Arg24 and 26 of the D2 protein are important for photosystem II assembly and plastoquinol exchange in Synechocystis sp. PCC 6803

Photosystem II (PS II) assembly is a stepwise process involving preassembly complexes or modules focused around four core PS II proteins. The current model of PS II assembly in cyanobacteria is derived from studies involving the deletion of one or more of these core subunits. Such deletions may destabilize other PS II assembly intermediates, making constructing a clear picture of the intermediate events difficult. Information on plastoquinone exchange pathways operating within PS II is also unclear and relies heavily on computer-aided simulations. Deletion of PsbX in [S. Biswas, J.J. Eaton-Rye, Biochim. Biophys. Acta - Bioenerg. 1863 (2022) 148519] suggested modified QB binding in PS II lacking this subunit. This study has indicated the phenotype of the ∆PsbX mutant arose by disrupting a conserved hydrogen bond between PsbX and the D2 (PsbD) protein. We mutated two conserved arginine residues (D2:Arg24 and D2:Arg26) to further understand the observations made with the ∆PsbX mutant. Mutating Arg24 disrupted the interaction between PsbX and D2, replicating the high-light sensitivity and altered fluorescence decay kinetics observed in the ∆PsbX strain. The Arg26 residue, on the other hand, was more important for either PS II assembly or for stabilizing the fully assembled complex. The effects of mutating both arginine residues to alanine or aspartate were severe enough to render the corresponding double mutants non-photoautotrophic. Our study furthers our knowledge of the amino-acid interactions stabilizing plastoquinone-exchange pathways while providing a platform to study PS II assembly and repair without the actual deletion of any proteins.

Energy deprivation affects nitrogen assimilation and fatty acid biosynthesis leading to leaf chlorosis under waterlogging stress in the endangered Abies koreana

Energy deprivation triggers various physiological, biochemical and molecular changes in plants under abiotic stress. We investigated the oxidative damages in the high altitude grown conifer Korean fir (Abies koreana) exposed to waterlogging stress. Our experimental results showed that waterlogging stress led to leaf chlorosis, 35 days after treatment. A significant decrease in leaf fresh weight, chlorophyll and sugar content supported this phenotypic change. Biochemical analysis showed a significant increase in leaf proline, lipid peroxidase and 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical content of waterlogged plants. To elucidate the molecular mechanisms, we conducted RNA-sequencing (RNA-seq) and de novo assembly. Using RNA-seq analysis approach and filtering (P < 0.05 and false discovery rate <0.001), we obtained 134 unigenes upregulated and 574 unigenes downregulated. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis placed the obtained differentially expressed unigenes in α-linoleic pathway, fatty acid degradation, glycosis, glycolipid metabolism and oligosaccharide biosynthesis process. Mapping of unigenes with Arabidopsis using basic local alignment search tool for nucleotides showed several critical genes in photosynthesis and carbon metabolism downregulated. Following this, we found the repression of multiple nitrogen (N) assimilation and nucleotide biosynthesis genes including purine metabolism. In addition, waterlogging stress reduced the levels of polyunsaturated fatty acids with a concomitant increase only in myristic acid. Together, our results indicate that the prolonged snowmelt may cause inability of A. koreana seedlings to lead the photosynthesis normally due to the lack of root intercellular oxygen and emphasizes a detrimental effect on the N metabolic pathway, compromising this endangered tree's ability to be fully functional under waterlogging stress.

Integrated eco-physiological, biochemical, and molecular biological analyses of selenium fortification mechanism in alfalfa

