Oxygen (O2) availability in plant tissues is dynamically shaped by photosynthesis and respiration and is linked to stress responses and development. While mitochondria are the primary consumers of cellular O2, their impact on chloroplast functions under low-oxygen conditions remains insufficiently understood. Mitochondrial retrograde signaling activates expression of nuclear genes encoding alternative oxidases and other respiratory components, and high abundance of these enzymes coincides not only with changes in respiration but also with alterations in chloroplast functions. For example, plants with induced mitochondrial signaling are tolerant to methyl viologen, which catalyzes the chloroplastic Mehler reaction. The mechanism of this inter-organelle interaction remains unclear. Here, we investigated respiration, photosynthesis, and in vivo O2 levels in Arabidopsis (Arabidopsis thaliana) mutants and transgenic lines with perturbations in diverse mitochondrial functions, including defects in respiratory complex I, ATP synthase, mitochondrial protein processing, transcription, nucleoid organization, and organelle architecture, as well as in lines with altered mitochondrial signaling, alternative oxidase activities, and nitric oxide metabolism. Increased abundance and capacity of alternative oxidases correlated with elevated O2 consumption in darkness, slower O2 re-accumulation in light, and reduced effects of methyl viologen on chloroplasts. The changes are likely mediated by multiple stress-induced alternative respiratory components. Our results support the hypothesis that enhanced mitochondrial O2 consumption under stress lowers tissue O2 levels, thereby modifying chloroplastic electron transfer and ROS metabolism. These data provide insights into the establishment and sensing of hypoxia in plants, plant adaptation to mitochondrial stress and low-oxygen environments, and the roles of chloroplasts in these processes.

Light intensity activation of alternative electron transport mechanisms in the moss Physcomitrium patens.

The study evaluates the accuracy of two FvCB model sub-models (I and II) in estimating the maximum electron transport rate for CO2 assimilation (JA-max) by comparing estimated values with observed maximum electron transport rates (Jf-max) in four C3 species: Triticum aestivum L., Silphium perfoliatum L., Lolium perenne L., and Trifolium pratense L. Significant discrepancies were found between JA-max estimates from sub-model I and observed Jf-max values for T. aestivum, S. perfoliatum, and T. pratense (p < 0.05), with sub-model I overestimating JA-max for T. aestivum. Sub-model II consistently produced higher JA-max estimates than sub-model I. This study highlights limitations in the FvCB sub-models, particularly their tendency to overestimate JA-max when accounting for electron consumption by photorespiration (JO), nitrate reduction (JNit), and the Mehler reaction (JMAP). An alternative empirical model provided more accurate Jf-max estimates, suggesting the need for improved approaches to model photosynthetic electron transport. These findings have important implications for crop yield prediction, ecological modeling, and climate change adaptation strategies, emphasizing the need for more accurate estimation methods in plant physiology research.

The functioning of the photosynthetic electron transport chain and the proceeding of accompanying processes were studied in Arabidopsis thaliana plants acclimated during 2weeks to reduced (150ppm) or elevated (1000ppm) CO2 concentrations in air. Measured at ambient CO2 , the quantum yields of both photosystems were lower in plants acclimated to these CO2 concentrations as compared with control plants grown at ambient CO2 . The difference was more pronounced at the beginning of the illumination. It is discussed that this difference resulted from the difference in Rubisco content, which at both reduced and elevated CO2 in air was lower than in control plants. The quantum yield of regulated non-photochemical energy loss in photosystem II under both reduced and elevated CO2 was lower than in control plants. This correlated with reduced expression of the PsbS protein gene. H2 O2 content in the leaves increased during the first days of plant adaptation to 150ppm CO2 , but then decreased. The increase resulted from enhanced rates of both photorespiration and Mehler reaction, while the following decrease resulted from enhancing contents of ascorbate peroxidases in all cell compartments.

Regulation of photosynthetic electron transfer is fundamental for energetic efficiency and coping with changing environmental factors. All plant groups, except angiosperms, use flavodiiron proteins (FDPs) on the acceptor side of photosystem I (PSI), putatively protecting from PSI photoinhibition under fluctuating light. An alternative electron flow during photosynthesis is the direct reduction of O2 with consequential production of reactive oxygen species (ROS; the Mehler reaction), but to what extent FDPs prevent this is unknown. Here, we quantified O2-dependent electron flow, photosystem activity and the Mehler reaction in Chlamydomonas reinhardtii. Near-infra red absorbance measurement of PSI reaction centre (P700) showed that FDPs remain active long after a dark-to-light transition and their activity increased under hyperoxia. Light-induced hydrogen peroxide (H2O2) production, as a marker of the Mehler reaction, was influenced by O2 concentration, and was up to 67 % higher in an FDP-deficient mutant (flvb) than in the wild-type under saturating constant light. In cultures kept under sub-saturating constant light, flvb produced 315 % more H2O2 and had lower PSII efficiency than wild-type. Inhibiting electron transfer out of photosystem II (PSII) with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) only partially blocked H2O2 production, particularly under hyperoxia, indicating that PSII was an additional ROS source. P700+ reduction in the dark in the presence of DCMU was faster in flvb than wild-type, revealing enhanced cyclic electron flow, which may also have led to PSI mediated Mehler reaction. We conclude that FDPs remain active in constant light and can prevent PSI mediated Mehler reaction, with relevance to PSII photoinhibition.

