Abstract
High light intensity constitutes a critical abiotic stress factor that profoundly affects the structural and functional integrity of the photosynthetic apparatus. Excessive irradiance triggers accelerated degradation of the PsbA polypeptide, increases susceptibility to photoinhibition, and promotes overproduction of reactive oxygen species (ROS), thereby inducing oxidative damage to proteins, lipids, and nucleic acids. Among the chloroplast-localized site-2 proteases of Arabidopsis thaliana, S2P2 remains the least characterized. In this study, our analyses revealed a pronounced upregulation of the S2P2 (AT1G05140) gene and a concomitant accumulation of the S2P2 protein under high light conditions. Functional characterization using two independent S2P2 insertional mutant lines lacking the protease demonstrated that loss of S2P2 significantly exacerbates photoinhibition. Mutants exhibited reduced photosystem II (PSII) efficiency, accompanied by accelerated degradation of the PSII core proteins PsbA, PsbD, and PsbC, as well as elevated ROS generation. These findings provide the first direct evidence that S2P2 plays a pivotal role in maintaining the stoichiometric balance of PSII core components and conferring resilience of the photosynthetic machinery to high light stress. This work expands the functional repertoire of chloroplast site-2 proteases and underscores S2P2 as a potential target for improving stress tolerance in plants.