The HIRA-interacting histone chaperone CABIN1 negatively regulates plant immunity mediated by immune receptor gene SNC1

Plant intracellular immune receptors, mostly nucleotide-binding leucine-rich repeat (NLR) proteins, are tightly regulated to maintain low expression levels under non-infection conditions and to allow induced expression by pathogens. The extensively studied NLR gene SUPPRESSOR OF NPR1, CONSTITUTIVE 1 (SNC1) is transcriptionally regulated by transcription factors, histone-modifying enzymes, and chromatin remodelers in Arabidopsis (Arabidopsis thaliana). Here, we identified a putative histone chaperone protein, CABIN1 (CALCINEURIN BINDING PROTEIN 1), as a negative regulator of SNC1-mediated plant immunity through a sensitized genetic screen. The cabin1 mutant exhibited enhanced resistance to the bacterial pathogen Pseudomonas syringae pv. tomato (Pst) DC3000. This was accompanied by elevated expression of SNC1 and many defense genes even in the absence of pathogen infection. The enhanced disease resistance in the cabin1 mutant depended on a functional SNC1 gene, and CABIN1 physically bound to the SNC1 promoter region, indicating that SNC1 directly mediates autoimmunity in cabin1. The Arabidopsis CABIN1 protein interacted with HISTONE REGULATOR A (HIRA), similarly in animals. The hira mutant also exhibited enhanced disease resistance and a higher SNC1 expression than the wild type; however, the resistance enhancement did not depend on SNC1 alone. This work identifies regulators of plant immunity and implicates histone chaperones in the regulation of immune gene expression.