HOS15 and HDA9 negatively regulate immunity through histone deacetylation of intracellular immune receptor NLR genes in Arabidopsis

• Plant immune responses need to be tightly controlled for growth–defense balance. The mechanism underlying this tight control is not fully understood. Here we identify epigenetic regulation of nucleotide-binding leucine rich repeat or Nod-Like Receptor (NLR) genes as an important mechanism for immune responses. • Through a sensitized genetic screen and molecular studies, we identified and characterized HOS15 and its associated protein HDA9 as negative regulators of immunity and NLR gene expression. • The loss-of-function of HOS15 or HDA9 confers enhanced resistance to pathogen infection accompanied with increased expression of one-third of the 207 NLR genes in Arabidopsis thaliana. HOS15 and HDA9 are physically associated with some of these NLR genes and repress their expression likely through reducing the acetylation of H3K9 at these loci. In addition, these NLR genes are repressed by HOS15 under both pathogenic and nonpathogenic conditions but by HDA9 only under infection condition. • Together, this study uncovers a previously uncharacterized histone deacetylase complex in plant immunity and highlights the importance of epigenetic regulation of NLR genes in modulating growth–defense balance.