Foliar Se (IV) application at 100 mg/kg can act as a positive bio-stimulator of redox, photosynthesis, and nutrient metabolism in alfalfa via phenotypes, nutritional compositions, biochemistry, combined with transcriptome analysis. Selenium (Se) is an essential element for mammals, and plants are the primary source of dietary Se. However, Se usually has dual (beneficial/toxic) effects on the plant itself. Alfalfa (Medicago sativa L.) is one of the most important forage resources in the world due to its high nutritive value. In this study, we have investigated the effects of sodium selenite (Se (IV)) (0, 100, 200, 300, and 500 mg/kg) on eco-physiological, biochemical, and transcriptional mechanisms in alfalfa. The phenotypic and nutritional composition alterations revealed that lower Se (IV) (100 mg/kg) levels positively affected alfalfa; it enhanced the antioxidant activity, which may contribute to redox homeostasis and chloroplast function. At 100 mg/kg Se (IV) concentration, the H₂O_2, and malondialdehyde (MDA) contents decreased by 36.72% and 22.62%, respectively, whereas the activity of glutathione peroxidase (GPX) increased by 31.10%. Se supplementation at 100 mg/kg increased the plant pigments contents, the light-harvesting capacity of PSII (Fv/Fm) and PSI (ΔP700max), and the carbon fixation efficiency, which was demonstrated by enhanced photosynthesis (37.6%). Furthermore, alfalfa shifted carbon flux to protein synthesis to improve quality at 100 mg/kg of Se (IV) by upregulating carbohydrate and amino acid metabolic genes. On the contrary, at 500 mg/kg, Se (IV) became toxic. Higher Se (IV) disordered the plant antioxidant system, increasing H₂O₂ and MDA by 14.2 and 4.3%, respectively. Moreover, photosynthesis was inhibited by 20.2%, and more structural substances, such as lignin, were synthesized. These results strongly suggest that Se (IV) at a concentration of 100 mg/kg act as the positive bio-stimulator of redox metabolism, photosynthesis, and nutrient in alfalfa.

Overexpression of PagSTOMAGEN, a Positive Regulator of Stomatal Density, Promotes Vegetative Growth in Poplar

Poplar is an important fast-growing tree, and its photosynthetic capacity directly affects its vegetative growth. Stomatal density is closely related to photosynthetic capacity and growth characteristics in plants. Here, we isolated PagSTOMAGEN from the hybrid poplar (Populus alba × Populus glandulosa) clone 84K and investigated its biological function in vegetative growth. PagSTOMAGEN was expressed predominantly in young tissues and localized in the plasma membrane. Compared with wild-type 84K poplars, PagSTOMAGEN-overexpressing plants displayed an increased plant height, leaf area, internode number, basal diameter, biomass, IAA content, IPR content, and stomatal density. Higher stomatal density improved the net photosynthetic rate, stomatal conductance, intercellular CO₂ concentration, and transpiration rate in transgenic poplar. The differential expression of genes related to stomatal development showed a diverged influence of PagSTOMAGEN at different stages of stomatal development. Finally, transcriptomic analysis showed that PagSTOMAGEN affected vegetative growth by affecting the expression of photosynthesis and plant hormone-related genes (such as SAUR75, PQL2, PSBX, ERF1, GNC, GRF5, and ARF11). Taken together, our data indicate that PagSTOMAGEN could positively regulate stomatal density and increase the photosynthetic rate and plant hormone content, thereby promoting vegetative growth in poplar. Our study is of great significance for understanding the relationship between stoma, photosynthesis, and yield breeding in poplar.

Properties of Photosystem II lacking the PsbJ subunit

Photosystem II (PSII), the oxygen-evolving enzyme, consists of 17 trans-membrane and 3 extrinsic membrane proteins. Other subunits bind to PSII during assembly, like Psb27, Psb28, and Tsl0063. The presence of Psb27 has been proposed (Zabret et al. in Nat Plants 7:524-538, 2021; Huang et al. Proc Natl Acad Sci USA 118:e2018053118, 2021; Xiao et al. in Nat Plants 7:1132-1142, 2021) to prevent the binding of PsbJ, a single transmembrane α-helix close to the quinone Q_B binding site. Consequently, a PSII rid of Psb27, Psb28, and Tsl0034 prior to the binding of PsbJ would logically correspond to an assembly intermediate. The present work describes experiments aiming at further characterizing such a ∆PsbJ-PSII, purified from the thermophilic Thermosynechococcus elongatus, by means of MALDI-TOF spectroscopy, thermoluminescence, EPR spectroscopy, and UV-visible time-resolved spectroscopy. In the purified ∆PsbJ-PSII, an active Mn₄CaO₅ cluster is present in 60-70% of the centers. In these centers, although the forward electron transfer seems not affected, the Em of the Q_B/Q_B^- couple increases by ≥ 120 mV , thus disfavoring the electron coming back on Q_A. The increase of the energy gap between Q_A/Q_A^- and Q_B/Q_B^- could contribute in a protection against the charge recombination between the donor side and Q_B^-, identified at the origin of photoinhibition under low light (Keren et al. in Proc Natl Acad Sci USA 94:1579-1584, 1997), and possibly during the slow photoactivation process.

