Role of photolyase in stress tolerance and virulence of plant-pathogenic bacterium Pseudomonas cichorii JBC1

The cryptochrome/photolyase family (CPF) comprises structurally conserved flavoproteins with diverse roles in light-dependent DNA repair and stress adaptation. Here, we identified and functionally characterized a class I cyclobutane pyrimidine dimer (CPD) photolyase, Pc-Phr, from the plant-pathogenic bacterium Pseudomonas cichorii JBC1 (PcJBC1). Spectroscopic analyses revealed that Pc-Phr binds flavin adenine dinucleotide (FAD) and undergoes characteristic blue light–induced photoreduction from the oxidized state (FAD_OX) to catalytically active reduced forms. Pc-Phr exhibited robust photoreactivation activity, repairing UV-induced CPDs in vitro and restoring the viability of UV-C–irradiated cells in vivo, whereas this activity was abolished in a phr deletion mutant. Transcriptional analyses showed that phr expression is induced by UV-C and further enhanced by blue light, consistent with a light-dependent DNA damage response. Beyond DNA repair, Pc-Phr promoted bacterial survival, growth, and adhesion under UV-C, high-intensity blue light, and oxidative stress conditions, and was essential for maintaining intracellular reactive oxygen species (ROS) homeostasis. Deletion of phr resulted in a drastic ROS accumulation and significantly reduced survival under both UV-C and H₂O₂ stress. Importantly, Pc-Phr contributed to virulence in host plants, with the mutant exhibiting markedly reduced pathogenicity. These results demonstrate that efficient photorepair is a critical determinant of environmental fitness and pathogenicity in P. cichorii, highlighting the importance of light-responsive DNA repair systems in plant-associated bacteria.