psbA Transcripts Research Articles

Transcriptomic insights into the dominance of two phototrophs throughout the water column of a tropical hypersaline-alkaline crater lake (Dziani Dzaha, Mayotte).

Saline-alkaline lakes often shelter high biomasses despite challenging conditions, owing to the occurrence of highly adapted phototrophs. Dziani Dzaha (Mayotte) is one such lake characterized by the stable co-dominance of the cyanobacterium Limnospira platensis and the picoeukaryote Picocystis salinarum throughout its water column. Despite light penetrating only into the uppermost meter, the prevailing co-dominance of these species persists even in light- and oxygen-deprived zones. Here, a depth profile of phototrophs metatranscriptomes, annotated using genomic data from isolated strains, is employed to identify expression patterns of genes related to carbon processing pathways including photosynthesis, transporters and fermentation. The findings indicate a prominence of gene expression associated with photosynthesis, with a peak of expression around 1 m below the surface, although the light intensity is very low and only red and dark red wavelengths can reach it, given the very high turbidity linked to the high biomass of L. platensis. Experiments on strains confirmed that both species do grow under these wavelengths, at rates comparable to those obtained under white light. A decrease in the expression of photosynthesis-related genes was observed in L. platensis with increasing depth, whereas P. salinarum maintained a very high pool of psbA transcripts down to the deepest point as a possible adaptation against photodamage, in the absence and/or very low levels of expression of genes involved in protection. In the aphotic/anoxic zone, expression of genes involved in fermentation pathways suggests active metabolism of reserve or available dissolved carbon compounds. Overall, L. platensis seems to be adapted to the uppermost water layer, where it is probably maintained thanks to gas vesicles, as evidenced by high expression of the gvpA gene. In contrast, P. salinarum occurs at similar densities throughout the water column, with a peak in abundance and gene expression levels which suggests a better adaptation to lower light intensities. These slight differences may contribute to limited inter-specific competition, favoring stable co-dominance of these two phototrophs.

Transcriptome characterization of Pocillopora grandis transplanted into reefs with different health conditions: potential stress indicators at the holobiont level

Understanding the potential of coral to adapt to environmental stressors that cause coral bleaching is urgent. The molecular responses of the coral holobiont to such stress conditions determine the success of symbiosis. Therefore, by characterizing molecular stress responses at the holobiont level, we can develop better tools to diagnose its health and resilience. However, only some genomic-scale resources are available for reefbuilding corals from the Eastern Tropical Pacific. This study aimed to perform a transcriptomic characterization of the Pocillopora grandis holobiont following transplantation into environments with different health conditions in Colima, Mexico. Healthy specimens of two color morphotypes (green and brown) were collected in Carrizales, a reef in good condition. Coral fragments from the two morphotypes were relocated within the source location (local transplant stress). In contrast, similar fragments were translocated into another reef with a poorer health state, La Boquita. After 24 h, the transplanted fragments were collected, and RNA-seq was performed with the Illumina system. De novo transcriptome assembly, functional annotation, identification of co-expression modules, and enrichment analysis of molecular pathways were performed. The analysis of rRNA LSU, rRNA SSU, and COI sequences confirmed the coral species, whereas analysis of Rubisco, psbA, and psaA transcripts revealed that the dominant endosymbiont was Durusdinium sp. Gene expression patterns observed across samples suggest that the transplantation to a reef with a poorer state of health affected processes such as photosynthesis, calcium homeostasis, and immune response. The transcriptomic indicators proposed here are valuable for further studies examining the adaptation of Pocillopora corals to global changes.

Ferredoxin C2 is required for chlorophyll biosynthesis and accumulation of photosynthetic antennae in Arabidopsis.

Ferredoxins (Fd) are small iron-sulphur proteins, with sub-types that have evolved for specific redox functions. Ferredoxin C2 (FdC2) proteins are essential Fd homologues conserved in all photosynthetic organisms and a number of different FdC2 functions have been proposed in angiosperms. Here we use RNAi silencing in Arabidopsis thaliana to generate a viable fdC2 mutant line with near-depleted FdC2 protein levels. Mutant leaves have ~50% less chlorophyll a and b, and chloroplasts have poorly developed thylakoid membrane structure. Transcriptomics indicates upregulation of genes involved in stress responses. Although fdC2 antisense plants show increased damage at photosystem II (PSII) when exposed to high light, PSII recovers at the same rate as wild type in the dark. This contradicts literature proposing that FdC2 regulates translation of the D1 subunit of PSII, by binding to psbA transcript. Measurement of chlorophyll biosynthesis intermediates revealed a build-up of Mg-protoporphyrin IX, the substrate of the aerobic cyclase. We localise FdC2 to the inner chloroplast envelope and show that the FdC2 RNAi line has a disproportionately lower protein abundance of antennae proteins, which are nuclear-encoded and must be refolded at the envelope after import.

