The HY5-NPR1 module governs light-dependent virulence of a plant bacterial pathogen

Light is essential for plant development, but its influence on pathogen virulence and immunity remains poorly understood. Here, we found that the Pseudomonas syringae DC3000 type III effector, AvrPtoB, exhibits virulence exclusively upon light exposure. This light-dependent regulation is controlled by the Arabidopsis transcription factor ELONGATED HYPOCOTYL 5 (HY5), a central regulator of photomorphogenesis. AvrPtoB targets HY5 in the nucleus, facilitating its ubiquitination and degradation. Genetic disruption of HY5 eliminates susceptibility to AvrPtoB and compromises plant immunity upon light exposure. HY5 enhances immunity by binding promoters of defense-related genes, activating their expression, and stabilizing the transcriptional coregulator NONEXPRESSOR OF PATHOGENESIS-RELATED (PR) GENES 1 (NPR1) by inhibiting its negative regulators NPR3/4. Both HY5-mediated immunity and light-dependent AvrPtoB virulence require NPR1. By contrast, during darkness, CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1)-mediated HY5 degradation suppresses AvrPtoB virulence and HY5-enhanced immunity. These findings elucidate a mechanism in which light modulates bacterial virulence and plant immunity via an HY5-NPR1 module, advancing our understanding of light-pathogen-host interactions. Overall design: Total RNA was extracted from leaves of 3-week-old Arabidopsis infiltrated with DC3000 at 0 or 24 hours post infiltration using TRIzol® Reagent