Chlororespiration revisited: Mitochondrial-plastid interactions in Chlamydomonas

Weak acids produced during anaerobic respiration suppress both photosynthesis and aerobic respiration

While photosynthesis transforms sunlight energy into sugar, aerobic and anaerobic respiration (fermentation) catabolizes sugars to fuel cellular activities. These processes take place within one cell across several compartments, however it remains largely unexplored how they interact with one another. Here we report that the weak acids produced during fermentation down-regulate both photosynthesis and aerobic respiration. This effect is mechanistically explained with an "ion trapping" model, in which the lipid bilayer selectively traps protons that effectively acidify subcellular compartments with smaller buffer capacities - such as the thylakoid lumen. Physiologically, we propose that under certain conditions, e.g., dim light at dawn, tuning down the photosynthetic light reaction could mitigate the pressure on its electron transport chains, while suppression of respiration could accelerate the net oxygen evolution, thus speeding up the recovery from hypoxia. Since we show that this effect is conserved across photosynthetic phyla, these results indicate that fermentation metabolites exert widespread feedback control over photosynthesis and aerobic respiration. This likely allows algae to better cope with changing environmental conditions.

Electron transfer via cytochrome b6f complex displays sensitivity to Antimycin A upon STT7 kinase activation

The cytochrome b6f complex (b6f) has been initially considered as the ferredoxin-plastoquinone reductase (FQR) during cyclic electron flow (CEF) with photosystem I that is inhibited by antimycin A (AA). The binding of AA to the b6f Qi-site is aggravated by heme-ci, which challenged the FQR function of b6f during CEF. Alternative models suggest that PROTON GRADIENT REGULATION5 (PGR5) is involved in a b6f-independent, AA-sensitive FQR. Here, we show in Chlamydomonas reinhardtii that the b6f is conditionally inhibited by AA in vivo and that the inhibition did not require PGR5. Instead, activation of the STT7 kinase upon anaerobic treatment induced the AA sensitivity of b6f which was absent in stt7-1. However, a lock in State 2 due to persisting phosphorylation in the phosphatase double mutant pph1;pbcp did not increase AA sensitivity of electron transfer. The latter required a redox poise, supporting the view that state transitions and CEF are not coercively coupled. This suggests that the b6f-interacting kinase is required for structure-function modulation of the Qi-site under CEF favoring conditions. We propose that PGR5 and STT7 independently sustain AA-sensitive FQR activity of the b6f. Accordingly, PGR5-mediated electron injection into an STT7-modulated Qi-site drives a Mitchellian Q cycle in CEF conditions.

Proton motive force in plant photosynthesis dominated by ΔpH in both low and high light

The proton motive force (pmf) across the thylakoid membrane couples photosynthetic electron transport and ATP synthesis. In recent years, the electrochromic carotenoid and chlorophyll absorption band shift (ECS), peaking ∼515 nm, has become a widely used probe to measure pmf in leaves. However, the use of this technique to calculate the parsing of the pmf between the proton gradient (ΔpH) and electric potential (Δψ) components remains controversial. Interpretation of the ECS signal is complicated by overlapping absorption changes associated with violaxanthin de-epoxidation to zeaxanthin (ΔA505) and energy-dependent nonphotochemical quenching (qE; ΔA535). In this study, we used Arabidopsis (Arabidopsis thaliana) plants with altered xanthophyll cycle activity and photosystem II subunit S (PsbS) content to disentangle these overlapping contributions. In plants where overlap among ΔA505, ΔA535, and ECS is diminished, such as npq4 (lacking ΔA535) and npq1npq4 (also lacking ΔA505), the parsing method implies the Δψ contribution is virtually absent and pmf is solely composed of ΔpH. Conversely, in plants where ΔA535 and ECS overlap is enhanced, such as L17 (a PsbS overexpressor) and npq1 (where ΔA535 is blue-shifted to 525 nm) the parsing method implies a dominant contribution of Δψ to the total pmf. These results demonstrate the vast majority of the pmf attributed by the ECS parsing method to Δψ is caused by ΔA505 and ΔA535 overlap, confirming pmf is dominated by ΔpH following the first 60 s of continuous illumination under both low and high light conditions. Further implications of these findings for the regulation of photosynthesis are discussed.

Disentangling chloroplast ATP synthase regulation by proton motive force and thiol modulation in Arabidopsis leaves

The chloroplast ATP synthase (CF₁F_o) contains a specific feature to the green lineage: a γ-subunit redox domain that contains a cysteine couple which interacts with the torque-transmitting βDELSEED-loop. This thiol modulation equips CF₁F_o with an important environmental fine-tuning mechanism. In vitro, disulfide formation in the γ-redox domain slows down the activity of the CF₁F_o at low transmembrane electrochemical proton gradient ( [Formula: see text] ), which agrees with its proposed role as chock based on recently solved structure. The γ-dithiol formation at the onset of light is crucial to maximize photosynthetic efficiency since it lowers the [Formula: see text] activation level for ATP synthesis in vitro. Here, we validate these findings in vivo by utilizing absorption spectroscopy in Arabidopsis thaliana. To do so, we monitored the [Formula: see text] present in darkness and identified its mitochondrial sources. By following the fate and components of light-induced extra [Formula: see text] , we estimated the ATP lifetime that lasted up to tens of minutes after long illuminations. Based on the relationship between [Formula: see text] and CF₁F_o activity, we conclude that the dithiol configuration in vivo facilitates photosynthesis by driving the same ATP synthesis rate at a significative lower [Formula: see text] than in the γ-disulfide state. The presented in vivo findings are an additional proof of the importance of CF₁F_o thiol modulation, reconciling biochemical in vitro studies and structural insights.

Diclofenac and atrazine restrict the growth of a synchronous Chlamydomonas reinhardtii population via various mechanisms

Non-steroidal anti-inflammatory drug diclofenac (DCF) is commonly found in freshwater bodies and can have adverse effects on non-target organisms. Among the studies on DCF toxicity, several ones have reported its harmful effects on plants and algae. To gain a better understanding of the mechanisms of DCF toxicity towards green algae, we used a synchronous Chlamydomonas reinhardtii cc-1690 culture and compared DCF (135 mg/L) effects with effects caused by atrazine (ATR; 77.6 μg/L), an herbicide with a well-known mechanism of toxic action. To achieve our goal, cell number and size, photosynthetic oxygen consumption/evolution, chlorophyll a fluorescence in vivo, H₂O₂ production by the cells, antioxidative enzymes encoding genes expression were analyzed during light phase of the cell cycle. We have found, that DCF and ATR affect C. reinhardtii through different mechanisms. ATR inhibited the photosynthetic electron transport chain and induced oxidative stress in chloroplast. Such chloroplastic energetics disruption indirectly influenced respiration, the intensification of which could partially mitigate low efficiency of photosynthetic energy production. As a result, ATR inhibited the growth of single cell leading to limitation in C. reinhardtii population development. In contrast to ATR-treated algae, in DCF-treated cells the fraction of active PSII reaction centers was diminished without drastic changes in electron transport or oxidative stress symptoms in chloroplast. However, significant increase in transcript level of gene encoding for mitochondria-located catalase indicates respiratory processes as a source of H₂O₂ overproduced in the DCF-treated cells. Because the single cell growth was not strongly affected by DCF, its adverse effect on progeny cell number seemed to be related rather to arresting of cell divisions. Concluding, although the DCF phytotoxic action appeared to be different from the action of the typical herbicide ATR, it can act as algal growth-inhibiting factor in the environment.

