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

Glycogen synthesis prevents metabolic imbalance and disruption of photosynthetic electron transport from photosystem II during transition to photomixotrophy in Synechocystis sp. PCC 6803

Some cyanobacteria can grow photoautotrophically or photomixotrophically by using simultaneously CO2 and glucose. The switch between these trophic modes and the role of glycogen, their main carbon storage macromolecule, was investigated. We analysed the effect of glucose addition on the physiology, metabolic and photosynthetic state of Synechocystis sp. PCC 6803 and mutants lacking phosphoglucomutase and ADP-glucose pyrophosphorylase, with limitations in glycogen synthesis. Glycogen acted as a metabolic buffer: glucose addition increased growth and glycogen reserves in the wild-type (WT), but arrested growth in the glycogen synthesis mutants. Already 30 min after glucose addition, metabolites from the Calvin-Benson-Bassham cycle and the oxidative pentose phosphate shunt increased threefold more in the glycogen synthesis mutants than the WT. These alterations substantially affected the photosynthetic performance of the glycogen synthesis mutants, as O2 evolution and CO2 uptake were both impaired. We conclude that glycogen synthesis is essential during transitions to photomixotrophy to avoid metabolic imbalance that induces inhibition of electron transfer from PSII and subsequently accumulation of reactive oxygen species, loss of PSII core proteins, and cell death. Our study lays foundations for optimising photomixotrophy-based biotechnologies through understanding the coordination of the crosstalk between photosynthetic electron transport and metabolism.

Indirect interactions involving the PsbM or PsbT subunits and the PsbO, PsbU and PsbV proteins stabilize assembly and activity of Photosystem II in Synechocystis sp. PCC 6803

The low-molecular-weight PsbM and PsbT proteins of Photosystem II (PS II) are both located at the monomer-monomer interface of the mature PS II dimer. Since the extrinsic proteins are associated with the final step of assembly of an active PS II monomer and, in the case of PsbO, are known to impact the stability of the PS II dimer, we have investigated the potential cooperativity between the PsbM and PsbT subunits and the PsbO, PsbU and PsbV extrinsic proteins. Blue-native polyacrylamide electrophoresis and western blotting detected stable PS II monomers in the ∆PsbM:∆PsbO and ∆PsbT:∆PsbO mutants that retained sufficient oxygen-evolving activity to support reduced photoautotrophic growth. In contrast, the ∆PsbM:∆PsbU and ∆PsbT:∆PsbU mutants assembled dimeric PS II at levels comparable to wild type and supported photoautotrophic growth at rates similar to those obtained with the corresponding ∆PsbM and ∆PsbT cells. Removal of PsbV was more detrimental than removal of PsbO. Only limited levels of dimeric PS II were observed in the ∆PsbM:∆PsbV mutant and the overall reduced level of assembled PS II in this mutant resulted in diminished rates of photoautotrophic growth and PS II activity below those obtained in the ∆PsbM:∆PsbO and ∆PsbT:∆PsbO strains. In addition, the ∆PsbT:∆PsbV mutant did not assemble active PS II centers although inactive monomers could be detected. The inability of the ∆PsbT:∆PsbV mutant to grow photoautotrophically, or to evolve oxygen, suggested a stable oxygen-evolving complex could not assemble in this mutant.

Transcriptomic and physiological analyses of Trichoderma citrinoviride HT-1 assisted phytoremediation of Cd contaminated water by Phragmites australis

Plant growth promoting microbe assisted phytoremediation is considered a more effective approach to rehabilitation than the single use of plants, but underlying mechanism is still unclear. In this study, we combined transcriptomic and physiological methods to explore the mechanism of plant growth promoting microbe Trichoderma citrinoviride HT-1 assisted phytoremediation of Cd contaminated water by Phragmites australis. The results show that the strain HT-1 significantly promoted P. australis growth, increased the photosynthetic rate, enhanced antioxidant enzyme activities. The chlorophyll content and the activity of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) were increased by 83.78%, 23.17%, 47.60%, 97.14% and 12.23% on average, and decreased the content of malondialdehyde (MDA) by 31.10%. At the same time, strain HT-1 improved the absorption and transport of Cd in P. australis, and the removal rate of Cd was increased by 7.56% on average. Transcriptome analysis showed that strain HT-1 induced significant up-regulated the expression of genes related to oxidative phosphorylation and ribosome pathways, and these upregulated genes promoted P. australis remediation efficiency and resistance to Cd stress. Our results provide a mechanistic understanding of plant growth promoting microbe assisted phytoremediation under Cd stress.

Proteomic and physiological responses of contrasting two different heat-resistant orchardgrass genotypes to heat stress

As an important forage crop worldwide, the growth and productivity of orchardgrass are greatly impacted by high temperatures. However, little information is known about how orchardgrass proteomic changes under heat conditions. Therefore, the present study investigated the proteomics and physiological changes in 667 [AKZ-NRGR667 (heat-tolerant)] and 7602 [PI237602 (heat-sensitive)] under heat stress (40/35 °C). In addition, the responses of translational regulating of heat stress in orchardgrass were analyzed through proteomic changes using the tandem mass tags (TMT) technique. Together, 410 differentially expressed proteins (DEPs) were identified from two orchardgrass genotypes under heat at 24 h. Proteomics analyses indicated that proteins related to substance metabolism, photosynthesis, and heat shock proteins (HSPs) were differentially expressed under heat stress and control conditions. Moreover, a large proportion of HSPs were expressed in the heat-tolerant genotype as compared to the heat-sensitive genotype. In conclusion, genotype 667 has higher adaptability and repairing capability due to stronger heat tolerance capacity that can make it more suited to sustaining its survival and growth than genotype 7602. These findings can provide the basis for genetic improvements in orchardgrass and other crops facing high-temperature stress or heat environment that may lead to heat resistance or tolerance.

Solar energy conversion by photosystem II: principles and structures

Photosynthetic water oxidation by Photosystem II (PSII) is a fascinating process because it sustains life on Earth and serves as a blue print for scalable synthetic catalysts required for renewable energy applications. The biophysical, computational, and structural description of this process, which started more than 50 years ago, has made tremendous progress over the past two decades, with its high-resolution crystal structures being available not only of the dark-stable state of PSII, but of all the semi-stable reaction intermediates and even some transient states. Here, we summarize the current knowledge on PSII with emphasis on the basic principles that govern the conversion of light energy to chemical energy in PSII, as well as on the illustration of the molecular structures that enable these reactions. The important remaining questions regarding the mechanism of biological water oxidation are highlighted, and one possible pathway for this fundamental reaction is described at a molecular level.

Million-atom molecular dynamics simulations reveal the interfacial interactions and assembly of plant PSII-LHCII supercomplex

Protein-protein interface interactions dictate efficient excitation energy transfer from light-harvesting antennas to the photosystem II (PSII) core. In this work, we construct a 1.2 million atom-scale model of plant C₂S₂-type PSII-LHCII supercomplex and perform microsecond-scale molecular dynamics (MD) simulations to explore the interactions and assembly mechanisms of the sizeable PSII-LHCII supercomplex. We optimize the nonbonding interactions of the PSII-LHCII cryo-EM structure using microsecond-scale MD simulations. Binding free energy calculations with component decompositions reveal that hydrophobic interactions predominantly drive antenna-core association and the antenna-antenna interactions are relatively weak. Despite the positive electrostatic interaction energies, hydrogen bonds and salt bridges mainly provide directional or anchoring forces for interface binding. Analysis of the roles of small intrinsic subunits of PSII suggests that LHCII and CP26 first interact with small intrinsic subunits and then bind to the core proteins, whereas CP29 adopts a one-step binding process to the PSII core without the assistance of other factors. Our study provides insights into the molecular underpinnings of the self-organization and regulation of plant PSII-LHCII. It lays the framework for deciphering the general assembly principles of photosynthetic supercomplexes and possibly other macromolecular structures. The finding also has implications for repurposing photosynthetic systems to enhance photosynthesis.

