Auxiliary functions of the PsbO, PsbP and PsbQ proteins of higher plant Photosystem II: a critical analysis

High and unique carbonic anhydrase activity of Photosystem II from Pisum sativum: Measurements by a new and very sensitive fluorescence method

Carbonic anhydrase (CA) activity, associated with Photosystem II (PSII), has been shown to enhance water oxidation. However, CA activity was thought to be a side effect or even a "contamination" of other CAs because of the relatively low rates of CA reactions in PSII measured previously. Here, by using 8-hydroxy-pyrene-1,3,6-trisulfonate (pyranine), a fluorescent dye, as a pH indicator, we show that PSII preparations (∗∗BBYs) from Pisum sativum have a high CA activity (as measured by HCO3- dehydration), which is close to that of highly active CAs. This fluorescence method is new for BBYs giving at least ten times higher activity than the other methods used earlier, as well as being highly sensitive and, thus, more convenient to use for BBYs than any other approach. We show here that the pH range of 5.0-7.5 is optimum for the pyranine measuring system, in general, and this pH range is suitable not only for the CA in BBYs but also for other CAs. Further, the CA activity of BBYs has the following unique properties: (1) low sensitivity to some known, and otherwise, effective CA inhibitors; (2) an opposite pH profile of HCO3- dehydration than observed in other known CAs. These findings indicate that the high CA activity, we have observed, belongs to BBYs, i.e., free of other CAs. At pH 6.5, CA activity of BBYs is shown to be directly correlated with that of photosynthetic O2 evolution. We propose that the CA activity may accelerate the removal of H+s during water oxidation. # Celebrating 80th birthday of Alan James Stemler, a pioneer on the role of bicarbonate on the electron donor side of Photosystem II. S.G. Vaklinova & associates (1982), and A.J. Stemler (1986) were the first who have measured carbonic anhydrase activity in Photosystem II preparations. ∗∗ BBYs stand for Photosystem II samples made by the procedure of Berthold (B), Babcock (B) and Yocum (Y); see Berthold et al. (1981).

Transcriptomic analysis of Virescentia guangxiensis (Rhodophyta: Batrachospermales) revealed differential expression of genes in gametophyte and chantransia life phases

BACKGROUND

The genus Virescentia is a significant member of the Batrachospermaceae, exhibiting distinctive life history characteristics defined by alternating generations. This group of taxa has specific environmental requirements for growth. This paper investigates Virescentia, which primarily thrives in freshwater environments, such as streams and springs, characterized by low light, low temperatures, and high dissolved oxygen levels. Currently, no laboratory simulations of their growth conditions have been reported in culture studies. Additionally, previous studies indicate that comparisons of photosynthetic strength across different life-history stages of the same species have not been conducted, mainly due to the challenges of simultaneously collecting algal strains at both life-history stages.

RESULTS

During the gametophyte stage, the chloroplast and mitochondrial genomes were measured at 184,899 bp and 26,867 bp, respectively. In the chantransia stage, the lengths of these genomes were 184,887 bp and 27,014 bp, respectively. A comparison of organellar genome covariation and phylogenetic reconstruction revealed that the chloroplast and mitochondrial genomes across different life history stages were highly conserved, with genetic distances of 0 and nucleotide variants of only 9-15 bp. The mitochondrial genome of gametophyte SXU-YN24005 was found to lack two tRNA-Leu (tag) genes compared to that of the chantransia strain. Additionally, a comparative analysis of KEGG pathway transcriptome data from the two life history stages showed that 33 genes related to the ribosomal pathway and 53 genes associated with the photosynthesis pathway exhibited a significant decline in expression during the gametophyte stage compared to the chantransia stage.

CONCLUSION

In this study, two samples of the same species at different life-history stages were collected from the same location for the first time. The analysis revealed a high degree of conservation between their organelle genomes. Additionally, transcriptome sequencing results indicated substantial differences in gene expression patterns between the two life-history stages. This research will provide reliable data to support the future histological database of freshwater red algae and will establish a theoretical basis for conserving rare and endangered species.

The mechanisms of photoinhibition and repair in plants under high light conditions and interplay with abiotic stressors

This review comprehensively examines the phenomenon of photoinhibition in plants, focusing mainly on the intricate relationship between photodamage and photosystem II (PSII) repair and the role of PSII extrinsic proteins and protein phosphorylation in these processes. In natural environments, photoinhibition occurs together with a suite of concurrent stress factors, including extreme temperatures, drought and salinization. Photoinhibition, primarily caused by high irradiance, results in a critical imbalance between the rate of PSII photodamage and its repair. Central to this process is the generation of reactive oxygen species (ROS), which not only impair the photosynthetic apparatus first PSII but also play a signalling role in chloroplasts and other cellulular structures. ROS generated under stress conditions inhibit the repair of photodamaged PSII by suppressing D1 protein synthesis and affecting PSII protein phosphorylation. Furthermore, this review considers how environmental stressors exacerbate PSII damage by interfering with PSII repair primarily by reducing de novo protein synthesis. In addition to causing direct damage, these stressors also contribute to ROS production by restricting CO2 fixation, which also reduces the intensity of protein synthesis. This knowledge has significant implications for agricultural practices and crop improvement under stressful conditions.

Response Mechanisms of Zelkova schneideriana Leaves to Varying Levels of Calcium Stress

Calcium stress can negatively impact plant growth, prompting plants to respond by mitigating this effect. However, the specific mechanisms underlying this response remain unclear. In this study, we used non-targeted metabolomics and transcriptomics to investigate the response mechanisms of Zelkova schneideriana leaves under varying degrees of calcium stress. Results revealed that calcium stress led to wilt in young leaves. When calcium stress exceeds the tolerance threshold of the leaf, it results in wilting of mature leaves, rupture of chloroplasts in palisade tissue, and extensive wrinkling and breakage of leaf cells. Transcriptomic analysis indicated that calcium stress inhibited photosynthesis by suppressing the expression of genes related to photosynthetic system II and electron transport. Leaf cells activate phenylpropanoid biosynthesis, flavonoid biosynthesis, and Vitamin B6 metabolism to resist calcium stress. When calcium accumulation gradually surpassed the tolerance threshold of the cells, this results in failure of conventional anti-calcium stress mechanisms, leading to cell death. Furthermore, excessive calcium stress inhibits the expression of CNGC and anti-pathogen genes. The results of the metabolomics study showed that five key metabolites increased in response to calcium stress, which may play an important role in countering calcium stress. This study provides insights into the response of Z. schneideriana leaves to different levels of calcium stress, which could provide a theoretical basis for cultivating Z. schneideriana in karst areas and enhance our understanding of plant responses to calcium stress.

Transcriptome profiling reveals the impact of various levels of biochar application on the growth of flue-cured tobacco plants

BACKGROUND

Biochar, a carbon-rich source and natural growth stimulant, is usually produced by the pyrolysis of agricultural biomass. It is widely used to enhance plant growth, enzyme activity, and crop productivity. However, there are no conclusive studies on how different levels of biochar application influence these systems.