Heat stress, especially when coupled with high light, accelerates the decline of tropical seagrass (Enhalus acoroides) meadows

Heat stress threatens the survival of seagrass, but its damage mechanisms are unclear. In this study, the results reveal that heat stress exceeding 36 °C in the dark caused inactivation of the PSII reaction center, damaging both the PSII donor and acceptor sides in Enhalus acoroides. High light further increased the damage to the photosynthetic apparatus under heat stress. The stronger the heat stress under high light, the harder the recovery of photosynthetic activity. Therefore, during ebb tide at noon in nature, heat stress combined with strong light would induce a significant, even irreversible decrease in photosynthetic activity. Moreover, the heat stress hindered the transcription of psbA and RuBisCO, enhanced respiratory O2, and induced severe peroxidation even if the SOD, APX, and GPX activities significantly improved. The results clearly suggest that heat stress, especially when coupled with high light, may be an important cause for the decline of E. acoroides meadows.

One-helix protein 2 is not required for the synthesis of photosystem II subunit D1 in Chlamydomonas.

In land plants and cyanobacteria, co-translational association of chlorophyll (Chl) to the nascent D1 polypeptide, a reaction center protein of photosystem II (PSII), requires a Chl binding complex consisting of a short-chain dehydrogenase (high chlorophyll fluorescence 244 [HCF244]/uncharacterized protein 39 [Ycf39]) and one-helix proteins (OHP1 and OHP2 in chloroplasts) of the light-harvesting antenna complex superfamily. Here, we show that an ohp2 mutant of the green alga Chlamydomonas (Chlamydomonas reinhardtii) fails to accumulate core PSII subunits, in particular D1 (encoded by the psbA mRNA). Extragenic suppressors arose at high frequency, suggesting the existence of another route for Chl association to PSII. The ohp2 mutant was complemented by the Arabidopsis (Arabidopsis thaliana) ortholog. In contrast to land plants, where psbA translation is prevented in the absence of OHP2, ribosome profiling experiments showed that the Chlamydomonas mutant translates the psbA transcript over its full length. Pulse labeling suggested that D1 is degraded during or immediately after translation. The translation of other PSII subunits was affected by assembly-controlled translational regulation. Proteomics showed that HCF244, a translation factor which associates with and is stabilized by OHP2 in land plants, still partly accumulates in the Chlamydomonas ohp2 mutant, explaining the persistence of psbA translation. Several Chl biosynthesis enzymes overaccumulate in the mutant membranes. Partial inactivation of a D1-degrading protease restored a low level of PSII activity in an ohp2 background, but not photoautotrophy. Taken together, our data suggest that OHP2 is not required for psbA translation in Chlamydomonas, but is necessary for D1 stabilization.

Development of C4 Biochemistry and Change in Expression of Markers for Photosystems I and II in the Single-Cell C4 Species, Bienertia sinuspersici