A novel protein domain is important for photosystem II complex assembly and photoautotrophic growth in angiosperms

Photosystem II (PSII) is a multi-subunit protein complex of the photosynthetic electron transport chain that is vital to photosynthesis. Although the structure, composition, and function of PSII have been extensively studied, its biogenesis mechanism remains less understood. Thylakoid rhodanese-like (TROL) provides an anchor for leaf-type ferredoxin:NADP+ oxidoreductase. Here, we report the chacterizaton of a second type of TROL protein, TROL2, encoded by seed plant genomes whose function has not previously been reported. We show that TROL2 is a PSII assembly cofactor with essential roles in the establishment of photoautotrophy. TROL2 contains a 45-amino-acid domain, termed the chlorotic lethal seedling (CLS) domain, that is both necessary and sufficient for TROL2 function in PSII assembly and photoautotrophic growth. Phylogenetic analyses suggest that TROL2 may have arisen from ancestral TROL1 via gene duplication before the emergence of seed plants and acquired the CLS domain via evolution of the sequence encoding its N-terminal portion. We further reveal that TROL2 (or CLS) forms an assembly cofactor complex with the intrinsic thylakoid membrane protein LOW PSII ACCUMULATION2 and interacts with small PSII subunits to facilitate PSII complex assembly. Collectively, our study not only shows that TROL2 (CLS) is essential for photoautotrophy in angiosperms but also reveals its mechanistic role in PSII complex assembly, shedding light on the molecular and evolutionary mechanisms of photosynthetic complex assemblyin angiosperms.

Effect of shading on physiological attributes and comparative transcriptome analysis of Camellia sinensis cultivar reveals tolerance mechanisms to low temperatures

Tea is a vital beverage crop all over the world, including in China. Low temperatures restrict its growth, development, and terrestrial distribution, and cold event variability worsens cold damage. However, the physiological and molecular mechanisms of Camellia sinensis under shade in winter remain unclear. In our study, tea leaves were utilized for physiological attributes and transcriptome analysis in November and December in three shading groups and no-shade control plants. When compared to the no-shade control plants, the shading group protected tea leaves from cold damage, increased photochemical efficiency (Fv/Fm) and soil plant analysis development (SPAD), and sustained chlorophyll a, chlorophyll b, chlorophyll, and carotenoid contents by physiological mean. Then, transcriptome analysis revealed 20,807 differentially expressed genes (DEGs) and transcription factors (TFs) in November and December. A comparative study of transcriptome resulted in 3,523 DEGs and many TFs under SD0% vs. SD30%, SD0% vs. SD60%, and SD0% vs. SD75% of shading in November and December. Statistically, 114 DEGs were downregulated and 72 were upregulated under SD0% vs. SD30%. SD0% vs. SD60% resulted in 154 DEGs, with 60 downregulated and 94 upregulated. Similarly, there were 505 DEGs of which 244 were downregulated and 263 were upregulated under SD0% vs. SD75% of shading throughout November. However, 279 DEGs were downregulated and 105 were upregulated under SD0% vs. SD30%. SD0% vs. SD60% resulted in 296 DEGs, with 172 downregulated and 124 upregulated. Finally, 2,173 DEGs were regulated in December, with 1,428 downregulated and 745 upregulated under SD0% vs. SD75%. These indicate that the number of downregulated DEGs in December was higher than the number of upregulated DEGs in November during low temperatures. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses of differentially expressed genes were highly regulated in the photosynthesis, plant hormone signal transduction, and mitogen-activated protein kinase (MAPK) signaling pathways. However, qRT-PCR and RNA-seq relative expression of photosynthetic (DEGs) Lhcb2 in both November and December, plant hormone (DEGs) BRI1 and JAZ in November and IAA and ERF1 in December, and key DEGs of MAPK signal transduction FLS2, CHIB, and MPK4 in November and RBOH, MKK4_5, and MEKK1 in December in three shading groups and no-shade control plants responded to tea cold tolerance. The enhanced expression of light-harvesting photosystem I gene Lhca5, light-harvesting photosystem II gene Lhcb2, and mitogen-activated protein kinases MEKK1 and MPK4/6 enhance the cold-tolerance mechanism of C. sinensis. These comprehensive transcriptomic findings are significant for furthering our understanding of the genes and underlying regulatory mechanisms of shade-mediated low-temperature stress tolerance in horticultural crops.

Structure of the Acidobacteria homodimeric reaction center bound with cytochrome c

Photosynthesis converts light energy to chemical energy to fuel life on earth. Light energy is harvested by antenna pigments and transferred to reaction centers (RCs) to drive the electron transfer (ET) reactions. Here, we present cryo-electron microscopy (cryo-EM) structures of two forms of the RC from the microaerophilic Chloracidobacterium thermophilum (CabRC): one containing 10 subunits, including two different cytochromes; and the other possessing two additional subunits, PscB and PscZ. The larger form contained 2 Zn-bacteriochlorophylls, 16 bacteriochlorophylls, 10 chlorophylls, 2 lycopenes, 2 hemes, 3 Fe₄S₄ clusters, 12 lipids, 2 Ca²⁺ ions and 6 water molecules, revealing a type I RC with an ET chain involving two hemes and a hybrid antenna containing bacteriochlorophylls and chlorophylls. Our results provide a structural basis for understanding the excitation energy and ET within the CabRC and offer evolutionary insights into the origin and adaptation of photosynthetic RCs.

ScCBF1 plays a stronger role in cold, salt and drought tolerance than StCBF1 in potato (Solanum tuberosum)

Solanum tuberosum (St) and Solanum commersonii (Sc) are two potato varieties with different freezing tolerance. Among them, St is a freezing-sensitive variety and. Sc is a cold-resistant wild potato. CBF/DREB family members mainly function in response to freezing stress. In order to explore the different roles of St C-Repeat Binding Factor1 (StCBF1) and Sc C-Repeat Binding Factor1 (ScCBF1) in potato plants (Solanum tuberosum) under stress conditions, two kinds of potato lines were obtained with ScCBF1 and StCBF1 overexpressing respectively. Phenotypes analysis showed that both overexpressing ScCBF1 and StCBF1 caused smaller leaves, and reduced tuber yield. While the limited phenotypes of StCBF1 lines were more severe than that of ScCBF lines. After freezing treatment, StCBF1 over expression plants grown better than WT plants and worse than ScCBF1 over expression plants. Specifically, compared with wild-type lines, overexpressing ScCBF1 could up-regulate fatty acid desaturase genes, key enzyme of Calvin cycle genes, and antioxidant enzyme genes. Both ScCBF1 and StCBF1 lines showed higher PSII activity, thus maintaining a higher photosynthetic rate under cold stress. In addition, we also found that overexpression ScCBF1 and StCBF1 could also enhance the drought and salt tolerance in potato. In summary, ScCBF1 plays a stronger role in cold, salt, and drought tolerance than StCBF1 in potato (Solanum tuberosum).

Dynamic transcriptome and network-based analysis of yellow leaf mutant Ginkgo biloba

BACKGROUND

Golden leaf in autumn is a prominent feature of deciduous tree species like Ginkgo biloba L., a landscape tree widely cultivated worldwide. However, little was known about the molecular mechanisms of leaf yellowing, especially its dynamic regulatory network. Here, we performed a suite of comparative physiological and dynamic transcriptional analyses on the golden-leaf cultivar and the wild type (WT) ginkgo to investigate the underlying mechanisms of leaf yellowing across different seasons.