METHODS AND RESULTS

The present study elucidated the dose-dependent effects of biochar application on the physiological performance, enzyme activity, and dry matter accumulation of tobacco plants via field experiments. In addition, transcriptome analysis was performed on 60-day-old (early growth stage) and 100-day-old (late growth stage) tobacco leaves to determine the changes in transcript levels at the molecular level under various biochar application levels (0, 600, and 1800 kg/ha). The results demonstrated that optimum biochar application enhances plant growth, regulates enzymatic activity, and promotes biomass accumulation in tobacco plants, while higher biochar doses had adverse effects. Furthermore, transcriptome analysis revealed a total of 6561 differentially expressed genes (DEGs) that were up- or down-regulated in the groupwise comparison under different treatments. KEGG pathways analysis demonstrated that carbon fixation in photosynthetic organisms (ko00710), photosynthesis (ko00195), and starch and sucrose metabolism (ko00500) pathways were significantly up-regulated under the optimal biochar dosage (600 kg/ha) and down-regulated under the higher biochar dosage (1800 kg/ha).

CONCLUSION

Collectively, these results indicate that biochar application at an optimal rate (600 kg/ha) could positively affect photosynthesis and carbon fixation, which in turn increased the synthesis and accumulation of sucrose and starch, thus promoting the growth and dry matter accumulation of tobacco plants. However, a higher biochar dosage (1800 kg/ha) disturbs the crucial source-sink balance of organic compounds and inhibits the growth of tobacco plants.

Transcriptome analysis of the bloom-forming dinoflagellate Prorocentrum donghaiense exposed to Ginkgo biloba leaf extract, with an emphasis on photosynthesis

Ginkgo biloba leaf extract (GBE) can effectively treat bloom-forming freshwater algae. However, there is limited information about the underlying suppression mechanism of the marine bloom-forming Prorocentrum donghaiense-the most dominant algal bloom species in the East China Sea. We investigated the effect of GBE on P. donghaiense in terms of its response to photosynthesis at the molecular/omic level. In total, 93,743 unigenes were annotated using six functional databases. Furthermore, 67,203 differentially expressed genes (DEGs) were identified in algae treated with 1.8 g∙L-1 GBE. Among these DEGs, we identified the genes involved in photosynthesis. PsbA, PsbB and PsbD in photosystem II, PsaA in photosystem I, and PetB and PetD in the cytochrome b6/f complex were downregulated. Other related genes, such as PsaC, PsaE, and PsaF in photosystem I; PetA in the cytochrome b6/f complex; and atpA, atpD, atpH, atpG, and atpE in the F-type H+-ATPase were upregulated. These results suggest that the structure and activity of the complexes were destroyed by GBE, thereby inhibiting the electron flow between the primary and secondary quinone electron acceptors, primary quinone electron acceptor, and oxygen-evolving complex in the PSII complex, and interrupting the electron flow between PSII and PSI, ultimately leading to a decline in algal cell photosynthesis. These findings provide a basis for understanding the molecular mechanisms underlying P. donghaiense exposure to GBE and a theoretical basis for the prevention and control of harmful algal blooms.

Structure of native photosystem II assembly intermediate from Chlamydomonas reinhardtii

Chlamydomonas reinhardtii Photosystem II (PSII) is a dimer consisting of at least 13 nuclear-encoded and four chloroplast-encoded protein subunits that collectively function as a sunlight-driven oxidoreductase. In this study, we present the inaugural structure of a green alga PSII assembly intermediate (pre-PSII-int). This intermediate was isolated from chloroplast membranes of the temperature-sensitive mutant TSP4, cultivated for 14 hours at a non-permissive temperature. The assembly state comprises a monomer containing subunits A, B, C, D, E, F, H, I, K, and two novel assembly factors, Psb1 and Psb2. Psb1 is identified as a novel transmembrane helix located adjacent to PsbE and PsbF (cytochrome b559). The absence of PsbJ, typically found in mature PSII close to this position, indicates that Psb1 functions as an assembly factor. Psb2 is an eukaryotic homolog of the cyanobacterial assembly factor Psb27. The presence of iron, coupled with the absence of QA, QB, and the manganese cluster, implies a protective mechanism against photodamage and provides insights into the intricate assembly process.

Transcriptome analyses reveal photosynthesis-related genes involved in photosynthetic regulation under low temperature stress in Lavandula angustifolia Mill

In order to reveal the mechanisms of photosynthetic regulation of Lavandula angustifolia Mill. under low temperature stress, photosynthesis-related genes were screened and the molecular mechanism were analyzed for this species growing in Harbin, northeast of China. RNA-seq technique and photosynthetic physiology measurement were performed under 20°C, 10°C, and 0°C in this study. The results showed that the observing modified rectangular hyperbola mode could accurately reflect the light-response processes under low temperature stress and the low temperature reduced the light energy utilization of L. angustifolia. The stomatal conductance decreased with the temperature dropping, which was associated with the up-regulation of LaBAM1s, LaMPK4-1 and LaMMK2. The up-regulation of LaMPK4-1 and LaMMK2 was beneficial for ROS scavenging. The improvement of cold resistance in L. angustifolia was related to the up-regulated expression of LaFBA and LaOMTs and down-regulated expression of LaGAPAs, LaGOX, and LaTKL1s with the temperature decreasing. The up-expression of LaPSY at 10°C than it at 20°C could protect the photosynthetic organs from oxidative damage. Moreover, the photosynthetic rates at 10°C and 0°C were close to the measured values, which was related to the interactions of RCA with SBPase and Rubisco with SBPase. These findings could provide a theoretical reference for further exploring the cold tolerance mechanism of L. angustifolia, as an important aromatic plant resource, and promoting its cultivation and distribution in the northeast of China.

Responses of Trollius chinensis to drought stress and rehydration: From photosynthetic physiology to gene expression

Drought stress occurs more frequently in recent years due to the global climate change. Widely distributed in northern China, Mongolia, and Russia, Trollius chinensis Bunge has high medicinal and ornamental values and is often exposed to drought stress, while the mechanism underlying its drought response is still unclear. In this study, we applied 74-76% (control, CK), 49-51% (mild drought), 34-36% (moderate drought), and 19-21% (severe drought, SD) of the soil gravimetric water content to T. chinensis, and measured leaf physiological characteristics on the 0, 5th, 10th, 15th day after the soil reaching the set drought severities, and on the 10th day after rehydration. The results showed that many physiological parameters, such as chlorophyll contents, Fv/Fm, ΦPSⅡ, Pn, and gs decreased with the deepening of severity and duration of drought stress and recovered to some extent after rehydration. On the 10th day of drought stress, leaves in SD and CK were selected for RNA-Seq, and 1649 differentially expressed genes (DEGs) were found, including 548 up-regulated and 1101 down-regulated DEGs. Gene Ontology enrichment found that the DEGs were mainly enriched in catalytic activity and thylakoid. Koyto Encyclopedia of Genes and Genomes enrichment found that DEGs were enriched in some metabolic pathways such as carbon fixation and photosynthesis. Among them, the differential expression of genes related to photosynthesis process, ABA biosynthesis and signaling pathway, such as NCED, SnRK2, PsaD, PsbQ, and PetE, might explain why T. chinensis could tolerate and recover from as long as 15 days of severe drought conditions.