PsbX maintains efficient electron transport in Photosystem II and reduces susceptibility to high light in Synechocystis sp. PCC 6803

PsbX is a 4.1 kDa intrinsic Photosystem II (PS II) protein, found together with the low-molecular-weight proteins, PsbY and PsbJ, in proximity to cytochrome b₅₅₉. The function of PsbX is not yet fully characterized but PsbX may play a role in the exchange of the secondary plastoquinone electron acceptor Q_B with the quinone pool in the thylakoid membrane. To study the role of PsbX, we have constructed a PsbX-lacking strain of Synechocystis sp. PCC 6803. Our studies indicate that the absence of PsbX causes sensitivity to high light and impairs electron transport within PS II. In addition to a change in the Q_B-binding pocket, PsbX-lacking cells exhibited sensitivity to sodium formate, suggesting altered binding of the bicarbonate ligand to the non-heme iron between the sequential plastoquinone electron acceptors Q_A and Q_B. Experiments using ³⁵S-methionine revealed high-light-treated PsbX-lacking cells restore PS II activity during recovery under low light by an increase in the turnover of PS II-associated core proteins. These labeling experiments indicate the recovery after exposure to high light requires both selective removal and replacement of the D1 protein and de novo PS II assembly.

Neoxanthin affects the stability of the C2 S2 M2 -type photosystem II supercomplexes and the kinetics of state transition in Arabidopsis

Neoxanthin (Neo), which is only bound to the peripheral antenna proteins of photosystem (PS) II, is a conserved carotenoid in all green plants. It has been demonstrated that Neo plays an important role in photoprotection and its deficiency fails to impact LHCII stability in vitro and indoor plant growth in vivo. Whether Neo is involved in maintaining the PSII complex structure or adaptive mechanisms for the everchanging environment has not yet been elucidated. In this study, the role of Neo in maintaining the structure and function of the PSII-LHCII supercomplexes was studied using Neo deficient Arabidopsis mutants. Our results show that Neo deficiency had little effect on the electron transport capacity and the plant fitness, but the PSII-LHCII supercomplexes were significantly impacted by the lack of Neo. In the absence of Neo, the M-type LHCII trimer cannot effectively associate with the C₂ S₂ -type PSII-LHCII supercomplexes even in moderate light conditions. Interestingly, Neo deficiency also leads to decreased PSII protein phosphorylation but rapid transition from state 1 to state 2. We suggest that Neo might enforce the interactions between LHCII and the minor antennas and that the absence of Neo makes M-type LHCII disassociate from the PSII complex, leading to the disassembly of the PSII-LHCII C₂ S₂ M₂ supercomplexes, which results in alterations in the phosphorylation patterns of the thylakoid photosynthetic proteins and the kinetics of state transition.

Genetic dissection of the gene coexpression network underlying photosynthesis in Populus

Photosynthesis is a key reaction that ultimately generates the carbohydrates needed to form woody tissues in trees. However, the genetic regulatory network of protein-encoding genes (PEGs) and regulatory noncoding RNAs (ncRNAs), including microRNAs (miRNAs) and long noncoding RNAs (lncRNAs), underlying the photosynthetic pathway is unknown. Here, we integrated data from coexpression analysis, association studies (additive, dominance and epistasis), and expression quantitative trait nucleotide (eQTN) mapping to dissect the causal variants and genetic interaction network underlying photosynthesis in Populus. We initially used 30 PEGs, 6 miRNAs and 12 lncRNAs to construct a coexpression network based on the tissue-specific gene expression profiles of 15 Populus samples. Then, we performed association studies using a natural population of 435 unrelated Populus tomentosa individuals, and identified 72 significant associations (P ≤ 0.001, q ≤ 0.05) with diverse additive and dominance patterns underlying photosynthesis-related traits. Analysis of epistasis and eQTNs revealed that the complex genetic interactions in the coexpression network contribute to phenotypes at various levels. Finally, we demonstrated that heterologously expressing the most highly linked gene (PtoPsbX1) in this network significantly improved photosynthesis in Arabidopsis thaliana, pointing to the functional role of PtoPsbX1 in the photosynthetic pathway. This study provides an integrated strategy for dissecting a complex genetic interaction network, which should accelerate marker-assisted breeding efforts to genetically improve woody plants.

