Excess Light Conditions Research Articles

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.

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.

Transcriptome analysis of the cyanobacterium Synechocystis sp. PCC 6803 and mechanisms of photoinhibition tolerance under extreme high light conditions

Photoinhibition, or cell damage caused by excessively intense light is a major issue for the industrial use of cyanobacteria. To investigate the mechanism of responses to extreme high light intensity, gene expression analysis was performed using the model cyanobacterium Synechocystis sp. PCC 6803 (PCC 6803) cultured under various light intensities. The culture profile data demonstrated that, in contrast to the slow cell growth observed under low light intensities (30 and 50μmolm-2s-1), maximal cell growth was observed under mid light conditions (300 and 1000μmolm-2s-1). PCC 6803cells exhibited photoinhibition when cultured under excessive high light intensities of 1100 and 1300μmolm-2s-1. From the low to the mid light conditions, the expression of genes related to light harvesting systems was repressed, whereas that of CO2 fixation and of D1 protein turnover-related genes was induced. Gene expression data also revealed that the down-regulation of genes related to flagellum synthesis (pilA2), pyridine nucleotide transhydrogenase (pntA and pntB), and sigma factor (sigA and sigF) represents the key responses of PCC 6803 under excessive high light conditions. The results obtained in this study provide further understanding of high light tolerance mechanisms and should help to improve the productivity of bioprocess using cyanobacteria.

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.

Large-scale in vitro production, refolding and dimerization of PsbS in different microenvironments

Plants adapt to fluctuating light conditions by a process called non-photochemical quenching (NPQ), where membrane protein PsbS plays a crucial role and transforms a change in the pH-gradient across the thylakoid membrane under excess light conditions into a photoprotective state, leading to de-excitation of antenna chlorophylls. The PsbS activation mechanism is elusive and has been proposed to involve a monomerization step and protonation of specific residues. To elucidate its function, it is essential to produce PsbS in large quantities, stabilize PsbS in a membrane-mimicking environment and analyze its pH-dependent conformational structure. We present an approach for large-scale in-vitro production and spectroscopic characterization of PsbS under controlled, non-crystalline conditions. We produced PsbS of the moss Physcomitrella patens in milligram quantities in E. coli, refolded PsbS in several detergent types and analyzed its conformation at neutral and low pH by Dynamic Light Scattering and NMR spectroscopy. Our results reveal that at both pH conditions, PsbS exist as dimers or in apparent monomer-dimer equilibria. Lowering of the pH induces conformational changes, destabilizes the dimer state and shifts the equilibria towards the monomeric form. In vivo, a similar response upon thylakoid lumen acidification may tune PsbS activity in a gradual manner.

Tetratricopeptide repeat protein Pyg7 is essential for photosystem I assembly by interacting with PsaC in Arabidopsis.

Although progress has been made in determining the structure and understanding the function of photosystem I (PSI), the PSI assembly process remains poorly understood. PsaC is an essential subunit of PSI and participates in the transfer of electrons to ferredoxin. However, how PsaC is assembled during accumulation of the PSI complex is unknown. In the present study, we showed that Pyg7 localized to the stromal thylakoid and associated with the PSI complex. We also showed that Pyg7 interacted with PsaC. Furthermore, we found that the PSI assembly process was blocked following formation of the PsaAB heterodimer in the pyg7 mutant. In addition, the analyses of PSI stability in Pyg7RNAi plants showed that Pyg7 is involved in maintaining the assembled PSI complex under excess-light conditions. Moreover, we demonstrated that decreased Pyg7 content resulted in decreased efficiency of PSI assembly in Pyg7RNAi plants. These findings suggest that the role of Pyg7 in PSI biogenesis has evolved as an essential assembly factor by interacting with PsaC in Arabidopsis, in addition to being a stability factor for PSI as seen in Synechocystis.

Photoprotection in a monophyletic branch of chlorophyte algae is independent of energy-dependent quenching (qE).