High light stress decreases the photosynthetic rate in plants due to photooxidative damage to photosynthetic apparatus, photoinhibition of PSII, and/or damage to PSI. The dissipation of excess energy by nonphotochemical quenching and degradation of the D1 protein of PSII and its repair cycle help against photooxidative damage. Light stress also activates stress-responsive nuclear genes through the accumulation of phosphonucleotide-3'-phosphoadenosine-5'-phosphate, methylerythritol cyclodiphosphate, and reactive oxygen species which comprise the chloroplast retrograde signaling pathway. Additionally, hormones, such as abscisic acid, cytokinin, brassinosteroids, and gibberellins, play a role in acclimation to light fluctuations. Several alternate electron flow mechanisms, which offset the excess of electrons, include activation of plastid or plastoquinol terminal oxidase, cytochrome b 6/f complex, cyclic electron flow through PSI, Mehler ascorbate peroxidase pathway or water-water cycle, mitochondrial alternative oxidase pathway, and photorespiration. In this review, we provided insights into high light stress-mediated damage to photosynthetic apparatus and strategies to mitigate the damage by decreasing antennae size, enhancing NPQ through the introduction of mutants, expression of algal proteins to improve photosynthetic rates and engineering ATP synthase.

In nature, photosynthetic organisms are exposed to fluctuating light, and their physiological systems must adapt to this fluctuation. To maintain homeostasis, these organisms have a light fluctuation photoprotective mechanism, which functions in both photosystems and metabolism. Although the photoprotective mechanisms functioning in the photosystem have been studied, it is unclear how metabolism responds to light fluctuations within a few seconds. In the present study, we investigated the metabolic response of Synechocystis sp. PCC 6803 to light fluctuations using 13 C-metabolic flux analysis. The light intensity and duty ratio were adjusted such that the total number of photons or the light intensity during the low-light phase was equal. Light fluctuations affected cell growth and photosynthetic activity under the experimental conditions. However, metabolic flux distributions and cofactor production rates were not affected by the light fluctuations. Furthermore, the estimated ATP and NADPH production rates in the photosystems suggest that NADPH-consuming electron dissipation occurs under fluctuating light conditions. Although we focused on the water-water cycle as the electron dissipation path, no growth effect was observed in an flv3-disrupted strain under fluctuating light, suggesting that another path contributes to electron dissipation under these conditions.

ABSTRACTThe open ocean is an extremely competitive environment, partially due to the dearth of nutrients. Trichodesmium erythraeum, a marine diazotrophic cyanobacterium, is a keystone species in the ocean due to its ability to fix nitrogen and leak 30 to 50% into the surrounding environment, providing a valuable source of a necessary macronutrient to other species. While there are other diazotrophic cyanobacteria that play an important role in the marine nitrogen cycle, Trichodesmium is unique in its ability to fix both carbon and nitrogen simultaneously during the day without the use of specialized cells called heterocysts to protect nitrogenase from oxygen. Here, we use the advanced modeling framework called multiscale multiobjective systems analysis (MiMoSA) to investigate how Trichodesmium erythraeum can reduce dimolecular nitrogen to ammonium in the presence of oxygen. Our simulations indicate that nitrogenase inhibition is best modeled as Michealis-Menten competitive inhibition and that cells along the filament maintain microaerobia using high flux through Mehler reactions in order to protect nitrogenase from oxygen. We also examined the effect of location on metabolic flux and found that cells at the end of filaments operate in distinctly different metabolic modes than internal cells despite both operating in a photoautotrophic mode. These results give us important insight into how this species is able to operate photosynthesis and nitrogen fixation simultaneously, giving it a distinct advantage over other diazotrophic cyanobacteria because they can harvest light directly to fuel the energy demand of nitrogen fixation.IMPORTANCETrichodesmium erythraeum is a marine cyanobacterium responsible for approximately half of all biologically fixed nitrogen, making it an integral part of the global nitrogen cycle. Interestingly, unlike other nitrogen-fixing cyanobacteria, Trichodesmium does not use temporal or spatial separation to protect nitrogenase from oxygen poisoning; instead, it operates photosynthesis and nitrogen fixation reactions simultaneously during the day. Unfortunately, the exact mechanism the cells utilize to operate carbon and nitrogen fixation simultaneously is unknown. Here, we use an advanced metabolic modeling framework to investigate and identify the most likely mechanisms Trichodesmium uses to protect nitrogenase from oxygen. The model predicts that cells operate in a microaerobic mode, using both respiratory and Mehler reactions to dramatically reduce intracellular oxygen concentrations.