Structure of Chlorella ohadii Photosystem II Reveals Protective Mechanisms against Environmental Stress

Green alga Chlorella ohadii is known for its ability to carry out photosynthesis under harsh conditions. Using cryogenic electron microscopy (cryoEM), we obtained a high-resolution structure of PSII at 2.72 Å. This structure revealed 64 subunits, which encompassed 386 chlorophylls, 86 carotenoids, four plastoquinones, and several structural lipids. At the luminal side of PSII, a unique subunit arrangement was observed to protect the oxygen-evolving complex. This arrangement involved PsbO (OEE1), PsbP (OEE2), PsbB, and PsbU (a homolog of plant OEE3). PsbU interacted with PsbO, PsbC, and PsbP, thereby stabilizing the shield of the oxygen-evolving complex. Significant changes were also observed at the stromal electron acceptor side. PsbY, identified as a transmembrane helix, was situated alongside PsbF and PsbE, which enclosed cytochrome b559. Supported by the adjacent C-terminal helix of Psb10, these four transmembrane helices formed a bundle that shielded cytochrome b559 from the surrounding solvent. Moreover, the bulk of Psb10 formed a protective cap, which safeguarded the quinone site and likely contributed to the stacking of PSII complexes. Based on our findings, we propose a protective mechanism that prevents QB (plastoquinone B) from becoming fully reduced. This mechanism offers insights into the regulation of electron transfer within PSII.

CryoEM PSII structure reveals adaptation mechanisms to environmental stress in Chlorella ohadii

Performing photosynthesis in the desert is a challenging task since it requires a fast adaptation to extreme illumination and temperature changes. To understand adaptive mechanisms, we purified Photosystem II (PSII) from Chlorella ohadii , a green alga from the desert soil surface, and identified structural elements that might enable the photosystem functioning under harsh conditions. The 2.72 Å cryogenic electron-microscopy (cryoEM) structure of PSII exhibited 64 subunits, encompassing 386 chlorophylls, 86 carotenoids, four plastoquinones, and several structural lipids. At the luminal side of PSII, the oxygen evolving complex was protected by a unique subunit arrangement - PsbO (OEE1), PsbP (OEE2), CP47, and PsbU (plant OEE3 homolog). PsbU interacted with PsbO, CP43, and PsbP, thus stabilising the oxygen evolving shield. Substantial changes were observed on the stromal electron acceptor side - PsbY was identified as a transmembrane helix situated alongside PsbF and PsbE enclosing cytochrome b559, supported by the adjacent C-terminal helix of Psb10. These four transmembrane helices bundled jointly, shielding cytochrome b559 from the solvent. The bulk of Psb10 formed a cap protecting the quinone site and probably contributed to the PSII stacking. So far, the C. ohadii PSII structure is the most complete description of the complex, suggesting numerous future experiments. A protective mechanism that prevented Q B from rendering itself fully reduced is proposed.

The lifetime of the oxygen-evolving complex subunit PSBO depends on light intensity and carbon availability in Chlamydomonas

PSBO is essential for the assembly of the oxygen-evolving complex in plants and green algae. Despite its importance, we lack essential information on its lifetime and how it depends on the environmental conditions. We have generated nitrate-inducible PSBO amiRNA lines in the green alga Chlamydomonas reinhardtii. Transgenic strains grew normally under non-inducing conditions, and their photosynthetic performance was comparable to the control strain. Upon induction of the PSBO amiRNA constructs, cell division halted. In acetate-containing medium, cellular PSBO protein levels decreased by 60% within 24 h in the dark, by 75% in moderate light, and in high light, the protein completely degraded. Consequently, the photosynthetic apparatus became strongly damaged, probably due to 'donor-side-induced photoinhibition', and cellular ultrastructure was also severely affected. However, in the absence of acetate during induction, PSBO was remarkably stable at all light intensities and less substantial changes occurred in photosynthesis. Our results demonstrate that the lifetime of PSBO strongly depends on the light intensity and carbon availability, and thus, on the metabolic status of the cells. We also confirm that PSBO is required for photosystem II stability in C. reinhardtii and demonstrate that its specific loss also entails substantial changes in cell morphology and cell cycle.

Effects of abiotic stress on photosystem II proteins

Photosystem II (PSII) represents the most vulnerable component of the photosynthetic machinery and its response in plants subjected to abiotic stress has been widely studied over many years. PSII is a thylakoid membrane-located multiprotein pigment complex that catalyses the light-induced electron transfer from water to plastoquinone with the concomitant production of oxygen. PSII is rich in intrinsic (PsbA and PsbD, namely D1 and D2, CP47 or PsbB and CP43 or PsbC) but also extrinsic proteins. The first ones are more largely conserved from cyanobacteria to higher plants while the extrinsic proteins are different among species. It has been found that extrinsic proteins involved in oxygen evolution change dramatically the PSII efficiency and PSII repair systems. However, little information is available on the effects of abiotic stress on their function and structure.

Transcriptome analysis provides novel insights into the soil amendments induced response in continuously cropped Codonopsis tangshen

Codonopsis tangshen Oliv (C. tangshen) is an important Chinese traditional medicinal plant with various health benefits. However, the growth of C. tangshen are seriously affected by continuous cropping, which led to the decrease of the yield and quality. A field experiment was conducted to learn the effects of soil amendments on the growth of C. tangshen under continuous cropping condition, and the biological events which occurred at molecular level were investigated. The results indicated that the content of chlorophyll a (Chl a), chlorophyll b (Chl b), and carotenoid (Car) was significantly higher in SCPM (silicon-calcium-potassium-magnesium fertilizer), SCPMA (SCPM combined with azoxystrobin) and SCPMAOM (SCPM combined with azoxystrobin and organic manure) treatments. Moreover, the yield and the levels of alkaloid, polysaccharide, flavone and total protein in the treatments of SCPM, SCPMA and SCPMAOM were significantly higher than those in the control, and these indexes were all highest in the SCPMAOM treatment. RNA-sequencing (RNA-Seq) is an economical and efficient method to obtain genetic information for species with or without available genome data. In this study, RNA-Seq was performed to understand how continuously cropped C. tangshen responded to the soil amendments at the transcriptome level. The number of differentially expressed genes (DEGs) were as follows: CK vs. SCPM (719 up- and 1456 down-), CK vs. SCPMA (1302 up- and 1748 down-), CK vs. SCPMAOM (1274 up- and 1678 down-). The soil amendments affected the growth of C. tangshen mainly by regulating the genes involved in pathways of 'photosynthesis,' 'plant hormone signal transduction,' 'biosynthesis of unsaturated fatty acids,' 'phenylpropanoid biosynthesis,' and 'starch and sucrose metabolism,' etc. qRT-PCR was performed to validate the expressions of 10 target genes such as CP26, PsaF, and POX, etc., which verified the reliability of RNA-Seq results. Overall, this study revealed the roles and underlying mechanisms of the soil amendments in regulating the growth of continuously cropped C. tangshen at transcriptome level. These findings are beneficial for improving the continuous cropping tolerance and may be valuable for future genetic improvement of C. tangshen.

