8 research outputs found
ConsĂ©quences Fonctionnelles de lâOrganisation SupramolĂ©culaire de la ChaĂźne PhotosynthĂ©tique et Commutation Entre Transferts dâElectrons Cyclique et LinĂ©aire
The photosynthetic process relies on an electron flow involving several complexes in the thylakoid membranes of photosynthetic organisms. This flux can follow two possibly competing pathways: the linear electron transfer through which electrons are transferred from water (which is oxidized) to NADP+ (which is reduced), which is coupled to the generation of a transmembrane potential difference allowing the synthesis of ATP (Allen 2002); the cyclic pathway (around PSI and Cytochrome b6f complex) which only allows the production of ATP. These two pathways are thought to be essential for the reduction of CO2 and must likely coexist to allow the photosynthetic ATP/NADPH ratio to meet the requirement of the reduction of CO2 into carbohydrates (Seelert, Poetsch et al. 2000 ; Munekage, Hashimoto et al. 2004). This mere statement raises the question of the mechanisms that prevail in the implication of the same actors, within the same membrane, in either one of the two functional modes. In the green algae Chlamydomonas reinhardtii, our results show that the regulation of cyclic electron transfer is controlled by the redox poise and not by the lateral migration of antennae (Takahashi, Clowez et al. 2013), and disprove with the conclusion drawn from previous studies (BultĂ©, RebeillĂ© et al. 1990 ; Finazzi, Rappaport et al. 2002) according to which state transition would determine this switch. The association of these antennae to Photosystem I would promote the sequestration, within a single unit, of all the actors of the cyclic mode. Functional studies, in vitro, of supercomplex formation under anoxic conditions, questions on their functional capacities. This PhD work presents also the characterization of transient ââacceptor side limitationââ of PSI, upon the onset of anoxia where it is not possible to observe an oxidation of P700 in 705 nm. This phenomenon due to the charge recombination is created by an accumulation of NADPH. The spontaneous oxidation of the PSI acceptor pool, after some time under anoxia, involves the hydrogenase induction, accepting the electrons from NADPH. Itâs also possible to induce this PSI oxidation as soon as cells are constantly under illumination, involving chloroplast ATP pathway. ATP synthesised in the light, allow the consumption of NADPH through Benson-Calvin cycle.Le processus photosynthĂ©tique se traduit par un flux dâĂ©lectron impliquant diffĂ©rents complexes de la membrane thylacoĂŻdale. Ce flux peut adopter deux chemins diffĂ©rents : le transfert dâĂ©lectron linĂ©aire (Merchant, Prochnik et al. 2007) Ă travers lequel les Ă©lectrons sont transfĂ©rĂ©s de lâeau oxydĂ©e au niveau du PhotosystĂšme II (PSII), au NADPH rĂ©duit par le PSI ; et le transfert dâĂ©lectron cyclique autour du PhotosystĂšme I (PSI) et du complexe cytochrome b6f. Ces flux dâĂ©lectrons sont couplĂ©s Ă un pompage de proton du stroma vers le lumen gĂ©nĂ©rant une diffĂ©rence de potentiel transmembranaire, permettant la synthĂšse dâATP (Allen 2002). La coexistence de ces deux flux est considĂ©rĂ© comme nĂ©cessaire Ă la fixation et la mĂ©tabolisation des molĂ©cules de dioxyde de carbone (Seelert, Poetsch et al. 2000 ; Munekage, Hashimoto et al. 2004) dans un rapport stricte ATP / NADPH. Cette coexistence qui semble essentiel soulĂšve la question des mĂ©canismes qui prĂ©valent Ă lâimplication des mĂȘmes acteurs photosynthĂ©tiques, dans une mĂȘme membrane, dans lâun ou lâautre mode de transfert dâĂ©lectron. Chez lâalgue verte Chlamydomonas reinhardtii, nous avons dĂ©montrĂ© que la commutation entre les deux transferts Ă©tait dĂ©pendante de lâĂ©tat redox des cellules, mais contrairement Ă ce qui avait Ă©tĂ© suggĂ©rĂ© dans les Ă©tudes prĂ©cĂ©dentes (BultĂ©, RebeillĂ© et al. 1990 ; Finazzi, Rappaport et al. 2002) indĂ©pendante du