The PsbP and PsbQ family proteins in the photosynthetic machinery of chloroplasts

Growth phase-dependent reorganization of cryptophyte photosystem I antennae

Photosynthetic cryptophytes are eukaryotic algae that utilize membrane-embedded chlorophyll a/c binding proteins (CACs) and lumen-localized phycobiliproteins (PBPs) as their light-harvesting antennae. Cryptophytes go through logarithmic and stationary growth phases, and may adjust their light-harvesting capability according to their particular growth state. How cryptophytes change the type/arrangement of the photosynthetic antenna proteins to regulate their light-harvesting remains unknown. Here we solve four structures of cryptophyte photosystem I (PSI) bound with CACs that show the rearrangement of CACs at different growth phases. We identify a cryptophyte-unique protein, PsaQ, which harbors two chlorophyll molecules. PsaQ specifically binds to the lumenal region of PSI during logarithmic growth phase and may assist the association of PBPs with photosystems and energy transfer from PBPs to photosystems.

Photoheterotroph improved the growth and nutrient levels of Chlorella vulgaris and the related molecular mechanism

Microalgae are rich in fatty acids, proteins, and other nutrients, which have gained the general attention of researchers all over the world. For the development of Chlorella vulgaris in food and feed industry, this study was conducted to investigate the differences in C. vulgaris' growth and nutritional components under different culture conditions (autotrophic, heterotrophic, photoheterotrophic) and the internal factors through cell counting in combination with transcriptome and nutrient analyses. The results showed that, under the photoheterotrophic condition, Chlorella's growth and the contents of lipid and protein were significantly higher than that under the heterotrophic condition, and the moisture content was lower than that under the heterotrophic condition. The saturated fatty acid content under the photoheterotrophic condition was the lowest, while the polyunsaturated fatty acid content was significantly higher than those under the other two conditions. There were 46,583 differentially expressed genes (DEGs), including 33,039 up-regulated DEGs (70.93%) and 13,544 down-regulated DEGs (29.07%), under the photoheterotrophic condition in comparison with the autotrophic condition. The fold change between the two conditions of samples of up-regulated genes was higher than that of the down-regulated genes. The KEGG enrichment showed that the up-regulated DEGs in the photoheterotrophic condition were significantly enriched in 5 pathways, including protein processing in endoplasmic reticulum pathway, photosynthesis pathway, photosynthesis-antenna protein pathway, endocytosis pathway, and phosphonate and phosphinate metabolism pathway. DEGs related to fatty acid metabolic pathways were significantly enriched in the fatty acid biosynthesis pathway and the biosynthesis of unsaturated fatty acid pathway. The qPCR analysis showed that the expression pattern of the selected genes was consistent with that of transcriptome analysis. The results of this study lay a theoretical foundation for the large-scale production of Chlorella and its application in food, feed, and biodiesel. KEY POINTS: • Nutrient levels under photoheterotrophic condition were higher than other conditions. • Six important pathways were discovered that affect changes in nutritional composition. • Explored genes encode important enzymes in the differential metabolic pathways.

Na2CO3-responsive mechanism insight from quantitative proteomics and SlRUB gene function in Salix linearistipularis seedlings

Mongolian willow (Salix linearistipularis) is a naturally occurring woody dioecious plant in the saline soils of north-eastern China, which has a high tolerance to alkaline salts. Although transcriptomics studies have identified a large number of salinity-responsive genes, the mechanism of salt tolerance in Mongolian willow is not clear. Here, we found that in response to Na2CO3 stress, Mongolian willow regulates osmotic homeostasis by accumulating proline and soluble sugars and scavenges reactive oxygen species (ROS) by antioxidant enzymes and non-enzymatic antioxidants. Our quantitative proteomics study identified 154 salt-sensitive proteins mainly involved in maintaining the stability of the photosynthetic system and ROS homeostasis to cope with Na2CO3 stress. Among them, Na2CO3-induced rubredoxin (RUB) was predicted to be associated with 122 proteins for the modulation of these processes. The chloroplast-localized S. linearistipularis rubredoxin (SlRUB) was highly expressed in leaves and was significantly induced under Na2CO3 stress. Phenotypic analysis of overexpression, mutation and complementation materials of RUB in Arabidopsis suggests that SlRUB is critical for the regulation of photosynthesis, ROS scavenging and other metabolisms in the seedlings of Mongolian willow to cope with Na2CO3 stress. This provides more clues to better understand the alkali-responsive mechanism and RUB functions in the woody Mongolian willow.

Toxicity of tigecycline on the freshwater microalga Scenedesmus obliquus: Photosynthetic and transcriptional responses

Tigecycline (TGC) is a new tetracycline antibiotic medication against multidrug-resistant bacteria. However, the toxicity of TGC to microalgae remains largely unknown. In this study, the toxicity of TGC on Scenedesmus obliquus was examined, focusing on changes in algal growth, photosynthetic activity, and transcriptome. According to an acute toxicity test, the IC10 and IC50 values were 0.72 mg/L and 4.15 mg/L, respectively. Analyses of photosynthetic efficiency and related parameters, such as light absorption, energy capture, and electron transport, identified a 35% perturbation in the IC50 group, while the IC10 group remained largely unaffected. Transcriptomic analysis showed that in the IC10 and IC50 treatment groups, there were 874 differentially expressed genes (DEGs) (220 upregulated and 654 downregulated) and 4289 DEGs (2660 upregulated and 1629 downregulated), respectively. Gene Ontology enrichment analysis showed that TGC treatment markedly affected photosynthesis, electron transport, and chloroplast functions. In the IC50 group, a clear upregulation of genes related to photosynthesis and chloroplast functions was observed, which could be an adaptive stress response. In the IC10 group, significant downregulation of DEGs involved in ribosomal pathways and peptide biosynthesis processes was observed. Kyoto Encyclopedia of Gene and Genomes enrichment analysis showed that treatment with TGC also disrupted energy production, protein synthesis, and metabolic processes in S. obliquus. Significant downregulation of key proteins related to Photosystem II was observed under the IC10 TGC treatment. Conversely, IC50 TGC treatment resulted in substantial upregulation across a broad array of photosystem-related proteins from both Photosystems II and I. IC10 and IC50 TGC treatments differentially influenced proteins involved in the photosynthetic electron transport process. This study emphasizes the potential risks of TGC pollution to microalgae, which contributes to a better understanding of the effects of antibiotic contamination in aquatic ecosystems.

