Inhibition of plastocyanin to P(700)(+) electron transfer in Chlamydomonas reinhardtii by hyperosmotic stress

Combined high light and salt stress enhances accumulation of PsbS and zeaxanthin in Chlamydomonas reinhardtii

The performance and acclimation strategies of Chlamydomonas reinhardtii under stress conditions are typically studied in response to single stress factors. Under natural conditions, however, organisms rarely face only one stressor at a time. Here, we investigated the impact of combined salt and high light stress on the photoprotective response of C. reinhardtii. Compared to the single stress factors, the combination of both stressors decreased the photosynthetic performance, while the activation of energy dissipation remained unaffected. However, the PsbS protein was strongly accumulated and the conversion of violaxanthin to zeaxanthin was enhanced. These results support an important photoprotective function of PsbS and zeaxanthin independently of energy dissipation under combined salt and high light stress in C. reinhardtii.

Structure of Photosystem I Supercomplex Isolated from a Chlamydomonas reinhardtii Cytochrome b6f Temperature-Sensitive Mutant

The unicellular green alga, Chlamydomonas reinhardtii, has been widely used as a model system to study photosynthesis. Its possibility to generate and analyze specific mutants has made it an excellent tool for mechanistic and biogenesis studies. Using negative selection of ultraviolet (UV) irradiation–mutated cells, we isolated a mutant (TSP9) with a single amino acid mutation in the Rieske protein of the cytochrome b6f complex. The W143R mutation in the petC gene resulted in total loss of cytochrome b6f complex function at the non-permissive temperature of 37 °C and recovery at the permissive temperature of 25 °C. We then isolated photosystem I (PSI) and photosystem II (PSII) supercomplexes from cells grown at the non-permissive temperature and determined the PSI structure with high-resolution cryogenic electron microscopy. There were several structural alterations compared with the structures obtained from wild-type cells. Our structural data suggest that the mutant responded by excluding the Lhca2, Lhca9, PsaL, and PsaH subunits. This structural alteration prevents state two transition, where LHCII migrates from PSII to bind to the PSI complex. We propose this as a possible response mechanism triggered by the TSP9 phenotype at the non-permissive temperature.

Long- and short-term acclimation of the photosynthetic apparatus to salinity in Chlamydomonas reinhardtii. The role of Stt7 protein kinase

Salt stress triggers an Stt7-mediated LHCII-phosphorylation signaling mechanism similar to light-induced state transitions. However, phosphorylated LHCII, after detaching from PSII, does not attach to PSI but self-aggregates instead. Salt is a major stress factor in the growth of algae and plants. Here, our study mainly focuses on the organization of the photosynthetic apparatus to the long-term responses of Chlamydomonas reinhardtii to elevated NaCl concentrations. We analyzed the physiological effects of salt treatment at a cellular, membrane, and protein level by microscopy, protein profile analyses, transcripts, circular dichroism spectroscopy, chlorophyll fluorescence transients, and steady-state and time-resolved fluorescence spectroscopy. We have ascertained that cells that were grown in high-salinity medium form palmelloids sphere-shaped colonies, where daughter cells with curtailed flagella are enclosed within the mother cell walls. Palmelloid formation depends on the presence of a cell wall, as it was not observed in a cell-wall-less mutant CC-503. Using the stt7 mutant cells, we show Stt7 kinase-dependent phosphorylation of light-harvesting complex II (LHCII) in both short- and long-term treatments of various NaCl concentrations-demonstrating NaCl-induced state transitions that are similar to light-induced state transitions. The grana thylakoids were less appressed (with higher repeat distances), and cells grown in 150 mM NaCl showed disordered structures that formed diffuse boundaries with the flanking stroma lamellae. PSII core proteins were more prone to damage than PSI. At high salt concentrations (100-150 mM), LHCII aggregates accumulated in the thylakoid membranes. Low-temperature and time-resolved fluorescence spectroscopy indicated that the stt7 mutant was more sensitive to salt stress, suggesting that LHCII phosphorylation has a role in the acclimation and protection of the photosynthetic apparatus.

Determining photosynthetic control, a probe for the balance between electron transport and Calvin-Benson cycle activity, with the DUAL-KLAS-NIR

Photosynthetic Control is defined as the control imposed on photosynthetic electron transport by the lumen-pH-sensitive re-oxidation of plastoquinol (PQH₂) by cytochrome b₆f. Photosynthetic Control leads at higher actinic light intensities to an electron transport chain with a (relatively) reduced photosystem (PS) II and PQ pool and a (relatively) oxidized PS I. Making Light Curves of more than 33 plant species with the recently introduced DUAL-KLAS-NIR (Chl a fluorescence + the redox states of plastocyanin (PC), P700, and ferredoxin (Fd)) the light intensity-dependent induction of Photosynthetic Control was probed and characterized. It was observed that PC became completely oxidized at light intensities ≤ 400 µmol photons m^-2 s^-1 (at lower light intensities in shade than in sun leaves). The relationship between qP and P700(red) was used to determine the extent of Photosynthetic Control. Instead of measuring the whole Light Curve, it was shown that a single moderate light intensity can be used to characterize the status of a leaf relative to that of other leaves. It was further found that in some shade-acclimated leaves Fd becomes again more oxidized at high light intensities indicating that electron transfer from the PQ pool to P700 cannot keep up with the outflow of electrons on the acceptor side of PS I. It was observed as well that for NPQ-induction a lower light intensity (less acidified lumen) was needed than for the induction of Photosynthetic Control. The measurements were also used to make a comparison between the parameters qP and qL, a comparison suggesting that qP was the more relevant parameter.

