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.