Phototrophic organisms need to ensure high photosynthetic performance whilst suppressing reactive oxygen species (ROS)-induced stress occurring under excess light conditions. The xanthophyll cycle (XC), related to the high-energy quenching component (qE) of the nonphotochemical quenching (NPQ) of excitation energy, is considered to be an obligatory component of photoprotective mechanisms. The pigment composition of at least one representative of each major clade of Ulvophyceae (Chlorophyta) was investigated. We searched for a light-dependent conversion of pigments and investigated the NPQ capacity with regard to the contribution of XC and the qE component when grown under different light conditions. A XC was found to be absent in a monophyletic group of Ulvophyceae, the Bryopsidales, when cultivated under low light, but was triggered in one of the 10 investigated bryopsidalean species, Caulerpa cf. taxifolia, when cultivated under high light. Although Bryopsidales accumulate zeaxanthin (Zea) under high-light (HL) conditions, NPQ formation is independent of a XC and not related to qE. qE- and XC-independent NPQ in the Bryopsidales contradicts the common perception regarding its ubiquitous occurrence in Chloroplastida. Zea accumulation in HL-acclimated Bryopsidales most probably represents a remnant of a functional XC. The existence of a monophyletic algal taxon that lacks qE highlights the need for broad biodiversity studies on photoprotective mechanisms.

Fluorescent Labeling Preserving OCP Photoactivity Reveals Its Reorganization during the Photocycle

Orange carotenoid protein (OCP), responsible for the photoprotection of the cyanobacterial photosynthetic apparatus under excessive light conditions, undergoes significant rearrangements upon photoconversion and transits from the stable orange to the signaling red state. This is thought to involve a 12-Å translocation of the carotenoid cofactor and separation of the N- and C-terminal protein domains. Despite clear recent progress, the detailed mechanism of the OCP photoconversion and associated photoprotection remains elusive. Here, we labeled the OCP of Synechocystis with tetramethylrhodamine-maleimide (TMR) and obtained a photoactive OCP-TMR complex, the fluorescence of which was highly sensitive to the protein state, showing unprecedented contrast between the orange and red states and reflecting changes in protein conformation and the distances from TMR to the carotenoid throughout the photocycle. The OCP-TMR complex was sensitive to the light intensity, temperature, and viscosity of the solvent. Based on the observed Förster resonance energy transfer, we determined that upon photoconversion, the distance between TMR (donor) bound to a cysteine in the C-terminal domain and the carotenoid (acceptor) increased by 18 Å, with simultaneous translocation of the carotenoid into the N-terminal domain. Time-resolved fluorescence anisotropy revealed a significant decrease of the OCP rotation rate in the red state, indicating that the light-triggered conversion of the protein is accompanied by an increase of its hydrodynamic radius. Thus, our results support the idea of significant structural rearrangements of OCP, providing, to our knowledge, new insights into the structural rearrangements of OCP throughout the photocycle and a completely novel approach to the study of its photocycle and non-photochemical quenching. We suggest that this approach can be generally applied to other photoactive proteins.

Converted carotenoid production in Dunaliella salina by using cyclization inhibitors 2-methylimidazole and 3-amino-1,2,4-triazole

Carotenoids are vital for most photosynthetic organisms because of their crucial role in prevention of the damage caused by excess light or stress conditions like heat or nutrient deprivation. Some of them are also valuable for biotechnology with respect to their colorful feature and antioxidant properties. In this study, a natural β-carotene producer, green microalga Dunaliella salina, was evaluated for its potential to produce different valuable carotenoids through the inhibition of cyclization reactions in the carotenoid pathway by the chemical inhibitors 2-methylimidazole (2MI) and 3-amino-1,2,4-triazole (Amitrol). 2MI was shown to be effective on lutein accumulation in D. salina cultures grown at high temperature, without affecting cell viability. Addition of 2MI to the culture at 1 mM concentration resulted a 1.7-fold increase in lutein content in the cell with the highest amount of 3.45 pg/cell, and an associated decrease in β-carotene content suggesting that this inhibitor is more effective on lycopene beta-cyclase activity, thus shifting the pathway from β-carotene arm to the α-carotene direction. The results of this study may give the opportunity to use D. salina for the production of valuable carotenoids other than β-carotene.

