Oxidation of P700 in Photosystem I Is Essential for the Growth of Cyanobacteria

Photo-oxidative damage of photosystem I by repetitive flashes and chilling stress in cucumber leaves

Photosystem I (PSI) is an essential protein complex for oxygenic photosynthesis and is also known to be an important source of reactive oxygen species (ROS) in the light. When ROS are generated within PSI, the photosystem can be damaged. The so-called PSI photoinhibition is a lethal event for oxygenic phototrophs, and it is prevented by keeping the reaction center chlorophyll (P700) oxidized in excess light conditions. Whereas regulatory mechanisms for controlling P700 oxidation have been discovered already, the molecular mechanism of PSI photoinhibition is still unclear. Here, we characterized the damage mechanism of PSI photoinhibition by in vitro transient absorption and electron paramagnetic resonance (EPR) spectroscopy in isolated PSI from cucumber leaves that had been subjected to photoinhibition treatment. Photodamage to PSI was induced by two different light treatments: 1. continuous illumination with high light at low (chilling) temperature (C/LT) and 2. repetitive flashes at room temperature (F/RT). These samples were compared to samples that had been illuminated with high light at room temperature (C/RT). The [FeS] clusters FX and (FA FB) were destructed in C/LT but not in F/RT. Transient absorption spectroscopy indicated that half of the charge separation was impaired in F/RT, however, low-temperature EPR revealed the light-induced FX signal at a similar size as in the case of C/RT. This indicates that the two branches of electron transfer in PSI were affected differently. Electron transfer at the A-branch was inhibited in F/RT and also partially in C/LT, while the B-branch remained active.

Restricting electron flow at cytochrome b6f when downstream electron acceptors are severely limited

Photosynthetic organisms frequently experience abiotic stress that restricts their growth and development. Under such circumstances, most absorbed solar energy cannot be used for CO2 fixation and can cause the photoproduction of reactive oxygen species (ROS) that can damage the photosynthetic reaction centers of PSI and PSII, resulting in a decline in primary productivity. This work describes a biological "switch" in the green alga Chlamydomonas reinhardtii that reversibly restricts photosynthetic electron transport (PET) at the cytochrome b6f (Cyt b6f) complex when the capacity for accepting electrons downstream of PSI is severely limited. We specifically show this restriction in STARCHLESS6 (sta6) mutant cells, which cannot synthesize starch when they are limited for nitrogen (growth inhibition) and subjected to a dark-to-light transition. This restriction represents a form of photosynthetic control that causes diminished electron flow to PSI and thereby prevents PSI photodamage but does not appear to rely on a ΔpH. Furthermore, when electron flow is restricted, the plastid alternative oxidase (PTOX) becomes active, functioning as an electron valve that dissipates some excitation energy absorbed by PSII and allows the formation of a proton motive force (PMF) that would drive some ATP production (potentially sustaining PSII repair and nonphotochemical quenching [NPQ]). The restriction at the Cyt b6f complex can be gradually relieved with continued illumination. This study provides insights into how PET responds to a marked reduction in availability of downstream electron acceptors and the protective mechanisms involved.

Electron transport in cyanobacterial thylakoid membranes: Are cyanobacteria simple models for photosynthetic organisms?

Cyanobacteria are structurally the simplest oxygenic phototrophs, which makes it difficult to understand the regulation of photosynthesis because the photosynthetic and respiratory processes share the same thylakoid membranes and cytosolic space. This review aimed to summarise the molecular mechanisms and in vivo activities of electron transport in cyanobacterial thylakoid membranes based on the latest progress in photosynthesis research in cyanobacteria. Photosynthetic linear electron transport for CO2 assimilation has the dominant electron flux in the thylakoid membranes. The capacity of O2 photoreduction in cyanobacteria is comparable to the photosynthetic CO2 assimilation, which is mediated by flavodiiron proteins. Additionally, cyanobacterial thylakoid membranes harbour the significant electron flux of respiratory electron transport through a homologue of respiratory complex I, which is also recognized as the part of cyclic electron transport chain if it is coupled with photosystem I in the light. Further, O2-independent alternative electron transports through hydrogenase and nitrate reductase function with reduced ferredoxin as the electron donor. Whereas all these electron transports are recently being understood one by one, the complexity as the whole regulatory system remains to be uncovered in near future.

Diversity of responses to nitrogen deficiency in distinct wheat genotypes reveals the role of alternative electron flows in photoprotection

Nitrogen (N) deficiency represents an important limiting factor affecting photosynthetic productivity and the yields of crop plants. Significant reported differences in N use efficiency between the crop species and genotypes provide a good background for the studies of diversity of photosynthetic and photoprotective responses associated with nitrogen deficiency. Using distinct wheat (Triticum aestivum L.) genotypes with previously observed contrasting responses to nitrogen nutrition (cv. Enola and cv. Slomer), we performed advanced analyses of CO₂ assimilation, PSII, and PSI photochemistry, also focusing on the heterogeneity of the stress responses in the different leaf levels. Our results confirmed the loss of photosynthetic capacity and enhanced more in lower positions. Non-stomatal limitation of photosynthesis was well reflected by the changes in PSII and PSI photochemistry, including the parameters derived from the fast-fluorescence kinetics. Low photosynthesis in N-deprived leaves, especially in lower positions, was associated with a significant decrease in the activity of alternative electron flows. The exception was the cyclic electron flow around PSI that was enhanced in most of the samples with a low photosynthetic rate. We observed significant genotype-specific responses. An old genotype Slomer with a lower CO₂ assimilation rate demonstrated enhanced alternative electron flow and photorespiration capacity. In contrast, a modern, highly productive genotype Enola responded to decreased photosynthesis by a significant increase in nonphotochemical dissipation and cyclic electron flow. Our results illustrate the importance of alternative electron flows for eliminating the excitation pressure at the PSII acceptor side. The decrease in capacity of electron acceptors was balanced by the structural and functional changes of the components of the electron transport chain, leading to a decline of linear electron transport to prevent the overreduction of the PSI acceptor side and related photooxidative damage of photosynthetic structures in leaves exposed to nitrogen deficiency.

The ability of P700 oxidation in photosystem I reflects chilling stress tolerance in cucumber

Low temperature inhibits photosynthesis and negatively affects plant growth. Cucumber (Cucumis sativus L.) is a chilling-sensitive plant, and its greenhouse production requires considerable energy during the winter. Therefore, a useful stress marker for selecting chilling-tolerant cucumber cultivars is desirable. In this study, we evaluated chilling-stress damage in different cucumber cultivars by measuring photosynthetic parameters. The majority of cultivars showed decreases in the quantum yield of photosystem (PS) II [Fv/Fm and Y(II)] and the quantity of active PS I (Pm) after chilling stress. In contrast, Y(ND)-the ratio of the oxidized state of PSI reaction center chlorophyll P700 (P700⁺)-differed among cultivars and was perfectly inversely correlated with Y(NA)-the ratio of the non-photooxidizable P700. It has been known that P700⁺ accumulates under stress conditions and protects plants to suppress the generation of reactive oxygen species. In fact, cultivars unable to induce Y(ND) after chilling stress showed growth retardation with reductions in chlorophyll content and leaf area. Therefore, Y(ND) can be a useful marker to evaluate chilling-stress tolerance in cucumber.

