Arabidopsis CBP60b is a central transcriptional activator of immunity

Plants use a dual defense system to cope with microbial pathogens. The first involves pathogen-associated molecular pattern-triggered immunity which is conferred by membrane receptors, and the second involves effector-triggered immunity (ETI), which is conferred by disease-resistance proteins (nucleotide-binding leucine-rich repeat-containing proteins; NLRs). Calmodulin-Binding Protein 60 (CBP60) family transcription factors are crucial for pathogen defense: CBP60g and Systemic Acquired Resistance Deficient 1 (SARD1) positively regulate immunity, whereas CBP60a negatively regulates immunity. The roles of other Arabidopsis (Arabidopsis thaliana) CBP60s remain unclear. We report that CBP60b positively regulates immunity and is redundant with-yet distinct from-CBP60g and SARD1. By combining chromatin immunoprecipitation-PCRs and luciferase reporter assays, we demonstrate that CBP60b is a transcriptional activator of immunity genes. Surprisingly, CBP60b loss-of-function results in autoimmunity, exhibiting a phenotype similar to that of CBP60b gain-of-function. Mutations at the ENHANCED DISEASE SUSCEPTIBILITY 1-PHYTOALEXIN DEFICIENT 4-dependent ETI pathway fully suppressed the defects of CBP60b loss-of-function but not those of CBP60b gain-of-function, suggesting that CBP60b is monitored by NLRs. Functional loss of SUPPRESSOR OF NPR1-1, CONSTITUTIVE 1, an R-gene, partially rescued the phenotype of cbp60b, further supporting that CBP60b is a protein targeted by pathogen effectors, that is, a guardee. Unlike CBP60g and SARD1, CBP60b is constitutively and highly expressed in unchallenged plants. Transcriptional and genetic studies further suggest that CBP60b plays a role redundant with CBP60g and SARD1 in pathogen-induced defense, whereas CBP60b has a distinct role in basal defense, partially via direct regulation of CBP60g and SARD1.