Energy conversion processes and related gene expression in a sunflower mutant with altered salicylic acid metabolism

Salicylic acid (SA) is involved in several responses associated with plant development and defence against biotic and abiotic stress, but its role on photosynthetic regulation is still under debate. This work investigated energy conversion processes and related gene expression in the brachytic mutant of sunflower lingering hope (linho). This mutant was characterized by a higher ratio between the free SA form and its conjugate form SA O-β-D-glucoside (SAG) compared to wild type (WT), without significant changes in the endogenous level of abscisic acid and hydrogen peroxide. The mutant showed an inhibition of photosynthesis due to a combination of both stomatal and non-stomatal limitations, although the latter seemed to play a major role. The reduced carboxylation efficiency was associated with a down-regulation of the gene expression for both the large and small subunits of Rubisco and the Rubisco activase enzyme. Moreover, linho showed an alteration of photosystem II (PSII) functionality, with reduced PSII photochemistry, increased PSII excitation pressure and decreased thermal energy dissipation of excessive light energy. These responses were associated with a lower photosynthetic pigments concentration and a reduced expression of genes encoding for light-harvesting chlorophyll a/b binding proteins (i.e. HaLhcA), chlorophyll binding subunits of PSII proteins (i.e. HaPsbS and HaPsbX), phytoene synthase enzyme and a different expression level for genes related to PSII repair cycle, such as HaPsbA and HaPsbD. The concomitant stimulation of respiratory metabolism, suggests that linho activated a coordinate modulation of chloroplast and mitochondria activities to compensate the energy imbalance and regulate energy conversion processes.

A thylakoid membrane-bound and redox-active rubredoxin (RBD1) functions in de novo assembly and repair of photosystem II

Photosystem II (PSII) catalyzes the light-driven oxidation of water in photosynthesis, supplying energy and oxygen to many life-forms on earth. During PSII assembly and repair, PSII intermediate complexes are prone to photooxidative damage, requiring mechanisms to minimize this damage. Here, we report the functional characterization of RBD1, a PSII assembly factor that interacts with PSII intermediate complexes to ensure their functional assembly and repair. We propose that the redox activity of RBD1 participates together with the cytochrome b₅₅₉ to protect PSII from photooxidation. This work not only improves our understanding of cellular protection mechanisms for the vital PSII complex but also informs genetic engineering strategies for protection of PSII repair to increase agricultural productivity.

Photosystem II (PSII) undergoes frequent photooxidative damage that, if not repaired, impairs photosynthetic activity and growth. How photosynthetic organisms protect vulnerable PSII intermediate complexes during de novo assembly and repair remains poorly understood. Here, we report the genetic and biochemical characterization of chloroplast-located rubredoxin 1 (RBD1), a PSII assembly factor containing a redox-active rubredoxin domain and a single C-terminal transmembrane α-helix (TMH) domain. RBD1 is an integral thylakoid membrane protein that is enriched in stroma lamellae fractions with the rubredoxin domain exposed on the stromal side. RBD1 also interacts with PSII intermediate complexes containing cytochrome b₅₅₉. Complementation of the Chlamydomonas reinhardtii (hereafter Chlamydomonas) RBD1-deficient 2pac mutant with constructs encoding RBD1 protein truncations and site-directed mutations demonstrated that the TMH domain is essential for de novo PSII assembly, whereas the rubredoxin domain is involved in PSII repair. The rubredoxin domain exhibits a redox midpoint potential of +114 mV and is proficient in 1-electron transfers to a surrogate cytochrome c in vitro. Reduction of oxidized RBD1 is NADPH dependent and can be mediated by ferredoxin-NADP⁺ reductase (FNR) in vitro. We propose that RBD1 participates, together with the cytochrome b₅₅₉, in the protection of PSII intermediate complexes from photooxidative damage during de novo assembly and repair. This role of RBD1 is consistent with its evolutionary conservation among photosynthetic organisms and the fact that it is essential in photosynthetic eukaryotes.