Dissection of respiratory and cyclic electron transport in Synechocystis sp. PCC 6803

Cyclic electron transport (CET) is an attractive hypothesis for regulating photosynthetic electron transport and producing the additional ATP in oxygenic phototrophs. The concept of CET has been established in the last decades, and it is proposed to function in the progenitor of oxygenic photosynthesis, cyanobacteria. The in vivo activity of CET is frequently evaluated either from the redox state of the reaction center chlorophyll in photosystem (PS) I, P700, in the absence of PSII activity or by comparing PSI and PSII activities through the P700 redox state and chlorophyll fluorescence, respectively. The evaluation of CET activity, however, is complicated especially in cyanobacteria, where CET shares the intersystem chain, including plastoquinone, cytochrome b₆/f complex, plastocyanin, and cytochrome c₆, with photosynthetic linear electron transport (LET) and respiratory electron transport (RET). Here we sought to distinguish the in vivo electron transport rates in RET and CET in the cyanobacterium Synechocystis sp. PCC 6803. The reduction rate of oxidized P700 (P700⁺) decreased to less than 10% when PSII was inhibited, indicating that PSII is the dominant electron source to PSI but P700⁺ is also reduced by electrons derived from other sources. The oxidative pentose phosphate (OPP) pathway functions as the dominant electron source for RET, which was found to be inhibited by glycolaldehyde (GA). In the condition where the OPP pathway and respiratory terminal oxidases were inhibited by GA and KCN, the P700⁺ reduction rate was less than 1% of that without any inhibitors. This study indicate that the electron transport to PSI when PSII is inhibited is dominantly derived from the OPP pathway in Synechocystis sp. PCC 6803.

Coral symbionts evolved a functional polycistronic flavodiiron gene

Photosynthesis in cyanobacteria, green algae, and basal land plants is protected against excess reducing pressure on the photosynthetic chain by flavodiiron proteins (FLV) that dissipate photosynthetic electrons by reducing O₂. In these organisms, the genes encoding FLV are always conserved in the form of a pair of two-type isozymes (FLVA and FLVB) that are believed to function in O₂ photo-reduction as a heterodimer. While coral symbionts (dinoflagellates of the family Symbiodiniaceae) are the only algae to harbor FLV in photosynthetic red plastid lineage, only one gene is found in transcriptomes and its role and activity remain unknown. Here, we characterized the FLV genes in Symbiodiniaceae and found that its coding region is composed of tandemly repeated FLV sequences. By measuring the O₂-dependent electron flow and P700 oxidation, we suggest that this atypical FLV is active in vivo. Based on the amino-acid sequence alignment and the phylogenetic analysis, we conclude that in coral symbionts, the gene pair for FLVA and FLVB have been fused to construct one coding region for a hybrid enzyme, which presumably occurred when or after both genes were inherited from basal green algae to the dinoflagellate. Immunodetection suggested the FLV polypeptide to be cleaved by a post-translational mechanism, adding it to the rare cases of polycistronic genes in eukaryotes. Our results demonstrate that FLV are active in coral symbionts with genomic arrangement that is unique to these species. The implication of these unique features on their symbiotic living environment is discussed.

Recent advances in single-molecule spectroscopy studies on light-harvesting processes in oxygenic photosynthesis

Photosynthetic light-harvesting complexes (LHCs) play a crucial role in concentrating the photon energy from the sun that otherwise excites a typical pigment molecule, such as chlorophyll-a, only several times a second. Densely packed pigments in the complexes ensure efficient energy transfer to the reaction center. At the same time, LHCs have the ability to switch to an energy-quenching state and thus play a photoprotective role under excessive light conditions. Photoprotection is especially important for oxygenic photosynthetic organisms because toxic reactive oxygen species can be generated through photochemistry under aerobic conditions. Because of the extreme complexity of the systems in which various types of pigment molecules strongly interact with each other and with the surrounding protein matrixes, there has been long-standing difficulty in understanding the molecular mechanisms underlying the flexible switching between the light-harvesting and quenching states. Single-molecule spectroscopy studies are suitable to reveal the conformational dynamics of LHCs reflected in the fluorescence properties that are obscured in ordinary ensemble measurements. Recent advanced single-molecule spectroscopy studies have revealed the dynamical fluctuations of LHCs in their fluorescence peak position, intensity, and lifetime. The observed dynamics seem relevant to the conformational plasticity required for the flexible activations of photoprotective energy quenching. In this review, we survey recent advances in the single-molecule spectroscopy study of the light-harvesting systems of oxygenic photosynthesis.

Heat-induced down-regulation of photosystem II protects photosystem I in honeysuckle (Lonicera japonica)

Honeysuckle (Lonicera japonica Thunb.) is a traditional medicinal plant in China which is often threatened by high temperature at midday during summer. Heat-induced effects on the photosynthetic apparatus in honeysuckle are associated with a depression of the photosystem II (PSII) photochemical efficiency. However, very limited information is available on regulation of photosynthetic electron flow in PSI photoprotection in heat-stressed honeysuckle. Simultaneous analyses of chlorophyll fluorescence and the change in absorbance of P700 showed that energy transformation and electron transfer activity in PSII decreased under heat stress, but the fraction of photo-oxidizable PSI (Pm) remained stable. With treatments at 38 and 42 °C, the photochemical electron transport in PSII was suppressed, whereas the cyclic electron flow (CEF) around PSI was induced. In addition, the levels of high energy state quenching (qE) and P700 oxidation increased significantly with increasing temperature. However, a decline of qE in antimycin A (AA)- or 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)-treated leaves after heat treatment was observed, while P700 oxidation decreased only in the presence of AA. The results indicate that heat-induced inhibition of PSII and induction of CEF cooperatively protect PSI from ROS damages through moderate down-regulation of photosynthetic electron flow from PSII to PSI.

Down-Regulation of Photosynthetic Electron Transport and Decline in CO2 Assimilation under Low Frequencies of Pulsed Lights

The decline in CO2 assimilation in leaves exposed to decreasing frequencies of pulsed light is well characterized, in contrast to the regulation of photosynthetic electron transport under these conditions. Thus, we exposed sunflower leaves to pulsed lights of different frequencies but with the same duty ratio (25%) and averaged light intensity (575 μmoles photons m−2 s−1). The rates of net photosynthesis Pn were constant from 125 to 10 Hz, and declined by 70% from 10 to 0.1 Hz. This decline coincided with (1) a marked increase in nonphotochemical quenching (NPQ), and (2) the completion after 25 ms of illumination of the first phase of P700 photooxidation, the primary electron donor of PSI. Under longer light pulses (<5 Hz), there was a slower and larger P700 photooxidation phase that could be attributed to the larger NPQ and to a resistance of electron flow on the PSI donor side indicated by 44% slower kinetics of a P700+ dark reduction. In addition, at low frequencies, the decrease in quantum yield of photochemistry was 2.3-times larger for PSII than for PSI. Globally, our results indicate that the decline in CO2 assimilation at 10 Hz and lower frequencies coincide with the formation of NPQ and a restriction of electron flows toward PSI, favoring the accumulation of harmless P700+.

Photosynthetic Linear Electron Flow Drives CO2 Assimilation in Maize Leaves

Photosynthetic organisms commonly develop the strategy to keep the reaction center chlorophyll of photosystem I, P700, oxidized for preventing the generation of reactive oxygen species in excess light conditions. In photosynthesis of C₄ plants, CO₂ concentration is kept at higher levels around ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) by the cooperation of the mesophyll and bundle sheath cells, which enables them to assimilate CO₂ at higher rates to survive under drought stress. However, the regulatory mechanism of photosynthetic electron transport for P700 oxidation is still poorly understood in C₄ plants. Here, we assessed gas exchange, chlorophyll fluorescence, electrochromic shift, and near infrared absorbance in intact leaves of maize (a NADP-malic enzyme C₄ subtype species) in comparison with mustard, a C₃ plant. Instead of the alternative electron sink due to photorespiration in the C₃ plant, photosynthetic linear electron flow was strongly suppressed between photosystems I and II, dependent on the difference of proton concentration across the thylakoid membrane (ΔpH) in response to the suppression of CO₂ assimilation in maize. Linear relationships among CO₂ assimilation rate, linear electron flow, P700 oxidation, ΔpH, and the oxidation rate of ferredoxin suggested that the increase of ΔpH for P700 oxidation was caused by the regulation of proton conductance of chloroplast ATP synthase but not by promoting cyclic electron flow. At the scale of intact leaves, the ratio of PSI to PSII was estimated almost 1:1 in both C₃ and C₄ plants. Overall, the photosynthetic electron transport was regulated for P700 oxidation in maize through the same strategies as in C₃ plants only except for the capacity of photorespiration despite the structural and metabolic differences in photosynthesis between C₃ and C₄ plants.

