Arabidopsis STN7/STN8 alleviates oxidative damages via different regulatory patterns under drought stress.

STN7 and STN8 kinases phosphorylate several proteins of the photosynthetic apparatus in response to changes in environmental conditions. Phosphorylation is linked to the dynamic arrangement of both the 3D structure of the thylakoid membrane and thylakoid protein complexes, thereby controlling excitation energy transfer between photosystems and the repair cycle of photosystem II (PSII). However, mutants with phosphorylation of PSII core proteins show only minor changes compared with the wild-type (WT) under conditions tested so far, and thus the physiological significance of reversible thylakoid protein phosphorylation by STN8 remains largely unresolved. In this study, we examined the impact of STN7 and STN8 on osmotic stress and consequent changes in the structure of the thylakoid membrane in Arabidopsis thaliana using physiological, cellular, and biochemical assays. Our results demonstrate that Arabidopsis plants lacking STN7 or STN8 exhibited slow growth and impaired regulation of excitation energy transfer, as well as low photosynthetic efficiency, excessive accumulation of ROS, and more severe cell death in response to osmotic stress compared with the WT. Clear effects of osmotic stress were observed in both stn7 and stn8 mutants but were more pronounced in the stn7stn8 double mutant. Our data strongly suggest that phosphorylation of the PSII reaction center proteins, regulated by STN8, is important for maintaining the functional balance of the photosynthetic apparatus under osmotic stress-induced changes in the thylakoid membrane and thus protects it from oxidative stress.