NbPsbO1 Interacts Specifically with the Bamboo Mosaic Virus (BaMV) Subgenomic RNA (sgRNA) Promoter and Is Required for Efficient BaMV sgRNA Transcription

Many positive-strand (+) RNA viruses produce subgenomic RNAs (sgRNAs) in the infection cycle through the combined activities of viral replicase and host proteins. However, knowledge about host proteins involved in direct sgRNA promoter recognition is limited. Here, in the partially purified replicase complexes from Bamboo mosaic virus (BaMV)-infected tissue, we have identified the Nicotiana benthamiana photosystem II oxygen-evolving complex protein, NbPsbO1, which specifically interacted with the promoter of sgRNA but not that of genomic RNA (gRNA). Silencing of NbPsbO1 expression suppressed BaMV accumulation in N. benthamiana protoplasts without affecting viral gRNA replication. Overexpression of wild-type NbPsbO1 stimulated BaMV sgRNA accumulation. Fluorescent microscopy examination revealed that the fluorescence associated with NbPsbO1 was redistributed from chloroplast granal thylakoids to stroma in BaMV-infected cells. Overexpression of a mislocalized mutant of NbPsbO1, dTPPsbO1-T7, inhibited BaMV RNA accumulation in N. benthamiana, whereas overexpression of an NbPsbO1 derivative, sPsbO1-T7, designed to be targeted to chloroplast stroma, upregulated the sgRNA level. Furthermore, depletion of NbPsbO1 in BaMV RdRp preparation significantly inhibited sgRNA synthesis in vitro but exerted no effect on (+) or (-) gRNA synthesis, which indicates that NbPsbO1 is required for efficient sgRNA synthesis. These results reveal a novel role for NbPsbO1 in the selective enhancement of BaMV sgRNA transcription, most likely via direct interaction with the sgRNA promoter. IMPORTANCE Production of subgenomic RNAs (sgRNAs) for efficient translation of downstream viral proteins is one of the major strategies adapted for viruses that contain a multicistronic RNA genome. Both viral genomic RNA (gRNA) replication and sgRNA transcription rely on the combined activities of viral replicase and host proteins, which recognize promoter regions for the initiation of RNA synthesis. However, compared to the cis-acting elements involved in the regulation of sgRNA synthesis, the host factors involved in sgRNA promoter recognition mostly remain to be elucidated. Here, we found a chloroplast protein, NbPsbO1, which specifically interacts with Bamboo mosaic virus (BaMV) sgRNA promoter. We showed that NbPsbO1 is relocated to the BaMV replication site in BaMV-infected cells and demonstrated that NbPsbO1 is required for efficient BaMV sgRNA transcription but exerts no effect on gRNA replication. This study provides a new insight into the regulating mechanism of viral gRNA and sgRNA synthesis.

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.

Advances in understanding the physiological role and locations of carbonic anhydrases in C3 plant cells

The review describes the structures of plant carbonic anhydrases (CAs), enzymes catalyzing the interconversion of inorganic carbon forms and belonging to different families, as well as the interaction of inhibitors and activators of CA activity with the active sites of CAs in representatives of these families. We outline the data that shed light on the location of CAs in green cells of C3 plants, algae and angiosperms, with the emphasis on the recently obtained data. The proven and proposed functions of CAs in these organisms are listed. The possibility of the involvement of several chloroplast CAs in acceleration of the conversion of bicarbonate to CO₂ and in supply of CO₂ for fixation by Rubisco is particularly considered. Special attention is paid to CAs in various parts of thylakoids and to discussion about current knowledge of their possible physiological roles. The review states that, despite the significant progress in application of the mutants with suppressed CAs synthesis, the approach based on the use of the inhibitors of CA activity in some cases remains quite effective. Combination of these two approaches, namely determining the effect of CA activity inhibitors in plants with certain knocked-out CA genes, turns out to be very useful for understanding the functions of other CAs.

Stem Photosynthesis-A Key Element of Grass Pea (Lathyrus sativus L.) Acclimatisation to Salinity

Grass pea (Lathyrus sativus) is a leguminous plant of outstanding tolerance to abiotic stress. The aim of the presented study was to describe the mechanism of grass pea (Lathyrus sativus L.) photosynthetic apparatus acclimatisation strategies to salinity stress. The seedlings were cultivated in a hydroponic system in media containing various concentrations of NaCl (0, 50, and 100 mM), imitating none, moderate, and severe salinity, respectively, for three weeks. In order to characterise the function and structure of the photosynthetic apparatus, Chl a fluorescence, gas exchange measurements, proteome analysis, and Fourier-transform infrared spectroscopy (FT-IR) analysis were done inter alia. Significant differences in the response of the leaf and stem photosynthetic apparatus to severe salt stress were observed. Leaves became the place of harmful ion (Na+) accumulation, and the efficiency of their carboxylation decreased sharply. In turn, in stems, the reconstruction of the photosynthetic apparatus (antenna and photosystem complexes) activated alternative electron transport pathways, leading to effective ATP synthesis, which is required for the efficient translocation of Na+ to leaves. These changes enabled efficient stem carboxylation and made them the main source of assimilates. The observed changes indicate the high plasticity of grass pea photosynthetic apparatus, providing an effective mechanism of tolerance to salinity stress.

High light intensity aggravates latent manganese deficiency in maize

Manganese (Mn) plays an important role in the oxygen-evolving complex, where energy from light absorption is used for water splitting. Although changes in light intensity and Mn status can interfere with the functionality of the photosynthetic apparatus, the interaction between these two factors and the underlying mechanisms remain largely unknown. Here, maize seedlings were grown hydroponically and exposed to two different light intensities under Mn-sufficient or -deficient conditions. No visual Mn deficiency symptoms appeared even though the foliar Mn concentration in the Mn-deficient treatments was reduced to 2 µg g-1. However, the maximum quantum yield efficiency of PSII and the net photosynthetic rate declined significantly, indicating latent Mn deficiency. The reduction in photosynthetic performance by Mn depletion was further aggravated when plants were exposed to high light intensity. Integrated transcriptomic and proteomic analyses showed that a considerable number of genes encoding proteins in the photosynthetic apparatus were only suppressed by a combination of Mn deficiency and high light, thus indicating interactions between changes in Mn nutritional status and light intensity. We conclude that high light intensity aggravates latent Mn deficiency in maize by interfering with the abundance of PSII proteins.