Understanding photosynthesis in a spatial-temporal multiscale: The need for a systemic view

In October 2020, at the peak of the COVID-19 pandemic, a group of young Brazilian photosynthesis researchers organized the 1st Brazilian Symposium on Photosynthesis. The event was free and online, with the presence of important guest speakers from all over the world, who discussed their recent works on topics related to the future and perspectives of photosynthesis research. Summarizing the expectations of this symposium we highlighted the importance of adopting a systemic perspective for a better understanding of photosynthesis as a complex and dynamic process. Plants are modular and self-regulating presenting metabolic redundancy and functional degeneration. Among the various biological processes, photosynthesis plays a crucial role in promoting the direct conversion of light energy into carbon skeletons for support growth and productivity. In the past decades, significant advances have been made in photosynthesis at the biophysical, biochemical, and molecular levels. However, this myriad of knowledge has been insufficient to answer crucial questions, such as: how can we understand and eventually increase photosynthetic efficiency and yield in crops subjected to adverse environment related to climate-changing? We believe that a crucial limitation to the whole comprehension of photosynthesis is associated with a vastly widespread classic reductionist view. Moreover, this perspective is commonly accompanied by non-integrative, simplistic, and descriptive approaches to investigate a complex and dynamic process as photosynthesis. Herein, we propose the use of new approaches, mostly based on the Systems Theory, which certainly comes closer to the real world, such as the complex systems that the plants represent.

Physiological Roles of Flavodiiron Proteins and Photorespiration in the Liverwort Marchantia polymorpha

Against the potential risk in oxygenic photosynthesis, that is, the generation of reactive oxygen species, photosynthetic electron transport needs to be regulated in response to environmental fluctuations. One of the most important regulations is keeping the reaction center chlorophyll (P700) of photosystem I in its oxidized form in excess light conditions. The oxidation of P700 is supported by dissipating excess electrons safely to O₂, and we previously found that the molecular mechanism of the alternative electron sink is changed from flavodiiron proteins (FLV) to photorespiration in the evolutionary history from cyanobacteria to plants. However, the overall picture of the regulation of photosynthetic electron transport is still not clear in bryophytes, the evolutionary intermediates. Here, we investigated the physiological roles of FLV and photorespiration for P700 oxidation in the liverwort Marchantia polymorpha by using the mutants deficient in FLV (flv1) at different O₂ partial pressures. The effective quantum yield of photosystem II significantly decreased at 2kPa O₂ in flv1, indicating that photorespiration functions as the electron sink. Nevertheless, it was clear from the phenotype of flv1 that FLV was dominant for P700 oxidation in M. polymorpha. These data suggested that photorespiration has yet not replaced FLV in functioning for P700 oxidation in the basal land plant probably because of the lower contribution to lumen acidification, compared with FLV, as reflected in the results of electrochromic shift analysis.

Characterization of Light-Enhanced Respiration in Cyanobacteria

In eukaryotic algae, respiratory O₂ uptake is enhanced after illumination, which is called light-enhanced respiration (LER). It is likely stimulated by an increase in respiratory substrates produced during photosynthetic CO₂ assimilation and function in keeping the metabolic and redox homeostasis in the light in eukaryotic cells, based on the interactions among the cytosol, chloroplasts, and mitochondria. Here, we first characterize LER in photosynthetic prokaryote cyanobacteria, in which respiration and photosynthesis share their metabolisms and electron transport chains in one cell. From the physiological analysis, the cyanobacterium Synechocystis sp. PCC 6803 performs LER, similar to eukaryotic algae, which shows a capacity comparable to the net photosynthetic O₂ evolution rate. Although the respiratory and photosynthetic electron transports share the interchain, LER was uncoupled from photosynthetic electron transport. Mutant analyses demonstrated that LER is motivated by the substrates directly provided by photosynthetic CO₂ assimilation, but not by glycogen. Further, the light-dependent activation of LER was observed even with exogenously added glucose, implying a regulatory mechanism for LER in addition to the substrate amounts. Finally, we discuss the physiological significance of the large capacity of LER in cyanobacteria and eukaryotic algae compared to those in plants that normally show less LER.

Intrinsic Fluctuations in Transpiration Induce Photorespiration to Oxidize P700 in Photosystem I

Upon exposure to environmental stress, the primary electron donor in photosystem I (PSI), P700, is oxidized to suppress the production of reactive oxygen species that could oxidatively inactivate the function of PSI. The illumination of rice leaves with actinic light induces intrinsic fluctuations in the opening and closing of stomata, causing the net CO2 assimilation rate to fluctuate. We examined the effects of these intrinsic fluctuations on electron transport reactions. Under atmospheric O2 conditions (21 kPa), the effective quantum yield of photosystem II (PSII) (Y(II)) remained relatively high while the net CO2 assimilation rate fluctuated, which indicates the function of alternative electron flow. By contrast, under low O2 conditions (2 kPa), Y(II) fluctuated. These results suggest that photorespiration primarily drove the alternative electron flow. Photorespiration maintained the oxidation level of ferredoxin (Fd) throughout the fluctuation of the net CO2 assimilation rate. Moreover, the relative activity of photorespiration was correlated with both the oxidation level of P700 and the magnitude of the proton gradient across the thylakoid membrane in 21 kPa O2 conditions. These results show that photorespiration oxidized P700 by stimulating the proton gradient formation when CO2 assimilation was suppressed by stomatal closure.

Functional redundancy between flavodiiron proteins and NDH-1 in Synechocystis sp. PCC 6803

In oxygenic photosynthetic organisms, excluding angiosperms, flavodiiron proteins (FDPs) catalyze light-dependent reduction of O₂ to H₂ O. This alleviates electron pressure on the photosynthetic apparatus and protects it from photodamage. In Synechocystis sp. PCC 6803, four FDP isoforms function as hetero-oligomers of Flv1 and Flv3 and/or Flv2 and Flv4. An alternative electron transport pathway mediated by the NAD(P)H dehydrogenase-like complex (NDH-1) also contributes to redox hemostasis and the photoprotection of photosynthesis. Four NDH-1 types have been characterized in cyanobacteria: NDH-1₁ and NDH-1₂ , which function in respiration; and NDH-1₃ and NDH-1₄ , which function in CO₂ uptake. All four types are involved in cyclic electron transport. Along with single FDP mutants (∆flv1 and Δflv3) and the double NDH-1 mutants (∆d1d2, which is deficient in NDH-1_1,2 and ∆d3d4, which is deficient in NDH-1_3,4 ), we studied triple mutants lacking one of Flv1 or Flv3, and NDH-1_1,2 or NDH-1_3,4 . We show that the presence of either Flv1/3 or NDH-1_1,2 , but not NDH-1_3,4 , is indispensable for survival during changes in growth conditions from high CO₂ /moderate light to low CO₂ /high light. Our results show functional redundancy between FDPs and NDH-1_1,2 under the studied conditions. We suggest that ferredoxin probably functions as a primary electron donor to both Flv1/3 and NDH-1_1,2 , allowing their functions to be dynamically coordinated for efficient oxidation of photosystem I and for photoprotection under variable CO₂ and light availability.

