Diatom pyrenoids are encased in a protein shell that enables efficient CO2 fixation.

Pyrenoids are subcompartments of algal chloroplasts that increase the efficiency of Rubisco-driven CO2 fixation. Diatoms fix up to 20% of global CO2, but their pyrenoids remain poorly characterized. Here, we used in vivo photo-crosslinking to identify pyrenoid shell (PyShell) proteins, which we localized to the pyrenoid periphery of model pennate and centric diatoms, Phaeodactylum tricornutum and Thalassiosira pseudonana. In situ cryo-electron tomography revealed that pyrenoids of both diatom species are encased in a lattice-like protein sheath. Single-particle cryo-EM yielded a 2.4-Å-resolution structure of an in vitro TpPyShell1 lattice, which showed how protein subunits interlock. T. pseudonana TpPyShell1/2 knockout mutants had no PyShell sheath, altered pyrenoid morphology, and a high-CO2 requiring phenotype, with reduced photosynthetic efficiency and impaired growth under standard atmospheric conditions. The structure and function of the diatom PyShell provide a molecular view of how CO2 is assimilated in the ocean, a critical ecosystem undergoing rapid change.

Plastid terminal oxidase (PTOX) protects photosystem I and not photosystem II against photoinhibition in Arabidopsis thaliana and Marchantia polymorpha.

The plastid terminal oxidase PTOX controls the oxidation level of the plastoquinone pool in the thylakoid membrane and acts as a safety valve upon abiotic stress, but detailed characterization of its role in protecting the photosynthetic apparatus is limited. Here we used PTOX mutants in two model plants Arabidopsis thaliana and Marchantia polymorpha. In Arabidopsis, lack of PTOX leads to a severe defect in pigmentation, a so-called variegated phenotype, when plants are grown at standard light intensities. We created a green Arabidopsis PTOX mutant expressing the bacterial carotenoid desaturase CRTI and a double mutant in Marchantia lacking both PTOX isoforms, the plant-type and the alga-type PTOX. In both species, lack of PTOX affected the redox state of the plastoquinone pool. Exposure of plants to high light intensity showed in the absence of PTOX higher susceptibility of photosystem I to light-induced damage while photosystem II was more stable compared with the wild type demonstrating that PTOX plays both, a pro-oxidant and an anti-oxidant role in vivo. Our results shed new light on the function of PTOX in the protection of photosystem I and II.

Low-CO2-inducible bestrophins outside the pyrenoid sustain high photosynthetic efficacy in diatoms.

Anion transporters sustain a variety of physiological states in cells. Bestrophins (BSTs) belong to a Cl- and/or HCO3- transporter family conserved in bacteria, animals, algae, and plants. Recently, putative BSTs were found in the green alga Chlamydomonas reinhardtii, where they are upregulated under low CO2 (LC) conditions and play an essential role in the CO2-concentrating mechanism (CCM). The putative BST orthologs are also conserved in diatoms, secondary endosymbiotic algae harboring red-type plastids, but their physiological functions are unknown. Here, we characterized the subcellular localization and expression profile of BSTs in the marine diatoms Phaeodactylum tricornutum (PtBST1 to 4) and Thalassiosira pseudonana (TpBST1 and 2). PtBST1, PtBST2, and PtBST4 were localized at the stroma thylakoid membrane outside of the pyrenoid, and PtBST3 was localized in the pyrenoid. Contrarily, TpBST1 and TpBST2 were both localized in the pyrenoid. These BST proteins accumulated in cells grown in LC but not in 1% CO2 (high CO2 [HC]). To assess the physiological functions, we generated knockout mutants for the PtBST1 gene by genome editing. The lack of PtBST1 decreased photosynthetic affinity for dissolved inorganic carbon to the level comparable with the HC-grown wild type. Furthermore, non-photochemical quenching in LC-grown cells was 1.5 to 2.0 times higher in the mutants than in the wild type. These data suggest that HCO3- transport at the stroma thylakoid membranes by PtBST1 is a critical part of the CO2-evolving machinery of the pyrenoid in the fully induced CCM and that PtBST1 may modulate photoprotection under CO2-limited environments in P. tricornutum.

