Higher plants possess the ability to trigger a long-term acclimatory response to different environmental light conditions through the regulation of the light-harvesting antenna size of photosystem II. The present study provides an insight into the molecular nature of the signal which initiates the high light-mediated response of a reduction in antenna size. Using barley (Hordeum vulgare) plants, it is shown (i) that the light-harvesting antenna size is not reduced in high light with a low hydrogen peroxide content in the leaves; and (ii) that a decrease in the antenna size is observed in low light in the presence of an elevated concentration of hydrogen peroxide in the leaves. In particular, it has been demonstrated that the ability to reduce the antenna size of photosystem II in high light is restricted to photosynthetic apparatus with a reduced level of the plastoquinone pool and with a low hydrogen peroxide content. Conversely, the reduction of antenna size in low light is induced in photosynthetic apparatus possessing elevated hydrogen peroxide even when the reduction level of the plastoquinone pool is low. Hydrogen peroxide affects the relative abundance of the antenna proteins that modulate the antenna size of photosystem II through a down-regulation of the corresponding lhcb mRNA levels. This work shows that hydrogen peroxide contributes to triggering the photosynthetic apparatus response for the reduction of the antenna size of photosystem II by being the molecular signal for the long-term acclimation of plants to high light.
Changes in expression levels of genes encoding carbonic anhydrases α-CA1, α-CA2, α-CA4, β-CA1, β-CA2, β-CA3, β-CA4, β-CA5, and β-CA6 in Arabidopsis thaliana leaves after light increase from 80 to 400 µmol PAR quanta·m·s were investigated under short day (8 h) and long day (16 h) photoperiods. The expression of two forms of the gene, At3g01500.2 and At3g01500.3, encoding the most abundant carbonic anhydrase of leaves, β-CA1, situated in chloroplast stroma, was found. The content of At3g01500.3 transcripts was higher by approximately an order of magnitude compared to the content of At3g01500.2 transcripts. When plants were adapted to high light intensity under short day photoperiod, the expression level of both forms increased, whereas under long day photoperiod, the content of At3g01500.3 transcripts increased, and the content of transcripts of At3g01500.2 decreased. The expression levels of the At3g01500.3 gene and of genes encoding chloroplast carbonic anhydrases α-CA1, α-CA4, α-CA2 and cytoplasmic carbonic anhydrase β-CA2 increased significantly in response to increase in light intensity under short day, and these of the first three genes increased under long day as well. The expression level of the gene encoding α-CA2 under long day photoperiod as well as of genes of chloroplast β-CA5 and β-CA4 from plasma membranes and mitochondrial β-CA6 under both photoperiods depended insignificantly on light intensity. Hypotheses about the roles in higher plant metabolism of the studied carbonic anhydrases are discussed considering the effects of light intensity on expression levels of the correspondent genes.
Effect of knockout of the At4g20990 gene encoding α-carbonic anhydrase 4 (α-CA4) in Arabidopsis thaliana in plants grown in low light (LL, 80 μmol photons m s) or in high light (HL, 400 μmol photons m s) under long (LD, 16 h) or short (SD, 8 h) day length was studied. In α-CA4 knockout plants, under all studied conditions, the non-photochemical quenching was lower; the decrease was more pronounced under HL. This pointed to α-CA4 implication in the processes leading to energy dissipation in PSII antenna. In this context the content of major antenna proteins Lhcb1 and Lhcb2 was lower in α-CA4 knockouts than in wild-type (WT) plants under all growth conditions. The expression level of lhcb2 gene was also lower in mutants grown under LD, LL and HL in comparison to WT. At the same time, this level was higher in mutants grown under SD, LL and it was the same under SD, HL. Overall, the data showed that the knockout of the At4g20990 gene affected both the contents of proteins of PSII light-harvesting complex and the expression level of genes encoding these proteins, with peculiarities dependent on day length. These data together with the fact of a decrease of non-photochemical quenching of leaf chlorophyll a fluorescence in α-CA4-mut as compared with that in WT plants implied that α-CA4 participates in acclimation of photosynthetic apparatus to light intensity, possibly playing important role in the photoprotection. The role of this CA can be especially important in plants growing under high illumination conditions.
The expression of genes of two carbonic anhydrases (CA) belonging to the α-family, α-CA2 and α-CA4 (according to the nomenclature in N. Fabre et al. (2007) Plant Cell Environ., 30, 617-629), was studied in arabidopsis (Arabidopsis thaliana, var. Columbia) leaves. The expression of the At2g28210 gene coding α-CA2 decreased under increase in plant illumination, while the expression of the At4g20990 gene coding α-CA4 increased. Under conditions close to optimal for photosynthesis, in plants with gene At2g28210 knockout, the effective quantum yield of photosystem 2 and the light-induced accumulation of hydrogen peroxide in leaves were lower than in wild type plants, while the coefficient of non-photochemical quenching of leaf chlorophyll a fluorescence and the rate of CO2 assimilation in leaves were higher. In plants with At4g20990 gene knockout, the same characteristics changed in opposite ways relative to wild type. Possible mechanisms of the participation of α-CA2 and α-CA4 in photosynthetic reactions are discussed, taking into account that protons can be either consumed or released in the reactions they catalyze.
The review covers data representing the plastoquinone pool as the component integrated in plant antioxidant defense and plant signaling. The main goal of the review is to discuss the evidence describing the plastoquinone‐involved biochemical reactions, which are incorporated in maintaining the sustainability of higher plants to stress conditions. In this context, the analysis of the reactions of various redox forms of plastoquinone with oxygen species is presented. The review describes how these reactions can constitute both the antioxidant and signaling functions of the pool. Special attention is paid to the reaction of superoxide anion radicals with plastohydroquinone molecules, producing hydrogen peroxide as signal molecules. Attention is also given to the processes affecting the redox state of the plastoquinone pool because the redox state of the pool is of special importance for antioxidant defense and signaling.
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