Identification of the electron donor to flavodiiron proteins in Synechocystis sp. PCC 6803 by in vivo spectroscopy

Flavodiiron proteins (FDPs) of photosynthetic organisms play a photoprotective role by reducing oxygen to water and thus avoiding the accumulation of excess electrons on the photosystem I (PSI) acceptor side under stress conditions. In Synechocystis sp. PCC 6803 grown under high CO₂, both FDPs Flv1 and Flv3 are indispensable for oxygen reduction. We performed a detailed in vivo kinetic study of wild-type (WT) and Δflv1/3 strains of Synechocystis using light-induced NADPH fluorescence and near-infrared absorption of iron-sulfur clusters from ferredoxin and the PSI acceptors (F_AF_B), collectively named FeS. These measurements were performed under conditions where the Calvin-Benson cycle is inactive or poorly activated. Under such conditions, the NADPH decay following a short illumination decays in parallel in both strains and exhibits a time lag which is correlated to the presence of reduced FeS. On the contrary, reduced FeS decays much faster in WT than in Δflv1/3 (13 vs 2 s^-1). These data unambiguously show that reduced ferredoxin, or possibly reduced F_AF_B, is the direct electron donor to the Flv1/Flv3 heterodimer. Evidences for large reduction of (F_AF_B) and recombination reactions within PSI were also provided by near-infrared absorption. Mutants lacking either the NDH1-L complex, the homolog of complex I of respiration, or the Pgr5 protein show no difference with WT in the oxidation of reduced FeS following a short illumination. These observations question the participation of a significant cyclic electron flow in cyanobacteria during the first seconds of the induction phase of photosynthesis.

Chlamydomonas sp. UWO 241 Exhibits High Cyclic Electron Flow and Rewired Metabolism under High Salinity

The Antarctic green alga Chlamydomonas sp. UWO 241 (UWO 241) is adapted to permanent low temperatures, hypersalinity, and extreme shade. One of the most striking phenotypes of UWO 241 is an altered PSI organization and constitutive PSI cyclic electron flow (CEF). To date, little attention has been paid to CEF during long-term stress acclimation, and the consequences of sustained CEF in UWO 241 are not known. In this study, we combined photobiology, proteomics, and metabolomics to understand the underlying role of sustained CEF in high-salinity stress acclimation. High salt-grown UWO 241 exhibited increased thylakoid proton motive flux and an increased capacity for nonphotochemical quenching. Under high salt, a significant proportion of the up-regulated enzymes were associated with the Calvin-Benson-Bassham cycle, carbon storage metabolism, and protein translation. Two key enzymes of the shikimate pathway, 3-deoxy-d-arabinoheptulosonate 7-phosphate synthase and chorismate synthase, were also up-regulated, as well as indole-3-glycerol phosphate synthase, an enzyme involved in the biosynthesis of l-Trp and indole acetic acid. In addition, several compatible solutes (glycerol, Pro, and Suc) accumulated to high levels in high salt-grown UWO 241 cultures. We suggest that UWO 241 maintains constitutively high CEF through the associated PSI-cytochrome b ₆ f supercomplex to support robust growth and strong photosynthetic capacity under a constant growth regime of low temperatures and high salinity.

Photoprotection mechanisms under different CO2 regimes during photosynthesis in a green alga Chlorella variabilis

Oxygenic photosynthesis converts light energy into chemical energy via electron transport and assimilates CO₂ in the Calvin-Benson cycle with the chemical energy. Thus, high light and low CO₂ conditions induce the accumulation of electrons in the photosynthetic electron transport system, resulting in the formation of reactive oxygen species. To prevent the accumulation of electrons, oxygenic photosynthetic organisms have developed photoprotection mechanisms, including non-photochemical quenching (NPQ) and alternative electron flow (AEF). There are diverse molecular mechanisms underlying NPQ and AEF, and the corresponding molecular actors have been identified and characterized using a model green alga Chlamydomonas reinhardtii. In contrast, detailed information about the photoprotection mechanisms is lacking for other green algal species. In the current study, we examined the photoprotection mechanisms responsive to CO₂ in the green alga Chlorella variabilis by combining the analyses of pulse-amplitude-modulated fluorescence, O₂ evolution, and the steady-state and time-resolved fluorescence spectra. Under the CO₂-limited condition, ΔpH-dependent NPQ occurred in photosystems I and II. Moreover, O₂-dependent AEF was also induced. Under the CO₂-limited condition with carbon supplementation, NPQ was relaxed and light-harvesting chlorophyll-protein complex II was isolated from both photosystems. In C. variabilis, the O₂-dependent AEF and the mechanisms that instantly convert the light-harvesting functions of both photosystems may be important for maintaining efficient photosynthetic activities under various CO₂ conditions.

Molecular Mechanism of Oxidation of P700 and Suppression of ROS Production in Photosystem I in Response to Electron-Sink Limitations in C3 Plants

Photosynthesis fixes CO₂ and converts it to sugar, using chemical-energy compounds of both NADPH and ATP, which are produced in the photosynthetic electron transport system. The photosynthetic electron transport system absorbs photon energy to drive electron flow from Photosystem II (PSII) to Photosystem I (PSI). That is, both PSII and PSI are full of electrons. O₂ is easily reduced to a superoxide radical (O₂^-) at the reducing side, i.e., the acceptor side, of PSI, which is the main production site of reactive oxygen species (ROS) in photosynthetic organisms. ROS-dependent inactivation of PSI in vivo has been reported, where the electrons are accumulated at the acceptor side of PSI by artificial treatments: exposure to low temperature and repetitive short-pulse (rSP) illumination treatment, and the accumulated electrons flow to O₂, producing ROS. Recently, my group found that the redox state of the reaction center of chlorophyll P700 in PSI regulates the production of ROS: P700 oxidation suppresses the production of O₂^- and prevents PSI inactivation. This is why P700 in PSI is oxidized upon the exposure of photosynthesis organisms to higher light intensity and/or low CO₂ conditions, where photosynthesis efficiency decreases. In this study, I introduce a new molecular mechanism for the oxidation of P700 in PSI and suppression of ROS production from the robust relationship between the light and dark reactions of photosynthesis. The accumulated protons in the lumenal space of the thylakoid membrane and the accumulated electrons in the plastoquinone (PQ) pool drive the rate-determining step of the P700 photo-oxidation reduction cycle in PSI from the photo-excited P700 oxidation to the reduction of the oxidized P700, thereby enhancing P700 oxidation.

Growth under Fluctuating Light Reveals Large Trait Variation in a Panel of Arabidopsis Accessions

The oxidation of P700 in photosystem I (PSI) is a robust mechanism that suppresses the production of reactive oxygen species. We researched the contribution of photorespiration to the oxidation of P700 in wheat leaves. We analyzed the effects of changes in partial pressures of CO₂ and O₂ on photosynthetic parameters. The electron flux in photosynthetic linear electron flow (LEF) exhibited a positive linear relationship with an origin of zero against the dissipation rate (vH⁺) of electrochromic shift (ECS; ΔpH across thylakoid membrane), indicating that cyclic electron flow around PSI did not contribute to H⁺ usage in photosynthesis/photorespiration. The vH⁺ showed a positive linear relationship with an origin of zero against the H⁺ consumption rates in photosynthesis/photorespiration (JgH⁺). These two linear relationships show that the electron flow in LEF is very efficiently coupled with H⁺ usage in photosynthesis/photorespiration. Lowering the intercellular partial pressure of CO₂ enhanced the oxidation of P700 with the suppression of LEF. Under photorespiratory conditions, the oxidation of P700 and the reduction of the plastoquinone pool were stimulated with a decrease in JgH⁺, compared to non-photorespiratory conditions. These results indicate that the reduction-induced suppression of electron flow (RISE) suppresses the reduction of oxidized P700 in PSI under photorespiratory conditions. Furthermore, under photorespiratory conditions, ECS was larger and H⁺ conductance was lower against JgH⁺ than those under non-photorespiratory conditions. These results indicate that photorespiration enhances RISE and ΔpH formation by lowering H⁺ conductance, both of which contribute to keeping P700 in a highly oxidized state.