Light-dependent reactions of photosynthesis takes place in the thylakoids of chloroplasts where light energy harvested from the sun drives the synthesis of ATP and NADPH. The major pathway of photosynthetic chain is the linear electron transport (LET), in which both photosystems (PSI and PSII) are involved, and ATP and NADPH are produced. However, ratio in production of those components is insufficient to cover the Calvin cycle energy requirements, depending on the metabolism of the cell. Moreover, disturbance in metabolism homeostasis, caused by environmental stress conditions, increases ATP demand, which cannot be covered by LET. Thus, in photosynthetic apparatus must exist alternative electron transport pathways, these include: cyclic electron transport (CET) mediated by NDH complex or PGR5/PGRL1 proteins, water-water cycle and PTOX enzyme. Activity of alternative pathways can optimize ratio in production of ATP/NADPH, appropriately to requirements, which allows to achieve redox balance and ATP contents.

Chlorella vulgaris cultivation under super high light intensity: An application of the flashing light effect

Melatonin (MT), an important phytohormone synthesized naturally, was recently used to improve plant resistance against abiotic and biotic stresses. However, the effects of exogenous melatonin on photosynthetic performances have not yet been well clarified. We found that spraying of exogenous melatonin (100 μM) to leaves slightly affected the steady state values of CO2 assimilation rate (AN), stomatal conductance (gs) and mesophyll conductance (gm) under high light in tobacco leaves. However, this exogenous melatonin strongly delayed the induction kinetics of gs and gm, leading to the slower induction speed of AN. During photosynthetic induction, AN is mainly limited by biochemistry in the absence of exogenous melatonin, but by CO2 diffusion conductance in the presence of exogenous melatonin. Therefore, exogenous melatonin can aggravate photosynthetic carbon loss during photosynthetic induction and should be used with care for crop plants grown under natural fluctuating light. Within the first 10 min after transition from low to high light, photosynthetic electron transport rates (ETR) for AN and photorespiration were suppressed in the presence of exogenous melatonin. Meanwhile, an important alternative electron sink, namely water-water cycle, was enhanced to dissipate excess light energy. These results indicate that exogenous melatonin upregulates water-water cycle to facilitate photoprotection. Taking together, this study is the first to demonstrate that exogenous melatonin inhibits dynamic photosynthesis and improves photoprotection in higher plants.

Oxygen reduction in chloroplasts in the light was discovered by (Mehler Arch Biochem Biophys 33:65-77, 1951) as production of hydrogen peroxide. Later, it was shown that the primary product of the oxygen reduction is superoxide radical produced in thylakoids by one-electron transfer from reduced components of photosynthetic electron transport chain to O2 molecule. For a long time, the formation of hydrogen peroxide was considered to be a result of disproportionation of superoxide radicals in chloroplast stroma. Here, we overview a growing number of evidence indicating on another one, additional to disproportionation, pathway of hydrogen peroxide formation in chloroplasts, namely its formation in thylakoid membrane due to reaction of superoxide radical generated in the membrane with the reduced plastoquinone molecule, plastohydroquinone. Since various components of photosynthetic electron transport chain (primarily photosystem I) can supply superoxide radicals to this reaction, we refer this two-step O2 photoreduction to H2O2 as a cooperative process. The significance of hydrogen peroxide production via this pathway for redox signaling and scavenging of reactive oxygen species is discussed.

A key component of photosynthetic electron transport chain, photosystem I (PSI), is susceptible to the fluctuating light (FL) in angiosperms. Cyclic electron flow (CEF) around PSI and water-water cycle (WWC) are both used by the epiphytic orchid Dendrobium officinale to protect PSI under FL. This study examined whether the ontogenetic stage of leaf has an impact on the photoprotective mechanisms dealing with FL. Thus, chlorophyll fluorescence and P700 signals under FL were measured in D. officinale young and mature leaves. Upon transition from dark to actinic light, a rapid re-oxidation of P700 was observed in mature leaves but disappeared in young leaves, indicating that WWC existed in mature leaves but was lacking in young leaves. After shifting from low to high light, PSI over-reduction was clearly missing in mature leaves. By comparison, young leaves showed a transient PSI over-reduction within the first 30 s, which was accompanied with highly activation of CEF. Therefore, the effect of FL on PSI redox state depends on the leaf ontogenetic stage. In mature leaves, WWC is employed to avoid PSI over-reduction. In young leaves, CEF around PSI is enhanced to compensate for the lack of WWC and thus to prevent an uncontrolled PSI over-reduction induced by FL.

