Delicate tuning of H ₂ O ₂ by salicylic acid, catalase 2, and AtSAP5 incites plant immunity

Salicylic acid (SA) triggers plant immunity through redox state changes, initially oxidizing and then reducing. SA binds and inhibits the H ₂ O ₂ -scavenging enzyme catalase 2 (CAT2), promoting an initial oxidative state. However, a reduced state is crucial for a robust immune response. In Arabidopsis , the SA-induced A20/AN1 zinc finger protein, AtSAP5, competes with SA for CAT2 binding, maintaining CAT2 activity. This reduces H ₂ O ₂ levels and prevents CAT2 degradation through inhibition of H ₂ O ₂ -induced autophagy formation, which forms a positive feedback loop to CAT2 accumulation. Consequently, cells shift to a more reduced state, activating immune responses. Our findings reveal a nuanced regulatory mechanism through which H ₂ O ₂ levels are controlled by SA with cross-kingdom conserved proteins AtSAP5 and CAT2, managing redox states and plant immunity.