The disease resistance protein SNC1 represses the biogenesis of microRNAs and phased siRNAs

Plants evolved an array of disease resistance genes ( R genes) to fight pathogens. In the absence of pathogen infection, NBS-LRR genes, which comprise a major subfamily of R genes, are suppressed by a small RNA cascade involving microRNAs (miRNAs) that trigger the biogenesis of phased siRNAs (phasiRNAs) from R gene transcripts. However, whether or how R genes influence small RNA biogenesis is unknown. In this study, we isolate a mutant with global defects in the biogenesis of miRNAs and phasiRNAs in Arabidopsis thaliana and trace the defects to the over accumulation and nuclear localization of an R protein SNC1. We show that nuclear SNC1 represses the transcription of miRNA and phasiRNA loci, probably through the transcriptional corepressor TPR1. Intriguingly, nuclear SNC1 reduces the accumulation of phasiRNAs from three source R genes and concomitantly, the expression of a majority of the ~170 R genes is up-regulated. Taken together, this study suggests an R gene-miRNA-phasiRNA regulatory module that amplifies plant immune responses. A small RNA-based signaling cascade prevents the induction of plant resistance genes ( R -genes) in the absence of pathogen challenge. Here Cai et al . show that nuclear accumulation of the R protein SNC1 can activate immunity by suppressing small RNA production and releasing R -gene repression.