The roles of Arabidopsis proteins of Lhcb4, Lhcb5 and Lhcb6 in oxidative stress under natural light conditions

Under light conditions, highly reactive oxygen species (ROS) can be generated in the antenna systems and the reaction center of photosystems (PS). The protective roles of Lhcb4 (CP29), Lhcb5 (CP26) and Lhcb6 (CP24), three minor chlorophyll binding antenna proteins during photoinhibition have been well studied. However, their regulatory mechanisms against oxidative damages under natural light conditions remain unknown. Here we investigated their specific roles in oxidative stress responses and photosynthetic adaptation by using the Arabidopsis thaliana knockout lines grown in the field condition. All three mutant lines exhibited decreased energy-transfer efficiency from the LHCII (light-harvesting complex II) to the PSII reaction center. Oxygen evolution capacity decreased slightly in the plants lacking Lhcb4 (koLHCB4) and Lhcb6 (koLHCB6). Photosynthetic rates and fitness for the plants lacking Lhcb5 (koLHCB5) or koLHCB6 grown in the field were affected, but not in the plants lacking Lhcb4. Antioxidant analysis indicated the lowest antioxidant enzyme activities and the lowest levels of non-enzymatic antioxidants in koLHCB6 plants. In addition, koLHCB6 plants accumulated much higher levels of superoxide and hydrogen, and suffered more severe oxidative-damages in the field. Our results clearly demonstrate that Lhcb6 may be involved in alleviating oxidative stress and photoprotection under natural conditions.

Terrestrial Plants Evolve Highly Assembled Photosystem Complexes in Adaptation to Light Shifts

It has been known that PSI and PSII supercomplexes are involved in the linear and cyclic electron transfer, dynamics of light capture, and the repair cycle of PSII under environmental stresses. However, evolutions of photosystem (PS) complexes from evolutionarily divergent species are largely unknown. Here, we improved the blue native polyacrylamide gel electrophoresis (BN-PAGE) separation method and successfully separated PS complexes from all terrestrial plants. It is well known that reversible D1 protein phosphorylation is an important protective mechanism against oxidative damages to chloroplasts through the PSII photoinhibition-repair cycle. The results indicate that antibody-detectable phosphorylation of D1 protein is the latest event in the evolution of PS protein phosphorylation and occurs exclusively in seed plants. Compared to angiosperms, other terrestrial plant species presented much lower contents of PS supercomplexes. The amount of light-harvesting complexes II (LHCII) trimers was higher than that of LHCII monomers in angiosperms, whereas it was opposite in gymnosperms, pteridophytes, and bryophytes. LHCII assembly may be one of the evolutionary characteristics of vascular plants. In vivo chloroplast fluorescence measurements indicated that lower plants (bryophytes especially) showed slower changes in state transition and nonphotochemical quenching (NPQ) in response to light shifts. Therefore, the evolution of PS supercomplexes may be correlated with their acclimations to environments.

Sub-high Temperature and High Light Intensity Induced Irreversible Inhibition on Photosynthesis System of Tomato Plant (Solanum lycopersicum L.)