Methyl viologen-induced changes in the Arabidopsis proteome implicate PATELLIN 4 in oxidative stress responses

The photosynthesis-induced accumulation of reactive oxygen species in chloroplasts can lead to oxidative stress, triggering changes in protein synthesis, degradation, and the assembly/disassembly of protein complexes. Using shot-gun proteomics, we identified methyl viologen-induced changes in protein abundance in wild-type Arabidopsis and oxidative stress-hypersensitive fsd1-1 and fsd1-2 knockout mutants, which are deficient in IRON SUPEROXIDE DISMUTASE 1 (FSD1). The levels of proteins that are localized in chloroplasts and the cytoplasm were modified in all lines treated with methyl viologen. Compared with the wild-type, fsd1 mutants showed significant changes in metabolic protein and chloroplast chaperone levels, together with increased ratio of cytoplasmic, peroxisomal, and mitochondrial proteins. Different responses in proteins involved in the disassembly of photosystem II-light harvesting chlorophyll a/b binding proteins were observed. Moreover, the abundance of PATELLIN 4, a phospholipid-binding protein enriched in stomatal lineage, was decreased in response to methyl viologen. Reverse genetic studies using patl4 knockout mutants and a PATELLIN 4 complemented line indicate that PATELLIN 4 affects plant responses to oxidative stress by effects on stomatal closure.

Sensory plastid-associated PsbP DOMAIN-CONTAINING PROTEIN 3 triggers plant growth- and defense-related epigenetic responses

Sensory plastids are important in plant responses to environmental changes. Previous studies show that MutS HOMOLOG 1 (MSH1) perturbation in sensory plastids induces heritable epigenetic phenotype adjustment. Previously, the PsbP homolog DOMAIN-CONTAINING PROTEIN 3 (PPD3), a protein of unknown function, was postulated to be an interactor with MSH1. This study investigates the relationship of PPD3 with MSH1 and with plant environmental sensing. The ppd3 mutant displays a whole-plant phenotype variably altered in growth rate, flowering time, reactive oxygen species (ROS) modulation and response to salt, with effects on meristem growth. Present in both chloroplasts and sensory plastids, PPD3 colocalized with MSH1 in root tips but not in leaf tissues. The suppression or overexpression of PPD3 affected the plant growth rate and stress tolerance, and led to a heritable, heterogenous 'memory' state with both dwarfed and vigorous growth phenotypes. Gene expression and DNA methylome data sets from PPD3-OX and derived memory states showed enrichment in growth versus defense networks and meristem effects. Our results support a model of sensory plastid influence on nuclear epigenetic behavior and ppd3 as a second trigger, functioning within meristem plastids to recalibrate growth plasticity.

Transcriptomic Analysis Reveals Panicle Heterosis in an Elite Hybrid Rice ZZY10 and Its Parental Lines

Heterosis is the phenomenon in which some hybrid traits are superior to those of their parents. Most studies have analyzed the heterosis of agronomic traits of crops; however, heterosis of the panicles can improve yield and is important for crop breeding. Therefore, a systematic study of panicle heterosis is needed, especially during the reproductive stage. RNA sequencing (RNA Seq) and transcriptome analysis are suitable for further study of heterosis. Using the Illumina Nova Seq platform, the transcriptome of ZhongZheYou 10 (ZZY10), an elite rice hybrid, the maintainer line ZhongZhe B (ZZB), and the restorer line Z7-10 were analyzed at the heading date in Hangzhou, 2022. 581 million high-quality short reads were obtained by sequencing and were aligned against the Nipponbare reference genome. A total of 9000 differential expression genes were found between the hybrids and their parents (DG_HP). Of the DG_HP, 60.71% were up-regulated and 39.29% were down-regulated in the hybrid. Comparative transcriptome analysis revealed that 5235 and 3765 DG_HP were between ZZY10 and ZhongZhe B and between ZZY10 and Z7-10, respectively. This result is consistent with the transcriptome profile of ZZY10 and was similar to Z7-10. The expression patterns of DG_HP mainly exhibited over-dominance, under-dominance, and additivity. Among the DG_HP-involved GO terms, pathways such as photosynthesis, DNA integration, cell wall modification, thylakoid, and photosystem were significant. 21 DG_HP, which were involved in photosynthesis, and 17 random DG_HP were selected for qRT-PCR validation. The up-regulated PsbQ and down-regulated subunits of PSI and PSII and photosynthetic electron transport in the photosynthesis pathway were observed in our study. Extensive transcriptome data were obtained by RNA-Seq, providing a comprehensive overview of panicle transcriptomes at the heading stage in a heterotic hybrid.

Transcriptome analysis of Citrus limon infected with Citrus yellow vein clearing virus

BACKGROUND

Citrus yellow vein clearing virus (CYVCV) is the causative agent of citrus yellow vein clearing disease, and poses a serious threat to the lemon industry in Asia. The common symptoms of CYVCV-infected lemon plants are leaf crinkling, leaf chlorotic mottling, and yellow vein clearing. However, the molecular mechanisms underlying CYVCV-citrus interaction that responsible for symptom occurrence is still unclarified. In this study, RNA-seq was performed to analyze the gene expression patterns of 'Eureka' lemon (Citrus limon Burm. f.) plants in response to CYVCV infection.

RESULTS

There were 3691 differentially expressed genes (DEGs) identified by comparison between mock and CYVCV-infected lemon plants through RNA-seq. Bioinformatics analyses revealed that these DEGs were components of different pathways involved in phenylpropanoid biosynthesis, brassinosteroid biosynthesis, flavonoid biosynthesis and photosynthesis. Among these, the DEGs related to phytohormone metabolism and photosynthesis pathways were further enriched and analyzed. This study showed that different phytohormone-related genes had different responses toward CYVCV infection, however almost all of the photosynthesis-related DEGs were down-regulated in the CYVCV-infected lemon plants. The obtained RNA-seq data were validated by RT-qPCR using 12 randomly chosen genes, and the results of mRNA expression analysis were consistent with those of RNA-seq.

CONCLUSIONS

The phytohormone biosynthesis, signaling and photosynthesis-related genes of lemon plants were probably involved in systemic infection and symptom occurrence of CYVCV. Notably, CYVCV infection had regulatory effects on the biosynthesis and signaling of phytohormone, which likely improve systemic infection of CYVCV. Additionally, CYVCV infection could cause structural changes in chloroplast and inhibition of photosynthesis pathway, which probably contribute to the appearance of leaf chlorotic mottling and yellow vein clearing in CYVCV-infected lemon plants. This study illustrates the dynamic nature of the citrus-CYVCV interaction at the transcriptome level and provides new insights into the molecular mechanism underlying the pathogenesis of CYVCV in lemon plants.

NSvc4 Encoded by Rice Stripe Virus Targets Host Chloroplasts to Suppress Chloroplast-Mediated Defense

Our previous research found that NSvc4, the movement protein of rice stripe virus (RSV), could localize to the actin filaments, endoplasmic reticulum, plasmodesmata, and chloroplast, but the roles of NSvc4 played in the chloroplast were opaque. Here, we confirm the accumulation of NSvc4 in the chloroplasts and the N-terminal 1-73 amino acids of NSvc4 are sufficient to localize to chloroplasts. We provide evidence to show that chloroplast-localized NSvc4 can impair the chloroplast-mediated immunity. Expressing NSvc4 in Nicotiana benthamiana leaves results in the decreased expression of defense-related genes NbPR1, NbPR2, and NbWRKY12 and the inhibition of chloroplast-derived ROS production. In addition, generation of an infectious clone of potato virus X (PVX) carrying NSvc4 facilitates PVX infection in N. benthamiana plants. Moreover, we identify two chloroplast-related host factors, named NbGAPDH-A and NbPsbQ1, both of which can interact with NSvc4. Knockdown of NbGAPDH-A or NbPsbQ1 can both promote RSV infection. Our results decipher a detailed function of NSvc4 in the chloroplast.