A complex and dynamic redox network regulates oxygen reduction at photosystem I in Arabidopsis.

Thiol-dependent redox regulation of enzyme activities plays a central role in regulating photosynthesis. Beside the regulation of metabolic pathways, alternative electron transport is subjected to thiol-dependent regulation. We investigated the regulation of O2 reduction at photosystem I. The level of O2 reduction in leaves and isolated thylakoid membranes depends on the photoperiod in which plants are grown. We used a set of Arabidopsis (Arabidopsis thaliana) mutant plants affected in the stromal, membrane and lumenal thiol network to study the redox protein partners involved in regulating O2 reduction. Light-dependent O2 reduction was determined in leaves and in thylakoids of plants grown in short day and long day conditions using a spin-trapping electron paramagnetic resonance (EPR) assay. In wild type samples from short day conditions, reactive oxygen species (ROS) generation was double that of samples from long day conditions, while this difference was abolished in several redoxin mutants. An in vitro reconstitution assay showed that thioredoxin m, NADPH-dependent reductase C and NADPH are required for high O2 reduction levels in thylakoids from plants grown in long day conditions. Using isolated photosystem I, we also showed that reduction of a photosystem I protein is responsible for the increase in O2 reduction. Furthermore, differences in the membrane localization of m-type thioredoxins and 2-Cys peroxiredoxin were detected between thylakoids of short day and long day plants. Overall, we propose a model of redox regulation of O2 reduction according to the reduction power of the stroma and the ability of different thiol-containing proteins to form a network of redox interactions.

Light Wavelength as a Contributory Factor of Environmental Fitness in the Cyanobacterial Circadian Clock.

A circadian clock is an essential system that drives the 24-h expression rhythms for adaptation to day-night cycles. The molecular mechanism of the circadian clock has been extensively studied in cyanobacteria harboring the KaiC-based timing system. Nevertheless, our understanding of the physiological significance of the cyanobacterial circadian clock is still limited. In this study, we cultured wild-type Synechococcus elongatus PCC 7942 and circadian clock mutants in day-night cycles at different light qualities and found that the growth of the circadian clock mutants was specifically impaired during 12-h blue light/12-h dark (BD) cycles for the first time. The arrhythmic mutant kaiCAA was further analyzed by photosynthetic measurements. Compared with the wild type, the mutant exhibited decreases in the chlorophyll content, the ratio of photosystem I to II, net O2 evolution rate and efficiency of photosystem II photochemistry during BD cycles. These results indicate that the circadian clock is necessary for the growth and the maintenance of the optimum function of the photosynthetic apparatus in cyanobacteria under blue photoperiodic conditions.

Pyrenoid-core CO2-evolving machinery is essential for diatom photosynthesis in elevated CO2.

Marine diatoms are responsible for up to 20% of the annual global primary production by performing photosynthesis in seawater where CO2 availability is limited while HCO3- is abundant. Our previous studies have demonstrated that solute carrier 4 proteins at the plasma membrane of the diatom Phaeodactylum tricornutum facilitate the use of the abundant seawater HCO3-. There has been an unconcluded debate as to whether such HCO3- use capacity may itself supply enough dissolved inorganic carbon (DIC) to saturate the enzyme Rubisco. Here, we show that the θ-type carbonic anhydrase, Ptθ-CA1, a luminal factor of the pyrenoid-penetrating thylakoid membranes, plays an essential role in saturating photosynthesis of P. tricornutum. We isolated and analyzed genome-edited mutants of P. tricornutum defective in Ptθ-CA1. The mutants showed impaired growth in seawater aerated with a broad range of CO2 levels, from atmospheric to 1%. Independently of growth CO2 conditions, the photosynthetic affinity measured as K0.5 for DIC in mutants reached around 2 mm, which is about 10 times higher than K0.5[DIC] of high-CO2-grown wild-type cells that have repressed CO2-concentrating mechanism levels. The results clearly indicate that diatom photosynthesis is not saturated with either seawater-level DIC or even under a highly elevated CO2 environment unless the CO2-evolving machinery is at the core of the pyrenoid.