Proteomic profiles of young and mature cocoa leaves subjected to mechanical stress caused by wind

Cocoa is a perennial and arboreal species intolerant to strong and frequent winds and, for this reason, is usually grown with windbreaks of trees. The mechanical alterations caused by the wind in the field have a great impact on the growth, development and productivity of cocoa. The present work had a main objective to understand the molecular mechanisms of responses to mechanical stress, caused by the action of constant wind flow in young plants of cocoa through alterations of the proteomic profile in young (YL) and mature leaves (ML). Plants were exposed to constant wind (CW) at a speed of 4.5 m s^-1 for 12 h. There was a reduction in the accumulation of proteins in YL and a significant increase in ML submitted to CW in relation to the control. Differentially accumulated proteins, identified in YL and ML, belong to a broad functional group, related to energy production and carbon metabolism. Besides that, there was a higher efficiency in the protein relative abundance associated to energy production and the assimilation of carbon in the ML exposed to CW, in relation to the control. It was observed the appearance of new isoforms and, or post-transitional changes, which represent an acclimatization and tolerance response of these leaves to the stressor factor. In contrast, in YL, the energy production and the synthesis of gene products essential for their growth and development were affected by the mechanical stress caused by the wind, making them more intolerant.

Close relationship between the state of the oxygen evolving complex and rice cold stress tolerance

The results of the present work suggested a relationship between the growth stability and functional/structural parameters associated to the primary photochemistry and oxygen evolving complex (OEC) in tolerant rice plants under suboptimal low temperatures (SLT) stress. This was concluded from the absence of changes in net photosynthetic rate and in fraction of reaction centers to reduce quinone A, and very small changes in P680 efficiency to trap and donate electrons to quinone A and in fraction of active OEC in tolerant plants under cold stress but not in sensitive plants. The SLT stress also induced OEC activity limitations in both genotypes, but in a greater extent in sensitive plants. However, an assay using an artificial electron donor to replace OEC indicated that the P680⁺ capacity to accept electrons was not altered in both genotypes under SLT stress from the beginning of the stress treatment, suggesting that the OEC structure stability is related to rice SLT tolerance to sustain the photosynthesis. This hypothesis was also supported by the fact that tolerant plants but not sensitive plants did not alter the gene expression and protein content of PsbP under SLT stress, an OEC subunit with a role in stabilizing of OEC structure.

Barley cysteine protease PAP14 plays a role in degradation of chloroplast proteins

Chloroplast protein degradation is known to occur both inside chloroplasts and in the vacuole. Genes encoding cysteine proteases have been found to be highly expressed during leaf senescence. However, it remains unclear where they participate in chloroplast protein degradation. In this study HvPAP14, which belongs to the C1A family of cysteine proteases, was identified in senescing barley (Hordeum vulgare L.) leaves by affinity enrichment using the mechanism-based probe DCG-04 targeting cysteine proteases and subsequent mass spectrometry. Biochemical analyses and expression of a HvPAP14:RFP fusion construct in barley protoplasts was used to identify the subcellular localization and putative substrates of HvPAP14. The HvPAP14:RFP fusion protein was detected in the endoplasmic reticulum and in vesicular bodies. Immunological studies showed that HvPAP14 was mainly located in chloroplasts, where it was found in tight association with thylakoid membranes. The recombinant enzyme was activated by low pH, in accordance with the detection of HvPAP14 in the thylakoid lumen. Overexpression of HvPAP14 in barley revealed that the protease can cleave LHCB proteins and PSBO as well as the large subunit of Rubisco. HvPAP14 is involved in the normal turnover of chloroplast proteins and may have a function in bulk protein degradation during leaf senescence.

Ammonium triggered the response mechanism of lysine crotonylome in tea plants

BACKGROUND

Lysine crotonylation, as a novel evolutionarily conserved type of post-translational modifications, is ubiquitous and essential in cell biology. However, its functions in tea plants are largely unknown, and the full functions of lysine crotonylated proteins of tea plants in nitrogen absorption and assimilation remains unclear. Our study attempts to describe the global profiling of nonhistone lysine crotonylation in tea leaves and to explore how ammonium (NH4+) triggers the response mechanism of lysine crotonylome in tea plants.

RESULTS

Here, we performed the global analysis of crotonylome in tea leaves under NH4+ deficiency/resupply using high-resolution LC-MS/MS coupled with highly sensitive immune-antibody. A total of 2288 lysine crotonylation sites on 971 proteins were identified, of which contained in 15 types of crotonylated motifs. Most of crotonylated proteins were located in chloroplast (37%) and cytoplasm (33%). Compared with NH4+ deficiency, 120 and 151 crotonylated proteins were significantly changed at 3 h and 3 days of NH4+ resupply, respectively. Bioinformatics analysis showed that differentially expressed crotonylated proteins participated in diverse biological processes such as photosynthesis (PsbO, PsbP, PsbQ, Pbs27, PsaN, PsaF, FNR and ATPase), carbon fixation (rbcs, rbcl, TK, ALDO, PGK and PRK) and amino acid metabolism (SGAT, GGAT2, SHMT4 and GDC), suggesting that lysine crotonylation played important roles in these processes. Moreover, the protein-protein interaction analysis revealed that the interactions of identified crotonylated proteins diversely involved in photosynthesis, carbon fixation and amino acid metabolism. Interestingly, a large number of enzymes were crotonylated, such as Rubisco, TK, SGAT and GGAT, and their activities and crotonylation levels changed significantly by sensing ammonium, indicating a potential function of crotonylation in the regulation of enzyme activities.

CONCLUSIONS

The results indicated that the crotonylated proteins had a profound influence on metabolic process of tea leaves in response to NH4+ deficiency/resupply, which mainly involved in diverse aspects of primary metabolic processes by sensing NH4+, especially in photosynthesis, carbon fixation and amino acid metabolism. The data might serve as important resources for exploring the roles of lysine crotonylation in N metabolism of tea plants. Data were available via ProteomeXchange with identifier PXD011610.

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.

Carbonic Anhydrases in Photosynthesizing Cells of C3 Higher Plants

The review presents data on the location, nature, properties, number, and expression of carbonic anhydrase genes in the photosynthesizing cells of C3 plants. The available data about the presence of carbonic anhydrases in plasma membrane, cytoplasm, mitochondria, chloroplast stroma and thylakoids are scrutinized. Special attention was paid to the presence of carbonic anhydrase activities in the different parts of thylakoids, and on collation of sources of these activities with enzymes encoded by the established genes of carbonic anhydrases. The data are presented to show that the consistent incorporation of carbonic anhydrases belonging to different families of these enzymes forms a coherent system of CO2 molecules transport from air to chloroplasts in photosynthesizing cells, where they are included in organic molecules in the carboxylation reaction. It is discussed that the manifestation of the activity of a certain carbonic anhydrase depends on environmental conditions and the stage of ontogenesis.

Proteome of rice roots treated with exogenous proline

Proteomic analysis was conducted to identify the rice root proteins induced by exogenous proline and their involvement in root growth. Proteins were extracted from the root tissues grown under two conditions, T1 (control) and T2 (10 mM proline), and profiled by two-dimensional polyacrylamide gel electrophoresis. Seventeen of 30 differentially expressed proteins were identified by mass spectrometry. Proline-treated rice roots showed up-regulation and down-regulation of nine and eight proteins, respectively, when compared to those in the control. Among the differentially expressed proteins, the down-regulation of glutathione reductase and peroxidase could be involved in the regulation of cellular hydrogen peroxide and reactive oxygen species levels that modulate the root cell wall structure. Differentially expressed proteins identified as pathogenesis-related proteins might be related to stress adaptive mechanisms in response to exogenous proline treatment. In addition, differentially expressed protein identified as the fructose-bisphosphate aldolases and cytochrome c oxidase might be associated with energy metabolism, which is needed during root developmental process. This is the first attempt to study the changes in rice root proteome treated with proline. The acquired information could open new avenues for further functional studies on the involvement of proline in modulating root development and its relation to stress adaptation of plants.