Chloroplast function and ion regulation in plants growing on saline soils: lessons from halophytes

Salt stress impacts multiple aspects of plant metabolism and physiology. For instance it inhibits photosynthesis through stomatal limitation, causes excessive accumulation of sodium and chloride in chloroplasts, and disturbs chloroplast potassium homeostasis. Most research on salt stress has focused primarily on cytosolic ion homeostasis with few studies of how salt stress affects chloroplast ion homeostasis. This review asks the question whether membrane-transport processes and ionic relations are differentially regulated between glycophyte and halophyte chloroplasts and whether this contributes to the superior salt tolerance of halophytes. The available literature indicates that halophytes can overcome stomatal limitation by switching to CO2 concentrating mechanisms and increasing the number of chloroplasts per cell under saline conditions. Furthermore, salt entry into the chloroplast stroma may be critical for grana formation and photosystem II activity in halophytes but not in glycophytes. Salt also inhibits some stromal enzymes (e.g. fructose-1,6-bisphosphatase) to a lesser extent in halophyte species. Halophytes accumulate more chloride in chloroplasts than glycophytes and appear to use sodium in functional roles. We propose the molecular identities of candidate transporters that move sodium, chloride and potassium across chloroplast membranes and discuss how their operation may regulate photochemistry and photosystem I and II activity in chloroplasts.

The Plastid Terminal Oxidase is a Key Factor Balancing the Redox State of Thylakoid Membrane

Mitochondria possess oxygen-consuming respiratory electron transfer chains (RETCs), and the oxygen-evolving photosynthetic electron transfer chain (PETC) resides in chloroplasts. Evolutionarily mitochondria and chloroplasts are derived from ancient α-proteobacteria and cyanobacteria, respectively. However, cyanobacteria harbor both RETC and PETC on their thylakoid membranes. It is proposed that chloroplasts could possess a RETC on the thylakoid membrane, in addition to PETC. Identification of a plastid terminal oxidase (PTOX) in the chloroplast from the Arabidopsis variegation mutant immutans (im) demonstrated the presence of a RETC in chloroplasts, and the PTOX is the committed oxidase. PTOX is distantly related to the mitochondrial alternative oxidase (AOX), which is responsible for the CN-insensitive alternative RETC. Similar to AOX, an ubiquinol (UQH2) oxidase, PTOX is a plastoquinol (PQH2) oxidase on the chloroplast thylakoid membrane. Lack of PTOX, Arabidopsis im showed a light-dependent variegation phenotype; and mutant plants will not survive the mediocre light intensity during its early development stage. PTOX is very important for carotenoid biosynthesis, since the phytoene desaturation, a key step in the carotenoid biosynthesis, is blocked in the white sectors of Arabidopsis im mutant. PTOX is found to be a stress-related protein in numerous research instances. It is generally believed that PTOX can protect plants from various environmental stresses, especially high light stress. PTOX also plays significant roles in chloroplast development and plant morphogenesis. Global physiological roles played by PTOX could be a direct or indirect consequence of its PQH2 oxidase activity to maintain the PQ pool redox state on the thylakoid membrane. The PTOX-dependent chloroplast RETC (so-called chlororespiration) does not contribute significantly when chloroplast PETC is normally developed and functions well. However, PTOX-mediated RETC could be the major force to regulate the PQ pool redox balance in the darkness, under conditions of stress, in nonphotosynthetic plastids, especially in the early development from proplastids to chloroplasts.

A bestrophin-like protein modulates the proton motive force across the thylakoid membrane in Arabidopsis

During photosynthesis, photosynthetic electron transport generates a proton motive force (pmf) across the thylakoid membrane, which is used for ATP biosynthesis via ATP synthase in the chloroplast. The pmf is composed of an electric potential (ΔΨ) and an osmotic component (ΔpH). Partitioning between these components in chloroplasts is strictly regulated in response to fluctuating environments. However, our knowledge of the molecular mechanisms that regulate pmf partitioning is limited. Here, we report a bestrophin-like protein (AtBest), which is critical for pmf partitioning. While the ΔpH component was slightly reduced in atbest, the ΔΨ component was much greater in this mutant than in the wild type, resulting in less efficient activation of nonphotochemical quenching (NPQ) upon both illumination and a shift from low light to high light. Although no visible phenotype was observed in the atbest mutant in the greenhouse, this mutant exhibited stronger photoinhibition than the wild type when grown in the field. AtBest belongs to the bestrophin family proteins, which are believed to function as chloride (Cl^- ) channels. Thus, our findings reveal an important Cl^- channel required for ion transport and homeostasis across the thylakoid membrane in higher plants. These processes are essential for fine-tuning photosynthesis under fluctuating environmental conditions.

Transport Across Chloroplast Membranes: Optimizing Photosynthesis for Adverse Environmental Conditions

Chloroplasts are central to solar light harvesting and photosynthesis. Optimal chloroplast functioning is vitally dependent on a very intensive traffic of metabolites and ions between the cytosol and stroma, and should be attuned for adverse environmental conditions. This is achieved by an orchestrated regulation of a variety of transport systems located at chloroplast membranes such as porines, solute channels, ion-specific cation and anion channels, and various primary and secondary active transport systems. In this review we describe the molecular nature and functional properties of the inner and outer envelope and thylakoid membrane channels and transporters. We then discuss how their orchestrated regulation affects thylakoid structure, electron transport and excitation energy transfer, proton-motive force partition, ion homeostasis, stromal pH regulation, and volume regulation. We link the activity of key cation and anion transport systems with stress-specific signaling processes in chloroplasts, and discuss how these signals interact with the signals generated in other organelles to optimize the cell performance, with a special emphasis on Ca(2+) and reactive oxygen species signaling.

The slow S to M rise of chlorophyll a fluorescence reflects transition from state 2 to state 1 in the green alga Chlamydomonas reinhardtii

The green alga Chlamydomonas (C.) reinhardtii is a model organism for photosynthesis research. State transitions regulate redistribution of excitation energy between photosystem I (PS I) and photosystem II (PS II) to provide balanced photosynthesis. Chlorophyll (Chl) a fluorescence induction (the so-called OJIPSMT transient) is a signature of several photosynthetic reactions. Here, we show that the slow (seconds to minutes) S to M fluorescence rise is reduced or absent in the stt7 mutant (which is locked in state 1) in C. reinhardtii. This suggests that the SM rise in wild type C. reinhardtii may be due to state 2 (low fluorescence state; larger antenna in PS I) to state 1 (high fluorescence state; larger antenna in PS II) transition, and thus, it can be used as an efficient and quick method to monitor state transitions in algae, as has already been shown in cyanobacteria (Papageorgiou et al. 1999, 2007; Kaňa et al. 2012). We also discuss our results on the effects of (1) 3-(3,4-dichlorophenyl)-1,4-dimethyl urea, an inhibitor of electron transport; (2) n-propyl gallate, an inhibitor of alternative oxidase (AOX) in mitochondria and of plastid terminal oxidase in chloroplasts; (3) salicylhydroxamic acid, an inhibitor of AOX in mitochondria; and (4) carbonyl cyanide p-trifluoromethoxyphenylhydrazone, an uncoupler of phosphorylation, which dissipates proton gradient across membranes. Based on the data presented in this paper, we conclude that the slow PSMT fluorescence transient in C. reinhardtii is due to the superimposition of, at least, two phenomena: qE dependent non-photochemical quenching of the excited state of Chl, and state transitions.