Bienertia sinuspersici is one of four identified terrestrial plants that perform C4 photosynthesis within a single chlorenchyma cell via the compartmentation of organelles and photosynthetic enzymes. The patterns of accumulation of key photosynthetic enzymes and transcripts in developing leaves were examined using immunolocalization and in situ hybridization. The polypeptides of Rubisco large subunit (RbcL) and pyruvate Pi dikinase (PPDK) accumulated equally in all chloroplasts before the formation of two intracellular cytoplasmic compartments: the central (CCC) and peripheral (PCC) cytoplasmic compartments. The differential accumulation of these enzymes was not completed until the leaf had reached maturity, indicating that the transition from C3 to C4 photosynthesis occurred during leaf maturation. In mature chlorenchyma cells, RbcL accumulated 20-fold higher in the CCC than in the PCC, while PPDK exhibited a concentration gradient that was the lowest in the chloroplasts in the central region of the CCC and the highest in PCC chloroplasts. The pattern of rbcL transcript accumulation followed that of its polypeptides in developing leaves, suggesting that the expression of this gene was likely controlled by transcriptional and/or post-transcriptional processes. Immunocytochemical results examining the distribution of photosystems I and II in the chloroplasts of chlorenchyma cells from mature leaves showed that PSII is more abundant in chloroplasts of the central compartment, whereas PSI is higher in those of the peripheral compartment. The quantitative real-time PCR results of rbcL, psbA, and psaB transcripts from the isolated chloroplasts of each compartment further supported this observation. Our results suggest that multiple levels of regulation play a role in controlling the differential accumulation of photosynthetic gene expression in the dimorphic chloroplasts of single-cell C4 species during leaf development.

Omics-Inferred Partitioning and Expression of Diverse Biogeochemical Functions in a Low-O2 Cyanobacterial Mat Community.

ABSTRACTCyanobacterial mats profoundly influenced Earth’s biological and geochemical evolution and still play important ecological roles in the modern world. However, the biogeochemical functioning of cyanobacterial mats under persistent low-O2 conditions, which dominated their evolutionary history, is not well understood. To investigate how different metabolic and biogeochemical functions are partitioned among community members, we conducted metagenomics and metatranscriptomics on cyanobacterial mats in the low-O2, sulfidic Middle Island sinkhole (MIS) in Lake Huron. Metagenomic assembly and binning yielded 144 draft metagenome assembled genomes, including 61 of medium quality or better, and the dominant cyanobacteria and numerous Proteobacteria involved in sulfur cycling. Strains of a Phormidium autumnale-like cyanobacterium dominated the metagenome and metatranscriptome. Transcripts for the photosynthetic reaction core genes psaA and psbA were abundant in both day and night. Multiple types of psbA genes were expressed from each cyanobacterium, and the dominant psbA transcripts were from an atypical microaerobic type of D1 protein from Phormidium. Further, cyanobacterial transcripts for photosystem I genes were more abundant than those for photosystem II, and two types of Phormidium sulfide quinone reductase were recovered, consistent with anoxygenic photosynthesis via photosystem I in the presence of sulfide. Transcripts indicate active sulfur oxidation and reduction within the cyanobacterial mat, predominately by Gammaproteobacteria and Deltaproteobacteria, respectively. Overall, these genomic and transcriptomic results link specific microbial groups to metabolic processes that underpin primary production and biogeochemical cycling in a low-O2 cyanobacterial mat and suggest mechanisms for tightly coupled cycling of oxygen and sulfur compounds in the mat ecosystem.IMPORTANCE Cyanobacterial mats are dense communities of microorganisms that contain photosynthetic cyanobacteria along with a host of other bacterial species that play important yet still poorly understood roles in this ecosystem. Although such cyanobacterial mats were critical agents of Earth’s biological and chemical evolution through geological time, little is known about how they function under the low-oxygen conditions that characterized most of their natural history. Here, we performed sequencing of the DNA and RNA of modern cyanobacterial mat communities under low-oxygen and sulfur-rich conditions from the Middle Island sinkhole in Lake Huron. The results reveal the organisms and metabolic pathways that are responsible for both oxygen-producing and non-oxygen-producing photosynthesis as well as interconversions of sulfur that likely shape how much O2 is produced in such ecosystems. These findings indicate tight metabolic reactions between community members that help to explain the limited the amount of O2 produced in cyanobacterial mat ecosystems.

The effect of light quality on plant physiology, photosynthetic, and stress response in Arabidopsis thaliana leaves.