Dimeric and high-resolution structures of Chlamydomonas Photosystem I from a temperature-sensitive Photosystem II mutant

Water molecules play a pivotal functional role in photosynthesis, primarily as the substrate for Photosystem II (PSII). However, their importance and contribution to Photosystem I (PSI) activity remains obscure. Using a high-resolution cryogenic electron microscopy (cryo-EM) PSI structure from a Chlamydomonas reinhardtii temperature-sensitive photoautotrophic PSII mutant (TSP4), a conserved network of water molecules - dating back to cyanobacteria - was uncovered, mainly in the vicinity of the electron transport chain (ETC). The high-resolution structure illustrated that the water molecules served as a ligand in every chlorophyll that was missing a fifth magnesium coordination in the PSI core and in the light-harvesting complexes (LHC). The asymmetric distribution of the water molecules near the ETC branches modulated their electrostatic landscape, distinctly in the space between the quinones and FX. The data also disclosed the first observation of eukaryotic PSI oligomerisation through a low-resolution PSI dimer that was comprised of PSI-10LHC and PSI-8LHC.

Caspy et al. report the structure of PSI from a temperature-sensitive photoautotrophic PSII mutant of Chlamydomonas reinhardtii (TSP4), and report the distribution of conserved water molecules in the structure from cyanobacterial to higher plant PSI. They suggest that the asymmetric distribution of water molecules near the electron transfer chain modulates the electron transfer from quinones to FX.

Determinants, and implications, of the shape and size of thylakoids and cristae

Thylakoids are flattened sacs isolated from other membranes; cristae are attached to the rest of the inner mitochondrial membrane by the crista junction, but the crista lumen is separated from the intermembrane space. The shape of thylakoids and cristae involves membranes with small (5-30 nm) radii of curvature. While the mechanism of curvature is not entirely clear, it seems to be largely a function of Curt proteins in thylakoids and Mitochondrial Organising Site and Crista Organising Centre proteins and oligomeric F_OF₁ ATP synthase in cristae. A subordinate, or minimal, role is attributable to lipids with areas of their head group area greater (convex leaflet) or smaller (concave leaflet) than the area of the lipid tail; examples of the latter group are monogalactosyldiglyceride in thylakoids and cardiolipin in cristae. The volume per unit area on the lumen side of the membrane is less than that of the chloroplast stroma or cyanobacterial cytosol for thylakoids, and mitochondrial matrix for cristae. A low volume per unit area of thylakoids and cristae means a small lumen width that is the average of wider spaces between lipid parts of the membranes and the narrower gaps dominated by extra-membrane components of transmembrane proteins. These structural constraints have important implications for the movement of the electron carriers plastocyanin and cytochrome c₆ (thylakoids) and cytochrome c (cristae) and hence the separation of the membrane-associated electron donors to, and electron acceptors from, these water-soluble electron carriers. The donor/acceptor pairs, are the cytochrome fb₆Fe_nh complex and P₇₀₀⁺ in thylakoids, and Complex III and Complex IV of cristae. The other energy flux parallel to the membranes is that of the proton motive force generated by redox-powered H⁺ pumps into the lumen to the proton motive force use in ATP synthesis by H⁺ flux from the lumen through the ATP synthase. For both the electron transport and proton motive force movement, concentration differences of reduced and oxidised electron carriers and protonated and deprotonated pH buffers are involved. The need for diffusion along a congested route of these energy transfer agents may limit the separation of sources and sinks parallel to the membranes of thylakoids and cristae.

Are plastocyanin and ferredoxin specific electron carriers or generic redox capacitors? Classical and murburn perspectives on two photosynthetic proteins

In the light reaction of oxygenic photosynthesis, plastocyanin (PC) and ferredoxins (Fd) are small/diffusible redox-active proteins playing key roles in electron transfer/transport phenomena. In the Z-scheme mechanistic purview, they are considered as specific affinity binding-based electron-relay agents, linking the functions of Cytochrome b₆f (Cyt. b₆f), Photosystem I (PS I) and Fd:NADPH oxidoreductase (FNR). The murburn explanation for photolytic photophosphorylation deems PC/Fd as generic 'redox capacitors', temporally accepting and releasing one-electron equivalents in reaction milieu. Herein, we explore the two theories with respect to structural, distributional and functional aspects of PC/Fd. Amino acid residues located on the surface loci of key patches of PC/Fd vary in electrostatic/contour (topography) signatures. Crystal structures of four different complexes each of Cyt.f-PC and Fd-FNR show little conservation in the contact-surfaces, thereby discrediting 'affinity binding-based electron transfers (ET)' as an evolutionary logic. Further, thermodynamic and kinetic data of wildtype and mutant proteins interactions do not align with Z-scheme. Furthermore, micromolar physiological concentrations of PC and the non-conducive architecture of chloroplasts render the classical model untenable. In the murburn model, as PC is optional, the observation that plants lacking PC survive and grow is justified. Further, the low physiological concentration/distribution of PC in chloroplast lumen/stroma is supported by murburn equilibriums, as higher concentrations would limit electron transfers. Thus, structural evidence, interactive dynamics with redox partners and physiological distribution/role of PC/Fd support the murburn perspective that these proteins serve as generic redox-capacitors in chloroplasts.Communicated by Ramaswamy H. Sarma.

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.

Medium-Chain Polyprenols Influence Chloroplast Membrane Dynamics in Solanum lycopersicum

The widespread occurrence of polyprenols throughout the plant kingdom is well documented, yet their functional role is poorly understood. These lipophilic compounds are known to be assembled from isoprenoid precursors by a class of enzymes designated as cis-prenyltransferases (CPTs), which are encoded by small CPT gene families in plants. In this study, we report that RNA interference (RNAi)-mediated knockdown of one member of the tomato CPT family (SlCPT5) reduced polyprenols in leaves by about 70%. Assays with recombinant SlCPT5 produced in Escherichia coli determined that the enzyme synthesizes polyprenols of approximately 50-55 carbons (Pren-10, Pren-11) in length and accommodates a variety of trans-prenyldiphosphate precursors as substrates. Introduction of SlCPT5 into the polyprenol-deficient yeast Δrer2 mutant resulted in the accumulation of Pren-11 in yeast cells, restored proper protein N-glycosylation and rescued the temperature-sensitive growth phenotype that is associated with its polyprenol deficiency. Subcellular fractionation studies together with in vivo localization of SlCPT5 fluorescent protein fusions demonstrated that SlCPT5 resides in the chloroplast stroma and that its enzymatic products accumulate in both thylakoid and envelope membranes. Transmission electron microscopy images of polyprenol-deficient leaves revealed alterations in chloroplast ultrastructure, and anisotropy measurements revealed a more disordered state of their envelope membranes. In polyprenol-deficient leaves, CO2 assimilation was hindered and their thylakoid membranes exhibited lower phase transition temperatures and calorimetric enthalpies, which coincided with a decreased photosynthetic electron transport rate. Taken together, these results uncover a role for polyprenols in governing chloroplast membrane dynamics.