Disentangling protein and lipid interactions that control a molecular switch in photosynthetic light harvesting

In the photosynthetic apparatus of plants and algae, the major Light-Harvesting Complexes (LHCII) collect excitations and funnel these to the photosynthetic reaction center where charge separation takes place. In excess light conditions, remodeling of the photosynthetic membrane and protein conformational changes produces a photoprotective state in which excitations are rapidly quenched to avoid photodamage. The quenched states are associated with protein aggregation, however the LHCII complexes are also proposed to have an intrinsic capacity to shift between light harvesting and fluorescence-quenched conformational states. To disentangle the effects of protein-protein and protein-lipid interactions on the LHCII photoprotective switch, we compared the structural and fluorescent properties of LHCII lipid nanodiscs and proteoliposomes with very low protein-to-lipid ratios. We demonstrate that LHCII proteins adapta fully fluorescent state in nanodiscs and in proteoliposomes with highly diluted protein densities. Increasing the protein density induces a transition into a mildly-quenched state that reaches a plateau at a molar protein-to-lipid ratio of 0.001 and has a fluorescence yield reminiscent of the light-harvesting state in vivo. The low onset for quenching strongly suggests that LHCII-LHCII attractive interactions occur inside membranes. The transition at low protein densities does not involve strong changes in the excitonic circular-dichroism spectrum and is distinct from a transition occurring at very high protein densities that comprises strong fluorescence quenching and circular-dichroism spectral changes involving chlorophyll 611 and 612, correlating with proposed quencher sites of the photoprotective mechanisms.

A blue-light photoreceptor mediates the feedback regulation of photosynthesis.

In plants and algae, light serves both as the energy source for photosynthesis and a biological signal that triggers cellular responses via specific sensory photoreceptors. Red light is perceived by bilin-containing phytochromes and blue light by the flavin-containing cryptochromes and/or phototropins (PHOTs), the latter containing two photosensory light, oxygen, or voltage (LOV) domains. Photoperception spans several orders of light intensity, ranging from far below the threshold for photosynthesis to values beyond the capacity of photosynthetic CO2 assimilation. Excess light may cause oxidative damage and cell death, processes prevented by enhanced thermal dissipation via high-energy quenching (qE), a key photoprotective response. Here we show the existence of a molecular link between photoreception, photosynthesis, and photoprotection in the green alga Chlamydomonas reinhardtii. We show that PHOT controls qE by inducing the expression of the qE effector protein LHCSR3 (light-harvesting complex stress-related protein 3) in high light intensities. This control requires blue-light perception by LOV domains on PHOT, LHCSR3 induction through PHOT kinase, and light dissipation in photosystem II via LHCSR3. Mutants deficient in the PHOT gene display severely reduced fitness under excessive light conditions, indicating that the sensing, utilization, and dissipation of light is a concerted process that plays a vital role in microalgal acclimation to environments of variable light intensities.

Differential Roles of Carotenes and Xanthophylls in Photosystem I Photoprotection.