The impacts of wavelengths in 500–600 nm on plant response and their underlying mechanisms remain elusive and required further investigation. Here, we investigated the effect of light quality on leaf area growth, biomass, pigments content, and net photosynthetic rate (Pn) across three Arabidopsis thaliana accessions, along with changes in transcription, photosynthates content, and antioxidative enzyme activity. Eleven-leaves plants were treated with BL; 450 nm, AL; 595 nm, RL; 650 nm, and FL; 400–700 nm as control. RL significantly increased leaf area growth, biomass, and promoted Pn. BL increased leaf area growth, carotenoid and anthocyanin content. AL significantly reduced leaf area growth and biomass, while Pn remained unaffected. Petiole elongation was further observed across accessions under AL. To explore the underlying mechanisms under AL, expression of key marker genes involved in light-responsive photosynthetic reaction, enzymatic activity of antioxidants, and content of photosynthates were monitored in Col-0 under AL, RL (as contrast), and FL (as control). AL induced transcription of GSH2 and PSBA, while downregulated NPQ1 and FNR2. Photosynthates, including proteins and starches, showed lower content under AL. SOD and APX showed enhanced enzymatic activity under AL. These results provide insight into physiological and photosynthetic responses to light quality, in addition to identifying putative protective-mechanisms that may be induced to cope with lighting-stress in order to enhance plant stress tolerance.

The Role of Chloroplast Gene Expression in Plant Responses to Environmental Stress.

Chloroplasts are plant organelles that carry out photosynthesis, produce various metabolites, and sense changes in the external environment. Given their endosymbiotic origin, chloroplasts have retained independent genomes and gene-expression machinery. Most genes from the prokaryotic ancestors of chloroplasts were transferred into the nucleus over the course of evolution. However, the importance of chloroplast gene expression in environmental stress responses have recently become more apparent. Here, we discuss the emerging roles of the distinct chloroplast gene expression processes in plant responses to environmental stresses. For example, the transcription and translation of psbA play an important role in high-light stress responses. A better understanding of the connection between chloroplast gene expression and environmental stress responses is crucial for breeding stress-tolerant crops better able to cope with the rapidly changing environment.

The Chloroplast Ribonucleoprotein CP33B Quantitatively Binds the psbA mRNA

Chloroplast RNAs are stabilized and processed by a multitude of nuclear-encoded RNA-binding proteins, often in response to external stimuli like light and temperature. A particularly interesting RNA-based regulation occurs with the psbA mRNA, which shows light-dependent translation. Recently, the chloroplast ribonucleoprotein CP33B was identified as a ligand of the psbA mRNA. We here characterized the interaction of CP33B with chloroplast RNAs in greater detail using a combination of RIP-chip, quantitative dot-blot, and RNA-Bind-n-Seq experiments. We demonstrate that CP33B prefers psbA over all other chloroplast RNAs and associates with the vast majority of the psbA transcript pool. The RNA sequence target motif, determined in vitro, does not fully explain CP33B’s preference for psbA, suggesting that there are other determinants of specificity in vivo.

Chitooligosaccharides enhance cold tolerance by repairing photodamaged PS II in rice

AbstractChitooligosaccharides (COS) are multi-functional foods and nutrients and environmentally friendly biological abiotic-resistance inducing agents for plants. In the current study, the effects and possible mechanisms of COS on improving the cold resistance of rice (II YOU 1259) seedlings were investigated. Compared with the control, a COS pre-soaking treatment enhanced photosynthesis, reduced oxidation damage and led to accumulation of more osmotic regulation substances under chilling treatment. In addition, a novel Deg/HtrA family serine endopeptidase (DegQ) gene, related to COS enhanced rice cold resistance, was identified. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis revealed that transcription of DegQ and psbA (D1 protein encoding gene) were up-regulated in a time-dependent manner by COS treatment under cold stress. With increasing expression of the D1 protein, chlorophyll b content was enhanced correspondingly. The current results suggest that COS could enhance cold stress tolerance of rice by repairing the photodamaged photosystem II, altering osmotic regulation and reducing oxidation damage.

Toxicological sensitivity of Pennisetum americanum (L.) K. Schum to atrazine exposure

ABSTRACTThe tolerance of Pennisetum americanum (L.) K Schum (P. americanum) to the herbicide atrazine, as well as the atrazine accumulation in exposed plants, was investigated in this study. The germination of P. americanum seeds was not significantly inhibited by exposure to atrazine at concentrations below 100 mg·L−1. The roots of the seedlings were much more sensitive to atrazine than the shoots were, as shown by observations that shoot and root elongation were significantly inhibited by treatment with 100 and 50 mg·kg−1 of atrazine, respectively. In addition, significant differences were found in the dry weights of the seedling shoots and roots after exposure to 50 and 20 mg·kg−1 of atrazine, respectively. Atrazine accumulated readily in the roots of exposed seedlings, and the root ultrastructure was visibly damaged by exposure to lower levels of atrazine compared to the ultrastructure of the shoot. The contents of chlorophyll a and chlorophyll b and the transcription of psbA were inhibited by exposure to atrazine at concentrations above 20 mg·kg−1. Finally, the tolerance threshold of P. americanum to atrazine was about 20 mg·kg−1, indicating that the test plant exhibited some atrazine tolerance.

