Search

Plant Resistance (R) genes encode immune receptors that recognize pathogens and activate defense responses. Because of fitness costs associated with maintaining R protein-mediated resistance, expression levels of R genes have to be tightly regulated. However, mechanisms on how R-gene expression is regulated are poorly understood. Here we show that MODIFIER OF snc1, 1 (MOS1) regulates the expression of SUPPRESSOR OF npr1-1, CONSTITUTIVE1 (SNC1), which encodes a Toll/interleukin receptor-nucleotide binding site-leucine-rich repeat type of R protein in Arabidopsis (Arabidopsis thaliana). In the mos1 loss-of-function mutant plants, snc1 expression is repressed and constitutive resistance responses mediated by snc1 are lost. The repression of snc1 expression in mos1 is released by knocking out DECREASE IN DNA METHYLATION1. In mos1 mutants, DNA methylation in a region upstream of SNCI is altered. Furthermore, expression of snc1 transgenes using the native promoter does not require MOS1, indicating that regulation of SNC1 expression by MOS1 is at the chromatin level. Map-based cloning of MOS1 revealed that it encodes a novel protein with a HLA-B ASSOCIATED TRANSCRIPT2 (BAT2) domain that is conserved in plants and animals. Our study on MOS1 suggests that BAT2 domain-containing proteins may function in regulation of gene expression at chromatin level. [ABSTRACT FROM AUTHOR]

Plant nucleotide-binding leucine-rich repeat (NB-LRR) proteins serve as intracellular sensors to detect pathogen effectors and trigger immune responses. Transcription of the NB-LRR-encoding Resistance (R) genes needs to be tightly controlled to avoid inappropriate defense activation. How the expression of the NB-LRR R genes is regulated is poorly understood. The Arabidopsis (Arabidopsis thaliana) suppressor of npr1-1, constitutive 1 (snc1) mutant carries a gain-of-function mutation in a Toll/Interleukin1 receptor-like (TIR)-NB-LRR-encoding gene, resulting in the constitutive activation of plant defense responses. A snc1 suppress screen identified modifier of snc1,9 (mos9), which partially suppresses the autoimmune phenotypes of snc1. Positional cloning revealed that MOS9 encodes a plant-specific protein of unknown function. Expression analysis showed that MOS9 is required for the full expression of TIR-NB-LRR protein-encoding RECOGNITION OF PERONOSPORA PARASITICA 4 (RPP4) and SNC both of which reside in the RPP4 cluster. Coimmunoprecipitation and mass spectrometry analyses revealed that MOS9 associate with the Set1 class lysine 4 of histone 3 (H3K4) methyltransferase Arabidopsis Trithorax-Related7 (ATXR7). Like MOS9, ATXR7 is also required for the full expression of SNC1 and the autoimmune phenotypes in the snc1 mutant. In atxr7 mutant plants expression of RPP4 is similarly reduced, and resistance against Hyaloperonospora arabidopsidis Emwal is compromised. Consistent with the attenuated expression of SNC1 and RPP4, trimethylated H3K4 marks are reduced around the promoters of SNC1 and RPP4 in mos9 plants. Our data suggest that MOS9 functions together with ATXR7 to regulate the expression of SNC1 and RPP4 through H3K4 methylation, which plays an important role in fine-tuning their transcription levels and functions in plant defense. [ABSTRACT FROM AUTHOR]