Extra O2 evolution reveals an O2-independent alternative electron sink in photosynthesis of marine diatoms.

Following the principle of oxygenic photosynthesis, electron transport in the thylakoid membranes (i.e., light reaction) generates ATP and NADPH from light energy, which is subsequently utilized for CO2 fixation in the Calvin-Benson-Bassham cycle (i.e., dark reaction). However, light and dark reactions could discord when an alternative electron flow occurs with a rate comparable to the linear electron flow. Here, we quantitatively monitored O2 and total dissolved inorganic carbon (DIC) during photosynthesis in the pennate diatom Phaeodactylum tricornutum, and found that evolved O2 was larger than the consumption of DIC, which was consistent with 14CO2 measurements in literature. In our measurements, the stoichiometry of O2 evolution to DIC consumption was always around 1.5 during photosynthesis at different DIC concentrations. The same stoichiometry was observed in the cells grown under different CO2 concentrations and nitrogen sources except for the nitrogen-starved cells showing O2 evolution 2.5 times larger than DIC consumption. An inhibitor to nitrogen assimilation did not affect the extra O2 evolution. Further, the same physiological phenomenon was observed in the centric diatom Thalassiosira pseudonana. Based on the present dataset, we propose that the marine diatoms possess the metabolic pathway(s) functioning as the O2-independent electron sink under steady state photosynthesis that reaches nearly half of electron flux of the Calvin-Benson-Bassham cycle.

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

Cyanobacteria are structurally the simplest oxygenic phototrophs, but it is difficult to understand the regulation of their photosynthesis because the photosynthetic and respiratory processes share the same thylakoid membranes and cytosolic space. This review aims to summarize 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 is the dominant electron flux in the thylakoid membranes. The capacity for O2 photoreduction mediated by flavodiiron proteins is comparable to that for photosynthetic CO2 assimilation in cyanobacteria. 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 forming part of the cyclic electron transport chain if it is coupled with photosystem I in the light. Further, O2-independent alternative electron transport through hydrogenase and nitrate reductase function with reduced ferredoxin as the electron donor. Whereas all these electron transport chains are understood individually, the regulatory complexity of the whole system remains to be uncovered in the near future.

Mapping of subcellular local pH in the marine diatom Phaeodactylum tricornutum.

Diatoms are one of the most important phytoplankton on Earth. They comprise at least ten thousand species and contribute to up to 20% of the global primary production. Because of serial endosymbiotic events and horizontal gene transfers, diatoms have developed a "secondary plastid" bounded by four membranes containing a large phase-separated compartment, termed the pyrenoid. However, the physiological significance of this unique chloroplast morphology is poorly understood. Characterization of fundamental physiological parameters such as local pH in various subcellular compartments should facilitate a greater understanding of the physiological roles of the unique structure of the secondary plastid. A promising method to estimate local pH is the in situ expression of the pH-sensitive green fluorescent protein. Here, we first developed the molecular tool for the mapping of in situ local pH in the diatom Phaeodactylum tricornutum by heterologously expressing pHluorin2 in the cytosol, periplastidal compartment (PPC; the space in between two sets of outer and inner chloroplast envelopes), chloroplast stroma, and the pyrenoid matrix. Our data suggested that PPC and the pyrenoid matrix are more acidic than the adjacent areas, the cytosol and the chloroplast stroma. Finally, absolute pH values at each compartment were estimated from the ratiometric fluorescence of a recombinant pHluorin2 protein, giving pH values of approximately 7.9, 6.8, 8.0, and 7.5 respectively, for the cytosol, PPC, stroma, and pyrenoid of the P. tricornutum cells, indicating the occurrence of pH gradients and the associated electrochemical potentials at their boundary.