Exogenous application of 5-aminolevulinic acid on wheat seedlings under drought stress enhances the transcription of psbA and psbD genes and improves photosynthesis

The compound 5-aminolevulinic acid (ALA) is an essential precursor for the biosynthesis of porphyrins, including chlorophyll, heme, and cytochromes. The protective effects of ALA on photosynthesis and the expression of photosynthetic genes in wheat under drought stress are not well understood. Two wheat cultivars, drought-tolerant Aikang-58 and drought-sensitive Chinese Spring, were exposed to drought stress induced by 20% polyethylene glycol (PEG-6000) after foliar pretreatment with ALA for 3 days. The results showed that exogenous application of ALA protected the drought-stressed wheat seedlings by significantly inhibiting the decrease in relative water and chlorophyll contents. The ALA-mediated alleviation of stress was similar between the drought-tolerant and drought-sensitive wheat cultivars. Meanwhile, compared to seedlings under drought treatment alone, the ALA-pre-treated wheat seedlings under drought stress maintained higher photosystem II (PSII) functional indexes. The ALA pretreatment reduced the drought-driven accumulation of both H2O2 and ABA and increased the stomatal conductance. Real-time PCR analysis showed that the psbA and psbD gene transcripts were both upregulated under drought stress following ALA pretreatment. The present study suggests that the exogenous foliar application of ALA alleviates the drought stress on wheat seedlings, which is associated with the enhancement of PSII function by inducing chlorophyll synthesis and psbA and psbD transcription. Moreover, the protective effect of ALA pretreatment was not related to the decline in stomatal conductance caused by ABA or H2O2 accumulation.

PTAC10, an S1-domain-containing component of the transcriptionally active chromosome complex, is essential for plastid gene expression in Arabidopsis thaliana and is phosphorylated by chloroplast-targeted casein kinase II.

In higher plant chloroplasts, the plastid-encoded RNA polymerase (PEP) consists of four catalytic subunits and numerous nuclear-encoded accessory proteins, including pTAC10, an S1-domain-containing protein. In this study, pTAC10 knockout lines were characterized. Two ptac10 mutants had an albino phenotype and severely impaired chloroplast development. The pTAC10 genomic sequence fused to a four-tandem MYC tag driven by its own promoter functionally complemented the ptac10-1 mutant phenotype. pTAC10 was present in both the chloroplast stroma and thylakoids. Two-dimensional blue native polyacrylamide gel electrophoresis (BN-PAGE), and immunoblotting assays showed that pTAC10:MYC co-migrates with one of the PEP core subunits, RpoB. A comprehensive investigation of the plastid gene expression profiles by quantitative RT-PCR revealed that, compared with wild-type plants, the abundance of PEP-dependent plastid transcripts is severely decreased in the ptac10-1 mutant, while the amount of plastid transcripts exclusively transcribed by NEP either barely changes or even increases. RNA blot analysis confirmed that PEP-dependent chloroplast transcripts, including psaB, psbA and rbcL, substantially decrease in the ptac10-1 mutant. Immunoblotting showed reduced accumulation of most chloroplast proteins in the ptac10 mutants. These data indicate the essential role of pTAC10 in plastid gene expression and plastid development. pTAC10 interacts with chloroplast-targeted casein kinase 2 (cpCK2) in vitro and in vivo and can be phosphorylated by Arabidopsis cpCK2 in vitro at sites Ser95, Ser396 and Ser434. RNA-EMSA assays showed that pTAC10 is able to bind to the psbA, atpE and accD transcripts, suggesting a non-specific RNA-binding activity of pTAC10. The RNA affinity of pTAC10 was enhanced by phosphorylation and decreased by the amino acid substitution Ser434-Ala of pTAC10. These data show that pTAC10 is essential for plastid gene expression in Arabidopsis and that it can be phosphorylated by cpCK2.