Effects of Long-Term Non-Pruning on Main Quality Constituents in 'Dancong' Tea (Camellia sinensis) Leaves Based on Proteomics and Metabolomics Analysis

'Dancong' tea is a famous traditional Oolong tea. In order to keep the original taste of "ancient tea trees", most of the 'Dancong' tea plants are planted in a single plant pattern without pruning. The objective of this study was to explore the effects of long-term non-pruning on main quality constituents in 'Dancong' tea. The results showed that the contents of free amino acids, chlorophylls, and floral-honey aromatic substances in tea leaves of unpruned tea plants were higher than those in every year pruned tea plants, while the catechin content in leaves of pruned tea plants was higher than that in leaves of unpruned tea plants. Quantitative proteomics analysis showed that most enzymes involved in biosynthesis of catechins were downregulated in leaves of unpruned tea plants. Five proteins involved in chlorophyll metabolism and 12 proteins related to photosynthesis were upregulated, and the results suggested that higher chlorophyll content and more efficient photosynthetic energy conversion may be important for the higher accumulation of special quality components in leaves of unpruned tea plants. The findings of this study will advance our understanding of the mechanism of formation of different metabolites in leaves of unpruned and pruned tea plants.

Transcriptome Analysis of Lolium temulentum Exposed to a Combination of Drought and Heat Stress

Drought and heat are two major stresses predicted to increase in the future due to climate change. Plants exposed to multiple stressors elicit unique responses from those observed under individual stresses. A comparative transcriptome analysis of Lolium temulentum exposed to drought plus heat and non-stressed control plants revealed 20,221 unique up-regulated and 17,034 unique down-regulated differentially regulated transcripts. Gene ontology analysis revealed a strong emphasis on transcriptional regulation, protein folding, cell cycle/parts, organelles, binding, transport, signaling, oxidoreductase, and antioxidant activity. Differentially expressed genes (DEGs) encoding for transcriptional control proteins such as basic leucine zipper, APETALA2/Ethylene Responsive Factor, NAC, and WRKY transcription factors, and Zinc Finger (CCCH type and others) proteins were more often up-regulated, while DEGs encoding Basic Helix-Loop-Helix, MYB and GATA transcription factors, and C2H2 type Zinc Finger proteins were more often down-regulated. The DEGs encoding heat shock transcription factors were only up-regulated. Of the hormones, auxin-related DEGs were the most prevalent, encoding for auxin response factors, binding proteins, and efflux/influx carriers. Gibberellin-, cytokinin- and ABA-related DEGs were also prevalent, with fewer DEGs related to jasmonates and brassinosteroids. Knowledge of genes/pathways that grasses use to respond to the combination of heat/drought will be useful in developing multi-stress resistant grasses.

Fluorescent carbon-dots enhance light harvesting and photosynthesis by overexpressing PsbP and PsiK genes

BACKGROUND

Fluorescent carbon-dots (CDs) with multifaceted advantages have provided hope for improvement of crop growth. Near infrared (NIR) CDs would be more competitive and promising than short-wavelength emissive CDs, which are not directly utilized by chloroplast. The molecular targets and underlying mechanism of these stimulative effects are rarely mentioned.

RESULTS

NIR-CDs with good mono-dispersity and hydrophily were easily prepared by a one-step microwave-assisted carbonization manner, which showed obvious UV absorptive and far-red emissive properties. The chloroplast-CDs complexes could accelerate the electron transfer from photosystem II (PS II) to photosystem I (PS I). NIR-CDs exhibited a concentration-dependent promotion effect on N. benthamiana growth by strengthening photosynthesis. We firstly demonstrated that potential mechanisms behind the photosynthesis-stimulating activity might be related to up-regulated expression of the photosynthesis and chloroplast synthesis related genes, among which PsbP and PsiK genes are the key regulators.

CONCLUSION

These results illustrated that NIR-CDs showed great potential in the applications to increase crop yields through ultraviolet light harvesting and elevated photosynthesis efficiency. This work would provide a theoretical basis for the understanding and applications of the luminescent nanomaterials (not limited to CDs) in the sunlight conversion-related sustainable agriculture.

Characterization of the Free and Membrane-Associated Fractions of the Thylakoid Lumen Proteome in Arabidopsis thaliana

The thylakoid lumen houses proteins that are vital for photosynthetic electron transport, including water-splitting at photosystem (PS) II and shuttling of electrons from cytochrome b₆f to PSI. Other lumen proteins maintain photosynthetic activity through biogenesis and turnover of PSII complexes. Although all lumen proteins are soluble, these known details have highlighted interactions of some lumen proteins with thylakoid membranes or thylakoid-intrinsic proteins. Meanwhile, the functional details of most lumen proteins, as well as their distribution between the soluble and membrane-associated lumen fractions, remain unknown. The current study isolated the soluble free lumen (FL) and membrane-associated lumen (MAL) fractions from Arabidopsis thaliana, and used gel- and mass spectrometry-based proteomics methods to analyze the contents of each proteome. These results identified 60 lumenal proteins, and clearly distinguished the difference between the FL and MAL proteomes. The most abundant proteins in the FL fraction were involved in PSII assembly and repair, while the MAL proteome was enriched in proteins that support the oxygen-evolving complex (OEC). Novel proteins, including a new PsbP domain-containing isoform, as well as several novel post-translational modifications and N-termini, are reported, and bi-dimensional separation of the lumen proteome identified several protein oligomers in the thylakoid lumen.

Structural insights into a dimeric Psb27-photosystem II complex from a cyanobacterium Thermosynechococcus vulcanus

Photosystem II (PSII) is a multisubunit pigment-protein complex and catalyzes light-driven water oxidation, leading to the conversion of light energy into chemical energy and the release of molecular oxygen. Psb27 is a small thylakoid lumen-localized protein known to serve as an assembly factor for the biogenesis and repair of the PSII complex. The exact location and binding fashion of Psb27 in the intermediate PSII remain elusive. Here, we report the structure of a dimeric Psb27-PSII complex purified from a psbV deletion mutant (ΔPsbV) of the cyanobacterium Thermosynechococcus vulcanus, solved by cryo-electron microscopy. Our structure showed that Psb27 is associated with CP43 at the luminal side, with specific interactions formed between Helix 2 and Helix 3 of Psb27 and a loop region between Helix 3 and Helix 4 of CP43 (loop C) as well as the large, lumen-exposed and hydrophilic E-loop of CP43. The binding of Psb27 imposes some conflicts with the N-terminal region of PsbO and also induces some conformational changes in CP43, CP47, and D2. This makes PsbO unable to bind in the Psb27-PSII. Conformational changes also occurred in D1, PsbE, PsbF, and PsbZ; this, together with the conformational changes occurred in CP43, CP47, and D2, may prevent the binding of PsbU and induce dissociation of PsbJ. This structural information provides important insights into the regulation mechanism of Psb27 in the biogenesis and repair of PSII.

