Co‐occurrence of chloroplastic ROS production and salicylic acid induction in plant immunity

Summary Chloroplasts are important sites of metabolite biosynthesis, including precursors of the defense‐related phytohormone salicylic acid (SA). Chloroplastic reactive oxygen species (cROS) have previously been shown to contribute significantly to disease resistance in Arabidopsis thaliana L. (Heyn). To understand the function of cROS in plant immunity, we coupled confocal cROS imaging with gene expression, mass spectrometry, and hyperspectral imaging analysis. Here, we show that cROS production correlates with the onset of SA biosynthesis and that abrogating cROS production during immune elicitation prevents the initiation of SA‐dependent, but not SA‐independent, immune responses. Furthermore, we show that the Pseudomonas syringae effector proteins HopM1 and AvrE1 suppress cROS production, contributing to virulence by facilitating the induction of water‐soaking lesions through immune inhibition. We propose that certain ROS produced in the chloroplasts serve as triggers for the induction of the SA signaling cascade in plants and that they are targeted by a conserved family of pathogen effector proteins.