phĂ©nomĂšne de transition dâĂ©tat (Takahashi, Clowez et al. 2013), qui implique la migration latĂ©rale des complexes antennaires au sein de la membrane. Lâassociation de ces antennes au PhotosystĂšme I conduirait Ă la sĂ©questration, dans une mĂȘme entitĂ© biochimique, des diffĂ©rents acteurs du mode cyclique. Cette formation de supercomplexe dans les conditions anoxiques, Ă fait lâobjet dâune Ă©tude fonctionnelle in vitro, laissant quelques questions ouvertes sur leurs capacitĂ©s fonctionnelles. Ce travail de thĂšse prĂ©sente aussi la caractĂ©risation dâune limitation transitoire des accepteurs du PhotosystĂšme I, en dĂ©but dâanoxie pendant laquelle il nâest pas possible dâobserver dâoxydation de P700, Ă 705 nm. Ce phĂ©nomĂšne dĂ» Ă la recombinaison de charge est crĂ©Ă© par un engorgement du pool de NADPH. Lâoxydation spontanĂ©e du PSI au bout dâun certain temps dâanoxie implique lâinduction de lâhydrogĂ©nase, acceptant les Ă©lectrons du PSI. Il reste possible dâinduire cette Ă©volution de lâoxydation de P700 lorsque les cellules sont constamment sous illumination dans les conditions anoxiques, impliquant cette fois ci, la voie de lâATP chloroplastique. LâATP synthĂ©tisĂ© Ă la lumiĂšre permettrait la consommation de NADPH via le cycle de Benson Calvin
Functional consequences of thylakoid membranes reorganization in photosynthetic chain and switching between cyclic and linear electron transfer in green algae Chlamydomonas reinhardtii
Le processus photosynthĂ©tique se traduit par un flux dâĂ©lectron impliquant diffĂ©rents complexes de la membrane thylacoĂŻdale. Ce flux peut adopter deux chemins diffĂ©rents : le transfert dâĂ©lectron linĂ©aire (Merchant, Prochnik et al. 2007) Ă travers lequel les Ă©lectrons sont transfĂ©rĂ©s de lâeau oxydĂ©e au niveau du PhotosystĂšme II (PSII), au NADPH rĂ©duit par le PSI ; et le transfert dâĂ©lectron cyclique autour du PhotosystĂšme I (PSI) et du complexe cytochrome b6f. Ces flux dâĂ©lectrons sont couplĂ©s Ă un pompage de proton du stroma vers le lumen gĂ©nĂ©rant une diffĂ©rence de potentiel transmembranaire, permettant la synthĂšse dâATP (Allen 2002). La coexistence de ces deux flux est considĂ©rĂ© comme nĂ©cessaire Ă la fixation et la mĂ©tabolisation des molĂ©cules de dioxyde de carbone (Seelert, Poetsch et al. 2000 ; Munekage, Hashimoto et al. 2004) dans un rapport stricte ATP / NADPH. Cette coexistence qui semble essentiel soulĂšve la question des mĂ©canismes qui prĂ©valent Ă lâimplication des mĂȘmes acteurs photosynthĂ©tiques, dans une mĂȘme membrane, dans lâun ou lâautre mode de transfert dâĂ©lectron. Chez lâalgue verte Chlamydomonas reinhardtii, nous avons dĂ©montrĂ© que la commutation entre les deux transferts Ă©tait dĂ©pendante de lâĂ©tat redox des cellules, mais contrairement Ă ce qui avait Ă©tĂ© suggĂ©rĂ© dans les Ă©tudes prĂ©cĂ©dentes (BultĂ©, RebeillĂ© et al. 1990 ; Finazzi, Rappaport et al. 2002) indĂ©pendante du phĂ©nomĂšne de transition dâĂ©tat (Takahashi, Clowez et al. 2013), qui implique la migration latĂ©rale des complexes antennaires au sein de la membrane. Lâassociation de ces antennes au PhotosystĂšme I conduirait Ă la sĂ©questration, dans une mĂȘme entitĂ© biochimique, des diffĂ©rents acteurs du mode cyclique. Cette formation de supercomplexe dans les conditions anoxiques, Ă fait lâobjet dâune Ă©tude fonctionnelle in vitro, laissant quelques questions ouvertes sur leurs capacitĂ©s fonctionnelles. Ce travail de thĂšse prĂ©sente aussi la caractĂ©risation dâune limitation transitoire des accepteurs du PhotosystĂšme I, en dĂ©but dâanoxie pendant laquelle il nâest pas possible dâobserver dâoxydation de P700, Ă 705 nm. Ce phĂ©nomĂšne dĂ» Ă la recombinaison de charge est crĂ©Ă© par un engorgement du pool de NADPH. Lâoxydation spontanĂ©e du PSI au bout dâun certain temps dâanoxie implique lâinduction de lâhydrogĂ©nase, acceptant les Ă©lectrons du PSI. Il reste possible dâinduire cette Ă©volution de lâoxydation de P700 lorsque les cellules sont constamment