RESULTS

In the present study, we used the natural bud mutant cultivar with yellow leaves "Wannianjin" (YL) as materials. Physiological analysis revealed that higher ratios of chlorophyll a to chlorophyll b and carotenoid to chlorophyll b caused the leaf yellowing of YL. On the other hand, dynamic transcriptome analyses showed that genes related to chlorophyll metabolism played key a role in leaf coloration. Genes encoding non-yellow coloring 1 (NYC1), NYC1-like (NOL), and chlorophyllase (CLH) involved in the degradation of chlorophyll were up-regulated in spring. At the summer stage, down-regulated HEMA encoding glutamyl-tRNA reductase functioned in chlorophyll biosynthesis, while CLH involved in chlorophyll degradation was up-regulated, causing a lower chlorophyll accumulation. In carotenoid metabolism, genes encoding zeaxanthin epoxidase (ZEP) and 9-cis-epoxy carotenoid dioxygenase (NCED) showed significantly different expression levels in the WT and YL. Moreover, the weighted gene co-expression network analysis (WGCNA) suggested that the most associated transcriptional factor, which belongs to the AP2/ERF-ERF family, was engaged in regulating pigment metabolism. Furthermore, quantitative experiments validated the above results.

CONCLUSIONS

By comparing the golden-leaf cultivar and the wide type of ginkgo across three seasons, this study not only confirm the vital role of chlorophyll in leaf coloration of YL but also provided new insights into the seasonal transcriptome landscape and co-expression network. Our novel results pinpoint candidate genes for further wet-bench experiments in tree species.

Identification of Genes and Metabolic Pathways Involved in Resin Yield in Masson Pine by Integrative Analysis of Transcriptome, Proteome and Biochemical Characteristics

Masson pine (Pinus massoniana L.) is one of the most important resin-producing tree species in southern China. However, the molecular regulatory mechanisms of resin yield are still unclear in masson pine. In this study, an integrated analysis of transcriptome, proteome, and biochemical characteristics from needles of masson pine with the high and common resin yield was investigated. The results showed that chlorophyll a (Chl a), chlorophyll b (Chl b), total chlorophyll (Chl C), carotenoids (Car), glucose (Glu), gibberellin A9 (GA9), gibberellin A15 (GA15), and gibberellin A53 (GA53) were significantly increased, whereas fructose (Fru), jasmonic acid (JA), jasmonoyl-L-isoleucine (JA-ILE), gibberellin A1 (GA1), gibberellin A3 (GA3), gibberellin A19 (GA19), and gibberellin A24 (GA24) were significantly decreased in the high resin yield in comparison with those in the common one. The integrated analysis of transcriptome and proteome showed that chlorophyll synthase (chlG), hexokinase (HXK), sucrose synthase (SUS), phosphoglycerate kinase (PGK), dihydrolipoamide dehydrogenase (PDH), dihydrolipoamide succinyltransferase (DLST), 12-oxophytodienoic acid reductase (OPR), and jasmonate O-methyltransferases (JMT) were consistent at the transcriptomic, proteomic, and biochemical levels. The pathways of carbohydrate metabolism, terpenoid biosynthesis, photosynthesis, and hormone biosynthesis may play crucial roles in the regulation of resin yield, and some key genes involved in these pathways may be candidates that influence the resin yield. These results provide insights into the molecular regulatory mechanisms of resin yield and also provide candidate genes that can be applied for the molecular-assisted selection and breeding of high resin-yielding masson pine.

StATL2-like could affect growth and cold tolerance of plant by interacting with StCBFs

Our results confirmed that StATL2-like could interact with StCBFs and regulate plant growth. Meanwhile, StATL2-like acted as a negative regulator on low-temperature tolerance in plants. As important transcription factors for resisting many kinds of stresses, C-repeat-binding factors (CBF) play a key role in plant low-temperature tolerance by increasing COR genes expressions. Here, we report that StATL2-like, a RING-H2 E3 ubiquitin in Solanum tuberosum L., interacted with StCBF1 and StCBF4, respectively. AtATL2 is a highly homologous gene of StATL2-like in Arabidopsis thaliana. Under normal conditions, atl2 Arabidopsis mutant showed a growth inhibition phenotype while overexpressed StATL2-like in wild type Arabidopsis and atl2 mutant promoted plant growth. Besides, atl2 mutant had better low-temperature tolerance compared with wild type and StATL2-like transgenic lines which demonstrated that StATL2-like acted as a negatively regulator on low-temperature tolerance in plant. Moreover, atl2 mutant improved the scavenging capacity of reactive oxygen species (ROS) and alleviate the damage of photosynthetic system II (PSII) compared with StATL2-like transgenic lines under cold conditions. These results suggested a new component in CBF-dependent pathway to regulate plant growth and response to low-temperature stress in potato plants.

Integrated physiological, transcriptomic and metabolomic analysis of the response of Trifolium pratense L. to Pb toxicity

Lead (Pb) interferes with plant gene expression, alters metabolite contents and affects plant growth. However, the molecular mechanism underlying the plant response to Pb is not completely understood. In the present study, Trifolium pratense L. was exposed to Pb concentrations of 0 (Pb0), 500 (Pb500), 1000 (Pb1000), 2000 (Pb2000) and 3000 (Pb3000) mg/kg in soils. Pb stress affected the ability of T. pratense to accumulate and transport Pb, increased the activity of peroxidase (POD) and the contents of malondialdehyde (MDA) and proline, decreased the amount of photosynthetic pigments and soluble proteins, and led to changes in growth and biomass. Transcriptomic and metabolomic analyses showed that Pb mainly affected eight pathways, and LHC, flavonoids, organic acids, amino acids and carbohydrates were upregulated or downregulated. Moreover, Pb500 induced the upregulation of serA, promoted the synthesis of citric acid, maintained photosynthetic pigment levels, and ultimately promoted an increase in stem length. Pb3000 induced the upregulation of ARF, GH3 and SAUR genes, but the saccharide contents and stem length decreased in response to Pb stress. We used a variety of methods to provide a molecular perspective on the mechanism underlying the response of T. pratense to Pb stress.

Tyr244 of the D2 Protein Is Required for Correct Assembly and Operation of the Quinone-Iron-Bicarbonate Acceptor Complex of Photosystem II

Two plastoquinone electron acceptors, Q_A and Q_B, are present in Photosystem II (PS II) with their binding sites formed by the D2 and D1 proteins, respectively. A hexacoordinate non-heme iron is bound between Q_A and Q_B by D2 and D1, each providing two histidine ligands, and a bicarbonate that is stabilized via hydrogen bonds with D2-Tyr244 and D1-Tyr246. Both tyrosines and bicarbonate are conserved in oxygenic photosynthetic organisms but absent from the corresponding quinone-iron electron acceptor complex of anoxygenic photosynthetic bacteria. We investigated the role of D2-Tyr244 by introducing mutations in the cyanobacterium Synechocystis sp. PCC 6803. Alanine, histidine, and phenylalanine substitutions were introduced creating the Y244A, Y244H, and Y244F mutants. Electron transfer between Q_A and Q_B was impaired, the back-reaction with the S2 state of the oxygen-evolving complex was modified, and PS II assembly was disrupted, with the Y244A strain being more affected than the Y244F and Y244H mutants. The strains were also highly susceptible to photodamage in the presence of PS II-specific electron acceptors. Thermoluminescence and chlorophyll a fluorescence decay measurements indicated that the redox potential of the Q_A/Q_A^- couple became more positive in the Y244F and Y244H mutants, consistent with bicarbonate binding being impacted. The replacement of Tyr244 by alanine also led to an insertion of two amino acid repeats from Gln239 to Ala249 within the DE loop of D2, resulting in an inactive PS II complex that lacked PS II-specific variable fluorescence. The 66 bp insertion giving rise to the inserted amino acids therefore created an obligate photoheterotrophic mutant.

