In vivo interactions between photosynthesis, mitorespiration, and chlororespiration in Chlamydomonas reinhardtii

Genome-based approaches to understanding phosphorus deprivation responses and PSR1 control in Chlamydomonas reinhardtii

The Chlamydomonas reinhardtii transcription factor PSR1 is required for the control of activities involved in scavenging phosphate from the environment during periods of phosphorus limitation. Increased scavenging activity reflects the development of high-affinity phosphate transport and the expression of extracellular phosphatases that can cleave phosphate from organic compounds in the environment. A comparison of gene expression patterns using microarray analyses and quantitative PCRs with wild-type and psr1 mutant cells deprived of phosphorus has revealed that PSR1 also controls genes encoding proteins with potential "electron valve" functions--these proteins can serve as alternative electron acceptors that help prevent photodamage caused by overexcitation of the photosynthetic electron transport system. In accordance with this finding, phosphorus-starved psr1 mutants die when subjected to elevated light intensities; at these intensities, the wild-type cells still exhibit rapid growth. Acclimation to phosphorus deprivation also involves a reduction in the levels of transcripts encoding proteins involved in photosynthesis and both cytoplasmic and chloroplast translation as well as an increase in the levels of transcripts encoding stress-associated chaperones and proteases. Surprisingly, phosphorus-deficient psr1 cells (but not wild-type cells) also display expression patterns associated with specific responses to sulfur deprivation, suggesting a hitherto unsuspected link between the signal transduction pathways involved in controlling phosphorus and sulfur starvation responses. Together, these results demonstrate that PSR1 is critical for the survival of cells under conditions of suboptimal phosphorus availability and that it plays a key role in controlling both scavenging responses and the ability of the cell to manage excess absorbed excitation energy.

Distinct Roles of the Cytochrome Pathway and Alternative Oxidase in Leaf Photosynthesis

In illuminated leaves, mitochondria are thought to play roles in optimizing photosynthesis. However, the roles of the cytochrome pathway (CP) and alternative oxidase (AOX) in photosynthesis, in particular in the redox state of the photosynthetic electron transport chain, are not separately characterized. We examined the effects of specific inhibition of two respiratory pathways, CP and AOX, on photosynthetic oxygen evolution and the redox state of the photosynthetic electron transport chain in broad bean (Vicia faba L.) leaves under various light intensities. Under saturating photosynthetic photon flux density (PPFD; 700 micromol photon m(-2) s(-1)), inhibition of either pathway caused a decrease in the steady-state levels of the photosynthetic O(2) evolution rate and the PSII operating efficiency, Phi(II). Because these inhibitors, at the concentrations applied to the leaves, had little effect on photosynthesis in the intact chloroplasts, two respiratory pathways are essential for maintenance of high photosynthetic rates at saturating PPFD. CP or AOX inhibition affected to Chl fluorescence parameters (e.g. photochemical quenching and non-photochemical quenching) differently, suggesting that CP and AOX contribute to photosynthesis in different ways. At low PPFD (100 micromol photon m(-2) s(-1)), only the inhibition of AOX, not CP, lowered the photosynthetic rate and Phi(II). AOX inhibition also decreased the Phi(II)/Phi(I) ratio even at low PPFD levels. These data suggest that AOX inhibition caused the over-reduction of the photosynthetic electron transport chain and induced the cyclic electron flow around PSI (CEF-PSI) even at the low PPFD. Based on these results, we discuss possible roles for CP and AOX in the light.

Inhibitor studies on non-photochemical plastoquinone reduction and H(2) photoproduction in Chlamydomonas reinhardtii

In the absence of PSII, non-photochemical reduction of plastoquinones (PQs) occurs following NADH or NADPH addition in thylakoid membranes of the green alga Chlamydomonas reinhardtii. The nature of the enzyme involved in this reaction has been investigated in vitro by measuring chlorophyll fluorescence increase in anoxia and light-dependent O(2) uptake in the presence of methyl viologen. Based on the insensitivity of these reactions to rotenone, a type-I NADH dehydrogenase (NDH-1) inhibitor, and their sensitivity to flavoenzyme inhibitors and thiol blocking agents, we conclude to the involvement of a type-II NADH dehydrogenase (NDH-2) in PQ reduction. Intact Chlamydomonas cells placed in anoxia have the property to produce H(2) in the light by a Fe-hydrogenase which uses reduced ferredoxin as an electron donor. H(2) production also occurs in the absence of PSII thanks to the existence of a non-photochemical pathway of PQ reduction. From inhibitors effects, we suggest the involvement of a plastidial NDH-2 in PSII-independent H(2) production in Chlamydomonas. These results are discussed in relation to the absence of ndh genes in Chlamydomonas plastid genome and to the existence of 7 ORFs homologous to type-II NDHs in its nuclear genome.