Role of PsbV-Tyr137 in photosystem II studied by site-directed mutagenesis in the thermophilic cyanobacterium Thermosynechococcus vulcanus

PsbV (cytochrome c₅₅₀) is one of the three extrinsic proteins of photosystem II (PSII) and functions to maintain the stability and activity of the Mn₄CaO₅ cluster, the catalytic center for water oxidation. PsbV-Y137 is the C-terminal residue of PsbV and is located at the exit of a hydrogen-bond network mediated by the D1-Y161-H190 residue pair. In order to examine the function of PsbV-Y137, four mutants, PsbV-Y137A, PsbV-Y137F, PsbV-Y137G, and PsbV-Y137W, were generated with Thermosynechococcus vulcanus (T. vulcanus). These mutants showed growth rates similar to that of the wild-type strain (WT); however, their oxygen-evolving activities were different. At pH 6.5, the oxygen evolution rates of Y137F and Y137W were almost identical to that of WT, whereas the oxygen evolution rates of the Y137A, Y137G mutants were 64% and 61% of WT, respectively. However, the oxygen evolution in the latter two mutants decreased less at higher pHs, suggesting that higher pHs facilitated oxygen evolution probably by facilitating proton egress in these two mutants. Furthermore, thylakoid membranes isolated from the PsbV-Y137A, PsbV-Y137G mutants exhibited much lower levels of oxygen-evolving activity than that of WT, which was found to be caused by the release of PsbV. In addition, PSII complexes purified from the PsbV-Y137A and PsbV-Y137G mutants lost all of the three extrinsic proteins but instead bind Psb27, an assembly cofactor of PSII. These results demonstrate that the PsbV-Tyr137 residue is required for the stable binding of PsbV to PSII, and the hydrogen-bond network mediated by D1-Y161-H190 is likely to function in proton egress during water oxidation.

Na2CO3-responsive Photosynthetic and ROS Scavenging Mechanisms in Chloroplasts of Alkaligrass Revealed by Phosphoproteomics

Alkali-salinity exerts severe osmotic, ionic, and high-pH stresses to plants. To understand the alkali-salinity responsive mechanisms underlying photosynthetic modulation and reactive oxygen species (ROS) homeostasis, physiological and diverse quantitative proteomics analyses of alkaligrass (Puccinellia tenuiflora) under Na₂CO₃ stress were conducted. In addition, Western blot, real-time PCR, and transgenic techniques were applied to validate the proteomic results and test the functions of the Na₂CO₃-responsive proteins. A total of 104 and 102 Na₂CO₃-responsive proteins were identified in leaves and chloroplasts, respectively. In addition, 84 Na₂CO₃-responsive phosphoproteins were identified, including 56 new phosphorylation sites in 56 phosphoproteins from chloroplasts, which are crucial for the regulation of photosynthesis, ion transport, signal transduction, and energy homeostasis. A full-length PtFBA encoding an alkaligrass chloroplastic fructose-bisphosphate aldolase (FBA) was overexpressed in wild-type cells of cyanobacterium Synechocystis sp. Strain PCC 6803, leading to enhanced Na₂CO₃ tolerance. All these results indicate that thermal dissipation, state transition, cyclic electron transport, photorespiration, repair of photosystem (PS) II, PSI activity, and ROS homeostasis were altered in response to Na₂CO₃ stress, which help to improve our understanding of the Na₂CO₃-responsive mechanisms in halophytes.

PsbQ-Like Protein 3 Functions as an Assembly Factor for the Chloroplast NADH Dehydrogenase-Like Complex in Arabidopsis

Angiosperms have three PsbQ-like (PQL) proteins in addition to the PsbQ subunit of the oxygen-evolving complex of photosystem II. Previous studies have shown that two PQL proteins, PnsL2 and PnsL3, are subunits of the chloroplast NADH dehydrogenase-like (NDH) complex involved in the photosystem I (PSI) cyclic electron flow. In addition, another PsbQ homolog, PQL3, is required for the NDH activity; however, the molecular function of PQL3 has not been elucidated. Here, we show that PQL3 is an assembly factor, particularly for the accumulation of subcomplex B (SubB) of the chloroplast NDH. In the pql3 mutant of Arabidopsis thaliana, the amounts of NDH subunits in SubB, PnsB1 and PsnB4, were decreased, causing a severe reduction in the NDH-PSI supercomplex. Analysis using blue native polyacrylamide gel electrophoresis suggested that the incorporation of PnsL3 into SubB was affected in the pql3 mutant. Unlike other PsbQ homologs, PQL3 was weakly associated with thylakoid membranes and was only partially protected from thermolysin digestion. Consistent with the function as an assembly factor, PQL3 accumulated independently in other NDH mutants, such as pnsl1-3. Furthermore, PQL3 accumulated in young leaves in a manner similar to the accumulation of CRR3, an assembly factor for SubB. These results suggest that PQL3 has developed a distinct function as an assembly factor for the NDH complex during evolution of the PsbQ protein family in angiosperms.

Understanding Mechanisms of Salinity Tolerance in Barley by Proteomic and Biochemical Analysis of Near-Isogenic Lines

Salt stress is one of the major environmental factors impairing crop production. In our previous study, we identified a major QTL for salinity tolerance on chromosome 2H on barley (Hordeum vulgare L.). For further investigation of the mechanisms responsible for this QTL, two pairs of near-isogenic lines (NILs) differing in this QTL were developed. Sensitive NILs (N33 and N53) showed more severe damage after exposure to 300 mM NaCl than tolerant ones (T46 and T66). Both tolerant NILs maintained significantly lower Na⁺ content in leaves and much higher K⁺ content in the roots than sensitive lines under salt conditions, thus indicating the presence of a more optimal Na⁺/K⁺ ratio in plant tissues. Salinity stress caused significant accumulation of H₂O₂, MDA, and proline in salinity-sensitive NILs, and a greater enhancement in antioxidant enzymatic activities at one specific time or tissues in tolerant lines. One pair of NILs (N33 and T46) were used for proteomic studies using two-dimensional gel electrophoresis. A total of 53 and 51 differentially expressed proteins were identified through tandem mass spectrometry analysis in the leaves and roots, respectively. Proteins which are associated with photosynthesis, reactive oxygen species (ROS) scavenging, and ATP synthase were found to be specifically upregulated in the tolerant NIL. Proteins identified in this study can serve as a useful resource with which to explore novel candidate genes for salinity tolerance in barley.