Lichen symbiosis: nature's high yielding machines for induced hydrogen production

Hydrogen is a promising future energy source. Although the ability of green algae to produce hydrogen has long been recognized (since 1939) and several biotechnological applications have been attempted, the greatest obstacle, being the O2-sensitivity of the hydrogenase enzyme, has not yet been overcome. In the present contribution, 75 years after the first report on algal hydrogen production, taking advantage of a natural mechanism of oxygen balance, we demonstrate high hydrogen yields by lichens. Lichens have been selected as the ideal organisms in nature for hydrogen production, since they consist of a mycobiont and a photobiont in symbiosis. It has been hypothesized that the mycobiont's and photobiont's consumption of oxygen (increase of COX and AOX proteins of mitochondrial respiratory pathways and PTOX protein of chrolorespiration) establishes the required anoxic conditions for the activation of the phycobiont's hydrogenase in a closed system. Our results clearly supported the above hypothesis, showing that lichens have the ability to activate appropriate bioenergetic pathways depending on the specific incubation conditions. Under light conditions, they successfully use the PSII-dependent and the PSII-independent pathways (decrease of D1 protein and parallel increase of PSaA protein) to transfer electrons to hydrogenase, while under dark conditions, lichens use the PFOR enzyme and the dark fermentative pathway to supply electrons to hydrogenase. These advantages of lichen symbiosis in combination with their ability to survive in extreme environments (while in a dry state) constitute them as unique and valuable hydrogen producing natural factories and pave the way for future biotechnological applications.

Involvement of chlororespiration in chilling stress in the tropical species Spathiphyllum wallisii

Spathiphyllum wallisii plants were used to study the effect of chilling stress under high illumination on photosynthesis and chlororespiration. Leaves showed different responses that depended on root temperature. When stem, but not root, was chilled, photosystem II (PSII) was strongly photoinhibited. However, when the whole plant was chilled, the maximal quantum yield of PSII decreased only slightly below the normal values and cyclic electron transport was stimulated. Changes were also observed in the chlororespiration enzymes and PGR5. In whole plants chilled under high illumination, the amounts of NADH dehydrogenase (NDH) complex and plastid terminal oxidase (PTOX) remained similar to control and increased when only stem was chilled. In contrast, the amount of PGR5 polypeptide was higher in plants when both root and stem were chilled than in plants in which only stem was chilled. The results indicated that the contribution of chlororespiration to regulating photosynthetic electron flow is not relevant when the whole plant is chilled under high light, and that another pathway, such as cyclic electron flow involving PGR5 polypeptide, may be more important. However, when PSII activity is strongly photoinhibited in plants in which only stem is chilled, chlororespiration, together with other routes of electron input to the electron transfer chain, is probably essential.

The plastid terminal oxidase: its elusive function points to multiple contributions to plastid physiology

Plastids have retained from their cyanobacterial ancestor a fragment of the respiratory electron chain comprising an NADPH dehydrogenase and a diiron oxidase, which sustain the so-called chlororespiration pathway. Despite its very low turnover rates compared with photosynthetic electron flow, knocking out the plastid terminal oxidase (PTOX) in plants or microalgae leads to severe phenotypes that encompass developmental and growth defects together with increased photosensitivity. On the basis of a phylogenetic and structural analysis of the enzyme, we discuss its physiological contribution to chloroplast metabolism, with an emphasis on its critical function in setting the redox poise of the chloroplast stroma in darkness. The emerging picture of PTOX is that of an enzyme at the crossroads of a variety of metabolic processes, such as, among others, the regulation of cyclic electron transfer and carotenoid biosynthesis, which have in common their dependence on the redox state of the plastoquinone pool, set largely by the activity of PTOX in darkness.

Rethinking the existence of a steady-state Δψ component of the proton motive force across plant thylakoid membranes

Light-driven photosynthetic electron transport is coupled to the movement of protons from the chloroplast stroma to the thylakoid lumen. The resulting proton motive force that is generated is used to drive the conformational rotation of the transmembrane thylakoid ATPase enzyme which converts ADP (adenosine diphosphate) and Pi (inorganic phosphate) into ATP (adenosine triphosphate), the energy currency of the plant cell required for carbon fixation and other metabolic processes. According to Mitchell's chemiosmotic hypothesis, the proton motive force can be parsed into the transmembrane proton gradient (ΔpH) and the electric field gradient (Δψ), which are thermodynamically equivalent. In chloroplasts, the proton motive force has been suggested to be split almost equally between Δψ and ΔpH (Kramer et al., Photosynth Res 60:151-163, 1999). One of the central pieces of evidence for this theory is the existence of a steady-state electrochromic shift (ECS) absorption signal detected ~515 nm in plant leaves during illumination. The interpretation of this signal is complicated, however, by a heavily overlapping absorption change ~535 nm associated with the formation of photoprotective energy dissipation (qE) during illumination. In this study, we present new evidence that dissects the overlapping contributions of the ECS and qE-related absorption changes in wild-type Arabidopsis leaves using specific inhibitors of the ΔpH (nigericin) and Δψ (valinomycin) and separately using leaves of the Arabidopsis lut2npq1 mutant that lacks qE. In both cases, our data show that no steady-state ECS signal persists in the light longer than ~60 s. The consequences of our observations for the suggesting parsing of steady-state thylakoid proton motive force between (ΔpH) and the electric field gradient (Δψ) are discussed.

Photosynthesis in Chondrus crispus: the contribution of energy spill-over in the regulation of excitonic flux

Chondrus crispus is a species of red algae that grows on rocks from the middle intertidal into the subtidal zones of the North Atlantic coasts. As such, it has to cope with strongly variable abiotic conditions. Here we studied the response of the photosynthetic apparatus of this red alga to illumination. We found that, as previously described in the case of the unicellular alga Rhodella violacea (E. Delphin et al., Plant Physiol. 118 (1998) 103-113), a single multi-turnover saturating pulse of light is sufficient to induce a strong quenching of fluorescence. To elucidate the mechanisms underlying this fluorescence quenching, we combined room temperature and 77K fluorescence measurements with absorption spectroscopy to monitor the redox state of the different electron carriers in the chain. In addition, we studied the dependence of these various observables upon the excitation wavelength. This led us to identify energy spill-over from Photosystem II to Photosystem I rather than a qE-type non-photochemical quenching as the major source of fluorescence quenching that develops upon a series of 200ms pulses of saturating light results, in line with the conclusion of Ley and Butler (Biochim. Biophys. Acta 592 (1980) 349-363) from their studies of the unicellular red alga Porphyridium cruentum. In addition, we show that the onset of this spill-over is triggered by the reduction of the plastoquinone pool.