The exceptional photofermentative hydrogen metabolism of the green alga Chlamydomonas reinhardtii

The photosynthetic green alga Chlamydomonas reinhardtii is capable of performing a complex fermentative metabolism which is related to the mixed acid fermentation of bacteria such as Escherichia coli. The fermentative pattern includes the products formate, ethanol, acetate, glycerol, lactate, carbon dioxide and molecular hydrogen (H(2)). H(2) production is catalysed by an active [Fe]-hydrogenase (HydA) which is coupled with the photosynthetic electron-transport chain. The most important enzyme of the classic fermentation pathway is pyruvate formate-lyase, which is common in bacteria but seldom found in eukaryotes. An interaction between fermentation, photosynthesis and H(2) evolution allows the algae to overcome long periods of anaerobiosis. In the absence of sulphur, the cells establish a photofermentative metabolism and accumulate large amounts of H(2).

The photosynthesis of individual algal cells during the cell cycle of Scenedesmus quadricauda studied by chlorophyll fluorescence kinetic microscopy

A microscope for imaging of chlorophyll fluorescence kinetics was equipped with a chamber that allows the growth of an immobilised population of algae and their study under well-defined conditions. Single cells of the chlorococcal alga Scenedesmus quadricauda were grown and recorded for periods of whole cell cycles (up to 48 h) displaying a normal course of cell development. Heterogeneity in fluorescence yield among individual coenobia in the population and among different cells in one coenobium were analysed. Differences were observed both in the shape of Kautsky transients and in the modulation of fluorescence parameter values during the progress of the cell cycle. The extent of heterogeneity in fluorescence parameters was cell cycle dependent - in some phases of the cycle, the population was almost homogeneous, while distinct heterogeneity was observed, in particular between the protoplast division and the release of the daughter coenobia. The heterogeneity was not random but reflected developmental processes.

State transitions in Chlamydomonas reinhardtii. The role of the Mehler reaction in state 2-to-state 1 transition

The light intensity-dependent transition to state 1 of dark-adapted anaerobic state 2 Chlamydomonas reinhardtii cells is stimulated by oxygen and by other electron acceptors for photosystem I, such as oxaloacetate and methylviologen. This suggests that the transition to state 1 requires the oxidation of the intersystem chain by photosystem I photochemistry. On the other hand, the mere oxidation in the dark of the chain-by addition of O2-leads only to a slow and incomplete transition. The light-driven stimulation by O2 of the state 1 transition is saturated at an O2 concentration of 15 to 20 microm, definitely higher than that of respiration. We suggest that this may represent the affinity for oxygen of the Mehler reaction, a conclusion that is confirmed by the observations that mitochondrial respiration is apparently not involved in modulating state 2-to-state 1 transition. The catalysis of the state 2-to-state 1 transition upon illumination of anaerobically adapted algae might represent, therefore, a relevant physiological role of this process in C. reinhardtii.

Enhanced rates of P700(+) dark-reduction in leaves of Cucumis sativus L photoinhibited at chilling temperature

The changes in electron transport within photosystem I (PSI) were studied in detached leaves of Cucumis sativus L. during the course of irradiation with moderate white light (300 micromol photons m(-2) s(-1)) at 4 degrees C. When intact leaves were exposed to the combination of moderate light and low temperature, the amplitude of far-red light-induced P700 absorbance changes at 820 nm (deltaA(820)), a relative measure of PSI, progressively decreased as the light treatment time increased. Almost no oxidation of P700 was noticeable after 5 h. Methyl viologen accelerated the oxidation of P700 to a steady-state level and also increased the magnitudes of deltaA(820) changes in photoinhibited leaves, reflecting the rapid removal of electrons from native carriers. Photoinhibition under moderate light and chilling temperature also accelerated the rate of P700(+) reduction after far-red light excitation as the half-times of the two exponential components of P700(+) decay curves decreased relative to the control ones. A detailed analysis of the kinetics of P700(+) reduction using diuron alone or the combination of diuron and methyl viologen strongly favours an increased rate of electron donation from stromal reductants to PSI through the plastoquinone pool following photoinhibitory treatment. Importantly, the marked acceleration of P700(+) re-reduction is the consequence of the irradiation of leaf segments at low temperature and not caused by chilling stress alone.