Photosynthesis under fluctuating light in the CAM plant Vanilla planifolia

Photoinactivation of the oxygen-evolving complex regulates the photosynthetic strategy of the seagrass Zostera marina

Fluctuating light can cause selective photoinhibition of photosystem I (PSI) in angiosperms. Cyclic electron flow (CEF) around PSI and electron flux from water via the electron transport chain to oxygen (the water-water cycle) play important roles in coping with fluctuating light in angiosperms. However, it is unclear whether plant species in the same genus employ the same strategy to cope with fluctuating light. To answer this question, we measured P700 redox kinetics and chlorophyll fluorescence under fluctuating light in two Paphiopedilum (P.) Pftzer (Orchidaceae) species, P. dianthum and P. micranthum. After transition from dark to high light, P. dianthum displayed a rapid re-oxidation of P700, while P. micranthum displayed an over-reduction of P700. Furthermore, the rapid re-oxidation of P700 in P. dianthum was not observed when measured under anaerobic conditions. These results indicated that photo-reduction of O2 mediated by the water-water cycle was functional in P. dianthum but not in P. micranthum. Within the first few seconds after an abrupt transition from low to high light, PSI was highly oxidized in P. dianthum but was highly reduced in P. micranthum, indicating that the different responses of PSI to fluctuating light between P. micranthum and P. dianthum was attributed to the water-water cycle. In P. micranthum, the lack of the water-water cycle was partially compensated for by an enhancement of CEF. Taken together, P. dianthum and P. micranthum employed different strategies to cope with the abrupt change of light intensity, indicating the diversity of strategies for photosynthetic acclimation to fluctuating light in these two closely related orchid species.

Key photoprotective pathways of a shade-tolerant plant (Alpinia oxyphylla) for precipitation patterns change during the dry season: thermal energy dissipation and water-water cycle

Photosynthesis is a vital process, responsible for fixing carbon dioxide, and producing most of the organic matter on the planet. However, photosynthesis has some inherent limitations in utilizing solar energy, and a part of the energy absorbed is lost in the reduction of O2 to produce the superoxide radical (O2•-) via the Mehler reaction, which occurs principally within photosystem I (PSI). For decades, O2 reduction within PSI was assumed to take place solely in the distal iron-sulfur clusters rather than within the two asymmetrical cofactor branches. Here, we demonstrate that under high irradiance, O2 photoreduction by PSI primarily takes place at the phylloquinone of one of the branches (the A-branch). This conclusion derives from the light dependency of the O2 photoreduction rate constant in fully mature wild-type PSI from Chlamydomonas reinhardtii, complexes lacking iron-sulfur clusters, and a mutant PSI, in which phyllosemiquinone at the A-branch has a significantly longer lifetime. We suggest that the Mehler reaction at the phylloquinone site serves as a release valve under conditions where both the iron-sulfur clusters of PSI and the mobile ferredoxin pool are highly reduced.

Photosystem I (PSI) is the primary target of photoinhibition under fluctuating light (FL). Photosynthetic organisms employ alternative electron flows to protect PSI under FL. However, the understanding of the coordination of alternative electron flows under FL at temperature stresses is limited. To address this question, we measured the chlorophyll fluorescence, P700 redox state, and electrochromic shift signal in leaves of Dendrobium officinale exposed to FL at 42 °C, 25 °C, and 4 °C. Upon a sudden increase in illumination at 42 °C and 25 °C, the water–water cycle (WWC) consumed a significant fraction of the extra reducing power, and thus avoided an over-reduction of PSI. However, WWC was inactivated at 4 °C, leading to an over-reduction of PSI within the first seconds after light increased. Therefore, the role of WWC under FL is largely dependent on temperature conditions. After an abrupt increase in light intensity, cyclic electron flow (CEF) around PSI was stimulated at any temperature. Therefore, CEF and WWC showed different temperature responses under FL. Furthermore, the enhancement of CEF and WWC at 42 °C quickly generated a sufficient trans-thylakoid proton gradient (ΔpH). The inactivation of WWC at 4 °C was partially compensated for by an increased CEF activity. These findings indicate that CEF and WWC coordinate to protect PSI under FL at temperature stresses.

The water-water cycle is not a major alternative sink in fluctuating light at chilling temperature