The Arabidopsis chloroplast protein HHL1 regulates AvrRpt2‐triggered immunity via light‐dependent reactive oxygen species homeostasis

Chloroplasts are key organelles for capturing solar energy and establishing plant immunity. During photosynthesis and pathogen defense, highly redox-active reactions take place in chloroplasts and generate large amounts of reactive oxygen species (ROS). However, our knowledge of chloroplast-produced ROS biosynthesis in plant immunity under varying light conditions is limited. Here, we report that the chloroplast-localized protein HYPERSENSITIVE TO HIGH LIGHT 1 (HHL1) functions as a dual regulator of AvrRpt2-triggered immunity in Arabidopsis (Arabidopsis thaliana), by modulating levels of chloroplast-produced ROS to activate appropriate responses to pathogen infection under various light intensities. Under normal light conditions, HHL1 positively regulates AvrRpt2-triggered immunity by promoting AvrRpt2-induced chloroplast-produced ROS accumulation, initiating salicylic acid signaling, and inducing the expression of genes encoding ROS-scavenging enzymes. By contrast, under high light (HL) conditions, HHL1 has an opposite role, functioning as a repressor of these immune responses while HL stress attenuates AvrRpt2-triggered immunity. These findings reveal that HHL1 modulates AvrRpt2-triggered immunity by regulating ROS homeostasis in a light intensity-dependent manner. Collectively, our results offer insight into the role of chloroplasts in the crosstalk between plant immunity and light intensity.