Regulation of the generation of reactive oxygen species during photosynthetic electron transport.

Light capture by chlorophylls and photosynthetic electron transport bury the risk of the generation of reactive oxygen species (ROS) including singlet oxygen, superoxide anion radicals and hydrogen peroxide. Rapid changes in light intensity, electron fluxes and accumulation of strong oxidants and reductants increase ROS production. Superoxide is mainly generated at the level of photosystem I while photosystem II is the main source of singlet oxygen. ROS can induce oxidative damage of the photosynthetic apparatus, however, ROS are also important to tune processes inside the chloroplast and participate in retrograde signalling regulating the expression of genes involved in acclimation responses. Under most physiological conditions light harvesting and photosynthetic electron transport are regulated to keep the level of ROS at a non-destructive level. Photosystem II is most prone to photoinhibition but can be quickly repaired while photosystem I is protected in most cases. The size of the transmembrane proton gradient is central for the onset of mechanisms that protect against photoinhibition. The proton gradient allows dissipation of excess energy as heat in the antenna systems and it regulates electron transport. pH-dependent slowing down of electron donation to photosystem I protects it against ROS generation and damage. Cyclic electron transfer and photoreduction of oxygen contribute to the size of the proton gradient. The yield of singlet oxygen production in photosystem II is regulated by changes in the midpoint potential of its primary quinone acceptor. In addition, numerous antioxidants inside the photosystems, the antenna and the thylakoid membrane quench or scavenge ROS.

Immobilization of a Broad Range of Polypeptides on the Frustule of the Diatom Thalassiosira pseudonana.

Proteins immobilized on biosilica which have superior reactivity and specificity and are innocuous to natural environments could be useful biological materials in industrial processes. One recently developed technique, living diatom silica immobilization (LiDSI), has made it possible to immobilize proteins, including multimeric and redox enzymes, via a cellular excretion system onto the silica frustule of the marine diatom Thalassiosira pseudonana. However, the number of application examples so far is limited, and the type of proteins appropriate for the technique is still enigmatic. Here, we applied LiDSI to six industrially relevant polypeptides, including protamine, metallothionein, phosphotriesterase, choline oxidase, laccase, and polyamine synthase. Protamine and metallothionein were successfully immobilized on the frustule as protein fusions with green fluorescent protein (GFP) at the N terminus, indicating that LiDSI can be used for polypeptides which are rich in arginine and cysteine. In contrast, we obtained mutants for the latter four enzymes in forms without green fluorescent protein. Immobilized phosphotriesterase, choline oxidase, and laccase showed enzyme activities even after the purification of frustule in the presence of 1% (wt/vol) octylphenoxy poly(ethyleneoxy)ethanol. An immobilized branched-chain polyamine synthase changed the intracellular polyamine composition and silica nanomorphology. These results illustrate the possibility of LiDSI for industrial applications. IMPORTANCE Proteins immobilized on biosilica which have superior reactivity and specificity and are innocuous to natural environments could be useful biological materials in industrial processes. Living diatom silica immobilization (LiDSI) is a recently developed technique for in vivo protein immobilization on the diatom frustule. We aimed to explore the possibility of using LiDSI for industrial applications by successfully immobilizing six polypeptides: (i) protamine (Oncorhynchus keta), a stable antibacterial agent; (ii) metallothionein (Saccharomyces cerevisiae), a metal adsorption molecule useful for bioremediation; (iii) phosphotriesterase (Sulfolobus solfataricus), a scavenger for toxic organic phosphates; (iv) choline oxidase (Arthrobacter globiformis), an enhancer for photosynthetic activity and yield of plants; (v) laccase (Bacillus subtilis), a phenol oxidase utilized for delignification of lignocellulosic materials; and (vi) branched-chain polyamine synthase (Thermococcus kodakarensis), which produces branched-chain polyamines important for DNA and RNA stabilization at high temperatures. This study provides new insights into the field of applied biological materials.

