One of the adaptive mechanisms used by photosynthetic organisms in response to changing light conditions is redistribution of antenna complexes between the photosystems, a process known as state transitions (ST). This mechanism allows to regulate the amount of light energy absorbed by the photosystems. Numerous studies have reported inhibition of ST at the elevated light intensity; however, the mechanism underlying this process is still debated. We evaluated the effect of H₂O₂ at various concentrations on the ST process in functionally active thylakoids isolated from Arabidopsis thaliana leaves and investigated which stage of this process is affected by H₂O₂. To assess the extent of ST, we measured low-temperature chlorophyll a fluorescence spectra (650-780 nm) and calculated the F745/F685 ratio, whose changes can serve as an indicator of ST progression. H₂O₂ inhibited ST under the low-intensity light conditions and, furthermore, led to a decrease in the accumulation of phosphorylated Lhcb1 and Lhcb2 proteins involved in ST. This suggests that the observed ST inhibition resulted from the suppression of STN7 kinase activity. Importantly, H₂O₂ in the tested concentrations did not affect the electron transport rate, indicating that the inhibition of STN7 kinase activity was not associated with suppression of the photosynthetic electron transport chain (PETC) activity. The treatment with H₂O₂ did not reduce the level of phosphorylated D1 protein (a product of phosphorylation by the thylakoid STN8 kinase). Taken together, these results demonstrate for the first time the mechanism by which H₂O₂ inhibits STN7 kinase activity and, consequently, the process of ST.

The knockout of the At2g28210 gene encoding α-carbonic anhydrase 2 (α-CA2) in Arabidopsis thaliana (Columbia) led to alterations in photosynthetic processes. The effective quantum yields of both photosystem II (PSII) and photosystem I (PSI) were higher in α-carbonic anhydrase 2 knockout plants (α-CA2-KO), and the reduction state of plastoquinone pool was lower than in wild type (WT). The electron transport rate in the isolated thylakoids measured with methyl viologen was higher in α-CA2-KO plants. The amounts of reaction centers of PSII and PSI were similar in WT and α-CA2-KO plants. The non-photochemical quenching of chlorophyll a fluorescence in α-CA2-KO leaves was lower at the beginning of illumination, but became slightly higher than in WT leaves when the steady state was achieved. The degree of state transitions in the leaves was lower in α-CA2-KO than in WT plants. Measurements of the electrochromic carotenoid absorbance shift (ECS) revealed that the light-dependent pH gradient (ΔpH) across the thylakoid membrane was lower in the leaves of α-CA2-KO plants than in WT plants. The starch content in α-CA2-KO leaves was lower than in WT plants. The expression levels of the genes encoding chloroplast CAs in α-CA2-KO changed noticeably, whereas the expression levels of genes of cytoplasmic CAs remained almost the same. It is proposed that α-CA2 may be situated in the chloroplasts.