Effects of Salt Stress on Transcriptional and Physiological Responses in Barley Leaves with Contrasting Salt Tolerance

Salt stress negatively impacts crop production worldwide. Genetic diversity among barley (Hordeum vulgare) landraces adapted to adverse conditions should provide a valuable reservoir of tolerance genes for breeding programs. To identify molecular and biochemical differences between barley genotypes, transcriptomic and antioxidant enzyme profiles along with several morpho-physiological features were compared between salt-tolerant (Boulifa) and salt-sensitive (Testour) genotypes subjected to salt stress. Decreases in biomass, photosynthetic parameters, and relative water content were low in Boulifa compared to Testour. Boulifa had better antioxidant protection against salt stress than Testour, with greater antioxidant enzymes activities including catalase, superoxide dismutase, and guaiacol peroxidase. Transcriptome assembly for both genotypes revealed greater accumulation of differentially expressed transcripts in Testour compared to Boulifa, emphasizing the elevated transcriptional response in Testour following salt exposure. Various salt-responsive genes, including the antioxidant catalase 3, the osmoprotectant betaine aldehyde dehydrogenase 2, and the transcription factors MYB20 and MYB41, were induced only in Boulifa. By contrast, several genes associated with photosystems I and II, and light receptor chlorophylls A and B, were more repressed in Testour. Co-expression network analysis identified specific gene modules correlating with differences in genotypes and morpho-physiological traits. Overall, salinity-induced differential transcript accumulation underlies the differential morpho-physiological response in both genotypes and could be important for breeding salt tolerance in barley.

Advances in the Understanding of the Lifecycle of Photosystem II

Photosystem II is a light-driven water-plastoquinone oxidoreductase present in cyanobacteria, algae and plants. It produces molecular oxygen and protons to drive ATP synthesis, fueling life on Earth. As a multi-subunit membrane-protein-pigment complex, Photosystem II undergoes a dynamic cycle of synthesis, damage, and repair known as the Photosystem II lifecycle, to maintain a high level of photosynthetic activity at the cellular level. Cyanobacteria, oxygenic photosynthetic bacteria, are frequently used as model organisms to study oxygenic photosynthetic processes due to their ease of growth and genetic manipulation. The cyanobacterial PSII structure and function have been well-characterized, but its lifecycle is under active investigation. In this review, advances in studying the lifecycle of Photosystem II in cyanobacteria will be discussed, with a particular emphasis on new structural findings enabled by cryo-electron microscopy. These structural findings complement a rich and growing body of biochemical and molecular biology research into Photosystem II assembly and repair.

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.

Complexome profiling on the Chlamydomonas lpa2 mutant reveals insights into PSII biogenesis and new PSII associated proteins

While the composition and function of the major thylakoid membrane complexes are well understood, comparatively little is known about their biogenesis. The goal of this work was to shed more light on the role of auxiliary factors in the biogenesis of photosystem II (PSII). Here we have identified the homolog of LOW PSII ACCUMULATION 2 (LPA2) in Chlamydomonas. A Chlamydomonas reinhardtii lpa2 mutant grew slower in low light, was hypersensitive to high light, and exhibited aberrant structures in thylakoid membrane stacks. Chlorophyll fluorescence (Fv/Fm) was reduced by 38%. Synthesis and stability of newly made PSII core subunits D1, D2, CP43, and CP47 were not impaired. However, complexome profiling revealed that in the mutant CP43 was reduced to ~23% and D1, D2, and CP47 to ~30% of wild type levels. Levels of PSI and the cytochrome b6f complex were unchanged, while levels of the ATP synthase were increased by ~29%. PSII supercomplexes, dimers, and monomers were reduced to ~7%, ~26%, and ~60% of wild type levels, while RC47 was increased ~6-fold and LHCII by ~27%. We propose that LPA2 catalyses a step during PSII assembly without which PSII monomers and further assemblies become unstable and prone to degradation. The LHCI antenna was more disconnected from PSI in the lpa2 mutant, presumably as an adaptive response to reduce excitation of PSI. From the co-migration profiles of 1734 membrane-associated proteins, we identified three novel putative PSII associated proteins with potential roles in regulating PSII complex dynamics, assembly, and chlorophyll breakdown.

The PsbJ protein is required for photosystem II activity in centers lacking the PsbO and PsbV lumenal subunits

Photosystem II (PS II) of oxygenic photosynthesis is found in the thylakoid membranes of plastids and cyanobacteria. The mature PS II complex comprises a central core of four membrane proteins that bind the majority of the redox-active cofactors. In cyanobacteria the central core is surrounded by 13 low-molecular-weight (LMW) subunits which each consist of one or two transmembrane helices. Three additional hydrophilic subunits known as PsbO, PsbU and PsbV are found associated with hydrophilic loops belonging to the core proteins protruding into the thylakoid lumen. During biogenesis the majority of the LMW subunits are known to initially associate with individual pre-assembly complexes consisting of one or more of the core proteins; however, the point at which the PsbJ LMW subunit binds to PS II is not known. The majority of models for PS II biogenesis propose that the three extrinsic proteins and PsbJ bind in the final stages of PS II assembly. We have investigated the impact of creating the double mutants ∆PsbJ:∆PsbO, ∆PsbJ:∆PsbU and ∆PsbJ:∆PsbV to investigate potential cooperation between these subunits in the final stages of biogenesis. Our results indicate that PsbJ can bind to PS II in the absence of any one of the extrinsic proteins. However, unlike their respective single mutants, the ∆PsbJ:∆PsbO and ∆PsbJ:∆PsbV strains were not photoautotrophic and were unable to support oxygen evolution suggesting a functional oxygen-evolving complex could not assemble in these strains. In contrast, the PS II centers formed in the ∆PsbJ:∆PsbU strain were capable of photoautotrophic growth and could support oxygen evolution when whole-chain electron transport was supported by the addition of bicarbonate.

The availability of neither D2 nor CP43 limits the biogenesis of photosystem II in tobacco

The pathway of photosystem II (PSII) assembly is well understood, and multiple auxiliary proteins supporting it have been identified, but little is known about rate-limiting steps controlling PSII biogenesis. In the cyanobacterium Synechocystis PCC6803 and the green alga Chlamydomonas reinhardtii, indications exist that the biosynthesis of the chloroplast-encoded D2 reaction center subunit (PsbD) limits PSII accumulation. To determine the importance of D2 synthesis for PSII accumulation in vascular plants and elucidate the contributions of transcriptional and translational regulation, we modified the 5'-untranslated region of psbD via chloroplast transformation in tobacco (Nicotiana tabacum). A drastic reduction in psbD mRNA abundance resulted in a strong decrease in PSII content, impaired photosynthetic electron transport, and retarded growth under autotrophic conditions. Overexpression of the psbD mRNA also increased transcript abundance of psbC (the CP43 inner antenna protein), which is co-transcribed with psbD. Because translation efficiency remained unaltered, translation output of pbsD and psbC increased with mRNA abundance. However, this did not result in increased PSII accumulation. The introduction of point mutations into the Shine-Dalgarno-like sequence or start codon of psbD decreased translation efficiency without causing pronounced effects on PSII accumulation and function. These data show that neither transcription nor translation of psbD and psbC are rate-limiting for PSII biogenesis in vascular plants and that PSII assembly and accumulation in tobacco are controlled by different mechanisms than in cyanobacteria or in C. reinhardtii.