Trimeric organization of photosystem I is required to maintain the balanced photosynthetic electron flow in cyanobacterium Synechocystis sp. PCC 6803

In Synechocystis sp. PCC 6803 and some other cyanobacteria photosystem I reaction centres exist predominantly as trimers, with minor contribution of monomeric form, when cultivated at standard optimized conditions. In contrast, in plant chloroplasts photosystem I complex is exclusively monomeric. The functional significance of trimeric organization of cyanobacterial photosystem I remains not fully understood. In this study, we compared the photosynthetic characteristics of PSI in wild type and psaL knockout mutant. The results show that relative to photosystem I trimer in wild-type cells, photosystem I monomer in psaL^- mutant has a smaller P700⁺ pool size under low and moderate light, slower P700 oxidation upon dark-to-light transition, and slower P700⁺ reduction upon light-to-dark transition. The mutant also shows strongly diminished photosystem I donor side limitations [quantum yield Y(ND)] at low, moderate and high light, but enhanced photosystem I acceptor side limitations [quantum yield Y(NA)], especially at low light (22 µmol photons m^-2 s^-1). In line with these functional characteristics are the determined differences in the relative expression genes encoding of selected electron transporters. The psaL^- mutant showed significant (ca fivefold) upregulation of the photosystem I donor cytochrome c₆, and downregulation of photosystem I acceptors (ferredoxin, flavodoxin) and proteins of alternative electron flows originating in photosystem I acceptor side. Taken together, our results suggest that photosystem I trimerization in wild-type Synechocystis cells plays a role in the protection of photosystem I from photoinhibition via maintaining enhanced donor side electron transport limitations and minimal acceptor side electron transport limitations at various light intensities.

What Quantity of Photosystem I Is Optimum for Safe Photosynthesis?

PSI has the potential to generate reactive oxygen species and be oxidatively inactivated by the reactive oxygen species. The photo-oxidative damage of PSI (also called PSI photoinhibition) causes the inhibition of the plant growth and is a lethal event for plants. It has been reported that PSI photoinhibition does not occur as long as the reaction-center chlorophyll (P700) remains oxidized, even in excess light conditions. This process is termed P700 oxidation and is supported by various regulatory mechanisms and likely also by the stoichiometric quantities of photosynthetic apparatus. In this study, we assessed how decreased photochemically active PSI in Arabidopsis (Arabidopsis thaliana) affected a variety of photosynthetic parameters, including P700 oxidation. Inactivation of PSI was rapidly and selectively induced by repetitive short-pulse illumination. PSI photoinhibition correlated linearly with decreases in effective quantum yield of PSII and nonphotochemical quenching; however, the photosynthetic CO₂ assimilation rate was less affected, as exemplified by ∼50% of the normal CO₂ assimilation rate maintained with an 80% loss in PSI photochemical activity. In contrast, effective quantum yield of PSI was enhanced following PSI photoinhibition, mainly owing to a decrease in the electron donor-side limitation of PSI. Based on these results, we propose that the stoichiometric quantity of PSI is optimized to induce P700 oxidation for dissipating excess light energy in PSI, thus avoiding inhibition of photosynthetic CO₂ assimilation caused by PSI photoinhibition.

Comparative analysis of strategies to prepare electron sinks in aquatic photoautotrophs

While subject to illumination, photosystem I (PSI) has the potential to produce reactive oxygen species (ROS) that can cause photo-oxidative damage in oxygenic photoautotrophs. The reaction center chlorophyll in PSI (P700) is kept oxidized in excess light conditions to limit over-excitation of PSI and alleviate the production of ROS. Oxidation of P700 requires a sufficient electron sink for PSI, which is responsible for flavodiiron proteins (FLV) safely dissipating electrons to O₂ in cyanobacteria, green algae, and land plants except for angiosperms during short-pulse light (SP) illumination under which photosynthesis and photorespiration do not occur. This fact implies that O₂ usage is essential for P700 oxidation but also raises the question why angiosperms lost FLV. Here, we first found that aquatic photoautotrophs in red plastid lineage, in which no gene for FLV has been found, could keep P700 oxidized during SP illumination alleviating the photo-oxidative damage in PSI even without O₂ usage. We comprehensively assessed P700 oxidation during SP illumination in the presence and absence of O₂ in cyanobacteria (Cyanophyta), green algae (Chlorophyta), angiosperms (Streptophyta), red algae (Rhodophyta), and secondary algae (Cryptophyta, Haptophyta, and Heterokontophyta). A variety of dependencies of P700 oxidation on O₂ among these photoautotrophs clearly suggest that O₂ usage and FLV are not universally required to oxidize P700 for protecting PSI against ROS damage. Our results expand the understanding of the diverse strategies taken by oxygenic photoautotrophs to oxidize P700 and mitigate the risks of ROS.

Enhanced Production of D-Lactate in Cyanobacteria by Re-Routing Photosynthetic Cyclic and Pseudo-Cyclic Electron Flow

Cyanobacteria are promising chassis strains for the photosynthetic production of platform and specialty chemicals from carbon dioxide. Their efficient light harvesting and metabolic flexibility abilities have allowed a wide range of biomolecules, such as the bioplastic polylactate precursor D-lactate, to be produced, though usually at relatively low yields. In order to increase photosynthetic electron flow towards the production of D-lactate, we have generated several strains of the marine cyanobacterium Synechococcus sp. PCC 7002 (Syn7002) with deletions in genes involved in cyclic or pseudo-cyclic electron flow around photosystem I. Using a variant of the Chlamydomonas reinhardtii D-lactate dehydrogenase (LDH^SRT, engineered to efficiently utilize NADPH in vivo), we have shown that deletion of either of the two flavodiiron flv homologs (involved in pseudo-cyclic electron transport) or the Syn7002 pgr5 homolog (proposed to be a vital part of the cyclic electron transport pathway) is able to increase D-lactate production in Syn7002 strains expressing LDH^SRT and the Escherichia coli LldP (lactate permease), especially at low temperature (25°C) and 0.04% (v/v) CO₂, though at elevated temperatures (38°C) and/or high (1%) CO₂ concentrations, the effect was less obvious. The Δpgr5 background seemed to be particularly beneficial at 25°C and 0.04% (v/v) CO₂, with a nearly 7-fold increase in D-lactate accumulation in comparison to the wild-type background (≈1000 vs ≈150 mg/L) and decreased side effects in comparison to the flv deletion strains. Overall, our results show that manipulation of photosynthetic electron flow is a viable strategy to increase production of platform chemicals in cyanobacteria under ambient conditions.

Growth Light Environment Changes the Sensitivity of Photosystem I Photoinhibition Depending on Common Wheat Cultivars

Light is an important factor for determining photosynthetic performance in land plants. At high light intensity, land plants develop photosynthetic activity by increasing electron sinks, such as the Calvin cycle and photorespiration and photoprotective mechanisms in photosystem II (PSII), to effectively utilize light and protect them from photoinhibition. In addition to PSII, photosystem I (PSI) has a risk of undergoing photoinhibition under high light intensity because of the reactive oxygen species (ROS) produced within PSI. However, the acclimation response has hardly been evaluated in the relationship of PSI photoprotection to growth light. In this study, we studied the effect of growth light intensity on the photoprotective mechanisms in PSI using six wheat cultivars. To evaluate the susceptibility of PSI to its photoinhibition, we used the repetitive short-pulse (rSP) illumination method to cause O₂-dependent PSI photoinhibition. We found that PSI photoinhibition induced by rSP illumination was much more alleviated in wheat cultivars grown under high-light conditions compared to those grown under low-light conditions. Here, we observed that wheat plant grown under high-light conditions lowered the susceptibility of PSI to its photoinhibition compared to those grown under low-light conditions. Furthermore, the acclimation response toward PSI photoinhibition was significantly different among the studied wheat cultivars, although the quantum yields both of PSII and PSI were increased by high-light acclimation in all wheat cultivars as reported previously. Interestingly, we observed that total chlorophyll content in leaves correlated with the susceptibility of PSI to its photoinhibition. On the basis of these results, we suggest that high-light acclimation induces protection mechanisms against PSI photoinhibition in land plants, and the increase in the leaf chlorophyll content relates to the susceptibility of PSI photoinhibition in wheat plants.