Water deficit and heat affect the tolerance to high illumination in hibiscus plants

This work studies the effects of water deficit and heat, as well as the involvement of chlororespiration and the ferredoxin-mediated cyclic pathway, on the tolerance of photosynthesis to high light intensity in Hibiscus rosa-sinensis plants. Drought and heat resulted in the down–regulation of photosynthetic linear electron transport in the leaves, although only a slight decrease in variable fluorescence (F_v)/maximal fluorescence (F_m) was observed, indicating that the chloroplast was protected by mechanisms that dissipate excess excitation energy to prevent damage to the photosynthetic apparatus. The incubation of leaves from unstressed plants under high light intensity resulted in an increase of the activity of electron donation by nicotinamide adenine dinucleotide phosphate (NADPH) and ferredoxin to plastoquinone, but no increase was observed in plants exposed to water deficit, suggesting that cyclic electron transport was stimulated by high light only in control plants. In contrast, the activities of the chlororespiration enzymes (NADH dehydrogenase (NDH) complex and plastid terminal oxidase (PTOX)) increased after incubation under high light intensity in leaves of the water deficit plants, but not in control plants, suggesting that chlororespiration was stimulated in stressed plants. The results indicate that the relative importance of chlororespiration and the cyclic electron pathway in the tolerance of photosynthesis to high illumination differs under stress conditions. When plants were not subjected to stress, the contribution of chlororespiration to photosynthetic electron flow regulation was not relevant, and another pathway, such as the ferredoxin-mediated cyclic pathway, was more important. However, when plants were subjected to water deficit and heat, chlororespiration was probably essential.

Stimulation of chlororespiration by drought under heat and high illumination in Rosa meillandina

Rosa meillandina plants were used to study the effects of water deficit on photosynthesis and chlororespiration. Plants showed high tolerance to heat and high illumination in controlled conditions that ensured that there was no water deficit. However, when heat and high illumination were accompanied by low watering photosynthetic linear electron transport was down regulated, as indicated by the reduced photochemistry efficiency of PS II, which was associated with an increase in the non-photochemical quenching of fluorescence. In addition to the effects on the photosynthetic activity, changes were also observed in the plastidial NDH complex, PTOX and PGR5. In plants exposed to heat and high illumination without water deficit, the activities and amounts of the chlororespiration enzymes, NDH complex and PTOX, remained similar to the control and only increased in response to drought, high light and heat stress, applied together. In contrast, both the PS I activity and the amount of PGR5 polypeptide were higher in plants exposed to heat and high illumination without water deficit than in those with water deficit. The results indicated that in the conditions studied, the contribution of chlororespiration to regulating photosynthetic electron flow is not relevant when there is no water deficit, and another pathway, such as cyclic electron flow involving PGR5 polypeptide, may be more important. However, when PS II activity is inhibited by drought, chlororespiration, together with other routes of electron input to the electron transfer chain, is probably essential.

Cyclic electron transport around photosystem I and its relationship to non-photochemical quenching in the unicellular green alga Dunaliella salina under nitrogen deficiency

Electron transport in photosystem II (PSII) and photosystem I (PSI) was estimated in terms of chlorophyll fluorescence and changes in P700 redox, respectively, in the unicellular green alga Dunaliella salina in the presence or absence of a nitrogen source in the culture medium. In a nitrogen-containing medium, the quantum yield of PSII (Φ(II)) and that in PSI (Φ(I)) were at the same level in low light, but cyclic electron transport around photosystem I (CET-PSI) was induced under high light as estimated from an increase in Φ(I)/Φ(II). High light might further enhance the rate of electron transport in PSI by inducing the state 2 transition, in which the distribution of light energy is shifted to PSI at the expense of PSII. Nitrogen deficiency resulted in a decrease in Φ(II) and an increase in Φ(I). As a consequence, the rate of CET-PSI was expected to increase. The high CET-PSI under N deficiency was probably associated with a high level of energy quenching (qE) formation in PSII.

Anaerobiosis induced state transition: a non photochemical reduction of PQ pool mediated by NDH in Arabidopsis thaliana

Background

Non photochemical reduction of PQ pool and mobilization of LHCII between PSII and PSI are found to be linked under abiotic stress conditions. The interaction of non photochemical reduction of PQ pool and state transitions associated physiological changes are critically important under anaerobic condition in higher plants.

Methodology/Findings

The present study focused on the effect of anaerobiosis on non-photochemical reduction of PQ pool which trigger state II transition in Arabidopsis thaliana. Upon exposure to dark-anaerobic condition the shape of the OJIP transient rise is completely altered where as in aerobic treated leaves the rise is unaltered. Rise in F_o and F_J was due to the loss of oxidized PQ pool as the PQ pool becomes more reduced. The increase in F_o′ was due to the non photochemical reduction of PQ pool which activated STN7 kinase and induced LHCII phosphorylation under anaerobic condition. Further, it was observed that the phosphorylated LHCII is migrated and associated with PSI supercomplex increasing its absorption cross-section. Furthermore, evidences from crr2-2 (NDH mutant) and pgr5 mutants (deficient in non NDH pathway of cyclic electron transport) have indicated that NDH is responsible for non photochemical reduction of the PQ pool. We propose that dark anaerobic condition accelerates production of reducing equivalents (such as NADPH by various metabolic pathways) which reduce PQ pool and is mediated by NDH leading to state II transition.

Conclusions/Significance

Anaerobic condition triggers non photochemical reduction of PQ pool mediated by NDH complex. The reduced PQ pool activates STN7 kinase leading to state II transition in A. thaliana.

High yields of hydrogen production induced by meta-substituted dichlorophenols biodegradation from the green alga Scenedesmus obliquus

Hydrogen is a highly promising energy source with important social and economic implications. The ability of green algae to produce photosynthetic hydrogen under anaerobic conditions has been known for years. However, until today the yield of production has been very low, limiting an industrial scale use. In the present paper, 73 years after the first report on H(2)-production from green algae, we present a combinational biological system where the biodegradation procedure of one meta-substituted dichlorophenol (m-dcp) is the key element for maintaining continuous and high rate H(2)-production (>100 times higher than previously reported) in chloroplasts and mitochondria of the green alga Scenedesmus obliquus. In particular, we report that reduced m-dcps (biodegradation intermediates) mimic endogenous electron and proton carriers in chloroplasts and mitochondria, inhibit Photosystem II (PSII) activity (and therefore O(2) production) and enhance Photosystem I (PSI) and hydrogenase activity. In addition, we show that there are some indications for hydrogen production from sources other than chloroplasts in Scenedesmus obliquus. The regulation of these multistage and highly evolved redox pathways leads to high yields of hydrogen production and paves the way for an efficient application to industrial scale use, utilizing simple energy sources and one meta-substituted dichlorophenol as regulating elements.