Toxic and protective mechanisms of cyanobacterium Synechocystis sp. in response to titanium dioxide nanoparticles

An increasing production and use of titanium dioxide nanoparticles (TiO₂ NPs) pose a huge threat to phytoplankton since they are largely released into aquatic environments, which represent a sink for TiO₂ NPs. However, toxicity and protective mechanisms of cyanobacteria in response to TiO₂ NPs remain elusive. Here we investigated toxic effects of two sizes of TiO₂ NPs (50 and 10 nm) and one bulk TiO₂ (200 nm) on a cyanobacterium, Synechocystis sp. and their possible protective mechanisms. We found that 10 nm TiO₂ NPs caused significant growth and photosynthesis inhibition in Synechocystis sp. cells, largely reflected in decreased growth rate (38%), operational PSII quantum yields (40%), phycocyanin (51%) and allophycocyanin (63%), and increased reactive oxygen species content (245%), superoxide dismutase activity (46%). Also, transcriptomic analysis of Synechocystis sp. exposure to 10 nm TiO₂ NPs showed the up-regulation of D1 and D2 protein genes (psbA and psbD), ferredoxin gene (petF) and F-type ATPase genes (e.g., atpB), and the down-regulation of psbM and psb28-2 in PS II. We further proposed a conceptual model to explore possible toxic and protective mechanisms for Synechocystis sp. under TiO₂ nanoparticle exposure. This study provides mechanistic insights into our understanding of Synechocystis sp. responses to TiO₂ NPs. This is essential for more accurate environmental risk assessment approaches of nanoparticles in aquatic ecosystems by governmental environmental agencies worldwide.

Structural insights into photosystem II assembly

Biogenesis of photosystem II (PSII), nature's water-splitting catalyst, is assisted by auxiliary proteins that form transient complexes with PSII components to facilitate stepwise assembly events. Using cryo-electron microscopy, we solved the structure of such a PSII assembly intermediate from Thermosynechococcus elongatus at 2.94 Å resolution. It contains three assembly factors (Psb27, Psb28 and Psb34) and provides detailed insights into their molecular function. Binding of Psb28 induces large conformational changes at the PSII acceptor side, which distort the binding pocket of the mobile quinone (Q_B) and replace the bicarbonate ligand of non-haem iron with glutamate, a structural motif found in reaction centres of non-oxygenic photosynthetic bacteria. These results reveal mechanisms that protect PSII from damage during biogenesis until water splitting is activated. Our structure further demonstrates how the PSII active site is prepared for the incorporation of the Mn₄CaO₅ cluster, which performs the unique water-splitting reaction.

Slr0320 Is Crucial for Optimal Function of Photosystem II during High Light Acclimation in Synechocystis sp. PCC 6803

Upon exposure of photosynthetic organisms to high light (HL), several HL acclimation responses are triggered. Herein, we identified a novel gene, slr0320, critical for HL acclimation in Synechocystis sp. PCC 6803. The growth rate of the Δslr0320 mutant was similar to wild type (WT) under normal light (NL) but severely declined under HL. Net photosynthesis of the mutant was lower under HL, but maximum photosystem II (PSII) activity was higher under NL and HL. Immunodetection revealed the accumulation and assembly of PSII were similar between WT and the mutant. Chlorophyll fluorescence traces showed the stable fluorescence of the mutant under light was much higher. Kinetics of single flash-induced chlorophyll fluorescence increase and decay revealed the slower electron transfer from Q_A to Q_B in the mutant. These data indicate that, in the Δslr0320 mutant, the number of functional PSIIs was comparable to WT even under HL but the electron transfer between Q_A and Q_B was inefficient. Quantitative proteomics and real-time PCR revealed that expression profiles of psbL, psbH and psbI were significantly altered in the Δslr0320 mutant. Thus, Slr0320 protein plays critical roles in optimizing PSII activity during HL acclimation and is essential for PSII electron transfer from Q_A to Q_B.

The Interaction between PsbT and the DE Loop of D1 in Photosystem II Stabilizes the Quinone-Iron Electron Acceptor Complex

The X-ray-derived Photosystem II (PS II) structure from the thermophilic cyanobacterium Thermosynechococcus vulcanus (Protein Data Bank entry 4UB6) indicates Phe239 of the DE loop of the D1 protein forms a hydrophobic interaction with Pro27 and Ile29 at the C-terminus of the 5 kDa PsbT protein found at the monomer-monomer interface of the PS II dimer. To investigate the importance of this interaction, we created the F239A and F239L mutants in Synechocystis sp. PCC 6803 through targeted mutagenesis of the D1:Phe239 residue into either an alanine or a leucine. Under moderate-light conditions, the F239A strain displayed reduced rates of oxygen evolution and impaired rates of fluorescence decay following a single-turnover actinic flash, while the F239L strain behaved like the control; however, under high-light conditions, the F239A and F239L strains grew more slowly than the control. Our results indicate the quinone-iron acceptor complex becomes more accessible to exogenously added electron acceptors in the F239A mutant and a ΔPsbT strain when compared with the control and F239L strains. This led to the hypothesis that the interaction between D1:Phe239 and the PsbT subunit is required to restrict movement of the DE loop of the D1 subunit, and we suggest disruption of this interaction may perturb the binding of bicarbonate to the non-heme iron and contribute to the signal for PS II to undergo repair following photodamage.

Changes in photosynthesis and respiratory metabolism of maize seedlings growing under low temperature stress may be regulated by arbuscular mycorrhizal fungi

Arbuscular mycorrhizal fungi as an important soil microbe have been demonstrated to mitigate the harmful effects of stress on plants. However, little is known about the molecular mechanisms underlying the AM symbiotic response to low temperature. Here, differentially expressed genes (DEGs) in the maize seedlings were identified after inoculating AMF under low temperature conditions. A total of 10,400 DEGs were obtained among four treatments, including non-inoculated AMF under ambient temperature (NMA), inoculated AMF under ambient temperature (MA), non-inoculated with low temperature stress (NML), and inoculated with low temperature stress (ML). The relative expression of 858 genes increased and that of 497 genes decreased in AM plants under low temperature stress. 24 DEGs were identified related to photosynthesis and respiratory metabolism. Among these DEGs, 10 genes were upregulated, and 14 genes were downregulated. The results show that inoculating AMF might decrease the production and transmission of electrons under low temperature, and the cyclic electron flow process in chloroplasts was stimulated to protect plants against low temperature. The fungi also influenced transmission of electrons and production of phosphoric acid in mitochondria in response to low temperature. CO₂ assimilation capacity was affected and the tricarboxylic acid cycle was promoted by the adjustments in the glycolysis, pentose phosphate pathway, gamma-aminobutyric acid shunt pathway, and glyoxylic acid cycle to produce more adenosine triphosphate and raw materials for other metabolic pathways under low temperature. These findings provide new insight into low temperature tolerance induced by AMF, and help identify genes for further investigation and functional analyses.

Stabilization of Photosystem II by the PsbT protein impacts photodamage, repair and biogenesis

Photosystem II (PS II) catalyzes the light-driven process of water splitting in oxygenic photosynthesis. Four core membrane-spanning proteins, including D1 that binds the majority of the redox-active co-factors, are surrounded by 13 low-molecular-weight (LMW) proteins. We previously observed that deletion of the LMW PsbT protein in the cyanobacterium Synechocystis sp. PCC 6803 slowed electron transfer between the primary and secondary plastoquinone electron acceptors Q_A and Q_B and increased the susceptibility of PS II to photodamage. Here we show that photodamaged ∆PsbT cells exhibit unimpaired rates of oxygen evolution if electron transport is supported by HCO₃^- even though the cells exhibit negligible variable fluorescence. We find that the protein environment in the vicinity of Q_A and Q_B is altered upon removal of PsbT resulting in inhibition of Q_A^- oxidation in the presence of 2,5-dimethyl-1,4-benzoquinone, an artificial PS II-specific electron acceptor. Thermoluminescence measurements revealed an increase in charge recombination between the S₂ oxidation state of the water-oxidizing complex and Q_A^- by the indirect radiative pathway in ∆PsbT cells and this is accompanied by increased ¹O₂ production. At the protein level, both D1 removal and replacement, as well as PS II biogenesis, were accelerated in the ∆PsbT strain. Our results demonstrate that PsbT plays a key role in optimizing the electron acceptor complex of the acceptor side of PS II and support the view that repair and biogenesis of PS II share an assembly pathway that incorporates both de novo synthesis and recycling of the assembly modules associated with the core membrane-spanning proteins.