Arabidopsis thaliana mutants lacking cpFtsY or cpSRP54 exhibit different defects in photosystem II repair.

Photosystem II (PS II) is a multi subunit protein complex embedded in the thylakoid membrane of cyanobacteria and chloroplasts. As the PS II reaction center protein D1 is prone to a light induced damage that inhibits PS II function especially at elevated light intensities, a highly ordered repair process including synthesis, targeting and insertion of D1 has evolved. To elucidate the function of the chloroplast signal recognition particle subunits, cpSRP43 and cpSRP54, and the cpSRP-receptor cpFtsY in D1 biogenesis we investigated the efficiency of the PS II repair cycle in the corresponding mutants of Arabidopsis thaliana. Immunological analyses, PAM measurements and in vivo labeling experiments demonstrate an impaired replacement of damaged D1 in the cpftsy mutant, while the chaos and the ffc mutant lacking cpSRP43 and cpSRP54, respectively, were not or hardly affected. The defect in cpftsy was neither caused by an impaired psbA transcript accumulation, D1 translation initiation nor by an enhanced D1 degradation. Further experiments revealed a decreased amount of salt stable, thylakoid membrane-associated translating ribosomes in the cpftsy mutant, while the amount of membrane-associated translating ribosomes is unaltered in the chaos and the ffc mutants. Therefore, our data indicate that the lack of cpFtsY leads to an inefficient PS II repair cycle caused by an impaired binding of translating ribosomes to the thylakoid membrane.

Engineered chloroplast dsRNA silences cytochrome p450 monooxygenase, V-ATPase and chitin synthase genes in the insect gut and disrupts Helicoverpa zea larval development and pupation.

SummaryIn the past two decades, chloroplast genetic engineering has been advanced to achieve high‐level protein accumulation but not for down‐regulation of targeted genes. Therefore, in this report, lepidopteran chitin synthase (Chi), cytochrome P450 monooxygenase (P450) and V‐ATPase dsRNAs were expressed via the chloroplast genome to study RNA interference (RNAi) of target genes in intended hosts. PCR and Southern blot analysis confirmed homoplasmy and site‐specific integration of transgene cassettes into the chloroplast genomes. Northern blots and real‐time qRT‐PCR confirmed abundant processed and unprocessed dsRNA transcripts (up to 3.45 million copies of P450 dsRNAs/μg total RNA); the abundance of cleaved dsRNA was greater than the endogenous psbA transcript. Feeding of leaves expressing P450,Chi and V‐ATPase dsRNA decreased transcription of the targeted gene to almost undetectable levels in the insect midgut, likely after further processing of dsRNA in their gut. Consequently, the net weight of larvae, growth and pupation rates were significantly reduced by chloroplast‐derived dsRNAs. Taken together, successful expression of dsRNAs via the chloroplast genome for the first time opens the door to study RNA interference/processing within plastids. Most importantly, dsRNA expressed in chloroplasts can be utilized for gene inactivation to confer desired agronomic traits or for various biomedical applications, including down‐regulation of dysfunctional genes in cancer or autoimmune disorders, after oral delivery of dsRNA bioencapsulated within plant cells.

If Photoinhibition of Soybean Photosystem II Enhances the Hypersensitive Response, It is Not Solely Due to Blockage of Electron Transfer Flow at D1

Previous studies have suggested that photoinhibition, through inactivation of photosystem II (PSII), could be beneficial to plants during defense to pathogens through enhanced reactive oxygen (ROS), especially during the hypersensitive response (HR). In this study, we addressed this question by focusing on a possible role of turnover and inhibition of the PSII subunit D1, in defense to the compatible and incompatible strains of the bacterial pathogen Pseudomonas syringae in soybean leaves. Expression of the D1 encoding gene, psbA, as well as 14 other chloroplast encoded genes, was down regulated in response to P. syringae. This down regulation is consistent with reduced production of PSII components leading to increased photoinhibition of existing photocenters, and is also consistent with multiple studies showing a concerted down regulation of nuclear-encoded chloroplast genes during pathogen attack. However, although expression of the psbA transcript was reduced in response to pathogen within 8 hours of inoculation, the level of the psbA product, the D1 protein, showed no significant changes via western blots, and did not show any signs of degradation. Additionally, infiltrating leaves with the D1 inhibiting herbicide bentazon (competitive inhibitor of QB binding to D1, stopping photosynthesis by blocking electron transfer from QA to QB) together with P. syringae inoculation, showed that D1 inhibition did not enhance defense as expected (if photoinhibition enhanced defense), but actually rendered the host slightly more susceptible. The results reflect two possibilities. One is that PSII inhibition through blockage of electron flow through D1 of PSII, does not enhance resistance to P. syringae. The second possibility supported by the data is that the mechanism of photoinhibition during pathogen defense is not due solely to the blockage of electron flow, but through another means of stimulating photoinhibition, such as an inefficient degradation, removal, and replacement of damaged D1 from the PSII complex.