Thylakoid potassium channel is required for efficient photosynthesis in cyanobacteria

A potassium channel (SynK) of the cyanobacterium Synechocystis sp. PCC 6803, a photoheterotrophic model organism for the study of photosynthesis, has been recently identified and demonstrated to function as a potassium selective channel when expressed in a heterologous system and to be located predominantly to the thylakoid membrane in cyanobacteria. To study its physiological role, a SynK-less knockout mutant was generated and characterized. Fluorimetric experiments indicated that SynK-less cyanobacteria cannot build up a proton gradient as efficiently as WT organisms, suggesting that SynK might be involved in the regulation of the electric component of the proton motive force. Accordingly, measurements of flash-induced cytochrome b(6)f turnover and respiration pointed to a reduced generation of ΔpH and to an altered linear electron transport in mutant cells. The lack of the channel did not cause an altered membrane organization, but decreased growth and modified the photosystem II/photosystem I ratio at high light intensities because of enhanced photosensitivity. These data shed light on the function of a prokaryotic potassium channel and reports evidence, by means of a genetic approach, on the requirement of a thylakoid ion channel for optimal photosynthesis.

Interaction between starch breakdown, acetate assimilation, and photosynthetic cyclic electron flow in Chlamydomonas reinhardtii

Spectroscopic studies on photosynthetic electron transfer generally are based upon the monitoring of dark to light changes in the electron transfer chain. These studies, which focus on the light reactions of photosynthesis, also indirectly provide information on the redox or metabolic state of the chloroplast in the dark. Here, using the unicellular microalga Chlamydomonas reinhardtii, we study the impact of heterotrophic/mixotrophic acetate feeding on chloroplast carbon metabolism by using the spectrophotometric detection of P700(+), the photooxidized primary electron donor of photosystem I. We show that, when photosynthetic linear and cyclic electron flows are blocked (DCMU inhibiting PSII and methylviologen accepting electrons from PSI), the post-illumination reduction kinetics of P700(+) directly reflect the dark metabolic production of reductants (mainly NAD(P)H) in the stroma of chloroplasts. Such results can be correlated to other metabolic studies: in the absence of acetate, for example, the P700(+) reduction rate matches the rate of starch breakdown reported previously, confirming the chloroplast localization of the upstream steps of the glycolytic pathway in Chlamydomonas. Furthermore, the question of the interplay between photosynthetic and non-photosynthetic carbon metabolism can be addressed. We show that cyclic electron flow around photosystem I is twice as fast in a starchless mutant fed with acetate than it is in the WT, and we relate how changes in the flux of electrons from carbohydrate metabolism modulate the redox poise of the plastoquinone pool in the dark through chlororespiration.

Temperature responses of dark respiration in relation to leaf sugar concentration

Changes in leaf sugar concentrations are a possible mechanism of short-term adaptation to temperature changes, with natural fluctuations in sugar concentrations in the field expected to modify the heat sensitivity of respiration. We studied temperature-response curves of leaf dark respiration in the temperate tree Populus tremula (L.) in relation to leaf sugar concentration (1) under natural conditions or (2) leaves with artificially enhanced sugar concentration. Temperature-response curves were obtained by increasing the leaf temperature at a rate of 1°C min⁻¹. We demonstrate that respiration, similarly to chlorophyll fluorescence, has a break-point at high temperature, where respiration starts to increase with a faster rate. The average break-point temperature (T(RD) ) was 48.6 ± 0.7°C at natural sugar concentration. Pulse-chase experiments with ¹⁴CO₂ demonstrated that substrates of respiration were derived mainly from the products of starch degradation. Starch degradation exhibited a similar temperature-response curve as respiration with a break-point at high temperatures. Acceleration of starch breakdown may be one of the reasons for the observed high-temperature rise in respiration. We also demonstrate that enhanced leaf sugar concentrations or enhanced osmotic potential may protect leaf cells from heat stress, i.e. higher sugar concentrations significantly modify the temperature-response curve of respiration, abolishing the fast increase of respiration. Sugars or enhanced osmotic potential may non-specifically protect respiratory membranes or may block the high-temperature increase in starch degradation and consumption in respiratory processes, thus eliminating the break-points in temperature curves of respiration in sugar-fed leaves.

Interplay between non-photochemical plastoquinone reduction and re-oxidation in pre-illuminated Chlamydomonas reinhardtii: a chlorophyll fluorescence study

In photosynthetic eukaryotes, the redox state of the plastoquinone (PQ) pool is an important sensor for mechanisms that regulate the photosynthetic electron transport. In higher plants, a multimeric nicotinamide adenine dinucleotide (phosphate) (NAD(P))H dehydrogenase (NDH) complex and a plastid terminal oxidase (PTOX) are involved in PQ redox homeostasis in the dark. We recently demonstrated that in the microalgae Chlamydomonas reinhardtii, which lacks the multimeric NDH complex of higher plants, non-photochemical PQ reduction is mediated by a monomeric type-II NDH (Nda2). In this study, we further explore the nature and the importance of non-photochemical PQ reduction and oxidation in relation to redox homeostasis in this alga by recording the 'dark' chlorophyll fluorescence transients of pre-illuminated algal samples. From the observation that this fluorescence transient is modified by addition of propyl gallate, a known inhibitor of PTOX, and in a Nda2-deficient strain we conclude that it reflects post-illumination changes in the redox state of PQ resulting from simultaneous PTOX and Nda2 activity. We show that the post-illumination fluorescence transient can be used to monitor changes in the relative rates of the non-photochemical PQ reduction and reoxidation in response to different physiological situations. We study this fluorescence transient in algae acclimated to high light and in a mutant deficient in mitochondrial respiration. Some of our observations indicate that the chlororespiratory pathway participates in redox homeostasis in C. reinhardtii.

The interrelationship between the lower oxygen limit, chlorophyll fluorescence and the xanthophyll cycle in plants

The lower oxygen limit (LOL) in plants may be identified through the measure of respiratory gases [i.e. the anaerobic compensation point (ACP) or the respiratory quotient breakpoint (RQB)], but recent work shows it may also be identified by a sudden rise in dark minimum fluorescence (F(o)). The interrelationship between aerobic respiration and fermentative metabolism, which occur in the mitochondria and cytosol, respectively, and fluorescence, which emanates from the chloroplasts, is not well documented in the literature. Using spinach (Spinacia oleracea), this study showed that F(o) and photochemical quenching (q(P)) remained relatively unchanged until O(2) levels dropped below the LOL. An over-reduction of the plastoquinone (PQ) pool is believed to increase F(o) under dark + anoxic conditions. It is proposed that excess cytosolic reductant due to inhibition of the mitochondria's cytochrome oxidase under low-O(2), may be the primary reductant source. The maximum fluorescence (F(m)) is largely unaffected by low-O(2) in the dark, but was severely quenched, mirroring changes to the xanthophyll de-epoxidation state (DEPS), under even low-intensity light (≈4 μmol m(-2) s(-1)). In low light, the low-O(2)-induced increase in F(o) was also quenched, likely by non-photochemical and photochemical means. The degree of quenching in the light was negatively correlated with the level of ethanol fermentation in the dark. A discussion detailing the possible roles of cyclic electron flow, the xanthophyll cycle, chlororespiration and a pathway we termed 'chlorofermentation' were used to interpret fluorescence phenomena of both spinach and apple (Malus domestica) over a range of atmospheric conditions under both dark and low-light.