Salt stress effects on the photosynthetic electron transport chain in two chickpea lines differing in their salt stress tolerance

The main objective of this study was to evaluate the effects of salt stress on the photosynthetic electron transport chain using two chickpea lines (Cicer arietinum L.) differing in their salt stress tolerance at the germination stage (AKN 87 and AKN 290). Two weeks after sowing, seedlings were exposed to salt stress for 2 weeks and irrigated with 200 ml of 200 mM NaCl every 2 days. The polyphasic OJIP fluorescence transient and the 820-nm transmission kinetics (photosystem I) were used to evaluate the effects of salt stress on the functionality of the photosynthetic electron transport chain. It was observed that a signature for salt stress was a combination of a higher J step (VJ), a smaller IP amplitude, and little or no effect on the primary quantum yield of PSII (φPo). We observed for AKN 290 a shorter leaf life cycle, which may represent a mechanism to cope with salt stress. For severely salt-stressed leaves, an inhibition of electron flow between the PQ pool and P700 was found. The data also suggest that the properties of electron flow beyond PSI are affected by salt stress.

Hacking the thylakoid proton motive force for improved photosynthesis: modulating ion flux rates that control proton motive force partitioning into Δψ and ΔpH

There is considerable interest in improving plant productivity by altering the dynamic responses of photosynthesis in tune with natural conditions. This is exemplified by the 'energy-dependent' form of non-photochemical quenching (q_E), the formation and decay of which can be considerably slower than natural light fluctuations, limiting photochemical yield. In addition, we recently reported that rapidly fluctuating light can produce field recombination-induced photodamage (FRIP), where large spikes in electric field across the thylakoid membrane (Δψ) induce photosystem II recombination reactions that produce damaging singlet oxygen (¹O₂). Both q_E and FRIP are directly linked to the thylakoid proton motive force (pmf), and in particular, the slow kinetics of partitioning pmf into its ΔpH and Δψ components. Using a series of computational simulations, we explored the possibility of 'hacking' pmf partitioning as a target for improving photosynthesis. Under a range of illumination conditions, increasing the rate of counter-ion fluxes across the thylakoid membrane should lead to more rapid dissipation of Δψ and formation of ΔpH. This would result in increased rates for the formation and decay of q_E while resulting in a more rapid decline in the amplitudes of Δψ-spikes and decreasing ¹O₂ production. These results suggest that ion fluxes may be a viable target for plant breeding or engineering. However, these changes also induce transient, but substantial mismatches in the ATP : NADPH output ratio as well as in the osmotic balance between the lumen and stroma, either of which may explain why evolution has not already accelerated thylakoid ion fluxes. Overall, though the model is simplified, it recapitulates many of the responses seen in vivo, while spotlighting critical aspects of the complex interactions between pmf components and photosynthetic processes. By making the programme available, we hope to enable the community of photosynthesis researchers to further explore and test specific hypotheses.This article is part of the themed issue 'Enhancing photosynthesis in crop plants: targets for improvement'.

Ectopic expression of SsPETE2, a plastocyanin from Suaeda salsa, improves plant tolerance to oxidative stress

Accumulating evidence indicates that plant plastocyanin is involved in copper homeostasis, yet the physiological relevance remains elusive. In this study, we found that a plastocyanin gene (SsPETE2) from euhalophyte Suaeda salsa possessed a novel antioxidant function, which was associated with the copper-chelating activity of SsPETE2. In S. salsa, expression of SsPETE2 increased in response to oxidative stress and ectopic expression of SsPETE2 in Arabidopsis enhanced the antioxidant ability of the transgenic plants. SsPETE2 bound Cu ion and alleviated formation of hydroxyl radicals in vitro. Accordingly, SsPETE2 expression lowered the free Cu content that was associated with reduced H₂O₂ level under oxidative stress. Arabidopsis pete1 and pete2 mutants showed ROS-sensitive phenotypes that could be restored by expression of SsPETE2 or AtPETEs. In addition, SsPETE2-expressing plants exhibited more potent tolerance to oxidative stress than plants overexpressing AtPETEs, likely owing to the stronger copper-binding activity of SsPETE2 than AtPETEs. Taken together, these results demonstrated that plant PETEs play a novel role in oxidative stress tolerance by regulating Cu homeostasis under stress conditions, and SsPETE2, as an efficient copper-chelating PETE, potentially could be used in crop genetic engineering.

An easily reversible structural change underlies mechanisms enabling desert crust cyanobacteria to survive desiccation

Biological desert sand crusts are the foundation of desert ecosystems, stabilizing the sands and allowing colonization by higher order organisms. The first colonizers of the desert sands are cyanobacteria. Facing the harsh conditions of the desert, these organisms must withstand frequent desiccation-hydration cycles, combined with high light intensities. Here, we characterize structural and functional modifications to the photosynthetic apparatus that enable a cyanobacterium, Leptolyngbya sp., to thrive under these conditions. Using multiple in vivo spectroscopic and imaging techniques, we identified two complementary mechanisms for dissipating absorbed energy in the desiccated state. The first mechanism involves the reorganization of the phycobilisome antenna system, increasing excitonic coupling between antenna components. This provides better energy dissipation in the antenna rather than directed exciton transfer to the reaction center. The second mechanism is driven by constriction of the thylakoid lumen which limits diffusion of plastocyanin to P700. The accumulation of P700(+) not only prevents light-induced charge separation but also efficiently quenches excitation energy. These protection mechanisms employ existing components of the photosynthetic apparatus, forming two distinct functional modes. Small changes in the structure of the thylakoid membranes are sufficient for quenching of all absorbed energy in the desiccated state, protecting the photosynthetic apparatus from photoinhibitory damage. These changes can be easily reversed upon rehydration, returning the system to its high photosynthetic quantum efficiency.