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.

NADPH production in dark stages is critical for cyanobacterial photocurrent generation: a study using mutants deficient in oxidative pentose phosphate pathway.

Live cyanobacteria and algae integrated onto an extracellular electrode can generate a light-induced current (i.e., a photocurrent). Although the photocurrent is expected to be correlated with the redox environment of the photosynthetic cells, the relationship between the photocurrent and the cellular redox state is poorly understood. Here, we investigated the effect of the reduced nicotinamide adenine dinucleotide phosphate [NADP(H)] redox level of cyanobacterial cells (before light exposure) on the photocurrent using several mutants (Δzwf, Δgnd, and ΔglgP) deficient in the oxidative pentose phosphate (OPP) pathway, which is the metabolic pathway that produces NADPH in darkness. The NAD(P)H redox level and photocurrent in the cyanobacterium Synechocystis sp. PCC 6803 were measured noninvasively. Dysfunction of the OPP pathway led to oxidation of the photosynthetic NADPH pool in darkness. In addition, photocurrent induction was retarded and the current density was lower in Δzwf, Δgnd, and ΔglgP than in wild-type cells. Exogenously added glucose compensated the phenotype of ΔglgP and drove the OPP pathway in the mutant, resulting in an increase in the photocurrent. The results indicated that NADPH accumulated by the OPP pathway before illumination is a key factor for the generation of a photocurrent. In addition, measuring the photocurrent can be a non-invasive approach to estimate the cellular redox level related to NADP(H) pool in cyanobacteria.

ROS-derived lipid peroxidation is prevented in barley leaves during senescence.

Senescence in plants enables resource recycling from senescent leaves to sink organs. Under stress, increased production of reactive oxygen species (ROS) and associated signalling activates senescence. However, senescence is not always associated with stress since it has a prominent role in plant development, in which the role of ROS signalling is less clear. To address this, we investigated lipid metabolism and patterns of lipid peroxidation related to signalling during sequential senescence in first-emerging barley leaves grown under natural light conditions. Leaf fatty acid compositions were dominated by linolenic acid (75% of total), the major polyunsaturated fatty acid (PUFA) in galactolipids of thylakoid membranes, known to be highly sensitive to peroxidation. Lipid catabolism during senescence, including increased lipoxygenase activity, led to decreased levels of PUFA and increased levels of short-chain saturated fatty acids. When normalised to leaf area, only concentrations of hexanal, a product from the 13-lipoxygenase pathway, increased early upon senescence, whereas reactive electrophile species (RES) from ROS-associated lipid peroxidation, such as 4-hydroxynonenal, 4-hydroxyhexenal and acrolein, as well as ß-cyclocitral derived from oxidation of ß-carotene, decreased. However, relative to total chlorophyll, amounts of most RES increased at late-senescence stages, alongside increased levels of α-tocopherol, zeaxanthin and non-photochemical quenching, an energy dissipative pathway that prevents ROS production. Overall, our results indicate that lipid peroxidation derived from enzymatic oxidation occurs early during senescence in first barley leaves, while ROS-derived lipid peroxidation associates weaker with senescence.

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.

Quantification of NAD(P)H in cyanobacterial cells by a phenol extraction method.

