CBL1/9-CIPK6 complex negatively regulates Respiratory burst oxidase homolog D in Arabidopsis thaliana
Plant innate immune response must be a well-balanced process with positive and negative regulations for the plants to survive. Calcium signalling is essential for pathogen-associated molecular pattern (PAMP)-driven Respiratory burst oxidase homolog D (RbohD)-mediated reactive oxygen species (ROS) burst. We show 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. Plasma membrane localization of CBL1 and CBL9 and CIPK6 kinase activity were associated with the ROS production and immune response. CBL1/9-CIPK6 module interacts with RbohD at the plasma membrane and phosphorylates its N-terminal cytoplasmic domain at a non-conserved (Ser33) and a conserve (Ser39) Serine residue. While Ser39 phosphorylation increased RbohD activity, Ser33 phosphorylation drastically reduced it and superseded the effect of Ser39 phosphorylation. Replacement of Ser33 with alanine or aspartic acid made RbohD a super-active or low-active enzyme, respectively. Our study reports a direct mechanism of negative regulation of ROS production and plant immune response by a Calcium-signalling module in Arabidopsis. Overall, this study provides a novel insight into how calcium signaling integrates with immune regulation to prevent excessive ROS accumulation, ensuring a balanced plant immune response. One Sentence SummaryInactivation of RbohD by CBL1/CBL9-CIPK6 complex negatively regulates the plant immunity against Pst DC3000 infection in Arabidopsis.