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.

Structural basis of light-harvesting in the photosystem II core complex

Photosystem II (PSII) is a membrane-spanning, multi-subunit pigment-protein complex responsible for the oxidation of water and the reduction of plastoquinone in oxygenic photosynthesis. In the present review, the recent explosive increase in available structural information about the PSII core complex based on X-ray crystallography and cryo-electron microscopy is described at a level of detail that is suitable for a future structure-based analysis of light-harvesting processes. This description includes a proposal for a consistent numbering scheme of protein-bound pigment cofactors across species. The structural survey is complemented by an overview of the state of affairs in structure-based modeling of excitation energy transfer in the PSII core complex with emphasis on electrostatic computations, optical properties of the reaction center, the assignment of long-wavelength chlorophylls, and energy trapping mechanisms.

Chlorella vulgaris genome assembly and annotation reveals the molecular basis for metabolic acclimation to high light conditions

Chlorella vulgaris is a fast-growing fresh-water microalga cultivated on the industrial scale for applications ranging from food to biofuel production. To advance our understanding of its biology and to establish genetics tools for biotechnological manipulation, we sequenced the nuclear and organelle genomes of Chlorella vulgaris 211/11P by combining next generation sequencing and optical mapping of isolated DNA molecules. This hybrid approach allowed us to assemble the nuclear genome in 14 pseudo-molecules with an N50 of 2.8 Mb and 98.9% of scaffolded genome. The integration of RNA-seq data obtained at two different irradiances of growth (high light, HL versus low light, LL) enabled us to identify 10 724 nuclear genes, coding for 11 082 transcripts. Moreover, 121 and 48 genes, respectively, were found in the chloroplast and mitochondrial genome. Functional annotation and expression analysis of nuclear, chloroplast and mitochondrial genome sequences revealed particular features of Chlorella vulgaris. Evidence of horizontal gene transfers from chloroplast to mitochondrial genome was observed. Furthermore, comparative transcriptomic analyses of LL versus HL provided insights into the molecular basis for metabolic rearrangement under HL versus LL conditions leading to enhanced de novo fatty acid biosynthesis and triacylglycerol accumulation. The occurrence of a cytosolic fatty acid biosynthetic pathway could be predicted and its upregulation upon HL exposure was observed, consistent with the increased lipid amount under HL conditions. These data provide a rich genetic resource for future genome editing studies, and potential targets for biotechnological manipulation of Chlorella vulgaris or other microalgae species to improve biomass and lipid productivity.

A novel chlorophyll protein complex in the repair cycle of photosystem II

In oxygenic photosynthetic organisms, photosystem II (PSII) is a unique membrane protein complex that catalyzes light-driven oxidation of water. PSII undergoes frequent damage due to its demanding photochemistry. It must undergo a repair and reassembly process following photodamage, many facets of which remain unknown. We have discovered a PSII subcomplex that lacks 5 key PSII core reaction center polypeptides: D1, D2, PsbE, PsbF, and PsbI. This pigment-protein complex does contain the PSII core antenna proteins CP47 and CP43, as well as most of their associated low molecular mass subunits, and the assembly factor Psb27. Immunoblotting, mass spectrometry, and ultrafast spectroscopic results support the absence of a functional reaction center in this complex, which we call the "no reaction center" complex (NRC). Analytical ultracentrifugation and clear native PAGE analysis show that NRC is a stable pigment-protein complex and not a mixture of free CP47 and CP43 proteins. NRC appears in higher abundance in cells exposed to high light and impaired protein synthesis, and genetic deletion of PsbO on the PSII luminal side results in an increased NRC population, indicative that NRC forms in response to photodamage as part of the PSII repair process. Our finding challenges the current model of the PSII repair cycle and implies an alternative PSII repair strategy. Formation of this complex may maximize PSII repair economy by preserving intact PSII core antennas in a single complex available for PSII reassembly, minimizing the risk of randomly diluting multiple recycling components in the thylakoid membrane following a photodamage event.

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.

LcFIN2, a novel chloroplast protein gene from sheepgrass, enhances tolerance to low temperature in Arabidopsis and rice

Adverse environmental stresses affect plant growth and crop yields. Sheepgrass (Leymus chinensis (Trin.) Tzvel), an important forage grass that is widely distributed in the east of Eurasia steppe, has high tolerance to extreme low temperature. Many genes that respond to cold stress were identified in sheepgrass by RNA-sequencing, but more detailed studies are needed to dissect the function of those genes. Here, we found that LcFIN2, a sheepgrass freezing-induced protein 2, encoded a chloroplast-targeted protein. Expression of LcFIN2 was upregulated by freezing, chilling, NaCl and abscisic acid (ABA) treatments. Overexpression of LcFIN2 enhanced the survival rate of transgenic Arabidopsis after freezing stress. Importantly, heterologous expression of LcFIN2 in rice exhibited not only higher survival rate but also accumulated various soluble substances and reduced membrane damage in rice under chilling stress. Furthermore, the chlorophyll content, the quantum photochemistry efficiency of photosystem II (ΦPSII), the non-photochemical quenching (NPQ), the net photosynthesis rate (Pn) and the expression of some chloroplast ribosomal-related and photosynthesis-related genes were higher in the transgenic rice under chilling stress. These findings suggested that the LcFIN2 gene could potentially be used to improve low-temperature tolerance in crops.

Site-directed mutagenesis of two amino acid residues in cytochrome b559 α subunit that interact with a phosphatidylglycerol molecule (PG772) induces quinone-dependent inhibition of photosystem II activity

X-ray crystallographic analysis (1.9-Å resolution) of the cyanobacterial photosystem II (PSII) dimer showed the presence of five phosphatidylglycerol (PG) molecules per reaction center. One of the PG molecules, PG772, is located in the vicinity of the Q_B-binding site. To investigate the role of PG772 in PSII, we performed site-directed mutagenesis in the cytochrome (Cyt) b₅₅₉ α subunit of Synechocystis sp. PCC 6803 to change two amino acids, Thr-5 and Ser-11, which interact with PG772. The photosynthetic activity of intact cells was slightly lower in all mutants than that of cells in the control strain; however, the oxygen-evolving PSII activity was decreased markedly in cells of mutants, as measured using artificial quinones (such as p-benzoquinone). Furthermore, electron transport from Q_A to Q_B was inhibited in mutants incubated with quinones, particularly under high-intensity light conditions. Lipid analysis of purified PSII showed approximately one PG molecule per reaction center, presumably PG772, was lost in the PSII dimer from the T5A and S11A mutants compared with that in the PSII dimer from the control strain. In addition, protein analysis of monomer and dimer showed decreased levels of PsbV and PsbU extrinsic proteins in the PSII monomer purified from T5A and S11A mutants. These results suggest that site-directed mutagenesis of Thr-5 and Ser-11, which presumably causes the loss of PG772, induces quinone-dependent inhibition of PSII activity under high-intensity light conditions and destabilizes the binding of extrinsic proteins to PSII.

