Calmodulin‐binding transcription activator AtSR1 / CAMTA3 fine‐tunes plant immune response by transcriptional regulation of the salicylate receptor NPR1

Calcium (Ca 2+ ) signalling regulates salicylic acid (SA)‐mediated immune response through calmodulin‐meditated transcriptional activators, AtSRs/CAMTAs, but its mechanism is not fully understood. Here, we report an AtSR1/CAMTA3‐mediated regulatory mechanism involving the expression of the SA receptor, NPR1. Results indicate that the transcriptional expression of NPR1 was regulated by AtSR1 binding to a CGCG box in the NPR1 promotor. The atsr1 mutant exhibited resistance to the virulent strain of Pseudomonas syringae pv. tomato ( Pst ), however, was susceptible to an avirulent Pst strain carrying avrRpt2 , due to the failure of the induction of hypersensitive responses. These resistant/susceptible phenotypes in the atsr1 mutant were reversed in the npr1 mutant background, suggesting that AtSR1 regulates NPR1 as a downstream target during plant immune response. The virulent Pst strain triggered a transient elevation in intracellular Ca 2+ concentration, whereas the avirulent Pst strain triggered a prolonged change. The distinct Ca 2+ signatures were decoded into the regulation of NPR1 expression through AtSR1's IQ motif binding with Ca 2+ ‐free‐CaM2, while AtSR1's calmodulin‐binding domain with Ca 2+ ‐bound‐CaM2. These observations reveal a role for AtSR1 as a Ca 2+ ‐mediated transcription regulator in controlling the NPR1‐mediated plant immune response.