Carotenes and their oxygenated derivatives, xanthophylls, are structural elements of the photosynthetic apparatus and contribute to increasing both the light-harvesting and photoprotective capacity of the photosystems. β-Carotene is present in both the core complexes and light-harvesting system (LHCI) of Photosystem (PS) I, while xanthophylls lutein and violaxanthin bind exclusively to its antenna moiety; another xanthophyll, zeaxanthin, which protects chloroplasts against photooxidative damage, binds to the LHCI complexes under conditions of excess light. We functionally dissected various components of the xanthophyll- and carotene-dependent photoprotection mechanism of PSI by analyzing two Arabidopsis mutants: szl1 plants, with a carotene content lower than that of the wild type, and npq1, with suppressed zeaxanthin formation. When exposed to excess light, the szl1 genotype displayed PSI photoinhibition stronger than that of wild-type plants, while removing zeaxanthin had no such effect. The PSI-LHCI complex purified from szl1 was more photosensitive than the corresponding wild-type and npq1 complexes, as is evident from its faster photobleaching and increased rate of singlet oxygen release, suggesting that β-carotene is crucial in controlling chlorophyll triplet formation. Accordingly, fluorescence-detected magnetic resonance analysis showed an increase in the amplitude of signals assigned to chlorophyll triplets in β-carotene-depleted complexes. When PSI was fractioned into its functional moieties, it was revealed that the boost in the rate of singlet oxygen release caused by β-carotene depletion was greater in LHCI than in the core complex. We conclude that PSI-LHCI complex-bound β-carotene elicits a protective response, consisting of a reduction in the yield of harmful triplet excited states, while accumulation of zeaxanthin plays a minor role in restoring phototolerance.

Photoacclimatory Responses of Zostera marina in the Intertidal and Subtidal Zones.

Photoacclimatory responses of the seagrass Zostera marina in the intertidal and subtidal zones were investigated by measuring chlorophyll a fluorescence parameters, photosynthetic pigments, leaf δ13C values, and shoot morphology in two bay systems. Intertidal plants had higher carotenoid concentrations than subtidal plants to avoid photodamage under excess light conditions during the day. The maximum relative electron transport rate (rETRmax) and minimum saturation irradiance (Ek) of the intertidal plants were higher than those of the subtidal plants, whereas photosynthetic efficiency (α) and maximum quantum yield (Fv/Fm) were higher in subtidal plants. The intertidal plants also had significantly greater Stern–Volmer non-photochemical quenching (NPQ) than that of the subtidal plants. These results suggest that the subtidal plants photoacclimated to use limited light more efficiently, and the intertidal plants exhibited photosynthetic responses to minimize photodamage at excess irradiance. The δ13C values of leaf tissues were more negative in the intertidal plants than those in the subtidal plants, suggesting that the intertidal plants used atmospheric or dissolved CO2 for photosynthesis during emersion. Effective quantum yield (ΔF/Fm´) in the intertidal plants decreased more slowly after emersion than that in the subtidal plants, indicating higher desiccation tolerance of the intertidal plants. The intertidal plants also recovered more rapidly from desiccation damage than the subtidal plants, suggesting photosynthetic adaptation to desiccation stress. The photosynthetic plasticity of Z. marina in response to variable environmental conditions most likely allows this species to occur in the intertidal and subtidal zones.

Identification of pH-sensing Sites in the Light Harvesting Complex Stress-related 3 Protein Essential for Triggering Non-photochemical Quenching in Chlamydomonas reinhardtii

Light harvesting complex stress-related 3 (LHCSR3) is the protein essential for photoprotective excess energy dissipation (non-photochemical quenching, NPQ) in the model green alga Chlamydomonas reinhardtii. Activation of NPQ requires low pH in the thylakoid lumen, which is induced in excess light conditions and sensed by lumen-exposed acidic residues. In this work we have used site-specific mutagenesis in vivo and in vitro for identification of the residues in LHCSR3 that are responsible for sensing lumen pH. Lumen-exposed protonatable residues, aspartate and glutamate, were mutated to asparagine and glutamine, respectively. By expression in a mutant lacking all LHCSR isoforms, residues Asp117, Glu221, and Glu224 were shown to be essential for LHCSR3-dependent NPQ induction in C. reinhardtii. Analysis of recombinant proteins carrying the same mutations refolded in vitro with pigments showed that the capacity of responding to low pH by decreasing the fluorescence lifetime, present in the wild-type protein, was lost. Consistent with a role in pH sensing, the mutations led to a substantial reduction in binding the NPQ inhibitor dicyclohexylcarbodiimide.