Foliar application is an effective method for incorporating selenium into peanut leaf proteins with antioxidant activities

Proteins were extracted from Se-enriched peanut leaves, an agro-byproduct, and the foliar application of sodium selenite was indicated to be an effective method to incorporate Se into leaf selenoproteins with 75-80% incorporation rates. After trypsin digestion, the most abundant proteins from Se-enriched peanut leaf (PSPL) were identified as pathogenesis-related class 10 proteins, Ara h 8 allergen and its isoforms, using LC-MS/MS. The Se species in both the low Se PSPL and high Se PSPL were determined to be selenomethionine (SeMet), methylselenocysteine (MeSeCys) and selenocystine (SeCys₂) with SeMet (15.6 mg/g) dominated the high Se PSPL. Their antioxidant activities were also evaluated using free radical scavenging assay, reducing power assay and ferric thiocyanate (FTC) test. As results, the PSPL exhibited potent DPPH radical (96.2%) and superoxide anion radical (98.4%) scavenging activities and showed strong reducing power in a Se-concentration-dependent manner, indicating that PSPL can be used as antioxidants and Se sources to improve health.

Dynamic pH-induced conformational changes of the PsbO protein in the fluctuating acidity of the thylakoid lumen

The PsbO protein is an essential extrinsic subunit of photosystem II, the pigment-protein complex responsible for light-driven water splitting. Water oxidation in photosystem II supplies electrons to the photosynthetic electron transfer chain and is accompanied by proton release and oxygen evolution. While the electron transfer steps in this process are well defined and characterized, the driving forces acting on the liberated protons, their dynamics and their destiny are all largely unknown. It was suggested that PsbO undergoes proton-induced conformational changes and forms hydrogen bond networks that ensure prompt proton removal from the catalytic site of water oxidation, i.e. the Mn₄ CaO₅ cluster. This work reports the purification and characterization of heterologously expressed PsbO from green algae Chlamydomonas reinhardtii and two isoforms from the higher plant Solanum tuberosum (PsbO1 and PsbO2). A comparison to the spinach PsbO reveals striking similarities in intrinsic protein fluorescence and CD spectra, reflecting the near-identical secondary structure of the proteins from algae and higher plants. Titration experiments using the hydrophobic fluorescence probe ANS revealed that eukaryotic PsbO proteins exhibit acid-base hysteresis. This hysteresis is a dynamic effect accompanied by changes in the accessibility of the protein's hydrophobic core and is not due to reversible oligomerization or unfolding of the PsbO protein. These results confirm the hypothesis that pH-dependent dynamic behavior at physiological pH ranges is a common feature of PsbO proteins and causes reversible opening and closing of their β-barrel domain in response to the fluctuating acidity of the thylakoid lumen.

Photosystem II Extrinsic Proteins and Their Putative Role in Abiotic Stress Tolerance in Higher Plants

Abiotic stress remains one of the major challenges in managing and preventing crop loss. Photosystem II (PSII), being the most susceptible component of the photosynthetic machinery, has been studied in great detail over many years. However, much of the emphasis has been placed on intrinsic proteins, particularly with respect to their involvement in the repair of PSII-associated damage. PSII extrinsic proteins include PsbO, PsbP, PsbQ, and PsbR in higher plants, and these are required for oxygen evolution under physiological conditions. Changes in extrinsic protein expression have been reported to either drastically change PSII efficiency or change the PSII repair system. This review discusses the functional role of these proteins in plants and indicates potential areas of further study concerning these proteins.

Overexpression of alfalfa Orange gene in tobacco enhances carotenoid accumulation and tolerance to multiple abiotic stresses

The multifunctional Orange (Or) protein plays crucial roles in carotenoid homeostasis, photosynthesis stabilization, and antioxidant activity in plants under various abiotic stress conditions. The Or gene has been cloned in several crops but not in alfalfa (Medicago sativa L.). Alfalfa is widely cultivated across the world; however, its cultivation is largely limited by various abiotic stresses, including drought. In this study, we isolated the Or gene from alfalfa (MsOr) cv. Xinjiang Daye. The amino acid sequence of the deduced MsOr protein revealed that the protein contained two trans-membrane domains and a DnaJ cysteine-rich zinc finger domain, and showed a high level of similarity with the Or protein of other plants species. The MsOr protein was localized in leaf chloroplasts of tobacco. The expression of MsOr was the highest in mature leaves and was significantly induced by abiotic stresses, especially drought. To perform functional analysis of the MsOr gene, we overexpressed MsOr gene in tobacco (Nicotiana benthamiana). Compared with wild-type (WT) plants, transgenic tobacco lines showed higher carotenoid accumulation and increased tolerance to various abiotic stresses, including drought, heat, salt, and methyl viologen-mediated oxidative stress. Additionally, contents of hydrogen peroxide and malondialdehyde were lower in the transgenic lines than in WT plants, suggesting superior membrane stability and antioxidant capacity of TOR lines under multiple abiotic stresses. These results indicate the MsOr gene as a potential target for the development of alfalfa cultivars with enhanced carotenoid content and tolerance to multiple environmental stresses.

Determination of chemical identity and occupancy from experimental density maps

Three basic electronic properties of molecules, electron density (ED), charge density (CD), and electrostatic potentials (ESP), are dependent on both atomic mobility and occupancy of components in the molecules. Small protein subunits may bind large macromolecular complexes with a reduced occupancy or an increased atomic mobility or both due to affinity-based functional regulation, and so may substrates, products, cofactors, ions or solvent molecule to the active sites of enzymes. A quantitative theory is presented in this study that describes the dependence of atomic functions on atomic B-factor in Fourier transforms of the corresponding maps. An application of this theory is described to an experimental ED map at 1.73-Å resolution, and to an experimental CD map at 2.2-Å resolution. All the three density functions are linearly proportional to occupancy when the structure factor F(000) term of Fourier transforms of experimental density maps is included. Upon application of this theory to both experimental CD and ESP maps recently reported for photosystem II-light harvesting complex II supercomplex at 3.2-Å resolution, the occupancy of two extrinsic protein subunits PsbQ and PsbP is determined to be 20.4 ± 0.2%, and the negative mean ESP value of vitreous ice displaced by the supercomplex on electron scattering path is estimated to be 3% of the mean ESP value of protein α-helices.

In vivo system for analyzing the function of the PsbP protein using Chlamydomonas reinhardtii

The PsbP protein is an extrinsic subunit of photosystem II (PSII) specifically developed in green-plant species including land plants and green algae. The protein-protein interactions involving PsbP and its effect on oxygen evolution have been investigated in vitro using isolated PSII membranes. However, the importance of those interactions needs to be examined at the cellular level. To this end, we developed a system expressing exogenous PsbP in the background of the Chlamydomonas BF25 mutant lacking native PsbP. Expression of His-tagged PsbP successfully restored the oxygen-evolving activity and photoautotrophic growth of the mutant, while PsbP-∆15 lacking the N-terminal 15 residues, which are crucial for the oxygen-evolving activity of spinach PSII in vitro, only partially did. This demonstrated the importance of N-terminal sequence of PsbP for the photosynthetic activity in vivo. Furthermore, the PSII-LHCII supercomplex can be specifically purified from the Chlamydomonas cells having His-tagged PsbP using a metal affinity chromatography. This study provides a platform not only for the functional analysis of PsbP in vivo but also for structural analysis of the PSII-LHCII supercomplex from green algae.