In photosynthetic organisms, it is recognized that the intracellular redox ratio of NADPH is regulated within an appropriate range for the cooperative function of a wide variety of physiological processes. However, despite its importance, there is large variability in the values of the NADPH fraction [NADPH/(NADPH + NADP+)] quantitatively estimated to date. In the present study, the light response of the NADPH fraction was investigated by applying a novel NADP(H) extraction method using phenol / chloroform / isoamyl alcohol (PCI) in the cyanobacterium Synechocystis sp. PCC 6803. The light response of NADP(H) observed using PCI extraction was qualitatively consistent with the NAD(P)H fluorescence time course measured in vivo. Moreover, the results obtained by PCI extraction and the fluorescence-based methods were also consistent in a mutant lacking the ability to oxidize NAD(P)H in the respiratory chain, and exhibiting a unique NADPH light response. These observations indicate that the PCI extraction method allowed quantitative determination of NADP(H) redox. Notably, the PCI extraction method showed that not all NADP(H) was oxidized or reduced by light-dark transition. Specifically, the fraction of NADPH was 42% in the dark-adapted cell, and saturated at 68% in light conditions.

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.

Changes in Photosynthetic Electron Transport during Leaf Senescence in Two Barley Varieties Grown in Contrasting Growth Regimes.

Leaf senescence is an important process for plants to remobilize a variety of metabolites and nutrients to sink tissues, such as developing leaves, fruits and seeds. It has been suggested that reactive oxygen species (ROS) play an important role in the initiation of leaf senescence. Flag leaves of two different barley varieties, cv. Lomerit and cv. Carina, showed differences in the loss of photosystems and in the production of ROS at a late stage of senescence after significant loss of chlorophyll (Krieger-Liszkay et al. 2015). Here, we investigated photosynthetic electron transport and ROS production in primary leaves of these two varieties at earlier stages of senescence. Comparisons were made between plants grown outside in natural light and temperatures and plants grown in temperature-controlled growth chambers under low light intensity. Alterations in the content of photoactive P700, ferredoxin and plastocyanin (PC) photosynthetic electron transport were analyzed using in vivo near-infrared absorbance changes and chlorophyll fluorescence, while ROS were measured with spin-trapping electron paramagnetic resonance spectroscopy. Differences in ROS production between the two varieties were only observed in outdoor plants, whereas a loss of PC was common in both barley varieties regardless of growth conditions. We conclude that the loss of PC is the earliest detectable photosynthetic parameter of leaf senescence while differences in the production of individual ROS species occur later and depend on environmental factors.

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.

Near-infrared in vivo measurements of photosystem I and its lumenal electron donors with a recently developed spectrophotometer.

In photosynthesis research, non-invasive in vivo spectroscopic analyses have been used as a practical tool for studying photosynthetic electron transport. Klas-NIR spectrophotometer has been recently developed by Klughammer and Schreiber (Photosynth Res 128:195-214, 2016) for in vivo measurements of redox changes of P700, plastocyanin (Pcy) and ferredoxin (Fd). Here we show examples using the Klas-NIR spectrophotometer for the evaluation of the redox states and quantities of these components in plant leaves and cyanobacterial suspensions. The redox poise under light of the electron transport components is different in leaves from higher plants compared with cyanobacteria. During a short illumination with an actinic light, P700, Pcy, and Fd are kept reduced in barley leaves but are oxidized in cyanobacteria. During far-red light illumination, P700 and Pcy are mostly oxidized in the leaves but are partially kept reduced in cyanobacteria. In the cyanobacterium, Thermosynechococcus elongatus, which has no Pcy but uses cytochrome c6 (cyt c6) as the electron donor to photosystem I, a cyt c6 signal was detected in vivo. To show the potential of Klas-NIR spectrophotometer for studying different developmental stages of a leaf, we performed measurements on fully mature and early senescing barley leaves. Pcy content in leaves decreased during senescence at an early stage. The Pcy loss was quantitatively analyzed using Klas-NIR spectrophotometer, giving absolute ratios of Pcy to PSI of 2.5 and 1.6 in younger and older leaves, respectively. For quantification of the signals in vivo, in vitro data (Sétif et al. in Photosynth Res142:307-319, 2019) obtained with Klas-NIR spectrophotometer were used.