sous illumination dans les conditions anoxiques, impliquant cette fois ci, la voie de lâATP chloroplastique. LâATP synthĂ©tisĂ© Ă la lumiĂšre permettrait la consommation de NADPH via le cycle de Benson Calvin.The photosynthetic process relies on an electron flow involving several complexes in the thylakoid membranes of photosynthetic organisms. This flux can follow two possibly competing pathways: the linear electron transfer through which electrons are transferred from water (which is oxidized) to NADP+ (which is reduced), which is coupled to the generation of a transmembrane potential difference allowing the synthesis of ATP (Allen 2002); the cyclic pathway (around PSI and Cytochrome b6f complex) which only allows the production of ATP. These two pathways are thought to be essential for the reduction of CO2 and must likely coexist to allow the photosynthetic ATP/NADPH ratio to meet the requirement of the reduction of CO2 into carbohydrates (Seelert, Poetsch et al. 2000 ; Munekage, Hashimoto et al. 2004). This mere statement raises the question of the mechanisms that prevail in the implication of the same actors, within the same membrane, in either one of the two functional modes. In the green algae Chlamydomonas reinhardtii, our results show that the regulation of cyclic electron transfer is controlled by the redox poise and not by the lateral migration of antennae (Takahashi, Clowez et al. 2013), and disprove with the conclusion drawn from previous studies (BultĂ©, RebeillĂ© et al. 1990 ; Finazzi, Rappaport et al. 2002) according to which state transition would determine this switch. The association of these antennae to Photosystem I would promote the sequestration, within a single unit, of all the actors of the cyclic mode. Functional studies, in vitro, of supercomplex formation under anoxic conditions, questions on their functional capacities. This PhD work presents also the characterization of transient ââacceptor side limitationââ of PSI, upon the onset of anoxia where it is not possible to observe an oxidation of P700 in 705 nm. This phenomenon due to the charge recombination is created by an accumulation of NADPH. The spontaneous oxidation of the PSI acceptor pool, after some time under anoxia, involves the hydrogenase induction, accepting the electrons from NADPH. Itâs also possible to induce this PSI oxidation as soon as cells are constantly under illumination, involving chloroplast ATP pathway. ATP synthesised in the light, allow the consumption of NADPH through Benson-Calvin cycle
Functional consequences of thylakoid membranes reorganization in photosynthetic chain and switching between cyclic and linear electron transfer in green algae Chlamydomonas reinhardtii
Le processus photosynthĂ©tique se traduit par un flux dâĂ©lectron impliquant diffĂ©rents complexes de la membrane thylacoĂŻdale. Ce flux peut adopter deux chemins diffĂ©rents : le transfert dâĂ©lectron linĂ©aire (Merchant, Prochnik et al. 2007) Ă travers lequel les Ă©lectrons sont transfĂ©rĂ©s de lâeau oxydĂ©e au niveau du PhotosystĂšme II (PSII), au NADPH rĂ©duit par le PSI ; et le transfert dâĂ©lectron cyclique autour du PhotosystĂšme I (PSI) et du complexe cytochrome b6f. Ces flux dâĂ©lectrons sont couplĂ©s Ă un pompage de proton du stroma vers le lumen gĂ©nĂ©rant une diffĂ©rence de potentiel transmembranaire, permettant la synthĂšse dâATP (Allen 2002). La coexistence de ces deux flux est considĂ©rĂ© comme nĂ©cessaire Ă la fixation et la mĂ©tabolisation des molĂ©cules de dioxyde de carbone (Seelert, Poetsch et al. 2000 ; Munekage, Hashimoto et al. 2004) dans un rapport stricte ATP / NADPH. Cette coexistence qui semble essentiel soulĂšve la question des mĂ©canismes qui prĂ©valent Ă lâimplication des mĂȘmes acteurs photosynthĂ©tiques, dans une mĂȘme membrane, dans lâun ou lâautre mode de transfert dâĂ©lectron. Chez lâalgue verte Chlamydomonas reinhardtii, nous avons dĂ©montrĂ© que la commutation entre les deux transferts Ă©tait