Differential effects of ambient or diminished CO2 and O2 levels on thylakoid membrane structure in light-stressed plants

Over-reduction of the photosynthetic electron transport chain may severely damage the photosynthetic apparatus as well as other constituents of the chloroplast and the cell. Here, we exposed Arabidopsis leaves to saturating light either under normal atmospheric conditions or under CO2--and O2 -limiting conditions, which greatly increase excitation and electron pressures by draining terminal electron acceptors. The two treatments were found to have very different, often opposing, effects on the structure of the thylakoid membranes, including the width of the granal lumenal compartment. Modulation of the latter is proposed to be related to movements of ions across the thylakoid membrane, which alter the relative osmolarity of the lumen and stroma and affect the partitioning of the proton motive force into its electrical and osmotic components. The resulting changes in thylakoid organization and lumenal width should facilitate the repair of photodamaged photosystem II complexes in response to light stress under ambient conditions, but are expected to inhibit the repair cycle when the light stress occurs concurrently with CO2 and O2 depletion. Under the latter conditions, the changes in thylakoid structure are predicted to complement other processes that restrict the flow of electrons into the high-potential chain, thus moderating the production of deleterious reactive oxygen species at photosystem I.

In vivo spectroscopy and NMR metabolite fingerprinting approaches to connect the dynamics of photosynthetic and metabolic phenotypes in resurrection plant Haberlea rhodopensis during desiccation and recovery

The resurrection plant Haberlea rhodopensis was used to study dynamics of drought response of photosynthetic machinery parallel with changes in primary metabolism. A relation between leaf water content and photosynthetic performance was established, enabling us to perform a non-destructive evaluation of the plant water status during stress. Spectroscopic analysis of photosynthesis indicated that, at variance with linear electron flow (LEF) involving photosystem (PS) I and II, cyclic electron flow around PSI remains active till almost full dry state at the expense of the LEF, due to the changed protein organization of photosynthetic apparatus. We suggest that, this activity could have a photoprotective role and prevent a complete drop in adenosine triphosphate (ATP), in the absence of LEF, to fuel specific energy-dependent processes necessary for the survival of the plant, during the late states of desiccation. The NMR fingerprint shows the significant metabolic changes in several pathways. Due to the declining of LEF accompanied by biosynthetic reactions during desiccation, a reduction of the ATP pool during drought was observed, which was fully and quickly recovered after plants rehydration. We found a decline of valine accompanied by lipid degradation during stress, likely to provide alternative carbon sources for sucrose accumulation at late stages of desiccation. This accumulation, as well as the increased levels of glycerophosphodiesters during drought stress could provide osmoprotection to the cells.

Photosynthetic complex stoichiometry dynamics in higher plants: environmental acclimation and photosynthetic flux control

The composition of the photosynthetic apparatus of higher plants is dynamically adjusted to long-term changes in environmental conditions such as growth light intensity and light quality, and to changing metabolic demands for ATP and NADPH imposed by stresses and leaf aging. By changing photosynthetic complex stoichiometry, a long-term imbalance between the photosynthetic production of ATP and NADPH and their metabolic consumption is avoided, and cytotoxic side reactions are minimized. Otherwise, an excess capacity of the light reactions, relative to the demands of primary metabolism, could result in a disturbance of cellular redox homeostasis and an increased production of reactive oxygen species, leading to the destruction of the photosynthetic apparatus and the initiation of cell death programs. In this review, changes of the abundances of the different constituents of the photosynthetic apparatus in response to environmental conditions and during leaf ontogenesis are summarized. The contributions of the different photosynthetic complexes to photosynthetic flux control and the regulation of electron transport are discussed.

Alternative electron transports participate in the maintenance of violaxanthin De-epoxidase activity of Ulva sp. under low irradiance

The xanthophyll cycle (Xc), which involves violaxanthin de-epoxidase (VDE) and the zeaxanthin epoxidase (ZEP), is one of the most rapid and efficient responses of plant and algae to high irradiance. High light intensity can activate VDE to convert violaxanthin (Vx) to zeaxanthin (Zx) via antheraxanthin (Ax). However, it remains unclear whether VDE remains active under low light or dark conditions when there is no significant accumulation of Ax and Zx, and if so, how the ΔpH required for activation of VDE is built. In this study, we used salicylaldoxime (SA) to inhibit ZEP activity in the intertidal macro-algae Ulva sp. (Ulvales, Chlorophyta) and then characterized VDE under low light and dark conditions with various metabolic inhibitors. With inhibition of ZEP by SA, VDE remained active under low light and dark conditions, as indicated by large accumulations of Ax and Zx at the expense of Vx. When PSII-mediated linear electron transport systems were completely inhibited by SA and DCMU, alternative electron transport systems (i.e., cyclic electron transport and chlororespiration) could maintain VDE activity. Furthermore, accumulations of Ax and Zx decreased significantly when SA, DCMU, or DBMIB together with an inhibitor of chlororespiration (i.e., propyl gallate (PG)) were applied to Ulva sp. This result suggests that chlororespiration not only participates in the build-up of the necessary ΔpH, but that it also possibly influences VDE activity indirectly by diminishing the oxygen level in the chloroplast.