Whirly1 enhances tolerance to chilling stress in tomato via protection of photosystem II and regulation of starch degradation

In plants, the chilling response involves decreased photosynthetic capacity and increased starch accumulation in chloroplasts. However, the mechanisms that modulate these processes remain unclear. We found that the SlWHY1 gene is significantly induced by chilling stress (4°C) in tomato. Three SlWHY1 overexpression (OE) lines grew better than the wild type (WT) under chilling stress; the OE plants retained intact photosynthetic grana lamellae and showed enhanced hydrolysis of starch. By contrast, RNAi lines that inhibited SlWHY1 were more affected than the corresponding WT cultivar. Their grana lamellae were damaged and starch content increased. The psbA gene encodes the key photosystem II (PSII) protein D1. We show that SlWHY1 binds to the upstream region (A/GTTACCCT/A) of SlpsbA and enhances the de novo synthesis of D1 in chloroplasts. Additionally, SlWHY1 regulates the expression of the starch-degrading enzyme α-amylase (SlAMY3-L) and the starch synthesis-related enzyme isoamylase gene (SlISA2) in the nucleus, thus modulating the starch content in chloroplasts. We demonstrate that SlWHY1 enhances the resistance of tomato to chilling stress by maintaining the function of PSII and degrading starch. Thus, overexpression of WHY1 may be an effective strategy for enhancing resistance to chilling stress of chilling-sensitive crops in agricultural production.

Enhanced tolerance to a combination of heat stress and drought in Arabidopsis plants deficient in ICS1 is associated with modulation of photosynthetic reaction center proteins

Plants are exposed to multiple abiotic stresses that simultaneously occur under natural environmental conditions. Studies deciphering acclimation of plants to stress combinations are, however, still scarce. ISOCHORISMATE SYNTHASE 1 (ICS1) is known as a crucial enzyme required for synthesis of salicylic acid and phylloquinone, one of the components of the photosystem I complex. Although the significance of ICS1 in the regulation of abiotic stress response and pathogen defense in plants has been evidenced in previous studies, the role of this enzyme in the acclimation of plants to stress combinations is still largely unknown. In this study, we demonstrated the enhanced tolerance of Arabidopsis salicylic acid induction deficient 2-1(sid2-1) mutant deficient in ICS1 to a combination of heat stress and drought. H₂ O₂ -dependent stomatal closure and accumulation of total soluble sugars are associated with the enhanced tolerance of sid2-1 plants to this stress combination. In addition, sid2-1 plants showed higher accumulation of reaction center proteins (D1 and D2) in photosystem II accompanied by enhanced expression of transcripts involved in repair of these reaction center proteins. Furthermore, investigation of chlorophyll fluorescence indicated that mechanisms for dissipating the excess energy might be activated in sid2-1 plants specifically under a combination of heat stress and drought. Taken together, our findings suggest that maintenance of photosynthetic apparatus as well as prevention of excess water loss might enhance the tolerance of sid2-1 plants deficient in ICS1 to a combination of heat stress and drought.

Thylakoid proteome modulation in pea plants grown at different irradiances: quantitative proteomic profiling in a non-model organism aided by transcriptomic data integration

Plant thylakoid membranes contain hundreds of proteins that closely interact to cope with ever-changing environmental conditions. We investigated how Pisum sativum L. (pea) grown at different irradiances optimizes light-use efficiency through the differential accumulation of thylakoid proteins. Thylakoid membranes from plants grown under low (LL), moderate (ML) and high (HL) light intensity were characterized by combining chlorophyll fluorescence measurements with quantitative label-free proteomic analysis. Protein sequences retrieved from available transcriptomic data considerably improved thylakoid proteome profiling, increasing the quantifiable proteins from 63 to 194. The experimental approach used also demonstrates that this integrative omics strategy is powerful for unravelling protein isoforms and functions that are still unknown in non-model organisms. We found that the different growth irradiances affect the electron transport kinetics but not the relative abundance of photosystems (PS) I and II. Two acclimation strategies were evident. The behaviour of plants acclimated to LL was compared at higher irradiances: (i) in ML, plants turn on photoprotective responses mostly modulating the PSII light-harvesting capacity, either accumulating Lhcb4.3 or favouring the xanthophyll cycle; (ii) in HL, plants reduce the pool of light-harvesting complex II and enhance the PSII repair cycle. When growing at ML and HL, plants accumulate ATP synthase, boosting both cyclic and linear electron transport by finely tuning the ΔpH across the membrane and optimizing protein trafficking by adjusting the thylakoid architecture. Our results provide a quantitative snapshot of how plants coordinate light harvesting, electron transport and protein synthesis by adjusting the thylakoid membrane proteome in a light-dependent manner.

The stress-induced SCP/HLIP family of small light-harvesting-like proteins (ScpABCDE) protects Photosystem II from photoinhibitory damages in the cyanobacterium Synechocystis sp. PCC 6803

Small CAB-like proteins (SCPs) are single-helix light-harvesting-like proteins found in all organisms performing oxygenic photosynthesis. We investigated the effect of growth in moderate salt stress on these stress-induced proteins in the cyanobacterium Synechocystis sp. PCC 6803 depleted of Photosystem I (PSI), which expresses SCPs constitutively, and compared these cells with a PSI-less/ScpABCDE^- mutant. SCPs, by stabilizing chlorophyll-binding proteins and Photosystem II (PSII) assembly, protect PSII from photoinhibitory damages, and in their absence electrons accumulate and will lead to ROS formation. The presence of 0.2 M NaCl in the growth medium increased the respiratory activity and other PSII electron sinks in the PSI-less/ScpABCDE^- strain. We postulate that this salt-induced effect consumes the excess of PSII-generated electrons, reduces the pressure of the electron transport chain, and thereby prevents ¹O₂ production.

Cold-regulated protein (SlCOR413IM1) confers chilling stress tolerance in tomato plants

Chilling stress severely affects the growth, development and productivity of crops. Chloroplast, a photosynthesis site, is extremely sensitive to chilling stress. In this study, the functions of a gene encoding a cold-regulated protein (SlCOR413IM1) under chilling stress were investigated using sense and antisense transgenic tomatoes. Under chilling stress, SlCOR413IM1 expression was rapidly induced and the sense lines exhibited better growth state of seedlings and grown tomato plants. Overexpression of SlCOR413IM1 alleviated chilling-induced damage to the chloroplast membrane and structure, whereas suppression of SlCOR413IM1 aggravated the damage to chloroplast. Moreover, the net photosynthetic rate (Pn), maximum photochemical efficiency of photosystem II (PSII) (Fv/Fm), actual photochemical efficiency of PSII (ΦPSII) and the activities of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and stromal fructose-1, 6-bisphosphatase (sFBPase) were higher in the sense lines than those in the antisense lines. Hence, the inhibition of photosynthetic capacity was less severe in the sense lines but more severe in the antisense lines compared with that in wild-type (WT) plants. Taken together, overexpression of SlCOR413IM1 enhanced the chilling stress tolerance, whereas suppression of this gene increased the chilling sensitivity of tomato plants.

A Benzimidazole Proton Pump Inhibitor Increases Growth and Tolerance to Salt Stress in Tomato

Pre-treatment of tomato plants with micromolar concentrations of omeprazole (OP), a benzimidazole proton pump inhibitor in mammalian systems, improves plant growth in terms of fresh weight of shoot and roots by 49 and 55% and dry weight by 54 and 105% under salt stress conditions (200 mM NaCl), respectively. Assessment of gas exchange, ion distribution, and gene expression profile in different organs strongly indicates that OP interferes with key components of the stress adaptation machinery, including hormonal control of root development (improving length and branching), protection of the photosynthetic system (improving quantum yield of photosystem II) and regulation of ion homeostasis (improving the K+:Na+ ratio in leaves and roots). To our knowledge OP is one of the few known molecules that at micromolar concentrations manifests a dual function as growth enhancer and salt stress protectant. Therefore, OP can be used as new inducer of stress tolerance to better understand molecular and physiological stress adaptation paths in plants and to design new products to improve crop performance under suboptimal growth conditions. Highlight: Omeprazole enhances growth of tomato and increases tolerance to salinity stress through alterations of gene expression and ion uptake and transport.