Diurnal variation in the coupling of photosynthetic electron transport and carbon fixation in iron-limited phytoplankton in the NE subarctic Pacific

Abstract. Active chlorophyll a fluorescence approaches, including fast repetition rate fluorometry (FRRF), have the potential to provide estimates of phytoplankton primary productivity at an unprecedented spatial and temporal resolution. FRRF-derived productivity rates are based on estimates of charge separation in reaction center II (ETRRCII), which must be converted into ecologically relevant units of carbon fixation. Understanding sources of variability in the coupling of ETRRCII and carbon fixation provides physiological insight into phytoplankton photosynthesis and is critical for the application of FRRF as a primary productivity measurement tool. In the present study, we simultaneously measured phytoplankton carbon fixation and ETRRCII in the iron-limited NE subarctic Pacific over the course of a diurnal cycle. We show that rates of ETRRCII are closely tied to the diurnal cycle in light availability, whereas rates of carbon fixation appear to be influenced by endogenous changes in metabolic energy allocation under iron-limited conditions. Unsynchronized diurnal oscillations of the two rates led to 3.5-fold changes in the conversion factor between ETRRCII and carbon fixation (Kc / nPSII). Consequently, diurnal variability in phytoplankton carbon fixation cannot be adequately captured with FRRF approaches if a constant conversion factor is applied. Utilizing several auxiliary photophysiological measurements, we observed that a high conversion factor is associated with conditions of excess light and correlates with the increased expression of non-photochemical quenching (NPQ) in the pigment antenna, as derived from FRRF measurements. The observed correlation between NPQ and Kc / nPSII requires further validation but has the potential to improve estimates of phytoplankton carbon fixation rates from FRRF measurements alone.

Glutathione reductase 2 maintains the function of photosystem II in Arabidopsis under excess light

Glutathione reductase plays a crucial role in the elimination of H2O2 molecules via the ascorbate–glutathione cycle. In this study, we used transgenic Arabidopsis plants with decreased glutathione reductase 2 (GR2) levels to investigate whether this GR2 activity protects the photosynthetic machinery under excess light. The transgenic plants were highly sensitive to excess light and accumulated high levels of H2O2. Photosystem II (PSII) activity was significantly decreased in transgenic plants. Flash-induced fluorescence relaxation and thermoluminescence measurements demonstrated inhibition of electron transfer between QA and QB and decreased redox potential of QB in transgenic plants. Immunoblot and blue native gel analysis showed that the levels of PSII proteins and PSII complexes were decreased in transgenic plants. Analyses of the repair of photodamaged PSII and in vivo pulse labeling of thylakoid proteins showed that the repair of photodamaged PSII is inhibited due to the inhibition of the synthesis of the D1 protein de novo in transgenic plants. Taken together, our results suggest that under excess light conditions, GR2 plays an important role in maintaining both the function of the acceptor side of PSII and the repair of photodamaged PSII by preventing the accumulation of H2O2. In addition, our results provide details of the role of H2O2 in vivo accumulation in photoinhibition in plants.

Complexity of Chlorophyll Fluorescence Dynamic Response as an Indicator of Excessive Light Intensity

The controllability of LED lighting systems for greenhouses and plant factories offers a possibility for light induced diagnose of plant status. Here, a novel method for proximal remote detection of plant light tolerance is investigated. The method is based on an identification of a transfer function model for the measured chlorophyll fluorescence response to a small step variation in blue LED light. It is postulated that the least required model order decreases as the plants become light stressed due to saturation effects at excess light conditions. We apply this method to basil and lettuce plants under different background light intensities, and the results are compared to measured effective quantum yield (y(II)), relative electron transport rate through PSII (ETR(II)) and non-photochemical quenching (NPQ), all reflecting the photosynthetic performance. For both species it is indeed found that the required model order decreases with increasing background light intensity at the same time as the measured reference parameters indicates a decreased photosynthetic efficiency. It is suggested that the light intensity should be such that the chlorophyll fluorescence response requires a model order of 3 or higher to avoid ineffective irradiation of the plants.