What proteomics can reveal about plant-virus interactions? Photosynthesis-related proteins on the spotlight

Plant viruses are responsible for losses in worldwide production of numerous economically important food and fuel crops. As obligate cellular parasites with very small genomes, viruses rely on their hosts for replication, assembly, intra- and intercellular movement, and attraction of vectors for dispersal. Chloroplasts are photosynthesis and are the site of replication for several viruses. When viruses replicate in chloroplasts, photosynthesis, an essential process in plant physiology, is inhibited. The mechanisms underlying molecular and biochemical changes during compatible and incompatible plants-virus interactions, are only beginning to be elucidated, including changes in proteomic profiles induced by virus infections. In this review, we highlight the importance of proteomic studies to understand plant-virus interactions, especially emphasizing the changes in photosynthesis-related protein accumulation. We focus on: (a) chloroplast proteins that differentially accumulate during viral infection; (b) the significance with respect to chloroplast-virus interaction; and (c) alterations in plant's energetic metabolism and the subsequently the plant defense mechanisms to overcome viral infection.

Transcriptome Analysis of Watermelon Leaves Reveals Candidate Genes Responsive to Cucumber green mottle mosaic virus Infection

Cucumber green mottle mosaic virus (CGMMV) is a member of the genus Tobamovirus, which cause diseases in cucurbits, especially watermelon. In watermelon, symptoms develop on the whole plant, including leaves, stems, peduncles, and fruit. To better understand the molecular mechanisms of watermelon early responses to CGMMV infection, a comparative transcriptome analysis of 24 h CGMMV-infected and mock-inoculated watermelon leaves was performed. A total of 1641 differently expressed genes (DEGs) were identified, with 886 DEGs upregulated and 755 DEGs downregulated after CGMMV infection. A functional analysis indicated that the DEGs were involved in photosynthesis, plant⁻pathogen interactions, secondary metabolism, and plant hormone signal transduction. In addition, a few transcription factor families, including WRKY, MYB, HLH, bZIP and NAC, were responsive to the CGMMV-induced stress. To confirm the high-throughput sequencing results, 15 DEGs were validated by qRT-PCR analysis. The results provide insights into the identification of candidate genes or pathways involved in the responses of watermelon leaves to CGMMV infection.

The presence of the low molecular mass carbonic anhydrase in photosystem II of C3 higher plants

The carrier of carbonic anhydrase (CA) activity was detected in gel among low molecular mass proteins from pea, spinach and Arabidopsis, after nondenaturing electrophoresis in PAAG of the dodecyl-β-d-maltoside treated PSII membranes (the fragments of thylakoid membrane containing PSII complexes). The elimination of Mn-stabilizing protein PsbO by treatment of PSII membranes with salts, did not lead to a decrease in CA activity observed in the gel although it reduced the amount of this protein down to 25% compared to the original sample. The isolated protein PsbO did not demonstrated CA activity. The distinguished features of CA activity of PSII membranes were as follows: 1) resistance to heating, 2) high sensitivity to ethoxyzolamide, the specific inhibitor of CA, and 3) stimulation of this activity by acetazolamide, another specific inhibitor of CA at low concentration of the latter. CA activity was not stimulated by acetazolamide in the PSII membranes samples from Arabidopsis thaliana mutants with knocked out gene At4g20990 encoding αCA4 (according to the nomenclature by Fabre et al., 2007). Taking into account the above data and our previous findings that the energy-dependent part of nonphotochemical quenching of chlorophyll a fluorescence is highly suppressed in that mutant, we suppose that thylakoid membranes of higher plants contain in the vicinity of PSII complex a true CA belonging to the α family of CAs.

Physiological and structural adjustments of two ecotypes of Platanus orientalis L. from different habitats in response to drought and re-watering

Platanus orientalis covers a very fragmented area in Europe and, at the edge of its natural distribution, is considered a relic endangered species near extinction. In our study, it was hypothesized that individuals from the edge of the habitat, with stronger climate constrains (drier and warmer environment, Italy, IT ecotype), developed different mechanisms of adaptation than those growing under optimal conditions at the center of the habitat (more humid and colder environment, Bulgaria, BG ecotype). Indeed, the two P. orientalis ecotypes displayed physiological, structural and functional differences already under control (unstressed) conditions. Adaptation to a dry environment stimulated constitutive isoprene emission, determined active stomatal behavior, and modified chloroplast ultrastructure, ultimately allowing more effective use of absorbed light energy for photochemistry. When exposed to short-term acute drought stress, IT plants showed active stomatal control that enhanced instantaneous water use efficiency, and stimulation of isoprene emission that sustained photochemistry and reduced oxidative damages to membranes, as compared to BG plants. None of the P. orientalis ecotypes recovered completely from drought stress after re-watering, confirming the sensitivity of this mesophyte to drought. Nevertheless, the IT ecotype showed less damage and better stability at the level of chloroplast membrane parameters when compared to the BG ecotype, which we interpret as possible adaptation to hostile environments and improved capacity to cope with future, likely more recurrent, drought stress.

Physiological, ultrastructural and proteomic responses of tobacco seedlings exposed to silver nanoparticles and silver nitrate

Since silver nanoparticles (AgNPs) are a dominant nanomaterial in consumer products, there is growing concern about their impact on the environment. Although numerous studies on the effects of AgNPs on living organisms have been conducted, the interaction of AgNPs with plants has not been fully clarified. To reveal the plant mechanisms activated after exposure to AgNPs and to differentiate between effects specific to nanoparticles and ionic silver, we investigated the physiological, ultrastructural and proteomic changes in seedlings of tobacco (Nicotiana tabacum) exposed to commercial AgNPs and ionic silver (AgNO3) from the seed stage. A higher Ag content was measured in seedlings exposed to AgNPs than in those exposed to the same concentration of AgNO3. However, the results on oxidative stress parameters obtained revealed that, in general, higher toxicity was recorded in AgNO3-treated seedlings than in those exposed to nanosilver. Ultrastructural analysis of root cells confirmed the presence of silver in the form of nanoparticles, which may explain the lower toxicity of AgNPs. However, the ultrastructural changes of chloroplasts as well as proteomic study showed that both AgNPs and AgNO3 can affect photosynthesis. Moreover, the majority of the proteins involved in the primary metabolism were up-regulated after both types of treatments, indicating that enhanced energy production, which can be used to reinforce defensive mechanisms, enables plants to cope with silver-induced toxicity.

