ofnicotinamide adenine dinucleotide phosphate;PS,photosystem;PGR5/L1,proton gradient generation 5/like-1;I,III and IV,complexes I,III and IV of themitochondrial electron transport chain.New Phytologist(2023)240:?2023 The AuthorsNew Phytologist?2023 New Phytologist FoundationReviewTansley insightNewPhytologist2198 14698137,2023,6,Downloaded from https:/ by Xinjiang Branch Of C.A.S,Wiley Online Library on 16/11/2023.See the Terms and Conditions(https:/ Wiley Online Library for rules of use;OA articles are governed by the applicable Creative Commons Licensemolecular mechanisms involved.I then show functional overlapsand key quantitative differences of the main AEF mechanisms anduse the model green algae Chlamydomonas reinhardtii(hereafterChlamydomonas)inphotoautotrophic conditionstoillustrate theirvarying roles in response to environmental variations.II.Molecular mechanismsIn Chlamydomonas,three main AEF have been described:cyclic(CEF),pseudocyclic(PCEF)and chloroplast-to-mitochondria(CMEF)electron flow(Fig.1).The CEF recycles photosyntheticelectrons around PSI and involves two pathways:one regulated bythe proton gradient regulation(PGR)5(Munekage et al.,2002;Johnson et al.,2014)and PGR-Like 1(DalCorso et al.,2008;Tolleter et al.,2011;Fig.1)and another one involving a pathwaydependentonaNADPHdehydrogenase(NDA2)(Janset al.,2008)which reduces plastoquinone directly using NAD(P)H.Because the NDA2-dependent CEF activity likely represents avery small proportion of AEF during photosynthesis(Nawrockiet al.,2019b),this pathway of CEF will not be further discussedhere and the PGR5/L1-controlled CEF,when in Chlamydomonas,will be referred thereafter simply as CEF.Pseudocyclic electron flow(PCEF)involves the use of photo-syntheticelectronsgeneratedbyPSIItoreduceO2andismediateddownstream of PSI by flavodiiron proteins(FLVs;Helmanet al.,2003;Gerotto et al.,2016;Chaux et al.,2017;Shimakawaetal.,2017;Fig.1).Chlamydomonas,likealloxygenic phototrophshavingaPCEF,harboursapairnumberofFLV(Chauxetal.,2017;Shimakawa et al.,2022),with two different holoenzymes FLVAand FLVB.Note here that while plastid terminal oxidases areinvolvedinChlamydomonasinO2reductionbetweenPSIIandPSI,they are mostly involved in the dark(Nawrocki et al.,2019b)in aprocesscalledchlororespirationandthusarenotconsideredamajorpayer of PCEF here.Photosynthetic electrons can also be shuttled from thechloroplast to the mitochondria and used in the mitochondrialelectron transport chain(Raghavendra&Padmasree,2003).Thiselectron flow,which I term here CMEF(Fig.1),entails twopathways,one involving the mitochondrial alternative oxidase(AOX)and a second one involving the complexes III and IV,all ofthemreducing O2inthemitochondria(Fig.1).Crosstalk betweenthe chloroplast and the mitochondria can entail various molecularmechanisms to translocate the reductant between the twoorganelles including:the interconversion of NADPH to malatevia malate dehydrogenases(MDH)and the exchange of malate bymalate shuttles(Dao et al.,2022);transport of fixed carbon viatriose-phosphate translocators and their subsequent breakdown inthe cytosol or by the TCA cycle in mitochondria(Huanget al.,2023);or export of other carbon compounds like fumarate,succinate or glycolate(Johnson&Alric,2013).III.Physiologicalroleofalternativeelectrontransport,a quantitative point of viewAnimportantpart oftheliteraturehas been pointing toPCEF andtheAOXpathwayofCMEFasacting aselectronvalvesthatgetridof photosynthetic energy,protecting photosystems from over-reduction pressure by extracting electrons from the PSI acceptorside(Alboresietal.,2019;Vanlerbergheetal.,2020;Fig.2).Thoseelectronvalvesareeitheractiveunderfluctuatinglightconditionsinthe case of PCEF(Chaux et al.,2017;Jokel et al.,2018;Stortietal.,2019)orunderconditionsofhighlightstressinthecaseoftheAOX pathway of CMEF(Kaye et al.,2019).However,bymediatingprotogenicelectrontransportswithoutnetgenerationofreducingpower,CEF,PCEFandCMEFallcontributetothetrans-thylakoidal pmf(Fig.1),and by doing so,they are all involved in:complementing ATP production to which LEF does notsufficiently contribute to(Fig.2a;Peltier et al.,2023);generatingATPaseCyt b6/fH+H+Cyt b6/fPSIPSIINPQNPQPSICO2HCO3eeepHCA(a)(b)PSI acceptor sidePSI donor sidePCEFCMEFFig.2 Physiological roles of photosynthetic electron flows.(a)Schematic of the main regulations of photosynthesis induced by the trans-thylakoidal protongradient(DpH).Thoseinclude(1)poweringATPgenerationbytheATPase,(2)inductionofnonphotochemicalquenching(NPQ)atbothphotosystems(PS),(3)induction of photosynthetic control at the cytochrome b6/f level or(4)conversion of HCO3?into CO2by lumenal carbonic anhydrase(CA)in the lumen ofmicroalgae.