Niraj Kumar Vishwakarma, Shalini Yadav, Atish Sardar, Megha Choudhary, Debasis Chattopadhyay
SUMMARY Plant innate immune response is a well‐balanced process with positive and negative regulations for the plants to survive. Calcium signaling is essential for pathogen‐associated molecular pattern (PAMP)‐driven respiratory burst oxidase homolog D (RBOHD)‐mediated reactive oxygen species (ROS) burst. We show that calcium sensors calcineurin B like protein 1 (CBL1) and CBL9 and their interacting protein kinase CIPK6 negatively regulate RBOHD activity and immune response in Arabidopsis thaliana . Arabidopsis mutant cbl1cbl9 , like cipk6 , exhibited enhanced resistance and ROS production when infected with the bacterial pathogen Pseudomonas syringae pv. tomato ( Pst ). CBL1 and CBL9 enhanced kinase activity of CIPK6. CBL1/9‐CIPK6 module interacts with RBOHD at the plasma membrane. CIPK6 along with CBL1 reduces RBOHD activity in planta . CIPK6 phosphorylates the N‐terminal cytoplasmic domain of RBOHD at a non‐conserved (S 33 ) and a conserved (S 39 ) serine residue. While S 39 phosphorylation increased RBOHD activity, S 33 phosphorylation drastically reduced it and superseded the effect of S 39 phosphorylation. We propose a model that CIPK6 phosphorylates RBOHD at S 33 to suppress its activity to balance ROS generation in post‐PTI situation in Arabidopsis. Our study reports a direct mechanism of negative regulation of ROS production and plant immune response by a calcium‐signaling module in Arabidopsis thaliana .