Molecular Basis Underlying Leaf Variegation of a Moth Orchid Mutant (Phalaenopsis aphrodite subsp. formosana)

Leaf variegation is often the focus of plant breeding. Here, we studied a variegated mutant of Phalaenopsis aphrodite subsp. formosana, which is usually used as a parent of horticultural breeding, to understand its anatomic and genetic regulatory mechanisms in variegation. Chloroplasts with well-organized thylakoids and starch grains were found only in the mesophyll cells of green sectors but not of yellow sectors, confirming that the variegation belongs to the chlorophyll type. The two-dimensional electrophoresis and LC/MS/MS also reveal differential expressions of PsbP and PsbO between the green and yellow leaf sectors. Full-length cDNA sequencing revealed that mutant transcripts were caused by intron retention. When conditioning on the total RNA expression, we found that the functional transcript of PsbO and mutant transcript of PsbP are higher expressed in the yellow sector than in the green sector, suggesting that the post-transcriptional regulation of PsbO and PsbP differentiates the performance between green and yellow sectors. Because PsbP plays an important role in the stability of thylakoid folding, we suggest that the negative regulation of PsbP may inhibit thylakoid development in the yellow sectors. This causes chlorophyll deficiency in the yellow sectors and results in leaf variegation. We also provide evidence of the link of virus CymMV and the formation of variegation according to the differential expression of CymMV between green and yellow sectors.

Proteomics analysis reveals a dynamic diurnal pattern of photosynthesis-related pathways in maize leaves

Plant leaves exhibit differentiated patterns of photosynthesis rates under diurnal light regulation. Maize leaves show a single-peak pattern without photoinhibition at midday when the light intensity is maximized. This mechanism contributes to highly efficient photosynthesis in maize leaves. To understand the molecular basis of this process, an isobaric tag for relative and absolute quantitation (iTRAQ)-based proteomics analysis was performed to reveal the dynamic pattern of proteins related to photosynthetic reactions. Steady, single-peak and double-peak protein expression patterns were discovered in maize leaves, and antenna proteins in these leaves displayed a steady pattern. In contrast, the photosystem, carbon fixation and citrate pathways were highly controlled by diurnal light intensity. Most enzymes in the limiting steps of these pathways were major sites of regulation. Thus, maize leaves optimize photosynthesis and carbon fixation outside of light harvesting to adapt to the changes in diurnal light intensity at the protein level.

IbOr Regulates Photosynthesis under Heat Stress by Stabilizing IbPsbP in Sweetpotato

The Orange (Or) protein regulates carotenoid biosynthesis and environmental stress in plants. Previously, we reported that overexpression of the sweetpotato [Ipomoea batatas (L.) Lam] Or gene (IbOr) in transgenic Arabidopsis (referred to as IbOr-OX/At) increased the efficiency of photosystem II (PSII) and chlorophyll content after heat shock. However, little is known about the role of IbOr in PSII-mediated protection against abiotic stress. In this study, comparative proteomics revealed that expression of PsbP (an extrinsic subunit of PSII) is up-regulated in heat-treated IbOr-OX/At plants. We then identified and functionally characterized the PsbP-like gene (IbPsbP) from sweetpotato. IbPsbP is predominantly localized in chloroplast, and its transcripts are tissue-specifically expressed and up-regulated in response to abiotic stress. In addition, IbOr interacts with IbPsbP and protects it from heat-induced denaturation, consistent with the observation that transgenic sweetpotato overexpressing IbOr maintained higher PSII efficiency and chlorophyll content upon exposure to heat stress. These results indicate that IbOr can protect plants from environmental stress not only by controlling carotenoid biosynthesis but also by directly stabilizing PSII.

N-Terminal Lipid Modification Is Required for the Stable Accumulation of CyanoQ in Synechocystis sp. PCC 6803

The CyanoQ protein has been demonstrated to be a component of cyanobacterial Photosystem II (PS II), but there exist a number of outstanding questions concerning its physical association with the complex. CyanoQ is a lipoprotein; upon cleavage of its transit peptide by Signal Peptidase II, which targets delivery of the mature protein to the thylakoid lumenal space, the N-terminal cysteinyl residue is lipid-modified. This modification appears to tether this otherwise soluble component to the thylakoid membrane. To probe the functional significance of the lipid anchor, mutants of the CyanoQ protein have been generated in Synechocystis sp. PCC 6803 to eliminate the N-terminal cysteinyl residue, preventing lipid modification. Substitution of the N-terminal cysteinyl residue with serine (Q-C22S) resulted in a decrease in the amount of detectable CyanoQ protein to 17% that of the wild-type protein. Moreover, the physical properties of the accumulated Q-C22S protein were consistent with altered processing of the CyanoQ precursor. The Q-C22S protein was shifted to a higher apparent molecular mass and partitioned in the hydrophobic phase in TX-114 phase-partitioning experiments. These results suggest that the hydrophobic N-terminal 22 amino acids were not properly cleaved by a signal peptidase. Substitution of the entire CyanoQ transit peptide with the transit peptide of the soluble lumenal protein PsbO yielded the Q-SS mutant and resulted in no detectable accumulation of the modified CyanoQ protein. Finally, the CyanoQ protein was present at normal amounts in the PS II mutant strains ΔpsbB and ΔpsbO, indicating that an association with PS II was not a prerequisite for stable CyanoQ accumulation. Together these results indicate that CyanoQ accumulation in Synechocystis sp. PCC 6803 depends on the presence of the N-terminal lipid anchor, but not on the association of CyanoQ with the PS II complex.

Use of Protein Cross-Linking and Radiolytic Labeling To Elucidate the Structure of PsbO within Higher-Plant Photosystem II

We have used protein cross-linking with the zero-length cross-linker 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide, and radiolytic footprinting coupled with high-resolution tandem mass spectrometry, to examine the structure of higher-plant PsbO when it is bound to Photosystem II. Twenty intramolecular cross-linked residue pairs were identified. On the basis of this cross-linking data, spinach PsbO was modeled using the Thermosynechococcus vulcanus PsbO structure as a template, with the cross-linking distance constraints incorporated using the MODELLER program. Our model of higher-plant PsbO identifies several differences between the spinach and cyanobacterial proteins. The N-terminal region is particularly interesting, as this region has been suggested to be important for oxygen evolution and for the specific binding of PsbO to Photosystem II. Additionally, using radiolytic mapping, we have identified regions on spinach PsbO that are shielded from the bulk solvent. These domains may represent regions on PsbO that interact with other components, as yet unidentified, of the photosystem.

