Linking sensitivity of photosystem II to UV-B with chloroplast ultrastructure and UV-B absorbing pigments contents in A. thaliana L. phyAphyB double mutants

Our recent studies showed that A. thaliana L. phyAphyB double mutants (DM) grown under red light (RL) and white light (WL) had higher photosystem II (PSII) vulnerability to UV-B than wild type (WT). The present work was aimed at revealing the mechanistic basis for this difference by analyzing the content of UV-absorbing pigments (UAPs) and chloroplast ultrastructure in leaves of DM and WT grown under different light conditions, of different photoperiods (12, 16 and 24 h) and light quality (WL vs RL). The content of UAPs in leaves of WT plants grown under RL showed a strong dependence of photoperiod and decreased in a sequence 24 h > 16 h > 12 h. In all treatments, contents of UAPs were higher in WT compared to mutant plants. While 1 h of UV treatment had only a small impact on chloroplast ultrastructure, it substantially inhibited PSII activity (maximum and effective PSII quantum yields). In all treatments, the UV-induced decreases of PSII activities were compared with each other. At any photoperiod, decreases in PSII activity were smaller in WT compared to that in mutant plants. Higher UAPs contents led to lesser PSII inhibition, while low UAPs contents resulted in strong decline in PSII activity. The results demonstrate that content of UAPs significantly contributes to PSII resistance to short-time UV-B exposures, and decreased content of UAPs in phytochrome double mutant can explain the reduced PSII resistance of these plants to UV-B radiation.