(b)ImpactofAEFsonPSIreductionlevel.Ononehand,thephotosyntheticcontrolinducedbyDpHlimitsthePSIreductionlevelbylimitingelectronflow at the donor side.On the other hand,pseudocyclic electron flow(PCEF)and the chloroplast-to-mitochondria electron flow(CMEF)serve as terminalelectron acceptors,relieving the reduction pressure from PSI on the acceptor side.?2023 The AuthorsNew Phytologist?2023 New Phytologist FoundationNew Phytologist(2023)240:NewPhytologistTansley insightReview2199 14698137,2023,6,Downloaded from https:/ by Xinjiang Branch Of C.A.S,Wiley Online Library on 16/11/2023.See the Terms and Conditions(https:/ Wiley Online Library for rules of use;OA articles are governed by the applicable Creative Commons Licenseahighluminalprotonconcentrationwhichtriggersproton-sensingproteins involved in safe dissipation of excitation energy bynonphotochemical quenching(NPQ;Munekage et al.,2004;Chaux et al.,2017;Burlacot et al.,2022)at the level of bothphotosystems(Girolomoniet al.,2019;Fig.2a);create an electronflow limitation at the cytochrome b6/f level(Malone et al.,2021)and limit overreduction of the PSI donor side(Basso et al.,2022;Fig.2a,b)inaprocessreferredtoasphotosyntheticcontrol(Maloneet al.,2021);or convert HCO3?into CO2thanks to carbonicanhydrase in the lumen of microalgae(Burlacot et al.,2022;Fig.2a).Chloroplast-to-mitochondria electron flow(CMEF)canalso lead to the generation of ATP in mitochondria thanks to theactivity of the respiratory electron transport chain(Fig.1),whichhas recently beenproposedtofuelATP-requiring reactionsdistantfrom the chloroplast(Burlacot et al.,2022;Burlacot&Peltier,2023)and to contribute to energise CO2fixation(Peltieretal.,2023).Hence,CEF,PCEFandCMEFshowlargefunctionaloverlaps and can complement each other(Dang et al.,2014;Stortiet al.,2019,2020;Burlacot et al.,2022;Peltier et al.,2023).Importantly,one electron circulating through CEF,PCEF orboth AOX and complex III/IV pathways of CMEF will lead to thetranslocation of widely different number of protons acrossthe thylakoid and/or the inner mitochondrial membrane(Fig.1;Table 1)and thus likely contribute differently to ATP,NPQ andphotosynthetic control.Theoretically,for the same flow ofelectrons,thecomplexIII/IVpathwayofCMEFwouldtranslocateoverallfourtimesmoreprotonsthanCEF(Table1)andcontributemoretotheNPQandphotosyntheticcontrolthankstotheprotonstranslocated across the thylakoid membrane during the initialreductant production by PSII and PSI(Fig.1).Such theoreticalconsiderations were recently experimentally confirmed by measur-ing the efficacy of one electron passing through one AEF inenergising net photosynthesis(Peltier et al.,2023).Because PCEFand CMEF are theoretically more efficient than CEF to transferprotons(Table 1);for PCEF and CMEF to serve as electron valvesand get rid of excess energy,the thylakoid and/or mitochondrialmembranes should leak the pmf without the generation of ATP.While quantification of all AEF and their role is pivotal indeciphering the bioenergetic requirements and regulation ofphotosynthesis,the respective contributions of each AEF undervarious physiological conditions remain elusive.In Chlamydomo-nas,evaluations of electron flow passing through each alternativepathway are scarce.Recently,the flux through each AEF wasquantified depending on inorganic carbon(CO2and HCO3?referred to as Ci)availability(Burlacot et al.,2022).Expanding onthisevaluation,IproposeavisualframeworkinFig.3todissectthebioenergetic regulations of photosynthesis by AEF showingthe proportion of electron flow as an additional electron flowrelative to LEF(%-LEF)involved in their documented physiolo-gicalrolesconsideringCiavailabilityandlightaskeyenvironmentalvariables.CilevelsInChlamydomonas,CO2fixation dependsontheactivityofaCO2concentrating mechanism(CCM),which uses photosyntheticenergy to concentrate Ciat the vicinity of the CO2-fixing enzymeRubisco(Burlacot&Peltier,2023).The Chlamydomonas CCMincludes transport of Ciat the periplasmic membrane,chloroplastenvelopeandthylakoidmembrane andasubsequentconversionofHCO3?toCO2inthethylakoidlumen(Burlacot&Peltier,2023).When the CCM is present,and at saturating light intensity,bothCEFandPCEFarestimulatedatlowCiconcentration(Fig.3)andsynergistically generate a low lumenal pH necessary for theconversion of bicarbonate into CO2(Burlacot et al.,2022).Theactivi