Effect of Deficiency of Cryptochromes 1 and 2 on Photosynthetic Activity and Pro-Antioxidant Balance in Arabidopsis thaliana Leaves under the Action of UV-B

The influence of UV-B irradiation (1 W/m², 1 h) was examined on the photochemical activity of photosystem II (PSII), the rate of net photosynthesis (Pₙ), the content of photosynthetic and UV-absorbing pigments, the activity of antioxidant enzymes (catalase and peroxidase), and H₂O₂ content in 25-day-old wild-type (WT) Arabidopsis thaliana plants (ecotype Landsberg erecta) and in the cryptochrome 1 and 2-deficient mutant (cry1cry2) grown under red light (RL), white light (WL), and blue light (BL) at a photon flux density of 130 μmol quanta/(m² s). The expression of genes for the transcription factor (HY5), flavonoid biosynthesis (CHS, PAL), phytochromobiline synthase (HY2), protochlorophyllide oxidoreductase (POR-A), and the stromal (sAPX) and thylakoid (tAPX) ascorbate peroxidases was also investigated. UV-B irradiation reduced the Pₙ and PSII photochemical activity (chlorophyll fluorescence parameters FV/FM, PIABS) in WT and mutant plants grown under WL, RL, and BL. At the same time, in cry1cry2 plants grown under BL, unlike those grown under RL and WL, the inhibition of PSII activity and photosynthetic rate was stronger than in WT plants. The content of UV-absorbing pigments in cry1cry2 plants grown under BL was markedly lower than in BL-grown wild type, while no difference was found for the plants grown under RL and WL. In all the treatments, the content of UV-absorbing pigments did not change under the action of UV-B radiation. The activities of catalase and peroxidase in WT plants grown under BL were noticeably higher than in the mutant, both before and after UV-B irradiation. Peroxidase activity in WT plants grown under WL increased after irradiation, while this activity in the mutant decreased in parallel with the accumulation of Н₂О₂. In the mutant plants grown under BL, the transcription levels of tAPX, sAPX, PAL, HY5, HY2, and POR-A genes were lower than in the WT. The authors believe that the reduced resistance of photosynthetic apparatus to UV-B in cry1cry2 plants is determined by the low content of UV-absorbing pigments and by the decreased activities of peroxidase and catalase in leaves of the mutant plants.