dĂ©pendante de lâĂ©tat redox des cellules, mais contrairement Ă ce qui avait Ă©tĂ© suggĂ©rĂ© dans les Ă©tudes prĂ©cĂ©dentes (BultĂ©, RebeillĂ© et al. 1990 ; Finazzi, Rappaport et al. 2002) indĂ©pendante du phĂ©nomĂšne de transition dâĂ©tat (Takahashi, Clowez et al. 2013), qui implique la migration latĂ©rale des complexes antennaires au sein de la membrane. Lâassociation de ces antennes au PhotosystĂšme I conduirait Ă la sĂ©questration, dans une mĂȘme entitĂ© biochimique, des diffĂ©rents acteurs du mode cyclique. Cette formation de supercomplexe dans les conditions anoxiques, Ă fait lâobjet dâune Ă©tude fonctionnelle in vitro, laissant quelques questions ouvertes sur leurs capacitĂ©s fonctionnelles. Ce travail de thĂšse prĂ©sente aussi la caractĂ©risation dâune limitation transitoire des accepteurs du PhotosystĂšme I, en dĂ©but dâanoxie pendant laquelle il nâest pas possible dâobserver dâoxydation de P700, Ă 705 nm. Ce phĂ©nomĂšne dĂ» Ă la recombinaison de charge est crĂ©Ă© par un engorgement du pool de NADPH. Lâoxydation spontanĂ©e du PSI au bout dâun certain temps dâanoxie implique lâinduction de lâhydrogĂ©nase, acceptant les Ă©lectrons du PSI. Il reste possible dâinduire cette Ă©volution de lâoxydation de P700 lorsque les cellules sont constamment sous illumination dans les conditions anoxiques, impliquant cette fois ci, la voie de lâATP chloroplastique. LâATP synthĂ©tisĂ© Ă la lumiĂšre permettrait la consommation de NADPH via le cycle de Benson Calvin.The photosynthetic process relies on an electron flow involving several complexes in the thylakoid membranes of photosynthetic organisms. This flux can follow two possibly competing pathways: the linear electron transfer through which electrons are transferred from water (which is oxidized) to NADP+ (which is reduced), which is coupled to the generation of a transmembrane potential difference allowing the synthesis of ATP (Allen 2002); the cyclic pathway (around PSI and Cytochrome b6f complex) which only allows the production of ATP. These two pathways are thought to be essential for the reduction of CO2 and must likely coexist to allow the photosynthetic ATP/NADPH ratio to meet the requirement of the reduction of CO2 into carbohydrates (Seelert, Poetsch et al. 2000 ; Munekage, Hashimoto et al. 2004). This mere statement raises the question of the mechanisms that prevail in the implication of the same actors, within the same membrane, in either one of the two functional modes. In the green algae Chlamydomonas reinhardtii, our results show that the regulation of cyclic electron transfer is controlled by the redox poise and not by the lateral migration of antennae (Takahashi, Clowez et al. 2013), and disprove with the conclusion drawn from previous studies (BultĂ©, RebeillĂ© et al. 1990 ; Finazzi, Rappaport et al. 2002) according to which state transition would determine this switch. The association of these antennae to Photosystem I would promote the sequestration, within a single unit, of all the actors of the cyclic mode. Functional studies, in vitro, of supercomplex formation under anoxic conditions, questions on their functional capacities. This PhD work presents also the characterization of transient ââacceptor side limitationââ of PSI, upon the onset of anoxia where it is not possible to observe an oxidation of P700 in 705 nm. This phenomenon due to the charge recombination is created by an accumulation of NADPH. The spontaneous oxidation of the PSI acceptor pool, after some time under anoxia, involves the hydrogenase induction, accepting the electrons from NADPH. Itâs also possible to induce this PSI oxidation as soon as cells are constantly under illumination, involving chloroplast ATP pathway. ATP synthesised in the light, allow the consumption of NADPH through Benson-Calvin cycle
The involvement of hydrogen-producing and ATP-dependent NADPH-consuming pathways in setting the redox poise in the chloroplast of Chlamydomonas reinhardtii in anoxia.