Impact of chlororespiration on non-photochemical quenching of chlorophyll fluorescence and on the regulation of the diadinoxanthin cycle in the diatom Thalassiosira pseudonana

In diatoms, metabolic activity during long dark periods leads to a chlororespiratory electron flow, which is accompanied by the build-up of a proton gradient strong enough to activate the diadinoxanthin (Ddx) de-epoxidation reaction of the Ddx cycle. In the present study, the impact of chlororespiration on non-photochemical quenching (NPQ) of chlorophyll fluorescence and the regulation of the Ddx cycle in the diatom Thalassiosira pseudonana was investigated by manipulation of the redox state of the photosynthetic electron transport chain during darkness. The response of a transfer of T. pseudonana cells from growth light conditions to 60 min darkness was found to depend on oxygen: in its presence there was no significant reduction of the PQ pool and no de-epoxidation of Ddx to diatoxanthin (Dtx). Under anaerobic conditions a high reduction state of the electron transport chain and a slow but steady de-epoxidation of Ddx was observed, which resulted in a significant accumulation of Dtx after 60 min of anaerobiosis. Unexpectedly, this high concentration of Dtx did not induce a correspondingly high NPQ as it would have been observed with Dtx formed under high light conditions. However, the sensitivity of NPQ to Dtx in cells kept under dark anaerobic conditions increased during reoxygenation and far-red (FR) light illumination. The results are discussed with respect to the activation of the de-epoxidation reaction and the formation of NPQ and their dependence on the extent of the proton gradient across the thylakoid membrane.

Cyclic electron flow around photosystem I in unicellular green algae

Cyclic electron flow around PSI, or cyclic photophosphorylation, is the photosynthetic process which recycles the reducing equivalents produced by photosystem I in the stroma towards the plastoquinone pool. Through the activity of cytochrome b(6)f, which also transfers protons across the membrane, it promotes the synthesis of ATP. The literature dealing with cyclic electron flow in unicellular algae is far less abundant than it is for plants. However, in the chloroplast of algae such as Chlorella or Chlamydomonas, an efficient carbohydrate catabolism renders the redox poise much more reducing than in plant chloroplasts. It is therefore worthwhile highlighting the specific properties of unicellular algae because cyclic electron flow is highly dependent upon the accumulation of these stromal reducing equivalents. Such an increase of reducing power in the stroma stimulates the reduction of plastoquinones, which is the limiting step of cyclic electron flow. In anaerobic conditions in the dark, this reaction can lead to a fully reduced plastoquinone pool and induce state transitions, the migration of 80% of light harvesting complexes II and 20% of cytochrome b(6)f complex from the PSII-enriched grana to the PSI-enriched lamella. These ultrastructural changes have been proposed to further enhance cyclic electron flow by increasing PSI antenna size, and forming PSI-cyt b(6)f supercomplexes. These hypotheses are discussed in light of recently published data.

Chlororespiration and tolerance to drought, heat and high illumination

Sun (Chrysanthemum morifolium) and shade (Spathiphyllum wallisii) plants were used to study the effects of drought, heat and high illumination. The stress conditions caused a greater accumulation of hydrogen peroxide in Chrysanthemum morifolium than in Spathiphyllum wallisii leaves. They also resulted in down-regulation of linear electron transport in the leaves of both species, as indicated by a gradual reduction in the photochemistry efficiency of PS II, which was associated with an increase in the non-photochemical quenching of fluorescence. Only a slight decrease in F(v)/F(m) was observed under stress conditions in either plant species, suggesting that the chloroplast is protected by mechanisms that dissipate excess excitation energy to prevent damage to the photosynthetic apparatus. In addition to the effects on photosynthetic activity, changes were also observed by immunoblot analysis in the plastidial NADH DH complex, PTOX and PGR5. The quantities of the PTOX and NDH-H subunit of the thylakoidal NADH DH complex, and the NADH DH activity in the thylakoid membranes were similar in control plants of both species and increased in stressed plants, particularly in Spathiphyllum wallisii. The level of PGR5 polypeptide was higher in Chrysanthemum morifolium than in Spathiphyllum wallisii control plants, while after stress, the quantity of PGR5 increased significantly in Chrysanthemum morifolium and remained constant in Spathiphyllum wallisii. These results indicate that the relative importance of chlororespiration and the cyclic electron pathways in the tolerance to drought, heat and high illumination differs in sun and shade plants, indicating different adaptive mechanisms to the environment. In the conditions studied, the PGR5-dependent cyclic pathway is more active in Chrysanthemum morifolium, a sun species, whereas in Spathiphyllum wallisii, a shade species, other ways involving the NADH DH complex and PTOX are stimulated in response to stress, which results in lower levels of ROS accumulation in the leaves.

Enrichment of oxygen heavy isotopes during photosynthesis in phytoplankton

Some of the oxygen produced during oxygenic photosynthesis is consumed but little is known about the extent of the processes involved. We measured the (17)O/(16)O and (18)O/(16)O ratios in O(2) produced by certain marine and freshwater phytoplankton representing important groups of primary producers. When the cells were performing photosynthesis under very low dissolved oxygen concentrations (< 3 μM), we observed significant enrichment in both (18)O and (17)O with respect to the substrate water. The difference in δ(18)O between O(2) and water was about 4.5, 3, 5.5, and 7‱ in the diatom Phaeodactylum tricornutum, Nannochloropsis sp. (Eustigmatophyceae), the coccolithophore Emiliania huxleyi and the green alga Chlamydomonas reinhardtii, respectively. The difference in δ(17)O was about 0.52 that of δ(18)O. As explained, the observed enrichments most probably stem from considerable oxygen consumption during photosynthesis even when major O(2)-consuming reactions such as photorespiration were minimized. These enrichments increased linearly with rising O(2) levels but with different δ(17)O/δ(18)O slopes for the various organisms, suggesting engagements of different O(2)-consuming reactions with rising O(2) levels. Consumption of O(2) may be important for energy dissipation during photosynthesis. The isotope enrichment observed here, not accounted for in earlier assessments, closes an important gap in our understanding of the difference between the isotopic compositions of atmospheric oxygen and that of seawater, i.e., the Dole effect.

Analysis of non-photochemical energy dissipating processes in wild type Dunaliella salina (green algae) and in zea1, a mutant constitutively accumulating zeaxanthin

Generally there is a correlation between the amount of zeaxanthin accumulated within the chloroplast of oxygenic photosynthetic organisms and the degree of non-photochemical quenching (NPQ). Although constitutive accumulation of zeaxanthin can help protect plants from photo-oxidative stress, organisms with such a phenotype have been reported to have altered rates of NPQ induction. In this study, basic fluorescence principles and the routinely used NPQ analysis technique were employed to investigate excitation energy quenching in the unicellular green alga Dunaliella salina, in both wild type (WT) and a mutant, zea1, constitutively accumulating zeaxanthin under all growth conditions. The results showed that, in D. salina, NPQ is a multi-component process consisting of energy- or DeltapH-dependent quenching (qE), state-transition quenching (qT), and photoinhibition quenching (qI). Despite the vast difference in the amount of zeaxanthin in WT and the zea1 mutant grown under low light, the overall kinetics of NPQ induction were almost the same. Only a slight difference in the relative contribution of each quenching component could be detected. Of all the NPQ subcomponents, qE seemed to be the primary NPQ operating in this alga in response to short-term exposure to excessive irradiance. Whenever qE could not operate, i.e., in the presence of nigericin, or under conditions where the level of photon flux is beyond its quenching power, qT and/or qI could adequately compensate its photoprotective function.