Desiccation induces accumulations of antheraxanthin and zeaxanthin in intertidal macro-alga Ulva pertusa (Chlorophyta)

For plants and algae, exposure to high light levels is deleterious to their photosynthetic machineries. It also can accelerate water evaporation and thus potentially lead to drought stress. Most photosynthetic organisms protect themselves against high light caused photodamages by xanthophyll cycle-dependent thermal energy dissipation. It is generally accepted that high light activates xanthophyll cycle. However, the relationship between xanthophyll cycle and drought stress remains ambiguous. Herein, Ulva pertusa (Chlorophyta), a representative perennial intertidal macro-algae species with high drought-tolerant capabilities and simple structures, was used to investigate the operation of xanthophyll cycle during desiccation in air. The results indicate that desiccation under dim light induced accumulation of antheraxanthin (Ax) and zeaxanthin (Zx) at the expense of violaxanthin (Vx). This accumulation could be arrested by dithiothreitol completely and by uncoupler (carbonyl cyanide p-trifluoromethoxyphenylhydrazone) partially, implying the participation of Vx de-epoxidase in conversion of Vx to Ax and Zx. Treatment with inhibitors of electron transport along thylakoid membrane, e.g. DCMU, PG and DBMIB, did not significantly arrest desiccation-induced accumulation of Ax and Zx. We propose that for U. pertusa, besides excess light, desiccation itself could also induce accumulation of Ax and Zx. This accumulation could proceed without electron transport along thylakoid membrane, and is possibly resulting from the reduction of thylakoid lumen volume during desiccation. Considering the pleiotropic effects of Ax and Zx, accumulated Ax and Zx may function in protecting thylakoid membrane and enhancing thermal quenching during emersion in air.

Antimycin A effect on the electron transport in chloroplasts of two Chlamydomonas reinhardtii strains

The effects of antimycin A on the redox state of plastoquinone and on electron donation to photosystem I (PS I) were studied in sulfur-deprived Chlamydomonas reinhardtii cells of the strains cc406 and 137c. We found that this reagent suppresses cyclic electron flow around PS I in the cc406 strain, whereas this inhibitory effect was completely absent in the 137c strain. In the latter strain, antimycin A induced rapid reduction of plastoquinone in the dark and considerably enhanced the rate of electron donation to P700 (+) in the dark. Importantly, neither myxothiazol, an inhibitor of mitochondrial respiration, FCCP, a protonophore, nor propyl gallate, an inhibitor of the plastid terminal oxidase, induced such a strong effect like antimycin A. The results indicate that in the chloroplast of the 137c strain, antimycin A has a site of action outside of the machinery of cyclic electron flow.

Novel thylakoid membrane GreenCut protein CPLD38 impacts accumulation of the cytochrome b6f complex and associated regulatory processes

Based on previous comparative genomic analyses, a set of nearly 600 polypeptides was identified that is present in green algae and flowering and nonflowering plants but is not present (or is highly diverged) in nonphotosynthetic organisms. The gene encoding one of these “GreenCut” proteins, CPLD38, is in the same operon as ndhL in most cyanobacteria; the NdhL protein is part of a complex essential for cyanobacterial respiration. A cpld38 mutant of Chlamydomonas reinhardtii does not grow on minimal medium, is high light-sensitive under photoheterotrophic conditions, has lower accumulation of photosynthetic complexes, reduced photosynthetic electron flow to P700⁺, and reduced photochemical efficiency of photosystem II (ΦPSII); these phenotypes are rescued by a wild-type copy of CPLD38. Single turnover flash experiments and biochemical analyses demonstrated that cytochrome b₆f function was severely compromised, and the levels of transcripts and polypeptide subunits of the cytochrome b₆f complex were also significantly lower in the cpld38 mutant. Furthermore, subunits of the cytochrome b₆f complex in mutant cells turned over much more rapidly than in wild-type cells. Interestingly, PTOX2 and NDA2, two major proteins involved in chlororespiration, were more than 5-fold higher in mutants relative to wild-type cells, suggesting a shift in the cpld38 mutant from photosynthesis toward chlororespiratory metabolism, which is supported by experiments that quantify the reduction state of the plastoquinone pool. Together, these findings support the hypothesis that CPLD38 impacts the stability of the cytochrome b₆f complex and possibly plays a role in balancing redox inputs to the quinone pool from photosynthesis and chlororespiration.

Retrograde signaling and photoprotection in a gun4 mutant of Chlamydomonas reinhardtii

GUN4 is a regulatory subunit of Mg-chelatase involved in the control of tetrapyrrole synthesis in plants and cyanobacteria. Here, we report the first characterization of a gun4 insertion mutant of the unicellular green alga Chlamydomonas reinhardtii. The mutant contains 50% of chlorophyll as compared to wild-type and accumulates ProtoIX. In contrast to the increase in LHC transcription, the accumulation of most LHC proteins is drastically diminished, implying posttranscriptional down-regulation in the absence of transcriptional coordination. We found that 803 genes change their expression level in gun4 as compared to wild-type, by RNA-Seq, and this wide-ranging effect on transcription is apparent under physiological conditions. Besides LHCs, we identified transcripts encoding enzymes of the tetrapyrrole pathway and factors involved in signal transduction, transcription, and chromatin remodeling. Moreover, we observe perturbations in electron transport with a strongly decreased PSI-to-PSII ratio. This is accompanied by an enhanced activity of the plastid terminal oxidase (PTOX) that could have a physiological role in decreasing photosystem II excitation pressure.

Retrograde signaling and photoprotection in a gun4 mutant of Chlamydomonas reinhardtii

GUN4 is a regulatory subunit of Mg-chelatase involved in the control of tetrapyrrole synthesis in plants and cyanobacteria. Here, we report the first characterization of a gun4 insertion mutant of the unicellular green alga Chlamydomonas reinhardtii. The mutant contains 50% of chlorophyll as compared to wild-type and accumulates ProtoIX. In contrast to the increase in LHC transcription, the accumulation of most LHC proteins is drastically diminished, implying posttranscriptional down-regulation in the absence of transcriptional coordination. We found that 803 genes change their expression level in gun4 as compared to wild-type, by RNA-Seq, and this wide-ranging effect on transcription is apparent under physiological conditions. Besides LHCs, we identified transcripts encoding enzymes of the tetrapyrrole pathway and factors involved in signal transduction, transcription, and chromatin remodeling. Moreover, we observe perturbations in electron transport with a strongly decreased PSI-to-PSII ratio. This is accompanied by an enhanced activity of the plastid terminal oxidase (PTOX) that could have a physiological role in decreasing photosystem II excitation pressure.