High temperature and high light intensity is a common environment posing a great risk to organisms. This study aimed to elucidate the effects of sub-high temperature and high light intensity stress (HH, 35°C, 1000 μmol⋅m^-2⋅s^-1) and recovery on the photosynthetic mechanism, photoinhibiton of photosystem II (PSII) and photosystem I (PSI), and reactive oxygen (ROS) metabolism of tomato seedlings. The results showed that with prolonged stress time, net photosynthetic rate (Pn), Rubisco activity, maximal photochemistry efficiency (Fv/Fm), efficient quantum yield and electron transport of PSII [Y(II) and ETR(II)] and PSI [Y(I) and ETR(I)] decreased significantly whereas yield of non-regulated and regulated energy dissipation of PSII [Y(NO) and Y(NPQ)] increased sharply. The donor side limitation of PSI [Y(ND)] increased but the acceptor side limitation of PSI [Y(NA)] decreased. Content of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) were increased while activity of superoxide dismutase (SOD) and peroxidase (POD) were significantly inhibited compared with control. HH exposure affected photosynthetic carbon assimilation, multiple sites in PSII and PSI, ROS accumulation and elimination of Solanum lycopersicum L.

Comparison of methods for extracting thylakoid membranes of Arabidopsis plants

Robust and reproducible methods for extracting thylakoid membranes are required for the analysis of photosynthetic processes in higher plants such as Arabidopsis. Here, we compare three methods for thylakoid extraction using two different buffers. Method I involves homogenizing the plant material with a metal/glass blender; method II involves manually grinding the plant material in ice-cold grinding buffer with a mortar and method III entails snap-freezing followed by manual grinding with a mortar, after which the frozen powder is thawed in isolation buffer. Thylakoid membrane samples extracted using each method were analyzed with respect to protein and chlorophyll content, yields relative to starting material, oxygen-evolving activity, protein complex content and phosphorylation. We also examined how the use of fresh and frozen thylakoid material affected the extracts' contents of protein complexes. The use of different extraction buffers did not significantly alter the protein content of the extracts in any case. Method I yielded thylakoid membranes with the highest purity and oxygen-evolving activity. Method III used low amounts of starting material and was capable of capturing rapid phosphorylation changes in the sample at the cost of higher levels of contamination. Method II yielded thylakoid membrane extracts with properties intermediate between those obtained with the other two methods. Finally, frozen and freshly isolated thylakoid membranes performed identically in blue native-polyacrylamide gel electrophoresis experiments conducted in order to separate multimeric protein supracomplexes.

Dark-adapted spinach thylakoid protein heterogeneity offers insights into the photosystem II repair cycle

In higher plants, thylakoid membrane protein complexes show lateral heterogeneity in their distribution: photosystem (PS) II complexes are mostly located in grana stacks, whereas PSI and adenosine triphosphate (ATP) synthase are mostly found in the stroma-exposed thylakoids. However, recent research has revealed strong dynamics in distribution of photosystems and their light harvesting antenna along the thylakoid membrane. Here, the dark-adapted spinach (Spinacia oleracea L.) thylakoid network was mechanically fragmented and the composition of distinct PSII-related proteins in various thylakoid subdomains was analyzed in order to get more insights into the composition and localization of various PSII subcomplexes and auxiliary proteins during the PSII repair cycle. Most of the PSII subunits followed rather equal distribution with roughly 70% of the proteins located collectively in the grana thylakoids and grana margins; however, the low molecular mass subunits PsbW and PsbX as well as the PsbS proteins were found to be more exclusively located in grana thylakoids. The auxiliary proteins assisting in repair cycle of PSII were mostly located in stroma-exposed thylakoids, with the exception of THYLAKOID LUMEN PROTEIN OF 18.3 (TLP18.3), which was more evenly distributed between the grana and stroma thylakoids. The TL29 protein was present exclusively in grana thylakoids. Intriguingly, PROTON GRADIENT REGULATION5 (PGR5) was found to be distributed quite evenly between grana and stroma thylakoids, whereas PGR5-LIKE PHOTOSYNTHETIC PHENOTYPE1 (PGRL1) was highly enriched in the stroma thylakoids and practically missing from the grana cores. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.