Subunit and chlorophyll organization of the plant photosystem II supercomplex

Photosystem II (PSII) is a light-driven protein, involved in the primary reactions of photosynthesis. In plant photosynthetic membranes PSII forms large multisubunit supercomplexes, containing a dimeric core and up to four light-harvesting complexes (LHCs), which act as antenna proteins. Here we solved a three-dimensional (3D) structure of the C₂S₂M₂ supercomplex from Arabidopsis thaliana using cryo-transmission electron microscopy (cryo-EM) and single-particle analysis at an overall resolution of 5.3 Å. Using a combination of homology modelling and restrained refinement against the cryo-EM map, it was possible to model atomic structures for all antenna complexes and almost all core subunits. We located all 35 chlorophylls of the core region based on the cyanobacterial PSII structure, whose positioning is highly conserved, as well as all the chlorophylls of the LHCII S and M trimers. A total of 13 and 9 chlorophylls were identified in CP26 and CP24, respectively. Energy flow from LHC complexes to the PSII reaction centre is proposed to follow preferential pathways: CP26 and CP29 directly transfer to the core using several routes for efficient transfer; the S trimer is directly connected to CP43 and the M trimer can efficiently transfer energy to the core through CP29 and the S trimer.

Plants contain small families of UPF0016 proteins including the PHOTOSYNTHESIS AFFECTED MUTANT71 transporter

PHOTOSYNTHESIS AFFECTED MUTANT71 (PAM71) is an integral thylakoid membrane protein that functions in manganese uptake into the lumen. Manganese is needed in the thylakoid lumen to build up the inorganic Mn₄CaO₅ cluster, the catalytic center for water oxidation, and is hence indispensable for oxygen evolution. A recent study revealed that PAM71 is well conserved in plants and shares homology to GCR1 DEPENDENT TRANSLATION FACTOR1 (GDT1) and TRANSMEMBRANE PROTEIN 165 (TMEM165) in Saccharomyces cerevisiae and Homo sapiens, respectively. In most eukaryotes only single members of this family, designated "Uncharacterized Protein Family 0016" (UPF0016), are present; however, plant genomes contain genes for several UPF0016 proteins. In Arabidopsis thaliana, this protein family comprises 5 members, which mainly differ in their N-terminal regions. PAM71 and its closest homolog PAM71-HL possess chloroplast transit peptides at their N-terminus. Two of the remaining 3 members are derived from a segmental chromosomal duplication event and lack an N-terminal extension. Thus, plants have evolved UPF0016 members residing in various compartments of the cell, whereas in non-plant eukaryotes just a Golgi localization occurs. The identification of PAM71 as a candidate Mn²⁺ transporter opens the question on the function of the remaining plant members. Here we resume briefly our current knowledge of UPF0016 members in Arabidopsis in comparison to their yeast and human UPF0016 members.

Use of Nicotiana tabacum transplastomic plants engineered to express a His-tagged CP47 for the isolation of functional photosystem II core complexes

This work focuses on the development of a molecular tool for purification of Photosystem II (PSII) from Nicotiana tabacum (L.). To this end, the chloroplast psbB gene encoding the CP47 PSII subunit was replaced with an engineered version of the same gene containing a C-terminal His-tag. Molecular analyses assessed the effective integration of the recombinant gene and its expression. Despite not exhibiting any obvious phenotype, the transplastomic plants remained heteroplasmic even after three rounds of regeneration under antibiotic selection. However, the recombinant His-tagged CP47 protein associated in vivo to the other PSII subunits allowing the isolation of a functional PSII core complex, although with low yield of extraction. These results will open up possible perspectives for further spectroscopic and structural studies.

A Benzimidazole Proton Pump Inhibitor Increases Growth and Tolerance to Salt Stress in Tomato

Pre-treatment of tomato plants with micromolar concentrations of omeprazole (OP), a benzimidazole proton pump inhibitor in mammalian systems, improves plant growth in terms of fresh weight of shoot and roots by 49 and 55% and dry weight by 54 and 105% under salt stress conditions (200 mM NaCl), respectively. Assessment of gas exchange, ion distribution, and gene expression profile in different organs strongly indicates that OP interferes with key components of the stress adaptation machinery, including hormonal control of root development (improving length and branching), protection of the photosynthetic system (improving quantum yield of photosystem II) and regulation of ion homeostasis (improving the K⁺:Na⁺ ratio in leaves and roots). To our knowledge OP is one of the few known molecules that at micromolar concentrations manifests a dual function as growth enhancer and salt stress protectant. Therefore, OP can be used as new inducer of stress tolerance to better understand molecular and physiological stress adaptation paths in plants and to design new products to improve crop performance under suboptimal growth conditions. Highlight: Omeprazole enhances growth of tomato and increases tolerance to salinity stress through alterations of gene expression and ion uptake and transport.

Retaining individualities: the photodynamics of self-ordering porphyrin assemblies

Retained photochemical properties – a simple porphyrin–polyDMA conjugate with the ability to self assemble into large (∼1 μm) vesicles in water. The photodynamics are remarkably preserved despite the extensive aggregation.

The retention of photochemical properties of individual chromophores is a key feature of biological light harvesting complexes. This is achieved despite extensive aggregation of the chromophores, which in synthetic chromophore assemblies often yields a change in spectral characteristics. As an alternative approach towards mimicking biological light harvesting complexes, we report the synthesis of porphyrin assemblies which retained the photochemical properties of the individual chromophore units despite their substantial aggregation. These new materials highlight a new bottom-up approach towards the design and understanding of more complex biomimetic and naturally occurring biological systems.

Identification of manganese efficiency candidate genes in winter barley (Hordeum vulgare) using genome wide association mapping

BACKGROUND

Manganese (Mn) has several essential functions in plants, including a role as cofactor in the oxygen evolving complex (OEC) of photosystem II (PSII). Manganese deficiency is a major plant nutritional disorder in winter cereals resulting in significant yield reductions and winter kill in more severe cases. Among the winter cereals, genotypes of winter barley are known to differ considerably in tolerance to Mn deficiency, but the genes controlling the Mn deficiency trait remains elusive.

RESULTS

Experiments were conducted using 248 barley varieties, cultivated in six distinct environments prone to induce Mn deficiency. High-throughput phenotyping for Mn deficiency was performed by chlorophyll a (Chl a) fluorescence analysis to quantify the quantum yield efficiency of PSII. High-throughput phenotyping in combination with ICP-OES based multi-element analyses allowed detection of marker-trait associations by genome wide association (GWA) mapping. Several key candidate genes were identified, including PSII subunit proteins, germin like proteins and Mn superoxide dismutase. The putative roles of the encoded proteins in Mn dependent metabolic processes are discussed.

CONCLUSIONS

Fifty-four candidate genes were identified by Chl a fluorescence phenotyping and association genetics. Tolerance of plants to Mn deficiency, which is referred to as Mn efficiency, appeared to be a complex trait involving many genes. Moreover, the trait appeared to be highly dependent on the environmental conditions in field. This study provides the basis for an improved understanding of the parameters influencing Mn efficiency and is valuable in future plant breeding aiming at producing new varieties with improved tolerance to cultivation in soil prone to induce Mn deficiency.