Modulation of the multilamellar membrane organization and of the chiral macrodomains in the diatom Phaeodactylum tricornutum revealed by small-angle neutron scattering and circular dichroism spectroscopy

Diatoms possess effective photoprotection mechanisms, which may involve reorganizations in the photosynthetic machinery. We have shown earlier, by using circular dichroism (CD) spectroscopy, that in Phaeodactylum tricornutum the pigment-protein complexes are arranged into chiral macrodomains, which have been proposed to be associated with the multilamellar organization of the thylakoid membranes and shown to be capable of undergoing light-induced reversible reorganizations (Szabó et al. Photosynth Res 95:237, 2008). Recently, by using small-angle neutron scattering (SANS) on the same algal cells we have determined the repeat distances and revealed reversible light-induced reorganizations in the lamellar order of thylakoids (Nagy et al. Biochem J 436:225, 2011). In this study, we show that in moderately heat-treated samples, the weakening of the lamellar order is accompanied by the diminishment of the psi-type CD signal associated with the long-range chiral order of the chromophores (psi, polymer or salt-induced). Further, we show that the light-induced reversible increase in the psi-type CD is associated with swelling in the membrane system, with magnitudes larger in high light than in low light. In contrast, shrinkage of the membrane system, induced by sorbitol, brings about a decrease in the psi-type CD signal; this shrinkage also diminishes the non-photochemical quenching capability of the cells. These data shed light on the origin of the psi-type CD signal, and confirm that both CD spectroscopy and SANS provide valuable information on the macro-organization of the thylakoid membranes and their dynamic properties; these parameters are evidently of interest with regard to the photoprotection in whole algal cells.

Proteomic analysis of salinity-stressed Chlamydomonas reinhardtii revealed differential suppression and induction of a large number of important housekeeping proteins

Salinity stress is one of the most common abiotic stresses that hamper plant productivity worldwide. Successful plant adaptations to salt stress require substantial changes in cellular protein expression. In this work, we present a 2-DE-based proteomic analysis of a model unicellular green alga, Chlamydomonas reinhardtii, subjected to 300 mM NaCl for 2 h. Results showed that, in addition to the protein spots that showed partial up- or down-regulation patterns, a number of proteins were exclusively present in the proteome of the control cells, but were absent from the salinity-stressed samples. Conversely, a large number of proteins exclusively appeared in the proteome of the salinity-stressed samples. Of those exclusive proteins, we could successfully identify, via LC-MS/MS, 18 spots uniquely present in the control cells and 99 spots specific to NaCl-treated cells. Interestingly, among the salt-exclusive protein spots, we identified several important housekeeping proteins like molecular chaperones and proteins of the translation machinery, suggesting that they may originate from post-translational modifications rather than from de novo biosynthesis. The possible role and the salt-specific modification of these proteins by salinity stress are discussed.

Plastid terminal oxidase 2 (PTOX2) is the major oxidase involved in chlororespiration in Chlamydomonas

By homology with the unique plastid terminal oxidase (PTOX) found in plants, two genes encoding oxidases have been found in the Chlamydomonas genome, PTOX1 and PTOX2. Here we report the identification of a knockout mutant of PTOX2. Its molecular and functional characterization demonstrates that it encodes the oxidase most predominantly involved in chlororespiration in this algal species. In this mutant, the plastoquinone pool is constitutively reduced under dark-aerobic conditions, resulting in the mobile light-harvesting complexes being mainly, but reversibly, associated with photosystem I. Accordingly, the ptox2 mutant shows lower fitness than wild type when grown under phototrophic conditions. Single and double mutants devoid of the cytochrome b(6)f complex and PTOX2 were used to measure the oxidation rates of plastoquinols via PTOX1 and PTOX2. Those lacking both the cytochrome b(6)f complex and PTOX2 were more sensitive to light than the single mutants lacking either the cytochrome b(6)f complex or PTOX2, which discloses the role of PTOX2 under extreme conditions where the plastoquinone pool is overreduced. A model for chlororespiration is proposed to relate the electron flow rate through these alternative pathways and the redox state of plastoquinones in the dark. This model suggests that, in green algae and plants, the redox poise results from the balanced accumulation of PTOX and NADPH dehydrogenase.

New direct dynamic models of protein interactions coupled to photosynthetic electron transport reactions

This review covers the methods of computer simulation of protein interactions taking part in photosynthetic electron transport reactions. A direct multiparticle simulation method that simulates reactions describing interactions of ensembles of molecules in the heterogeneous interior of a cell is developed. In the models, protein molecules move according to the laws of Brownian dynamics, mutually orient themselves in the electrical field, and form complexes in the 3D scene. The method allows us to visualize the processes of molecule interactions and to calculate the rate constants for protein complex formation reactions in the solution and in the photosynthetic membrane. Three-dimensional multiparticle computer models for simulating the complex formation kinetics for plastocyanin with photosystem I and cytochrome bf complex, and ferredoxin with photosystem I and ferredoxin:NADP+-reductase are considered. Effects of ionic strength are featured for wild type and mutant proteins. The computer multiparticle models describe nonmonotonic dependences of complex formation rates on the ionic strength as the result of long-range electrostatic interactions.

Regulation of protein function: crystal packing interfaces and conformational dimerization

The accepted view of interprotein electron transport involves molecules diffusing between donor and acceptor redox sites. An emerging alternative hypothesis is that efficient long-range electron transport can be achieved through proteins arranged in supramolecular assemblies. In this study, we have investigated the crystal packing interfaces in three crystal forms of plastocyanin, an integral component of the photosynthetic electron transport chain, and discuss their potential relevance to in vivo supramolecular assemblies. Symmetry-related protein chains within these crystals have Cu-Cu separations of <25 A, a distance that readily supports electron transfer. In one structure, the plastocyanin molecule exists in two forms in which a backbone displacement coupled with side chain rearrangements enables the modulation of protein-protein interfaces.