Oxidation of P700 Ensures Robust Photosynthesis

In the light, photosynthetic cells can potentially suffer from oxidative damage derived from reactive oxygen species. Nevertheless, a variety of oxygenic photoautotrophs, including cyanobacteria, algae, and plants, manage their photosynthetic systems successfully. In the present article, we review previous research on how these photoautotrophs safely utilize light energy for photosynthesis without photo-oxidative damage to photosystem I (PSI). The reaction center chlorophyll of PSI, P700, is kept in an oxidized state in response to excess light, under high light and low CO2 conditions, to tune the light utilization and dissipate the excess photo-excitation energy in PSI. Oxidation of P700 is co-operatively regulated by a number of molecular mechanisms on both the electron donor and acceptor sides of PSI. The strategies to keep P700 oxidized are diverse among a variety of photoautotrophs, which are evolutionarily optimized for their ecological niche.

Interaction of high seawater temperature and light intensity on photosynthetic electron transport of eelgrass (Zostera marina L.)

The interaction of widely recognized causes of eelgrass decline (high seawater temperature and limited light intensity) on photosynthetic electron transport was investigated via chlorophyll fluorescence technique. High seawater temperature combined light intensity significantly increasing the relative maximum electron transport rate (rETR_max); at critical temperature of 30 °C, the rETR_max increased with the enhancement of light intensity, indicating the elevation of overall photosynthetic performance. Based on the magnitude of effect size (η²), light intensity was the predominant factor affecting the performance index (PI_ABS), indicating that photosystem II (PSII) was sensitive to light intensity. Moreover, the donor side was severely damaged as evidenced by the higher decrease amplitude of fast component and its subsequent incomplete recovery. The reaction center exhibited limited flexibility due to the slight decrease amplitude in maximum photochemical quantum yield. In contrast with PSII, photosystem I (PSI) was more sensitive to high seawater temperature, based on the magnitude of η² derived from the maximal decrease in slope. High seawater temperature significantly increased PSI activity, plastoquinol reoxidation capacity, and probability for electron transfer to final PSI electron acceptors. Moreover, it combined elevated light intensity significantly stimulated the activity of cyclic electron flow (CEF) around PSI. Higher activity of both PSI and CEF contributed to balancing the linear electron transport via alleviating the over-reduction of the plastoquinone pool, exhibiting flexible regulation of photosynthetic electron transport at critical temperature. Therefore, limited light intensity decreased the tolerance of eelgrass to critical temperature, which might be a factor contributing factor in the observed decline.

Response of the thylakoid proteome of Synechocystis sp. PCC 6803 to photohinibitory intensities of orange-red light

Photoautotrophic growth of Synechocystis sp. PCC 6803 in a flat-panel photobioreactor, run in turbidostat mode under increasing intensities of orange-red light (636 nm), showed a maximal growth rate (0.12 h^-1) at 300 μmol_photons m^-2 s^-1, whereas first signs of photoinhibition were detected above 800 μmol_photons m^-2 s^-1. To investigate the dynamic modulation of the thylakoid proteome in response to photoinhibitory light intensities, quantitative proteomics analyses by SWATH mass spectrometry were performed by comparing thylakoid membranes extracted from Synechocystis grown under low-intensity illumination (i.e. 50 μmol_photons m^-2 s^-1) with samples isolated from cells subjected to photoinhibitory light regimes (800, 950 and 1460 μmol_photons m^-2 s^-1). We identified and quantified 126 proteins with altered abundance in all three photoinhibitory illumination regimes. These data reveal the strategies by which Synechocystis responds to photoinibitory growth irradiances of orange-red light. The accumulation of core proteins of Photosystem II and reduction of oxygen-evolving-complex subunits in photoinhibited cells revealed a different turnover and repair rates of the integral and extrinsic Photosystem II subunits with variation of light intensity. Furthermore, Synechocystis displayed a differentiated response to photoinhibitory regimes also regarding Photosystem I: the amount of PsaD, PsaE, PsaJ and PsaM subunits decreased, while there was an increased abundance of the PsaA, PsaB, Psak2 and PsaL proteins. Photoinhibition with 636 nm light also elicited an increased capacity for cyclic electron transport, a lowering of the amount of phycobilisomes and an increase of the orange carotenoid protein content, all presumably as a photoprotective mechanism against the generation of reactive oxygen species.

PROTON GRADIENT REGULATION 5 supports linear electron flow to oxidize photosystem I

In higher plants, light drives the linear photosynthetic electron transport reaction from H₂ O to electron sinks, which is called the linear electron flow (LEF). LEF activity should be regulated depending on electron sinks; otherwise excess electrons accumulate in the thylakoid membranes and stimulate reactive oxygen species (ROS) production in photosystem I (PSI), which causes oxidative damage to PSI. To prevent ROS production in PSI, PSI should be oxidized during photosynthesis, and PROTON GRADIENT REGULATION 5 (PGR5) and PGR like 1 (PGRL1) are important for this oxidation. PGR5 and PGRL1 are recognized as a component of ferredoxin-dependent cyclic electron flow around PSI (Fd-CEF-PSI), however there is no direct evidence for the significant operation of Fd-CEF-PSI during photosynthesis in wild-type (WT) plants. Thus, electron distribution by PGR5 and PGRL1 between Fd-CEF-PSI and LEF is still elusive. Here, we show direct evidence that Fd-CEF-PSI activity is minor during steady-state photosynthesis by measuring the Fd redox state in vivo in Arabidopsis thaliana. We found that Fd oxidation rate is determined by LEF activity during steady-state photosynthesis in WT. On the other hand, pgr5 and pgrl1 showed lower electron transport efficiency from PSI to electron sinks through Fd during steady-state photosynthesis. These results demonstrate that electrons are exclusively consumed in electron sinks through Fd, and the phenotypes of pgr5 and pgrl1 are likely caused by the disturbance of the LEF between PSI and electron sinks. We suggest that PGR5 and PGRL1 modulate the LEF according to electron sink activities around PSI.

Effects of genetic manipulation of the activity of photorespiration on the redox state of photosystem I and its robustness against excess light stress under CO2-limited conditions in rice

Under CO₂-limited conditions such as during stomatal closure, photorespiration is suggested to act as a sink for excess light energy and protect photosystem I (PSI) by oxidizing its reaction center chlorophyll P700. In this study, this issue was directly examined with rice (Oryza sativa L.) plants via genetic manipulation of the amount of Rubisco, which can be a limiting factor for photorespiration. At low [CO₂] of 5 Pa that mimicked stomatal closure condition, the activity of photorespiration in transgenic plants with decreased Rubisco content (RBCS-antisense plants) markedly decreased, whereas the activity in transgenic plants with overproduction of Rubisco (RBCS-sense plants) was similar to that in wild-type plants. Oxidation of P700 was enhanced at [CO₂] of 5 Pa in wild-type and RBCS-sense plants. PSI was not damaged by excess light stress induced by repetitive saturated pulse-light (rSP) in the presence of strong steady-state light. On the other hand, P700 was strongly reduced in RBCS-antisense plants at [CO₂] of 5 Pa. PSI was also damaged by rSP illumination. These results indicate that oxidation of P700 and the robustness of PSI against excess light stress are hampered by the decreased activity of photorespiration as a result of genetic manipulation of Rubisco content. It is also suggested that overproduction of Rubisco does not enhance photorespiration as well as CO₂ assimilation probably due to partial deactivation of Rubisco.

Responses of the chloroplast glyoxalase system to high CO2 concentrations

Sugar metabolism pathways such as photosynthesis produce dicarbonyls, e.g. methylglyoxal (MG), which can cause cellular damage. The glyoxalase (GLX) system comprises two enzymes GLX1 and GLX2, and detoxifies MG; however, this system is poorly understood in the chloroplast, compared with the cytosol. In the present study, we determined GLX1 and GLX2 activities in spinach chloroplasts, which constituted 40% and 10%, respectively, of the total leaf glyoxalase activity. In Arabidopsis thaliana, five GFP-fusion GLXs were present in the chloroplasts. Under high CO₂ concentrations, where increased photosynthesis promotes the MG production, GLX1 and GLX2 activities in A. thaliana increased and the expression of AtGLX1-2 and AtGLX2-5 was enhanced. On the basis of these findings and the phylogeny of GLX in oxygenic phototrophs, we propose that the GLX system scavenges MG produced in chloroplasts during photosynthesis.