Energetics of Photosystem II charge recombination in Acaryochloris marina studied by thermoluminescence and flash-induced chlorophyll fluorescence measurements

We studied the charge recombination characteristics of Photosystem II (PSII) redox components in whole cells of the chlorophyll (Chl) d-dominated cyanobacterium, Acaryochloris marina, by flash-induced chlorophyll fluorescence and thermoluminescence measurements. Flash-induced chlorophyll fluorescence decay was retarded in the mus and ms time ranges and accelerated in the s time range in Acaryochloris marina relative to that in the Chl a-containing cyanobacterium, Synechocystis PCC 6803. In the presence of 3-(3,4-dichlorophenyl)-1, 1-dimethylurea, which blocks the Q(B) site, the relaxation of fluorescence decay arising from S(2)Q(A)(-) recombination was somewhat faster in Acaryochloris marina than in Synechocystis PCC 6803. Thermoluminescence intensity of the so called B band, arising from the recombination of the S(2)Q(B)(-) charge separated state, was enhanced significantly (2.5 fold) on the basis of equal amounts of PSII in Acaryochloris marina as compared with Synechocystis 6803. Our data show that the energetics of charge recombination is modified in Acaryochloris marina leading to a approximately 15 meV decrease of the free energy gap between the Q(A) and Q(B) acceptors. In addition, the total free energy gap between the ground state and the excited state of the reaction center chlorophyll is at least approximately 25-30 meV smaller in Acaryochloris marina, suggesting that the primary donor species cannot consist entirely of Chl a in Acaryochloris marina, and there is a contribution from Chl d as well.

The role of respiration in the activation of photosynthesis upon illumination of dark adapted Chlamydomonas reinhardtii

It is reported that O(2) is required for the activation of photosynthesis in dark adapted Chlamydomonas reinhardtii in State 1, under low light intensity. The concentration of dissolved O(2) of ca. 9 microM is sufficient to saturate the requirement. When the concentration of O(2) is 3 muM or below, the activation of photosynthesis is strongly inhibited by myxothiazol, a specific inhibitor of the mitochondrial cytochrome bc(1). The effect of this inhibitor decreases as the O(2) concentration is raised, to disappear completely above 50 muM. Low concentrations of uncouplers delay the activation of photosynthesis, but do not inhibit it when steady state is reached. It is concluded that in State 1 C. reinhardtii mitochondrial respiration is required for the activation of photosynthesis upon illumination of dark adapted cells only when the concentration of O(2) is too low (less than 5 muM) to allow an appreciable activity of the Mehler reaction. The role of respiration does not seem to be due to the synthesis of ATP by oxidative phosphorylation, because photosynthesis activation is not sensitive to oligomycin.

Quantification of the electrochemical proton gradient and activation of ATP synthase in leaves

We have developed a new method to quantify the transmembrane electrochemical proton gradient present in chloroplasts of dark-adapted leaves. When a leaf is illuminated by a short pulse of intense light, we observed that the light-induced membrane potential changes, measured by the difference of absorption (520 nm-546 nm), reach a maximum value (approximately 190 mV) determined by ion leaks that occur above a threshold level of the electrochemical proton gradient. After the light-pulse, the decay of the membrane potential follows a multiphasic kinetics. A marked slowdown of the rate of membrane potential decay occurs approximately 100 ms after the light-pulse, which has been previously interpreted as reflecting the switch from an activated to an inactivated state of the ATP synthase (Junge, W., Rumberg, B. and Schröder, H., Eur. J. Biochem. 14 (1970) 575-581). This transition occurs at approximately 110 mV, thereby providing a second reference level. On this basis, we have estimated the Delta micro (H(+)) level that pre-exists in the dark. Depending upon the physiological state of the leaf, this level varies from 40 to 70 mV. In the dark, the Delta micro (H(+)) collapses upon addition of inhibitors of the respiratory chain, thus showing that it results from the hydrolysis of ATP of mitochondrial origin. Illumination of the leaf for a period longer than several seconds induces a long-lived Delta micro (H(+)) increase (up to approximately 150 mV) that reflects the light-induced increase in ATP concentration. Following the illumination, Delta micro (H(+)) relaxes to its dark-adapted value according a multiphasic kinetics that is completed in more than 1 h. In mature leaf, the deactivation of the Benson-Calvin cycle follows similar kinetics as Delta micro (H(+)) decay, showing that its state of activation is mainly controlled by ATP concentration.

Chlororespiration is involved in the adaptation of Brassica plants to heat and high light intensity

Two species of Brassica were used to study their acclimation to heat and high illumination during the first stages of development. One, Brassica fruticulosa, is a wild species from south-east Spain and is adapted to both heat and high light intensity in its natural habitat, while the other, Brassica oleracea, is an agricultural species that is widely cultivated throughout the world. Growing Brassica plants under high irradiance and moderate heat was seen to affect the growth parameters and the functioning of the photosynthetic apparatus. The photosystem II (PSII) quantum yields and the capacity of photosynthetic electron transport, which were lower in B. fruticulosa than in B. oleracea, decreased in B. oleracea plants when grown under stress conditions, indicating inhibition of PSII. However, in B. fruticulosa, the values of these parameters were similar to the values of control plants. Photosystem I (PSI) activity was higher in B. fruticulosa than in B. oleracea, and in both species this activity increased in plants exposed to heat and high illumination. Immunoblot analysis of thylakoid membranes using specific antibodies raised against the NDH-K subunit of the thylakoidal NADH dehydrogenase complex (NADH DH) and against plastid terminal oxidase (PTOX) revealed a higher amount of both proteins in B. fruticulosa than in B. oleracea. In addition, PTOX activity in plastoquinone oxidation, and NADH DH activity in thylakoid membranes were higher in the wild species (B. fruticulosa) than in the agricultural species (B. oleracea). The results indicate that tolerance to high illumination and heat of the photosynthetic activity was higher in the wild species than in the agricultural species, suggesting that plant adaptation to these stresses in natural conditions favours subsequent acclimation, and that the chlororespiration process is involved in adaptation to heat and high illumination in Brassica.