Proteomic Insight into the Response of Arabidopsis Chloroplasts to Darkness

Chloroplast function in photosynthesis is essential for plant growth and development. It is well-known that chloroplasts respond to various light conditions. However, it remains poorly understood about how chloroplasts respond to darkness. In this study, we found 81 darkness-responsive proteins in Arabidopsis chloroplasts under 8 h darkness treatment. Most of the proteins are nucleus-encoded, indicating that chloroplast darkness response is closely regulated by the nucleus. Among them, 17 ribosome proteins were obviously reduced after darkness treatment. The protein expressional patterns and physiological changes revealed the mechanisms in chloroplasts in response to darkness, e.g., (1) inhibition of photosystem II resulted in preferential cyclic electron flow around PSI; (2) promotion of starch degradation; (3) inhibition of chloroplastic translation; and (4) regulation by redox and jasmonate signaling. The results have improved our understanding of molecular regulatory mechanisms in chloroplasts under darkness.

The extrinsic proteins of photosystem II: update

Recent investigations have provided important new insights into the structures and functions of the extrinsic proteins of Photosystem II. This review is an update of the last major review on the extrinsic proteins of Photosystem II (Bricker et al., Biochemistry 31:4623-4628 2012). In this report, we will examine advances in our understanding of the structure and function of these components. These proteins include PsbO, which is uniformly present in all oxygenic organisms, the PsbU, PsbV, CyanoQ, and CyanoP proteins, found in the cyanobacteria, and the PsbP, PsbQ and PsbR proteins, found in the green plant lineage. These proteins serve to stabilize the Mn4CaO5 cluster and optimize oxygen evolution at physiological calcium and chloride concentrations. The mechanisms used to perform these functions, however, remain poorly understood. Recently, important new findings have significantly advanced our understanding of the structures, locations and functions of these important subunits. We will discuss the biochemical, structural and genetic studies that have been used to elucidate the roles played by these proteins within the photosystem and their locations within the photosynthetic complex. Additionally, we will examine open questions needing to be addressed to provide a coherent picture of the role of these components within the photosystem.

The N-terminal sequence of the extrinsic PsbP protein modulates the redox potential of Cyt b559 in photosystem II

The PsbP protein, an extrinsic subunit of photosystem II (PSII) in green plants, is known to induce a conformational change around the catalytic Mn₄CaO₅ cluster securing the binding of Ca²⁺ and Cl^– in PSII. PsbP has multiple interactions with the membrane subunits of PSII, but how these affect the structure and function of PSII requires clarification. Here, we focus on the interactions between the N-terminal residues of PsbP and the α subunit of Cytochrome (Cyt) b₅₅₉ (PsbE). A key observation was that a peptide fragment formed of the first N-terminal 15 residues of PsbP, ‘pN15’, was able to convert Cyt b₅₅₉ into its HP form. Interestingly, addition of pN15 to NaCl-washed PSII membranes decreased PSII’s oxygen-evolving activity, even in the presence of saturating Ca²⁺ and Cl^– ions. In fact, pN15 reversibly inhibited the S₁ to S₂ transition of the OEC in PSII. These data suggest that pN15 can modulate the redox property of Cyt b₅₅₉ involved in the side-electron pathway in PSII. This potential change of Cyt b₅₅₉, in the absence of the C-terminal domain of PsbP, however, would interfere with any electron donation from the Mn₄CaO₅ cluster, leading to the possibility that multiple interactions of PsbP, binding to PSII, have distinct roles in regulating electron transfer within PSII.

Structural Coupling of Extrinsic Proteins with the Oxygen-Evolving Center in Photosystem II

Photosystem II (PSII), which catalyzes photosynthetic water oxidation, is composed of more than 20 subunits, including membrane-intrinsic and -extrinsic proteins. The PSII extrinsic proteins shield the catalytic Mn4CaO5 cluster from the outside bulk solution and enhance binding of inorganic cofactors, such as Ca(2+) and Cl(-), in the oxygen-evolving center (OEC) of PSII. Among PSII extrinsic proteins, PsbO is commonly found in all oxygenic organisms, while PsbP and PsbQ are specific to higher plants and green algae, and PsbU, PsbV, CyanoQ, and CyanoP exist in cyanobacteria. In addition, red algae and diatoms have unique PSII extrinsic proteins, such as PsbQ' and Psb31, suggesting functional divergence during evolution. Recent studies with reconstitution experiments combined with Fourier transform infrared spectroscopy have revealed how the individual PSII extrinsic proteins affect the structure and function of the OEC in different organisms. In this review, we summarize our recent results and discuss changes that have occurred in the structural coupling of extrinsic proteins with the OEC during evolutionary history.

MSH1 Is a Plant Organellar DNA Binding and Thylakoid Protein under Precise Spatial Regulation to Alter Development

As metabolic centers, plant organelles participate in maintenance, defense, and signaling. MSH1 is a plant-specific protein involved in organellar genome stability in mitochondria and plastids. Plastid depletion of MSH1 causes heritable, non-genetic changes in development and DNA methylation. We investigated the msh1 phenotype using hemi-complementation mutants and transgene-null segregants from RNAi suppression lines to sub-compartmentalize MSH1 effects. We show that MSH1 expression is spatially regulated, specifically localizing to plastids within the epidermis and vascular parenchyma. The protein binds DNA and localizes to plastid and mitochondrial nucleoids, but fractionation and protein-protein interactions data indicate that MSH1 also associates with the thylakoid membrane. Plastid MSH1 depletion results in variegation, abiotic stress tolerance, variable growth rate, and delayed maturity. Depletion from mitochondria results in 7%-10% of plants altered in leaf morphology, heat tolerance, and mitochondrial genome stability. MSH1 does not localize within the nucleus directly, but plastid depletion produces non-genetic changes in flowering time, maturation, and growth rate that are heritable independent of MSH1. MSH1 depletion alters non-photoactive redox behavior in plastids and a sub-set of mitochondrially altered lines. Ectopic expression produces deleterious effects, underlining its strict expression control. Unraveling the complexity of the MSH1 effect offers insight into triggers of plant-specific, transgenerational adaptation behaviors.

Novel Features of Eukaryotic Photosystem II Revealed by Its Crystal Structure Analysis from a Red Alga

Photosystem II (PSII) catalyzes light-induced water splitting, leading to the evolution of molecular oxygen indispensible for life on the earth. The crystal structure of PSII from cyanobacteria has been solved at an atomic level, but the structure of eukaryotic PSII has not been analyzed. Because eukaryotic PSII possesses additional subunits not found in cyanobacterial PSII, it is important to solve the structure of eukaryotic PSII to elucidate their detailed functions, as well as evolutionary relationships. Here we report the structure of PSII from a red alga Cyanidium caldarium at 2.76 Å resolution, which revealed the structure and interaction sites of PsbQ', a unique, fourth extrinsic protein required for stabilizing the oxygen-evolving complex in the lumenal surface of PSII. The PsbQ' subunit was found to be located underneath CP43 in the vicinity of PsbV, and its structure is characterized by a bundle of four up-down helices arranged in a similar way to those of cyanobacterial and higher plant PsbQ, although helices I and II of PsbQ' were kinked relative to its higher plant counterpart because of its interactions with CP43. Furthermore, two novel transmembrane helices were found in the red algal PSII that are not present in cyanobacterial PSII; one of these helices may correspond to PsbW found only in eukaryotic PSII. The present results represent the first crystal structure of PSII from eukaryotic oxygenic organisms, which were discussed in comparison with the structure of cyanobacterial PSII.

