Arabidopsis CALMODULIN-BINDING PROTEIN 60b plays dual roles in plant immunity

Arabidopsis SYSTEMIC ACQUIRED RESISTANCE DEFICIENT 1 (SARD1) and CALMODULIN-BINDING PROTEIN 60g (CBP60g) are two master transcription factors that regulate many defense-related genes in plant immunity. They are required for immunity downstream of the receptor-like protein SUPPRESSOR OF NPR1-1, CONSTITUTIVE 2 (SNC2). Constitutive defense responses in the gain-of-function autoimmune snc2-1D mutant are modestly affected by either sard1 or cbp60g single mutants, but completely suppressed by the sard1 cbp60g double mutant. Here we report that CBP60b, another member of the CBP60 family, also functions as a positive regulator of SNC2-mediated immunity. Loss-of-function mutations of CBP60b suppress the constitutive expression of SARD1 and enhanced disease resistance in cbp60g-1 snc2-1D, whereas over-expression of CBP60b leads to elevated SARD1 expression and constitutive defense responses. In addition, transient expression of CBP60b in Nicotiana benthamiana activates the expression of the pSARD1::luciferase reporter gene. Chromatin immunoprecipitation assay further showed that CBP60b is recruited to the promoter region of SARD1, suggesting that it directly regulates SARD1 expression. Interestingly, knocking out CBP60b in the wild type background leads to ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1)-dependent autoimmunity, suggesting that CBP60b is required for the expression of a guardee/decoy or a negative regulator in immunity mediated by receptors carrying an N-terminal TIR (Toll-interleukin-1 receptor-like) domain. Significance statementArabidopsis SARD1 serves as a master transcription factor in plant immunity. In this study, we showed that CBP60b positively regulates SARD1 expression, and TIR signaling is activated when CBP60b is inactivated.