Distinct physiological responses to a high light and low CO2 environment revealed by fluorescence quenching in photoautotrophically grown Chlamydomonas reinhardtii

Mechanisms for countering environmental stress are essential to photosynthetic organisms. Alteration of the photosynthetic apparatus, a mechanism for balancing the flux of light energy and carbon fixation, can be characterized by fluorescence properties. In this study, we have established a simple protocol to determine the extent of energy-dependent quenching (qE) and quenching by state transition (qT) in Chlamydomonas cells by examining their fluorescence properties under light fluctuations. We identified qE as the uncoupler-sensitive NPQ component that was rapidly relaxed upon transition to dark conditions. We characterized the qT component by determining low-temperature fluorescence spectra and analyzing a state-transition-less mutant. By these methods, we observed that similar abiotic stresses-high light conditions (where excess energy is supplied) and low CO2 conditions (where energy utilization is limited)-induced different types of NPQ. High light conditions induced mainly qE-quenching that increased gradually while low CO2 conditions induced mainly qT-quenching that peaked in 20 min and then decreased gradually. That high light and low carbon signals induced different physiological responses suggests that they triggered different genetic responses, which altered protein expression under each of the conditions.

Radiative and non-radiative charge recombination pathways in Photosystem II studied by thermoluminescence and chlorophyll fluorescence in the cyanobacterium Synechocystis 6803

The mechanism of charge recombination was studied in Photosystem II by using flash induced chlorophyll fluorescence and thermoluminescence measurements. The experiments were performed in intact cells of the cyanobacterium Synechocystis 6803 in which the redox properties of the primary pheophytin electron acceptor, Phe, the primary electron donor, P(680), and the first quinone electron acceptor, Q(A), were modified. In the D1Gln130Glu or D1His198Ala mutants, which shift the free energy of the primary radical pair to more positive values, charge recombination from the S(2)Q(A)(-) and S(2)Q(B)(-) states was accelerated relative to the wild type as shown by the faster decay of chlorophyll fluorescence yield, and the downshifted peak temperature of the thermoluminescence Q and B bands. The opposite effect, i.e. strong stabilization of charge recombination from both the S(2)Q(A)(-) and S(2)Q(B)(-) states was observed in the D1Gln130Leu or D1His198Lys mutants, which shift the free energy level of the primary radical pair to more negative values, as shown by the retarded decay of flash induced chlorophyll fluorescence and upshifted thermoluminescence peak temperatures. Importantly, these mutations caused a drastic change in the intensity of thermoluminescence, manifested by 8- and 22-fold increase in the D1Gln130Leu and D1His198Lys mutants, respectively, as well as by a 4- and 2.5-fold decrease in the D1Gln130Glu and D1His198Ala mutants, relative to the wild type, respectively. In the presence of the electron transport inhibitor bromoxynil, which decreases the redox potential of Q(A)/Q(A)(-) relative to that observed in the presence of DCMU, charge recombination from the S(2)Q(A)(-) state was accelerated in the wild type and all mutant strains. Our data confirm that in PSII the dominant pathway of charge recombination goes through the P(680)(+)Phe(-) radical pair. This indirect recombination is branched into radiative and non-radiative pathways, which proceed via repopulation of P(680)(*) from (1)[P(680)(+)Ph(-)] and direct recombination of the (3)[P(680)(+)Ph(-)] and (1)[P(680)(+)Ph(-)] radical states, respectively. An additional non-radiative pathway involves direct recombination of P(680)(+)Q(A)(-). The yield of these charge recombination pathways is affected by the free energy gaps between the Photosystem II electron transfer components in a complex way: Increase of DeltaG(P(680)(*)<-->P(680)(+)Phe(-)) decreases the yield of the indirect radiative pathway (in the 22-0.2% range). On the other hand, increase of DeltaG(P(680)(+)Phe(-)<-->P(680)(+)Q(A)(-)) increases the yield of the direct pathway (in the 2-50% range) and decreases the yield of the indirect non-radiative pathway (in the 97-37% range).

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.

Salt-induced redox-independent phosphorylation of light harvesting chlorophyll a/b proteins in Dunaliella salina thylakoid membranes

This study investigated the regulation of the major light harvesting chlorophyll a/b protein (LHCII) phosphorylation in Dunaliella salina thylakoid membranes. We found that both light and NaCl could induce LHCII phosphorylation in D. salina thylakoid membranes. Treatments with oxidants (ferredoxin and NADP) or photosynthetic electron flow inhibitors (DCMU, DBMIB, and stigmatellin) inhibited LHCII phosphorylation induced by light but not that induced by NaCl. Furthermore, neither addition of CuCl(2), an inhibitor of cytochrome b(6)f complex reduction, nor oxidizing treatment with ferricyanide inhibited light- or NaCl-induced LHCII phosphorylation, and both salts even induced LHCII phosphorylation in dark-adapted D. salina thylakoid membranes as other salts did. Together, these results indicate that the redox state of the cytochrome b(6)f complex is likely involved in light- but not salt-induced LHCII phosphorylation in D. salina thylakoid membranes.

Role of the PSII-H subunit in photoprotection: novel aspects of D1 turnover in Synechocystis 6803

Photosystem I-less Synechocystis 6803 mutants carrying modified PsbH proteins, derived from different combinations of wild-type cyanobacterial and maize genes, were constructed. The mutants were analyzed in order to determine the relative importance of the intra- and extramembrane domains of the PsbH subunit in the functioning of photosystem (PS) II, by a combination of biochemical, biophysical, and physiological approaches. The results confirmed and extended previously published data showing that, besides D1, the whole PsbH protein is necessary to determine the correct structure of a QB/herbicide-binding site. The different turnover of the D1 protein and chlorophyll photobleaching displayed by mutant cells in response to photoinhibitory treatment revealed for the first time the actual role of the PsbH subunit in photoprotection. A functional PsbH protein is necessary for (i) rapid degradation of photodamaged D1 molecules, which is essential to avoid further oxidative damage to the PSII core, and (ii) insertion of newly synthesized D1 molecules into the thylakoid membrane. PsbH is thus required for both initiation and completion of the repair cycle of the PSII complex in cyanobacteria.

Involvement of a plastid terminal oxidase in plastoquinone oxidation as evidenced by expression of the Arabidopsis thaliana enzyme in tobacco

Chlororespiration has been defined as a respiratory electron transport chain in interaction with photosynthetic electron transport involving both non-photochemical reduction and oxidation of plastoquinones. Different enzymatic activities, including a plastid-encoded NADH dehydrogenase complex, have been reported to be involved in the non-photochemical reduction of plastoquinones. However, the enzyme responsible for plasquinol oxidation has not yet been clearly identified. In order to determine whether the newly discovered plastid oxidase (PTOX) involved in carotenoid biosynthesis acts as a plastoquinol oxidase in higher plant chloroplasts, the Arabidopsis thaliana PTOX gene (At-PTOX) was expressed in tobacco under the control of a strong constitutive promoter. We showed that At-PTOX is functional in tobacco chloroplasts and strongly accelerates the non-photochemical reoxidation of plastoquinols; this effect was inhibited by propyl gallate, a known inhibitor of PTOX. During the dark to light induction phase of photosynthesis at low irradiances, At-PTOX drives significant electron flow to O(2), thus avoiding over-reduction of plastoquinones, when photo- synthetic CO(2) assimilation was not fully induced. We proposed that PTOX, by modulating the redox state of intersystem electron carriers, may participate in the regulation of cyclic electron flow around photosystem I.