Identification and Roles of Photosystem II Assembly, Stability, and Repair Factors in Arabidopsis

Photosystem II (PSII) is a multi-component pigment-protein complex that is responsible for water splitting, oxygen evolution, and plastoquinone reduction. Components of PSII can be classified into core proteins, low-molecular-mass proteins, extrinsic oxygen-evolving complex (OEC) proteins, and light-harvesting complex II proteins. In addition to these PSII subunits, more than 60 auxiliary proteins, enzymes, or components of thylakoid protein trafficking/targeting systems have been discovered to be directly or indirectly involved in de novo assembly and/or the repair and reassembly cycle of PSII. For example, components of thylakoid-protein-targeting complexes and the chloroplast-vesicle-transport system were found to deliver PSII subunits to thylakoid membranes. Various auxiliary proteins, such as PsbP-like (Psb stands for PSII) and light-harvesting complex-like proteins, atypical short-chain dehydrogenase/reductase family proteins, and tetratricopeptide repeat proteins, were discovered to assist the de novo assembly and stability of PSII and the repair and reassembly cycle of PSII. Furthermore, a series of enzymes were discovered to catalyze important enzymatic steps, such as C-terminal processing of the D1 protein, thiol/disulfide-modulation, peptidylprolyl isomerization, phosphorylation and dephosphorylation of PSII core and antenna proteins, and degradation of photodamaged PSII proteins. This review focuses on the current knowledge of the identities and molecular functions of different types of proteins that influence the assembly, stability, and repair of PSII in the higher plant Arabidopsis thaliana.

Regulation of Photosynthesis during Abiotic Stress-Induced Photoinhibition

Plants as sessile organisms are continuously exposed to abiotic stress conditions that impose numerous detrimental effects and cause tremendous loss of yield. Abiotic stresses, including high sunlight, confer serious damage on the photosynthetic machinery of plants. Photosystem II (PSII) is one of the most susceptible components of the photosynthetic machinery that bears the brunt of abiotic stress. In addition to the generation of reactive oxygen species (ROS) by abiotic stress, ROS can also result from the absorption of excessive sunlight by the light-harvesting complex. ROS can damage the photosynthetic apparatus, particularly PSII, resulting in photoinhibition due to an imbalance in the photosynthetic redox signaling pathways and the inhibition of PSII repair. Designing plants with improved abiotic stress tolerance will require a comprehensive understanding of ROS signaling and the regulatory functions of various components, including protein kinases, transcription factors, and phytohormones, in the responses of photosynthetic machinery to abiotic stress. Bioenergetics approaches, such as chlorophyll a transient kinetics analysis, have facilitated our understanding of plant vitality and the assessment of PSII efficiency under adverse environmental conditions. This review discusses the current understanding and indicates potential areas of further studies on the regulation of the photosynthetic machinery under abiotic stress.

Localization and functional characterization of the extrinsic subunits of photosystem II: an update

Photosystem II (PSII), which catalyzes photosynthetic water oxidation, is composed of more than 20 subunits, including membrane-intrinsic and -extrinsic proteins. The extrinsic proteins of PSII shield the catalytic Mn4CaO5 cluster from exogenous reductants and serve to optimize oxygen evolution at physiological ionic conditions. These proteins include PsbO, found in all oxygenic organisms, PsbP and PsbQ, specific to higher plants and green algae, and PsbU, PsbV, CyanoQ, and CyanoP in cyanobacteria. Furthermore, red algal PSII has PsbQ′ in addition to PsbO, PsbV, and PsbU, and diatoms have Psb31 in supplement to red algal-type extrinsic proteins, exemplifying the functional divergence of these proteins during evolution. This review provides an updated summary of recent findings on PSII extrinsic proteins and discusses their binding, function, and evolution within various photosynthetic organisms.

The Structure of Photosystem II and the Mechanism of Water Oxidation in Photosynthesis

Oxygenic photosynthesis forms the basis of aerobic life on earth by converting light energy into biologically useful chemical energy and by splitting water to generate molecular oxygen. The water-splitting and oxygen-evolving reaction is catalyzed by photosystem II (PSII), a huge, multisubunit membrane-protein complex located in the thylakoid membranes of organisms ranging from cyanobacteria to higher plants. The structure of PSII has been analyzed at 1.9-Å resolution by X-ray crystallography, revealing a clear picture of the Mn4CaO5 cluster, the catalytic center for water oxidation. This article provides an overview of the overall structure of PSII followed by detailed descriptions of the specific structure of the Mn4CaO5 cluster and its surrounding protein environment. Based on the geometric organization of the Mn4CaO5 cluster revealed by the crystallographic analysis, in combination with the results of a vast number of experimental studies involving spectroscopic and other techniques as well as various theoretical studies, the article also discusses possible mechanisms for water splitting that are currently under consideration.

Use of protein cross-linking and radiolytic footprinting to elucidate PsbP and PsbQ interactions within higher plant Photosystem II

Protein cross-linking and radiolytic footprinting coupled with high-resolution mass spectrometry were used to examine the structure of PsbP and PsbQ when they are bound to Photosystem II. In its bound state, the N-terminal 15-amino-acid residue domain of PsbP, which is unresolved in current crystal structures, interacts with domains in the C terminus of the protein. These interactions may serve to stabilize the structure of the N terminus and may facilitate PsbP binding and function. These interactions place strong structural constraints on the organization of PsbP when associated with the Photosystem II complex. Additionally, amino acid residues in the structurally unresolved loop 3A domain of PsbP ((90)K-(107)V), (93)Y and (96)K, are in close proximity (≤ 11.4 Å) to the N-terminal (1)E residue of PsbQ. These findings are the first, to our knowledge, to identify a putative region of interaction between these two components. Cross-linked domains within PsbQ were also identified, indicating that two PsbQ molecules can interact in higher plants in a manner similar to that observed by Liu et al. [(2014) Proc Natl Acad Sci 111(12):4638-4643] in cyanobacterial Photosystem II. This interaction is consistent with either intra-Photosystem II dimer or inter-Photosystem II dimer models in higher plants. Finally, OH(•) produced by synchrotron radiolysis of water was used to oxidatively modify surface residues on PsbP and PsbQ. Domains on the surface of both protein subunits were resistant to modification, indicating that they were shielded from water and appear to define buried regions that are in contact with other Photosystem II components.