Photosynthetic microalgae are exposed to changing environmental conditions. In particular, microbes found in ponds or soils often face hypoxia or even anoxia, and this severely impacts their physiology. Chlamydomonas reinhardtii is one among such photosynthetic microorganisms recognized for its unusual wealth of fermentative pathways and the extensive remodeling of its metabolism upon the switch to anaerobic conditions. As regards the photosynthetic electron transfer, this remodeling encompasses a strong limitation of the electron flow downstream of photosystem I. Here, we further characterize the origin of this limitation. We show that it stems from the strong reducing pressure that builds up upon the onset of anoxia, and this pressure can be relieved either by the light-induced synthesis of ATP, which promotes the consumption of reducing equivalents, or by the progressive activation of the hydrogenase pathway, which provides an electron transfer pathway alternative to the CO2 fixation cycle
Interdependent iron and phosphorus availability controls photosynthesis through retrograde signaling
International audienceAbstract Iron deficiency hampers photosynthesis and is associated with chlorosis. We recently showed that iron deficiency-induced chlorosis depends on phosphorus availability. How plants integrate these cues to control chlorophyll accumulation is unknown. Here, we show that iron limitation downregulates photosynthesis genes in a phosphorus-dependent manner. Using transcriptomics and genome-wide association analysis, we identify two genes, PHT4;4 encoding a chloroplastic ascorbate transporter and bZIP58 , encoding a nuclear transcription factor, which prevent the downregulation of photosynthesis genes leading to the stay-green phenotype under iron-phosphorus deficiency. Joint limitation of these nutrients induces ascorbate accumulation by activating expression of an ascorbate biosynthesis gene, VTC4 , which requires bZIP58. Furthermore, we demonstrate that chloroplastic ascorbate transport prevents the downregulation of photosynthesis genes under iron-phosphorus combined deficiency through modulation of ROS homeostasis. Our study uncovers a ROS-mediated chloroplastic retrograde signaling pathway to adapt photosynthesis to nutrient availability
State transition7-dependent phosphorylation is modulated by changing environmental conditions, and its absence triggers remodeling of photosynthetic protein complexes
In plants and algae, the serine/threonine kinase STN7/STT7, orthologous protein kinases in Chlamydomonas reinhardtii and Arabidopsis (Arabidopsis thaliana), respectively, is an important regulator in acclimation to changing light environments. In this work, we assessed STT7-dependent protein phosphorylation under high light in C. reinhardtii, known to fully induce the expression of light-harvesting complex stress-related protein3 (LHCSR3) and a nonphotochemical quenching mechanism, in relationship to anoxia where the activity of cyclic electron flow is stimulated. Our quantitative proteomics data revealed numerous unique STT7 protein substrates and STT7-dependent protein phosphorylation variations that were reliant on the environmental condition. These results indicate that STT7-dependent phosphorylation is modulated by the environment and point to an intricate chloroplast phosphorylation network responding in a highly sensitive and dynamic manner to environmental cues and alterations in kinase function. Functionally, the absence of the STT7 kinase triggered changes in protein expression and photoinhibition of photosystem I (PSI) and resulted in the remodeling of photosynthetic complexes. This remodeling initiated a pronounced association of LHCSR3 with PSI-light harvesting complex I (LHCI)-ferredoxin-NADPH oxidoreductase supercomplexes. Lack of STT7 kinase strongly diminished PSII-LHCII supercomplexes, while PSII core complex phosphorylation and accumulation were significantly enhanced. In conclusion, our study provides strong evidence that the regulation of protein phosphorylation is critical for driving successful acclimation to high light and anoxic growth environments and gives new insights into acclimation strategies to these environmental conditions