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

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 a large number of 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.