Glycine betaine application in grain filling wheat plants alleviates heat and high light-induced photoinhibition by enhancing the psbA transcription and stomatal conductance

Wheat (Triticum aestivum) plants often suffer from heat stress combined with high irradiance during the grain filling stage. Glycine betaine (GB) is a compatible solute that accumulates rapidly to enhance stress tolerance in wheat under abiotic stress. In this study, the effect of foliar application of GB on photosystem II (PSII) photochemistry in wheat leaves under the heat and high light stresses were investigated. The results showed that GB-supplemented wheat plants maintained higher chlorophyll content, photochemical activity of PSII and net photosynthetic rate (P n) than stressed only plants. Moreover, GB-treated plants could retard the decrease of psbA gene transcription and accelerate the endogenous accumulation of GB in leaves. The results suggested that the increasing stress tolerance by GB accumulation was associated with an improvement in D1 protein synthesis, which accelerated the repair of PSII following stress-enhanced photoinhibition. Moreover, the accumulation of abscisic acid under the heat and high light stresses was retarded by foliar application of GB, and the decrease of stomatal conductance was inhibited as well. The results suggested that GB accumulation in vivo was involved in the regulation of stomatal conductance. Further work is required to elucidate the mechanism of GB-induced stomatal movement and PSII photoprotection.

An integrated study of photochemical function and expression of a key photochemical gene (psbA) in photosynthetic communities of Lake Bonney (McMurdo Dry Valleys, Antarctica)

Lake Bonney is one of several permanently ice-covered lakes in the McMurdo Dry Valleys, Antarctica, which maintain the only year-round biological activity on the Antarctic continent. Vertically stratified populations of autotrophic microorganisms occupying the water columns are adapted to numerous extreme conditions, including very low light, hypersalinity, ultra-oligotrophy and low temperatures. In this study, we integrated molecular biology, microscopy, flow cytometry, and functional photochemical analyses of the photosynthetic communities residing in the east and west basins of dry valley Lake Bonney. Diversity and abundance of the psbA gene encoding a major protein of the photosystem II reaction center were monitored during the seasonal transition between Antarctic summer (24-h daylight) to winter (24-h darkness). Vertical trends through the photic zone in psbA abundance (DNA and mRNA) closely matched that of primary production in both lobes. Seasonal trends in psbA transcripts differed between the two lobes, with psbA expression in the west basin exhibiting a transient rise in early Fall. Last, using spectroscopic and flow cytometric analyses, we provide the first evidence that the Lake Bonney photosynthetic community is dominated by picophytoplankton that possess photosynthetic apparatus adapted to extreme shade.

The effect of aluminium-stress and exogenous spermidine on chlorophyll degradation, glutathione reductase activity and the photosystem II D1 protein gene (psbA) transcript level in lichen Xanthoria parietina

In this study, the effects of short-term aluminium toxicity and the application of spermidine on the lichen Xanthoria parietina were investigated at the physiological and transcriptional levels. Our results suggest that aluminium stress leads to physiological processes in a dose-dependent manner through differences in lipid peroxidation rate, chlorophyll content and glutathione reductase (EC 1.6.4.2) activity in aluminium and spermidine treated samples. The expression of the photosystem II D1 protein (psbA) gene was quantified using semi-quantitative RT-PCR. Increased glutathione reductase activity and psbA mRNA transcript levels were observed in the X. parietina thalli that were treated with spermidine before aluminium-stress. The results showed that the application of spermidine could mitigate aluminium-induced lipid peroxidation and chlorophyll degradation on lichen X. parietina thalli through an increase in psbA transcript levels and activity of glutathione reductase (GR) enzymes.