Changing frequency of fluctuating light reveals the molecular mechanism for P700 oxidation in plant leaves

Natural sunlight exceeds the demand of photosynthesis such that it can cause plants to produce reactive oxygen species (ROS), which subsequently cause photo-oxidative damage. Because photosystem I (PSI) is a major source of ROS, plants actively maintain the reaction center chlorophyll of PSI(P700) oxidized under excessive light conditions to alleviate the ROS production. P700 oxidation is universally recognized in photosynthetic organisms as a physiological response to excessive light. However, it is still poorly understood how P700 oxidation is induced in response to fluctuating light with a variety of frequencies. Here, we investigated the relationships of photosynthetic parameters with P700 oxidation in Arabidopsis thaliana under a sine fluctuating light with different frequencies. As the photon flux density of the light increased, P700 was oxidized concurrently with the chlorophyll fluorescence parameter qL unless the electron acceptor side of PSI was limited. Conversely, we did not observe a proportional relationship of non-photochemical quenching with P700 oxidation. The mutant crr-2, which lacks chloroplast NADPH dehydrogenase, was impaired in P700 oxidation during light fluctuation at high, but not low frequency, unlike the pgrl1 mutant deficient in PGR5 and PGRL1 proteins, which could not oxidize P700 during light fluctuation at both high and low frequencies. Taken together, our findings suggested that the changing frequency of fluctuating light reveals the tracking performance of molecular mechanisms underlying P700 oxidation.

Reduction-Induced Suppression of Electron Flow (RISE) Is Relieved by Non-ATP-Consuming Electron Flow in Synechococcus elongatus PCC 7942

Photosynthetic organisms oxidize P700 to suppress the production of reactive oxygen species (ROS) in photosystem I (PSI) in response to the lower efficiency of photosynthesis under high light and low CO₂ conditions. Previously, we found a positive relationship between reduction of plastoquinone (PQ) pool and oxidation of P700, which we named reduction-induced suppression of electron flow (RISE). In the RISE model, we proposed that the highly reduced state of the PQ pool suppresses Q-cycle turnover to oxidize P700 in PSI. Here, we tested whether RISE was relieved by the oxidation of the PQ pool, but not by the dissipation of the proton gradient (ΔpH) across the thylakoid membrane. Formation of ΔpH can also suppress electron flow to P700, because acidification on the luminal side of the thylakoid membrane lowers oxidation of reduced PQ in the cytochrome b₆/f complex. We drove photosynthetic electron transport using H₂O₂-scavenging peroxidase reactions. Peroxidase reduces H₂O₂ with electron donors regenerated along the photosynthetic electron transport system, thereby promoting the formation of ΔpH. Addition of H₂O₂ to the cyanobacterium Synechococcus elongatus PCC 7942 under low CO₂ conditions induced photochemical quenching of chlorophyll fluorescence, enhanced NADPH fluorescence and reduced P700. Thus, peroxidase reactions relieved the RISE mechanism, indicating that P700 oxidation can be induced only by the reduction of PQ to suppress the production of ROS in PSI. Overall, our data suggest that RISE regulates the redox state of P700 in PSI in cooperation with ΔpH regulation.

Medium-chain dehydrogenase/reductase and aldo-keto reductase scavenge reactive carbonyls in Synechocystis sp. PCC 6803

Reactive carbonyls (RCs), which are inevitably produced during respiratory and photosynthetic metabolism, have the potential to cause oxidative damage to photosynthetic organisms. Previously, we proposed a scavenging model for RCs in the cyanobacterium Synechocystis sp. PCC 6803 (S. 6803). In the current study, we constructed mutants deficient in the enzymes medium-chain dehydrogenase/reductase (ΔMDR) and aldo-keto reductase (ΔAKR) to investigate their contributions to RC scavenging in vivo. We found that treatment with the lipid-derived RC acrolein causes growth inhibition and promotes greater protein carbonylation in ΔMDR, compared with the wild-type and ΔAKR. In both ΔMDR and ΔAKR, photosynthesis is severely inhibited in the presence of acrolein. These results suggest that these enzymes function as part of the scavenging systems for RCs in S. 6803 in vivo.

A Carbon Dioxide Limitation-Inducible Protein, ColA, Supports the Growth of Synechococcus sp. PCC 7002

A limitation in carbon dioxide (CO₂), which occurs as a result of natural environmental variation, suppresses photosynthesis and has the potential to cause photo-oxidative damage to photosynthetic cells. Oxygenic phototrophs have strategies to alleviate photo-oxidative damage to allow life in present atmospheric CO₂ conditions. However, the mechanisms for CO₂ limitation acclimation are diverse among the various oxygenic phototrophs, and many mechanisms remain to be discovered. In this study, we found that the gene encoding a CO₂ limitation-inducible protein, ColA, is required for the cyanobacterium Synechococcus sp. PCC 7002 (S. 7002) to acclimate to limited CO₂ conditions. An S. 7002 mutant deficient in ColA (ΔcolA) showed lower chlorophyll content, based on the amount of nitrogen, than that in S. 7002 wild-type (WT) under ambient air but not high CO₂ conditions. Both thermoluminescence and protein carbonylation detected in the ambient air grown cells indicated that the lack of ColA promotes oxidative stress in S. 7002. Alterations in the photosynthetic O₂ evolution rate and relative electron transport rate in the short-term response, within an hour, to CO₂ limitation were the same between the WT and ΔcolA. Conversely, these photosynthetic parameters were mostly lower in the long-term response of a few days in ΔcolA than in the WT. These data suggest that ColA is required to sustain photosynthetic activity for living under ambient air in S. 7002. The unique phylogeny of ColA revealed diverse strategies to acclimate to CO₂ limitation among cyanobacteria.

Association of Ferredoxin:NADP+ oxidoreductase with the photosynthetic apparatus modulates electron transfer in Chlamydomonas reinhardtii

Ferredoxins (FDX) and the FDX:NADP⁺ oxidoreductase (FNR) represent a key junction of electron transport downstream of photosystem I (PSI). Dynamic recruitment of FNR to the thylakoid membrane has been considered as a potential mechanism to define the fate of photosynthetically derived electrons. In this study, we investigated the functional importance of the association of FNR with the photosynthetic apparatus in Chlamydomonas reinhardtii. In vitro assays based on NADP⁺ photoreduction measurements as well as NMR chemical shift perturbation analyses showed that FNR preferentially interacts with FDX1 compared to FDX2. Notably, binding of FNR to a PSI supercomplex further enhanced this preference for FDX1 over FDX2, suggesting that FNR is potentially capable of channelling electrons towards distinct routes. NADP⁺ photoreduction assays and immunoblotting revealed that the association of FNR with the thylakoid membrane including the PSI supercomplex is impaired in the absence of Proton Gradient Regulation 5 (PGR5) and/or Proton Gradient Regulation 5-Like photosynthetic phenotype 1 (PGRL1), implying that both proteins, directly or indirectly, contribute to the recruitment of FNR to the thylakoid membrane. As assessed via in vivo absorption spectroscopy and immunoblotting, PSI was the primary target of photodamage in response to high-light stress in the absence of PGR5 and/or PGRL1. Anoxia preserved the activity of PSI, pointing to enhanced electron donation to O₂ as the source of the observed PSI inactivation and degradation. These findings establish another perspective on PGR5/PGRL1 knockout-related phenotypes and potentially interconnect FNR with the regulation of photosynthetic electron transport and PSI photoprotection in C. reinhardtii.