TRANSCRIPTOMIC ANALYSIS OF GRACILARIA CHANGII (RHODOPHYTA) IN RESPONSE TO HYPER- AND HYPOOSMOTIC STRESSES(1)

Osmotic stress is one of the most significant natural abiotic stresses that occur in the intertidal zones. Seaweeds may physiologically acclimate to changing osmolarity by altering their transcriptome. Here, we investigated the transcriptomic changes of Gracilaria changii (B. M. Xia et I. A. Abbott) I. A. Abbott, J. Zhang et B. M. Xia in response to hyper- and hypoosmotic stresses using a cDNA microarray approach. Microarray analysis revealed that 199 and 200 genes from ∼3,300 genes examined were up- and down-regulated by >2-fold in seaweed samples treated at 50 parts per thousand (ppt) artificial seawater (ASW) compared with those at 30 ppt ASW, respectively. The number of genes that were up- and down-regulated by >2-fold in seaweed samples treated at 10 ppt ASW compared with those at 30 ppt ASW were 154 and 187, respectively. A majority of these genes were only differentially expressed under hyper- or hypoosmotic conditions, whereas 67 transcripts were affected by both stresses. The findings of this study have shed light on the expression profiles of many transcripts during the acclimation of G. changii to hyperosmotic and hypoosmotic conditions. This information may assist in the prioritization of genes to be examined in future studies.

Mutants, overexpressors, and interactors of Arabidopsis plastocyanin isoforms: revised roles of plastocyanin in photosynthetic electron flow and thylakoid redox state

Two homologous plastocyanin isoforms are encoded by the genes PETE1 and PETE2 in the nuclear genome of Arabidopsis thaliana. The PETE2 transcript is expressed at considerably higher levels and the PETE2 protein is the more abundant isoform. Null mutations in the PETE genes resulted in plants, designated pete1 and pete2, with decreased plastocyanin contents. However, despite reducing plastocyanin levels by over approximately 90%, a pete2 null mutation on its own affects rates of photosynthesis and growth only slightly, whereas pete1 knockout plants, with about 60-80% of the wild-type plastocyanin level, did not show any alteration. Hence, plastocyanin concentration is not limiting for photosynthetic electron flow under optimal growth conditions, perhaps implying other possible physiological roles for the protein. Indeed, plastocyanin has been proposed previously to cooperate with cytochrome c(6A) (Cyt c(6A)) in thylakoid redox reactions, but we find no evidence for a physical interaction between the two proteins, using interaction assays in yeast. We observed homodimerization of Cyt c(6A) in yeast interaction assays, but also Cyt c(6A) homodimers failed to interact with plastocyanin. Moreover, phenotypic analysis of atc6-1 pete1 and atc6-1 pete2 double mutants, each lacking Cyt c(6A) and one of the two plastocyanin-encoding genes, failed to reveal any genetic interaction. Overexpression of either PETE1 or PETE2 in the pete1 pete2 double knockout mutant background results in essentially wild-type photosynthetic performance, excluding the possibility that the two plastocyanin isoforms could have distinct functions in thylakoid electron flow.

Toxicity assessment of manufactured nanomaterials using the unicellular green alga Chlamydomonas reinhardtii

With the rapid development of nanotechnology, there is an increasing risk of human and environmental exposure to nanotechnology-based materials and products. As water resources are particularly vulnerable to direct and indirect contamination of nonomaterials (NMs), the potential toxicity and environmental implication of NMs to aquatic organisms must be evaluated. In this study, we assessed potential toxicity of two commercially used NMs, titanium dioxide (TiO(2)) and quantum dots (QDs), using the unicellular green alga Chlamydomonas reinhartii as a model system. The response of the organism to NMs was assessed at physiological, biochemical, and molecular genetic levels. Growth kinetics showed that growth inhibition occurred during the first two to three days of cultivation in the presence of TiO(2) or QDs. Measurements of lipid peroxidation measurement indicated that oxidative stress of the cells occurred as early as 6 h after exposure to TiO(2) or QDs. The transcriptional expression profiling of four stress response genes (sod1, gpx, cat, and ptox2) revealed that transient up-regulation of these genes occurred in cultures containing as low as 1.0 mg L(-1) of TiO(2) or 0.1 mg L(-1) of QDs, and the maximum transcripts of cat, sod1, gpx, and ptox2 occurred at 1.5, 3, 3, and 6 h, respectively, and were proportional to the initial concentration of the NMs. As the cultures continued, recovery in growth was observed and the extent of recovery, as indicated by the final cell concentration, was dosage-dependent. QDs were found to be more toxic to Chlamydomonas cells than TiO(2) under our experimental conditions.

Factors controlling the mobility of photosynthetic proteins

Protein diffusion in and around the photosynthetic membrane must play a crucial role in photosynthetic functions including electron transport, regulation of light-harvesting, and biogenesis, turnover and repair of membrane components. Protein mobility is controlled by a complex web of specific interactions, plus the viscosity of the environment and the extent of macromolecular crowding. I discuss the techniques that can be used to measure protein mobility in photosynthetic membranes. I then summarize what we know about the constraints on protein mobility imposed by macromolecular aggregation and crowding in and around the thylakoid membranes of green plants and cyanobacteria, with particular reference to the fluidity of the thylakoid membrane and the aqueous phases on either side of the membrane (the stroma/cytoplasm and the thylakoid lumen). Current indications are that the stroma/cytoplasm is a relatively fluid environment, whereas protein mobility in the lumen may be extremely restricted. The thylakoid membrane itself has an intermediate fluidity: some protein complexes are virtually immobile, probably due to their incorporation into large, stable macromolecular aggregates. However, there is sufficient free space to allow the long-range diffusion of some complexes. Finally, I discuss some future directions for research in this area.

Molecular and electrophysiological characterization of a mechanosensitive channel expressed in the chloroplasts of Chlamydomonas

MscS is a mechanosensitive channel that is ubiquitous among bacteria. Recent progress in the genome projects has revealed that homologs of MscS are also present in eukaryotes, but whether they operate as ion channels is unknown. In this study we cloned MSC1, a homolog of MscS in Chlamydomonas, and examined its function when expressed in Escherichia coli. Full-length MSC1 was not functional when expressed in E. coli cells. However, removal of the N-terminal signal sequence (DeltaN-MSC1) reversed this effect. DeltaN-MSC1 was found to open in response to membrane stretch and displayed a preference for anions over cations as permeable ions. DeltaN-MSC1 exhibited marked hysteretic behavior in response to ascending and descending stimuli. That is, channel gating occurred in response to significant stimuli but remained open until the stimulus was almost completely removed. Indirect immunofluorescence revealed that MSC1 is present as punctate spots in the cytoplasm and chloroplasts. Moreover, knockdown of MSC1 expression resulted in the abnormal localization of chlorophyll. These findings show that MSC1 is an intracellular mechanosensitive channel and is responsible for the organization of chloroplast in Chlamydomonas.