Effects of PSII Manganese-Stabilizing Protein Succinylation on Photosynthesis in the Model Cyanobacterium Synechococcus sp. PCC 7002

Lysine succinylation is a newly identified protein post-translational modification and plays important roles in various biological pathways in both prokaryotes and eukaryotes, but its extent and function in photosynthetic organisms remain largely unknown. Here, we performed the first systematic studies of lysine succinylation in cyanobacteria, which are the only prokaryotes capable of oxygenic photosynthesis and the established model organisms for studying photosynthetic mechanisms. By using mass spectrometry analysis in combination with the enrichment of succinylated peptides from digested cell lysates, we identified 1,704 lysine succinylation sites on 691 proteins in a model cyanobacterium Synechococcus sp. PCC 7002. Bioinformatic analysis revealed that a large proportion of the succinylation sites were present on proteins in photosynthesis and metabolism. Among all identified succinylated proteins involved in photosynthesis, the PSII manganese-stabilizing protein (PsbO) was found to be succinylated on Lys99 and Lys234. Functional studies of PsbO were performed by site-directed mutagenesis, and mutants mimicking either constitutively succinylated (K99E and K234E) or non-succinylated states (K99R and K234R) were constructed. The succinylation-mimicking K234E mutant exhibited a decreased oxygen evolution rate of the PSII center and the efficiency of energy transfer during the photosynthetic reaction. Molecular dynamics simulations suggested a mechanism that may allow succinylation to influence the efficiency of photosynthesis by altering the conformation of PsbO, thereby hindering the interaction between PsbO and the PSII core. Our findings suggest that reversible succinylation may be an important regulatory mechanism during photosynthesis in Synechococcus, as well as in other photosynthetic organisms.

ATP is a driving force in the repair of photosystem II during photoinhibition

Repair of photosystem II (PSII) during photoinhibition involves replacement of photodamaged D1 protein by newly synthesized D1 protein. In this review, we summarize evidence for the indispensability of ATP in the degradation and synthesis of D1 during the repair of PSII. Synthesis of one molecule of the D1 protein consumes more than 1,300 molecules of ATP equivalents. The degradation of photodamaged D1 by FtsH protease also consumes approximately 240 molecules of ATP. In addition, ATP is required for several other aspects of the repair of PSII, such as transcription of psbA genes. These requirements for ATP during the repair of PSII have been demonstrated by experiments showing that the synthesis of D1 and the repair of PSII are interrupted by inhibitors of ATP synthase and uncouplers of ATP synthesis, as well as by mutation of components of ATP synthase. We discuss the contribution of cyclic electron transport around photosystem I to the repair of PSII. Furthermore, we introduce new terms relevant to the regulation of the PSII repair, namely, "ATP-dependent regulation" and "redox-dependent regulation," and we discuss the possible contribution of the ATP-dependent regulation of PSII repair under environmental stress.

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.

Tobacco vein banding mosaic virus 6K2 Protein Hijacks NbPsbO1 for Virus Replication

Chloroplast-bound vesicles are key components in viral replication complexes (VRCs) of potyviruses. The potyviral VRCs are induced by the second 6 kDa protein (6K2) and contain at least viral RNA and nuclear inclusion protein b. To date, no chloroplast protein has been identified to interact with 6K2 and involve in potyvirus replication. In this paper, we showed that the Photosystem II oxygen evolution complex protein of Nicotiana benthamiana (NbPsbO1) was a chloroplast protein interacting with 6K2 of Tobacco vein banding mosaic virus (TVBMV; genus Potyvirus) and present in the VRCs. The first 6 kDa protein (6K1) was recruited to VRCs by 6K2 but had no interaction with NbPSbO1. Knockdown of NbPsbO1 gene expression in N. benthamiana plants through virus-induced gene silencing significantly decreased the accumulation levels of TVBMV and another potyvirus Potato virus Y, but not Potato virus X of genus Potexvirus. Amino acid substitutions in 6K2 that disrupted its interaction with NbPsbO1 also affected the replication of TVBMV. NbPsbP1 and NbPsbQ1, two other components of the Photosystem II oxygen evolution complex had no interaction with 6K2 and no effect on TVBMV replication. To conclude, 6K2 recruits 6K1 to VRCs and hijacks chloroplast protein NbPsbO1 to regulate potyvirus replication.

Manganese Deficiency in Plants: The Impact on Photosystem II

Manganese (Mn) is an essential plant micronutrient with an indispensable function as a catalyst in the oxygen-evolving complex (OEC) of photosystem II (PSII). Even so, Mn deficiency frequently occurs without visual leaf symptoms, thereby masking the distribution and dimension of the problem restricting crop productivity in many places of the world. Hence, timely alleviation of latent Mn deficiency is a challenge in promoting plant growth and quality. We describe here the key mechanisms of Mn deficiency in plants by focusing on the impact of Mn on PSII stability and functionality. We also address the mechanisms underlying the differential tolerance towards Mn deficiency observed among plant genotypes, which enable Mn-efficient plants to grow on marginal land with poor Mn availability.

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.

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.

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.

PGR5-PGRL1-Dependent Cyclic Electron Transport Modulates Linear Electron Transport Rate in Arabidopsis thaliana

Plants need tight regulation of photosynthetic electron transport for survival and growth under environmental and metabolic conditions. For this purpose, the linear electron transport (LET) pathway is supplemented by a number of alternative electron transfer pathways and valves. In Arabidopsis, cyclic electron transport (CET) around photosystem I (PSI), which recycles electrons from ferrodoxin to plastoquinone, is the most investigated alternative route. However, the interdependence of LET and CET and the relative importance of CET remain unclear, largely due to the difficulties in precise assessment of the contribution of CET in the presence of LET, which dominates electron flow under physiological conditions. We therefore generated Arabidopsis mutants with a minimal water-splitting activity, and thus a low rate of LET, by combining knockout mutations in PsbO1, PsbP2, PsbQ1, PsbQ2, and PsbR loci. The resulting Δ5 mutant is viable, although mature leaves contain only ∼ 20% of wild-type naturally less abundant PsbO2 protein. Δ5 plants compensate for the reduction in LET by increasing the rate of CET, and inducing a strong non-photochemical quenching (NPQ) response during dark-to-light transitions. To identify the molecular origin of such a high-capacity CET, we constructed three sextuple mutants lacking the qE component of NPQ (Δ5 npq4-1), NDH-mediated CET (Δ5 crr4-3), or PGR5-PGRL1-mediated CET (Δ5 pgr5). Their analysis revealed that PGR5-PGRL1-mediated CET plays a major role in ΔpH formation and induction of NPQ in C3 plants. Moreover, while pgr5 dies at the seedling stage under fluctuating light conditions, Δ5 pgr5 plants are able to survive, which underlines the importance of PGR5 in modulating the intersystem electron transfer.