The perilous state of seagrass in the British Isles.

Seagrass ecosystems face widespread threat from reduced water quality, coastal development and poor land use. In recent decades, their distribution has declined rapidly, and in the British Isles, this loss is thought to have been extensive. Given increasing knowledge of how these ecosystems support fisheries production, the understanding of their potential rapid loss, and the difficulty in restoring them, it is vital we develop an understanding of the risks they are under, so that management actions can be developed accordingly. Developing an understanding of their environmental status and condition is therefore critical to their long-term management. This study provided, to our knowledge, the first examination of the environmental health of seagrass meadows around the British Isles. This study used a bioindicator approach and involved collecting data on seagrass density and morphology alongside analysis of leaf biochemistry. Our study provides, to the best of our knowledge, the first strong quantitative evidence that seagrass meadows of the British Isles are mostly in poor condition in comparison with global averages, with tissue nitrogen levels 75% higher than global values. Such poor status places their long-term resilience in doubt. Elemental nutrient concentrations and morphological change suggest conditions of excess nitrogen and probable low light, placing many of the meadows sampled in a perilous state, although others, situated away from human populations were perceived to be healthy. Although some sites were of a high environmental health, all sites were considered at risk from anthropogenic impacts, particularly poor water quality and boating-based disturbances. The findings of this study provide a warning of the need to take action, with respect to water quality and disturbance, to prevent the further loss and degradation of these systems across the British Isles.

High light induced changes in organization, protein profile and function of photosynthetic machinery in Chlamydomonas reinhardtii

The green alga Chlamydomonas (C.) reinhardtii is used as a model organism to understand the efficiency of photosynthesis along with the organization and protein profile of photosynthetic apparatus under various intensities of high light exposure for 1h. Chlorophyll (Chl) a fluorescence induction, OJIPSMT transient was decreased with increase in light intensity indicating the reduction in photochemical efficiency. Further, circular dichroism studies of isolated thylakoids from high light exposed cells showed considerable change in the pigment–pigment interactions and pigment–proteins interactions. Furthermore, the organization of supercomplexes from thylakoids is studied, in which, one of the hetero-trimer of light harvesting complex (LHC) II is affected significantly in comparison to other complexes of LHC's monomers. Also, other supercomplexes, photosystem (PS)II reaction center dimer and PSI complexes are reduced. Additionally, immunoblot analysis of thylakoid proteins revealed that PSII core proteins D1 and D2 were significantly decreased during high light treatment. Similarly, the PSI core proteins PsaC, PsaD and PsaG were drastically changed. Further, the LHC antenna proteins of PSI and PSII were differentially affected. From our results it is clear that LHCs are damaged significantly, consequently the excitation energy is not efficiently transferred to the reaction center. Thus, the photochemical energy transfer from PSII to PSI is reduced. The inference of the study deciphers the structural and functional changes driven by light may therefore provide plants/alga to regulate the light harvesting capacity in excess light conditions.

Crystal structures of the PsbS protein essential for photoprotection in plants.

The photosystem II protein PsbS has an essential role in qE-type nonphotochemical quenching, which protects plants from photodamage under excess light conditions. qE is initiated by activation of PsbS by low pH, but the mechanism of PsbS action remains elusive. Here we report the low-pH crystal structures of PsbS from spinach in its free form and in complex with the qE inhibitor N,N'-dicyclohexylcarbodiimide (DCCD), revealing that PsbS adopts a unique folding pattern, and, unlike other members of the light-harvesting-complex superfamily, it is a noncanonical pigment-binding protein. Structural and biochemical evidence shows that both active and inactive PsbS form homodimers in the thylakoid membranes, and DCCD binding disrupts the lumenal intermolecular hydrogen bonds of the active PsbS dimer. Activation of PsbS by low pH during qE may involve a conformational change associated with altered lumenal intermolecular interactions of the PsbS dimer.