The dependence of algal H2 production on Photosystem II and O2 consumption activities in sulfur-deprived Chlamydomonas reinhardtii cells

Chlamydomonas reinhardtii cultures, deprived of inorganic sulfur, undergo dramatic changes during adaptation to the nutrient stress [Biotechnol. Bioeng. 78 (2002) 731]. When the capacity for Photosystem II (PSII) O(2) evolution decreases below that of respiration, the culture becomes anaerobic [Plant Physiol. 122 (2000) 127]. We demonstrate that (a) the photochemical activity of PSII, monitored by in situ fluorescence, also decreases slowly during the aerobic period; (b) at the exact time of anaerobiosis, the remaining PSII activity is rapidly down regulated; and (c) electron transfer from PSII to PSI abruptly decreases at that point. Shortly thereafter, the PSII photochemical activity is partially restored, and H(2) production starts. Hydrogen production, which lasts for 3-4 days, is catalyzed by an anaerobically induced, reversible hydrogenase. While most of the reductants used directly for H(2) gas photoproduction come from water, the remaining electrons must come from endogenous substrate degradation through the NAD(P)H plastoquinone (PQ) oxido-reductase pathway. We propose that the induced hydrogenase activity provides a sink for electrons in the absence of other alternative pathways, and its operation allows the partial oxidation of intermediate photosynthetic carriers, including the PQ pool, between PSII and PSI. We conclude that the reduced state of this pool, which controls PSII photochemical activity, is one of the main factors regulating H(2) production under sulfur-deprived conditions. Residual O(2) evolved under these conditions is probably consumed mostly by the aerobic oxidation of storage products linked to mitochondrial respiratory processes involving both the cytochrome oxidase and the alternative oxidase. These functions maintain the intracellular anaerobic conditions required to keep the hydrogenase enzyme in the active, induced form.

Alternative photosystem I-driven electron transport routes: mechanisms and functions

In addition to the linear electron transport, several alternative Photosystem I-driven (PS I) electron pathways recycle the electrons to the intersystem electron carriers mediated by either ferredoxin:NADPH reductase, NAD(P)H dehydrogenase, or putative ferredoxin:plastoquinone reductase. The following functions have been proposed for these pathways: adjustment of ATP/NADPH ratio required for CO(2) fixation, generation of the proton gradient for the down-regulation of Photosystem II (PS II), and ATP supply the active transport of inorganic carbon in algal cells. Unlike ferredoxin-dependent cyclic electron transport, the pathways supported by NAD(P)H can function in the dark and are likely involved in chlororespiratory-dependent energization of the thylakoid membrane. This energization may support carotenoid biosynthesis and/or maintain thylakoid ATPase in active state. Active operation of ferredoxin-dependent cyclic electron transport requires moderate reduction of both the intersystem electron carriers and the acceptor side of PS I, whereas the rate of NAD(P)H-dependent pathways under light depends largely on NAD(P)H accumulation in the stroma. Environmental stresses such as photoinhibition, high temperatures, drought, or high salinity stimulated the activity of alternative PS I-driven electron transport pathways. Thus, the energetic and regulatory functions of PS I-driven pathways must be an integral part of photosynthetic organisms and provides additional flexibility to environmental stress.

Redox control, redox signaling, and redox homeostasis in plant cells

Redox chemistry is a key feature of life. Oxidized substrates are reduced to synthesize functional molecules; reduced substrates are oxidized for energy supply. In addition, cells must fight against uncontrolled oxidation of essential constituents, a process that continuously occurs in an atmosphere of 21% O2. The redox situation is further complicated in plants with their highly reactive photosynthetic metabolism. To this end it is now well established that redox regulation is a central element in adjusting plant metabolism and development to the prevailing environmental conditions. This review introduces general redox chemistry and the main components of the cellular redox network, namely pyridine nucleotides, ascorbate, glutathione, lipoic acid, tocopherol, thioredoxins, glutaredoxins, peroxiredoxins, and other thiol proteins. Examples for redox sensing, transduction, redox-regulated enzymes and transcription, and the function of regulatory circuits are presented. Emphasis is placed on redox regulation of photosynthesis, which is the best understood metabolism governed by redox control on essentially all levels, ranging from gene transcription to translation, assembly and turnover, as well as short-term adaptation by state transition and enzyme activity. Increasing evidence shows the importance of redox regulation in the context of transport, plant development, and programmed cell death.