Manganese Compounds as Water-Oxidizing Catalysts: From the Natural Water-Oxidizing Complex to Nanosized Manganese Oxide Structures

All cyanobacteria, algae, and plants use a similar water-oxidizing catalyst for water oxidation. This catalyst is housed in Photosystem II, a membrane-protein complex that functions as a light-driven water oxidase in oxygenic photosynthesis. Water oxidation is also an important reaction in artificial photosynthesis because it has the potential to provide cheap electrons from water for hydrogen production or for the reduction of carbon dioxide on an industrial scale. The water-oxidizing complex of Photosystem II is a Mn-Ca cluster that oxidizes water with a low overpotential and high turnover frequency number of up to 25-90 molecules of O2 released per second. In this Review, we discuss the atomic structure of the Mn-Ca cluster of the Photosystem II water-oxidizing complex from the viewpoint that the underlying mechanism can be informative when designing artificial water-oxidizing catalysts. This is followed by consideration of functional Mn-based model complexes for water oxidation and the issue of Mn complexes decomposing to Mn oxide. We then provide a detailed assessment of the chemistry of Mn oxides by considering how their bulk and nanoscale properties contribute to their effectiveness as water-oxidizing catalysts.

Recent advances in the use of mass spectrometry to examine structure/function relationships in photosystem II

Tandem mass spectrometry often coupled with chemical modification techniques, is developing into increasingly important tool in structural biology. These methods can provide important supplementary information concerning the structural organization and subunit make-up of membrane protein complexes, identification of conformational changes occurring during enzymatic reactions, identification of the location of posttranslational modifications, and elucidation of the structure of assembly and repair complexes. In this review, we will present a brief introduction to Photosystem II, tandem mass spectrometry and protein modification techniques that have been used to examine the photosystem. We will then discuss a number of recent case studies that have used these techniques to address open questions concerning PS II. These include the nature of subunit-subunit interactions within the phycobilisome, the interaction of phycobilisomes with Photosystem I and the Orange Carotenoid Protein, the location of CyanoQ, PsbQ and PsbP within Photosystem II, and the identification of phosphorylation and oxidative modification sites within the photosystem. Finally, we will discuss some of the future prospects for the use of these methods in examining other open questions in PS II structural biochemistry.

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.

Parallel subfunctionalisation of PsbO protein isoforms in angiosperms revealed by phylogenetic analysis and mapping of sequence variability onto protein structure

BACKGROUND

PsbO, the manganese-stabilising protein, is an indispensable extrinsic subunit of photosystem II. It plays a crucial role in the stabilisation of the water-splitting Mn4CaO5 cluster, which catalyses the oxidation of water to molecular oxygen by using light energy. PsbO was also demonstrated to have a weak GTPase activity that could be involved in regulation of D1 protein turnover. Our analysis of psbO sequences showed that many angiosperm species express two psbO paralogs, but the pairs of isoforms in one species were not orthologous to pairs of isoforms in distant species.

RESULTS

Phylogenetic analysis of 91 psbO sequences from 49 land plant species revealed that psbO duplication occurred many times independently, generally at the roots of modern angiosperm families. In spite of this, the level of isoform divergence was similar in different species. Moreover, mapping of the differences on the protein tertiary structure showed that the isoforms in individual species differ from each other on similar positions, mostly on the luminally exposed end of the β-barrel structure. Comparison of these differences with the location of differences between PsbOs from diverse angiosperm families indicated various selection pressures in PsbO evolution and potential interaction surfaces on the PsbO structure.

CONCLUSIONS

The analyses suggest that similar subfunctionalisation of PsbO isoforms occurred parallelly in various lineages. We speculate that the presence of two PsbO isoforms helps the plants to finely adjust the photosynthetic apparatus in response to variable conditions. This might be mediated by diverse GTPase activity, since the isoform differences predominate near the predicted GTP-binding site.

Isolation of Plant Photosystem II Complexes by Fractional Solubilization

Photosystem II (PSII) occurs in different forms and supercomplexes in thylakoid membranes. Using a transplastomic strain of Nicotiana tabacum histidine tagged on the subunit PsbE, we have previously shown that a mild extraction protocol with β-dodecylmaltoside enriches PSII characteristic of lamellae and grana margins. Here, we characterize residual granal PSII that is not extracted by this first solubilization step. Using affinity purification, we demonstrate that this PSII fraction consists of PSII-LHCII mega- and supercomplexes, PSII dimers, and PSII monomers, which were separated by gel filtration and functionally characterized. Our findings represent an alternative demonstration of different PSII populations in thylakoid membranes, and they make it possible to prepare PSII-LHCII supercomplexes in high yield.

Isolation of Plant Photosystem II Complexes by Fractional Solubilization

Photosystem II (PSII) occurs in different forms and supercomplexes in thylakoid membranes. Using a transplastomic strain of Nicotiana tabacum histidine tagged on the subunit PsbE, we have previously shown that a mild extraction protocol with β-dodecylmaltoside enriches PSII characteristic of lamellae and grana margins. Here, we characterize residual granal PSII that is not extracted by this first solubilization step. Using affinity purification, we demonstrate that this PSII fraction consists of PSII-LHCII mega- and supercomplexes, PSII dimers, and PSII monomers, which were separated by gel filtration and functionally characterized. Our findings represent an alternative demonstration of different PSII populations in thylakoid membranes, and they make it possible to prepare PSII-LHCII supercomplexes in high yield.

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.

Modulations of physiological responses and possible involvement of defense-related secondary metabolites in acclimation of Artemisia annua L. against short-term UV-B radiation

UV - B radiation exposure for upto 3 h did not cause direct damage to physiology, but adjusted secondary metabolism and metabolites accumulation as an effective acclimation mechanism to mitigate the adverse effects of radiation. Artemisia annua L. plants were irradiated with UV-B radiation (280-315 nm; 2.8 Wm(-2)) for different short-term (1, 2, 3 and 4 h) durations. UV-B irradiation of 3 h reduced the photosynthetic rate, stomatal conductance and transpiration rate. However, F v/F m, a sensitive indicator of photosynthetic inhibition, remained stable (0.78) upto 3 h, thereafter it declined sharply (0.72). Interestingly, transcript level of LHCB1, PSBA and PSBO genes related to photosystem II (PSII) were induced under UV-B exposure. In addition, genes coding for Rubisco small (RBCS1B) and large (RBCL) subunits were also upregulated upto 3 h. To mitigate the adverse effects of UV-B radiation, plants tremendously induced defense-related secondary metabolites such as antioxidative phenolics, UV-B absorbing flavonoids, anthocyanins and protective terpenes. The GC-MS analysis of essential oils revealed relatively higher production of monoterpenes over sesquiterpenes as well as 1.2-folds higher total oil yield under UV-B radiation. Owing to its diverse biological activities, the altered quantity and quality of essential oil of A. annua may contribute towards improving its therapeutic properties. The results suggest that UV-B irradiation upto 3 h reduced photosynthesis, probably due to stomatal limitations rather than any direct injury to photosynthetic apparatus as evident from stable F v/F m value, upregulated genes and greater accumulation of their corresponding proteins which gauge PSII health, elevated UV-B absorbing compounds and other protective metabolites. Correlation analysis indicates a significant positive correlation of photosynthetic rate with stomatal conductance while a negative correlation with anthocyanin and monoterpene contents under UV-B radiation. The present study provides first hand information regarding photosynthesis, related physiological parameters and essential oil profiling in response to UV-B radiation in A. annua.