Diversity of strategies for escaping reactive oxygen species production within photosystem I among land plants: P700 oxidation system is prerequisite for alleviating photoinhibition in photosystem I

In land plants, photosystem I (PSI) photoinhibition limits carbon fixation and causes growth defects. In addition, recovery from PSI photoinhibition takes much longer than PSII photoinhibition when the PSI core-complex is degraded by oxidative damage. Accordingly, PSI photoinhibition should be avoided in land plants, and land plants should have evolved mechanisms to prevent PSI photoinhibition. However, such protection mechanisms have not yet been identified, and it remains unclear whether all land plants suffer from PSI photoinhibition in the same way. In the present study, we focused on the susceptibility of PSI to photoinhibition and investigated whether mechanisms of preventing PSI photoinhibition varied among land plant species. To assess the susceptibility of PSI to photoinhibition, we used repetitive short-pulse (rSP) illumination, which specifically induces PSI photoinhibition. Subsequently, we found that land plants possess a wide variety of tolerance mechanisms against PSI photoinhibition. In particular, gymnosperms, ferns and mosses/liverworts exhibited higher tolerance to rSP illumination-induced PSI photoinhibition than angiosperms, and detailed analyses indicated that the tolerance of these groups could be partly attributed to flavodiiron proteins, which protected PSI from photoinhibition by oxidizing the PSI reaction center chlorophyll (P700) as an electron acceptor. Furthermore, we demonstrate, for the first time, that gymnosperms, ferns and mosses/liverworts possess a protection mechanism against photoinhibition of PSI that differs from that of angiosperms.

Land plants drive photorespiration as higher electron-sink: comparative study of post-illumination transient O2 -uptake rates from liverworts to angiosperms through ferns and gymnosperms

In higher plants, the electron-sink capacity of photorespiration contributes to alleviation of photoinhibition by dissipating excess energy under conditions when photosynthesis is limited. We addressed the question at which point in the evolution of photosynthetic organisms photorespiration began to function as electron sink and replaced the flavodiiron proteins which catalyze the reduction of O₂ at photosystem I in cyanobacteria. Algae do not have a higher activity of photorespiration when CO₂ assimilation is limited, and it can therefore not act as an electron sink. Using land plants (liverworts, ferns, gymnosperms, and angiosperms) we compared photorespiration activity and estimated the electron flux driven by photorespiration to evaluate its electron-sink capacity at CO₂ -compensation point. In vivo photorespiration activity was estimated by the simultaneous measurement of O₂ -exchange rate and chlorophyll fluorescence yield. All C3-plants leaves showed transient O₂ -uptake after actinic light illumination (post-illumination transient O₂ -uptake), which reflects photorespiration activity. Post-illumination transient O₂ -uptake rates increased in the order from liverworts to angiosperms through ferns and gymnosperms. Furthermore, photorespiration-dependent electron flux in photosynthetic linear electron flow was estimated from post-illumination transient O₂ -uptake rate and compared with the electron flux in photosynthetic linear electron flow in order to evaluate the electron-sink capacity of photorespiration. The electron-sink capacity at the CO₂ -compensation point also increased in the above order. In gymnosperms photorespiration was determined to be the main electron-sink. C3-C4 intermediate species of Flaveria plants showed photorespiration activity, which intermediate between that of C3- and C4-flaveria species. These results indicate that in the first land plants, liverworts, photorespiration started to function as electron sink. According to our hypothesis, the dramatic increase in partial pressure of O₂ in the atmosphere about 0.4 billion years ago made it possible to drive photorespiration with higher activity in liverworts.

Chloroplastic ATP synthase builds up a proton motive force preventing production of reactive oxygen species in photosystem I

Over-reduction of the photosynthetic electron transport (PET) chain should be avoided, because the accumulation of reducing electron carriers produces reactive oxygen species (ROS) within photosystem I (PSI) in thylakoid membranes and causes oxidative damage to chloroplasts. To prevent production of ROS in thylakoid membranes the H⁺ gradient (ΔpH) needs to be built up across the thylakoid membranes to suppress the over-reduction state of the PET chain. In this study, we aimed to identify the critical component that stimulates ΔpH formation under illumination in higher plants. To do this, we screened ethyl methane sulfonate (EMS)-treated Arabidopsis thaliana, in which the formation of ΔpH is impaired and the PET chain caused over-reduction under illumination. Subsequently, we isolated an allelic mutant that carries a missense mutation in the γ-subunit of chloroplastic CF₀ CF₁ -ATP synthase, named hope2. We found that hope2 suppressed the formation of ΔpH during photosynthesis because of the high H⁺ efflux activity from the lumenal to stromal side of the thylakoid membranes via CF₀ CF₁ -ATP synthase. Furthermore, PSI was in a more reduced state in hope2 than in wild-type (WT) plants, and hope2 was more vulnerable to PSI photoinhibition than WT under illumination. These results suggested that chloroplastic CF₀ CF₁ -ATP synthase adjusts the redox state of the PET chain, especially for PSI, by modulating H⁺ efflux activity across the thylakoid membranes. Our findings suggest the importance of the buildup of ΔpH depending on CF₀ CF₁ -ATP synthase to adjust the redox state of the reaction center chlorophyll P700 in PSI and to suppress the production of ROS in PSI during photosynthesis.

The Liverwort, Marchantia, Drives Alternative Electron Flow Using a Flavodiiron Protein to Protect PSI

The diffusion efficiency of oxygen in the atmosphere, like that of CO₂, is approximately 10⁴ times greater than that in aqueous environments. Consequently, terrestrial photosynthetic organisms need mechanisms to protect against potential oxidative damage. The liverwort Marchantia polymorpha, a basal land plant, has habitats where it is exposed to both water and the atmosphere. Furthermore, like cyanobacteria, M. polymorpha has genes encoding flavodiiron proteins (FLV). In cyanobacteria, FLVs mediate oxygen-dependent alternative electron flow (AEF) to suppress the production of reactive oxygen species. Here, we investigated whether FLVs are required for the protection of photosynthesis in M. polymorpha A mutant deficient in the FLV1 isozyme (ΔMpFlv1) sustained photooxidative damage to photosystem I (PSI) following repetitive short-saturation pulses of light. Compared with the wild type (Takaragaike-1), ΔMpFlv1 showed the same photosynthetic oxygen evolution rate but a lower electron transport rate during the induction phase of photosynthesis. Additionally, the reaction center chlorophyll in PSI, P700, was highly reduced in ΔMpFlv1 but not in Takaragaike-1. These results indicate that the gene product of MpFlv1 drives AEF to oxidize PSI, as in cyanobacteria. Furthermore, FLV-mediated AEF supports the production of a proton motive force to possibly induce the nonphotochemical quenching of chlorophyll fluorescence and suppress electron transport in the cytochrome b₆/f complex. After submerging the thalli, a decrease in photosystem II operating efficiency was observed, particularly in ΔMpFlv1, which implies that species living in these sorts of habitats require FLV-mediated AEF.

Diverse strategies of O2 usage for preventing photo-oxidative damage under CO2 limitation during algal photosynthesis

Photosynthesis produces chemical energy from photon energy in the photosynthetic electron transport and assimilates CO2 using the chemical energy. Thus, CO2 limitation causes an accumulation of excess energy, resulting in reactive oxygen species (ROS) which can cause oxidative damage to cells. O2 can be used as an alternative energy sink when oxygenic phototrophs are exposed to high light. Here, we examined the responses to CO2 limitation and O2 dependency of two secondary algae, Euglena gracilis and Phaeodactylum tricornutum. In E. gracilis, approximately half of the relative electron transport rate (ETR) of CO2-saturated photosynthesis was maintained and was uncoupled from photosynthesis under CO2 limitation. The ETR showed biphasic dependencies on O2 at high and low O2 concentrations. Conversely, in P. tricornutum, most relative ETR decreased in parallel with the photosynthetic O2 evolution rate in response to CO2 limitation. Instead, non-photochemical quenching was strongly activated under CO2 limitation in P. tricornutum. The results indicate that these secondary algae adopt different strategies to acclimatize to CO2 limitation, and that both strategies differ from those utilized by cyanobacteria and green algae. We summarize the diversity of strategies for prevention of photo-oxidative damage under CO2 limitation in cyanobacterial and algal photosynthesis.