Release of oxidized plastocyanin from photosystem I limits electron transfer between photosystem I and cytochrome b6f complex in vivo

We used fast absorbance spectroscopy to investigate in vivo binding dynamics and electron transfer between plastocyanin (pc) and photosystem I (PSI), and cytochrome (cyt) f oxidation kinetics in Chlamydomonas reinhardtii mutants in which either the binding or the release of pc from PSI was diminished. Under single flash-excitation conditions, electron flow between PSI and the cyt complex was not affected by a 5-fold lowering of the binding affinity of pc to PSI, as induced by a mutation replacing the tryptophan-651 of the PsaA subunit by a serine residue (PsaA-W651S). On the other hand, electron flow from PSI to the cyt b(6)f complex was very sensitive to a 2- to 3-fold decrease in the rate of pc release from PSI, obtained by replacing the glutamic acid residue 613 of the PsaB subunit with glutamine (PsaB-E613N). Thus, our data indicate that under these experimental conditions the release of oxidized pc limits electron transfer between cyt b(6)f complex and PSI in vivo.

The role of plastocyanin in the adjustment of the photosynthetic electron transport to the carbon metabolism in tobacco

We investigated adaptive responses of the photosynthetic electron transport to a decline in the carbon assimilation capacity. Leaves of different ages from wild-type tobacco (Nicotiana tabacum) L. var Samsun NN and young mature leaves of tobacco transformants with impaired photoassimilate export were used. The assimilation rate decreased from 280 in young mature wild-type leaves to below 50 mmol electrons mol chlorophyll(-1) s(-1) in older wild-type leaves or in transformants. The electron transport capacity, measured in thylakoids isolated from the different leaves, closely matched the leaf assimilation rate. The numbers of cytochrome (cyt)-bf complexes and plastocyanin (PC) decreased with the electron transport and assimilation capacity, while the numbers of photosystem I (PSI), photosystem II, and plastoquinone remained constant. The PC to PSI ratio decreased from five in leaves with high assimilation rates, to values below one in leaves with low assimilation rates, and the PC versus flux correlation was strictly proportional. Redox kinetics of cyt-f, PC, and P700 suggest that in leaves with low electron fluxes, PC is out of the equilibrium with P700 and cyt-f and the cyt-f reoxidation rate is restricted. It is concluded that the electron flux is sensitive to variations in the number of PC, relative to PSI and cyt-bf, and PC, in concert with cyt-bf, is a key component that adjusts to control the electron transport rate. PC dependent flux control may serve to adjust the electron transport rate under conditions where the carbon assimilation is diminished and thereby protects PSI against over-reduction and reactive oxygen production.

Plastocyanin redox kinetics in spinach chloroplasts: evidence for disequilibrium in the high potential chain

Reduction kinetics of cytochrome f, plastocyanin (PC) and P(700) ('high-potential chain') in thylakoids from spinach were followed after pre-oxidation by a saturating light pulse. We describe a novel approach to follow PC redox kinetics from deconvolution of 810-860 nm absorption changes. The equilibration between the redox-components was analyzed by plotting the redox state of cytochrome f and PC against that of P(700). In thylakoids with (1) diminished electron transport rate (adjusted with a cytochrome bf inhibitor) or (2) de-stacked grana, cytochrome f and PC relaxed close to their thermodynamic equilibriums with P(700). In stacked thylakoids with non-inhibited electron transport, the equilibration plots were complex and non-hyperbolic, suggesting that during fast electron flux, the 'high-potential chain' does not homogeneously equilibrate throughout the membrane. Apparent equilibrium constants <5 were calculated, which are below the thermodynamic equilibrium known for the 'high potential chain'. The disequilibrium found in stacked thylakoids with high electron fluxes is explained by restricted long-range PC diffusion. We develop a model assuming that about 30% of Photosystem I mainly located in grana end-membranes and margins rapidly equilibrate with cytochrome f via short-distance transluminal PC diffusion, while long-range lateral PC migration between grana cores and distant stroma lamellae is restricted. Implications for the electron flux control are discussed.

Long-term hyposaline and hypersaline stresses produce distinct antioxidant responses in the marine alga Dunaliella tertiolecta

Tolerance to salinity stress in higher plants correlates to levels of antioxidant enzymes and/or substrates. Do hyperosmotic and hypoosmotic stress induce antioxidant responses in salt tolerant algae, and if so, are these responses the same for both excess and minimal salinity? To answer these questions, cultures of the marine alga Dunaliella tertiolecta (Chlorophyta) were grown in seven salinities covering a 60-fold range from 0.05 to 3.0 mol/L NaCl. Long-term effects of salinity on growth and antioxidant parameters were determined. Growth rates were reduced at the salinity extremes (0.05 mol/L NaCl and 3 mol/L NaCl) indicating the cultures were stressed. The levels of six antioxidant enzymes and three antioxidant substrates were quantified at these growth salinities. Compared to growth at optimum salinities (i.e. 0.2-0.5 mol/L NaCl), high salinities produced a 260% increase in monodehydroascorbate reductase, a doubling of ascorbate peroxidase activity and a three-fold increase in the rate of dark respiration. Cells acclimated to low growth salinities (hyposaline stress, i.e. < 0.2 mol/L NaCl) showed major increases in glutathione and alpha-tocopherol coupled with decreases in Fv/Fm ratios and in total and reduced ascorbate compared to moderate and high external salinities. Cell volumes remained unchanged, except at the lowest salinity where they doubled. Catalase, superoxide dismutase, dehydroascorbate reductase and glutathione reductase activities were not altered by extreme salinities. The involvement of oxidative stress at both salinity extremes is implied by the alterations in antioxidant enzymes and substrates, but the specific changes are very different between hypo and hypersaline stresses.