Antisense reductions in the PsbO protein of photosystem II leads to decreased quantum yield but similar maximal photosynthetic rates

Photosystem (PS) II is the multisubunit complex which uses light energy to split water, providing the reducing equivalents needed for photosynthesis. The complex is susceptible to damage from environmental stresses such as excess excitation energy and high temperature. This research investigated the in vivo photosynthetic consequences of impairments to PSII in Arabidopsis thaliana (ecotype Columbia) expressing an antisense construct to the PsbO proteins of PSII. Transgenic lines were obtained with between 25 and 60% of wild-type (WT) total PsbO protein content, with the PsbO1 isoform being more strongly reduced than PsbO2. These changes coincided with a decrease in functional PSII content. Low PsbO (less than 50% WT) plants grew more slowly and had lower chlorophyll content per leaf area. There was no change in content per unit area of cytochrome b6f, ATP synthase, or Rubisco, whereas PSI decreased in proportion to the reduction in chlorophyll content. The irradiance response of photosynthetic oxygen evolution showed that low PsbO plants had a reduced quantum yield, but matched the oxygen evolution rates of WT plants at saturating irradiance. It is suggested that these plants had a smaller pool of PSII centres, which are inefficiently connected to antenna pigments resulting in reduced photochemical efficiency.

Photosystem II, a growing complex: updates on newly discovered components and low molecular mass proteins

Photosystem II is a unique complex capable of absorbing light and splitting water. The complex has been thoroughly studied and to date there are more than 40 proteins identified, which bind to the complex either stably or transiently. Another special feature of this complex is the unusually high content of low molecular mass proteins that represent more than half of the proteins. In this review we summarize the recent findings on the low molecular mass proteins (<15kDa) and present an overview of the newly identified components as well. We have also performed co-expression analysis of the genes encoding PSII proteins to see if the low molecular mass proteins form a specific sub-group within the Photosystem II complex. Interestingly we found that the chloroplast-localized genes encoding PSII proteins display a different response to environmental and stress conditions compared to the nuclear localized genes. This article is part of a Special Issue entitled: Photosystem II.

The PsbW protein stabilizes the supramolecular organization of photosystem II in higher plants

PsbW, a 6.1-kDa low-molecular-weight protein, is exclusive to photosynthetic eukaryotes, and associates with the photosystem II (PSII) protein complex. In vivo and in vitro comparison of Arabidopsis thaliana wild-type plants with T-DNA insertion knock-out mutants completely lacking the PsbW protein, or with antisense inhibition plants exhibiting decreased levels of PsbW, demonstrated that the loss of PsbW destabilizes the supramolecular organization of PSII. No PSII-LHCII supercomplexes could be detected or isolated in the absence of the PsbW protein. These changes in macro-organization were accompanied by a minor decrease in the chlorophyll fluorescence parameter F(V) /F(M) , a strongly decreased PSII core protein phosphorylation and a modification of the redox state of the plastoquinone (PQ) pool in dark-adapted leaves. In addition, the absence of PsbW protein led to faster redox changes in the PQ pool, i.e. transitions from state 1 to state 2, as measured by changes in stationary fluorescence (F(S) ) kinetics, compared with the wild type. Despite these dramatic effects on macromolecular structure, the transgenic plants exhibited no significant phenotype under normal growth conditions. We suggest that the PsbW protein is located close to the minor antenna of the PSII complex, and is important for the contact and stability between several PSII-LHCII supercomplexes.

The Mg-chelatase H subunit of Arabidopsis antagonizes a group of WRKY transcription repressors to relieve ABA-responsive genes of inhibition

The phytohormone abscisic acid (ABA) plays a vital role in plant development and response to environmental challenges, but the complex networks of ABA signaling pathways are poorly understood. We previously reported that a chloroplast protein, the magnesium-protoporphyrin IX chelatase H subunit (CHLH/ABAR), functions as a receptor for ABA in Arabidopsis thaliana. Here, we report that ABAR spans the chloroplast envelope and that the cytosolic C terminus of ABAR interacts with a group of WRKY transcription factors (WRKY40, WRKY18, and WRKY60) that function as negative regulators of ABA signaling in seed germination and postgermination growth. WRKY40, a central negative regulator, inhibits expression of ABA-responsive genes, such as ABI5. In response to a high level of ABA signal that recruits WRKY40 from the nucleus to the cytosol and promotes ABAR-WRKY40 interaction, ABAR relieves the ABI5 gene of inhibition by repressing WRKY40 expression. These findings describe a unique ABA signaling pathway from the early signaling events to downstream gene expression.