Opposite Roles of Salicylic Acid Receptors NPR1 and NPR3/NPR4 in Transcriptional Regulation of Plant Immunity

Salicylic acid (SA) is a plant defense hormone required for immunity. Arabidopsis NPR1 and NPR3/NPR4 were previously shown to bind SA and all three proteins were proposed as SA receptors. NPR1 functions as a transcriptional co-activator, whereas NPR3/NPR4 were suggested to function as E3 ligases that promote NPR1 degradation. Here we report that NPR3/NPR4 function as transcriptional co-repressors and SA inhibits their activities to promote the expression of downstream immune regulators. npr4-4D, a gain-of-function npr4 allele that renders NPR4 unable to bind SA, constitutively represses SA-induced immune responses. In contrast, the equivalent mutation in NPR1 abolishes its ability to bind SA and promote SA-induced defense gene expression. Further analysis revealed that NPR3/NPR4 and NPR1 function independently to regulate SA-induced immune responses. Our study indicates that both NPR1 and NPR3/NPR4 are bona fide SA receptors, but play opposite roles in transcriptional regulation of SA-induced defense gene expression. [Display omitted] •SA receptors NPR3 and NPR4 function redundantly as transcriptional co-repressors•SA inhibits the transcriptional repression activities of SA receptors NPR3/NPR4•NPR1 and NPR4 have opposite roles in early defense gene expression in response to SA•NPR4 and NPR1 function in parallel to regulate SA-induced defense gene expression Salicylic acid receptors NPR1 and NPR3/NPR4 play opposite roles in the transcriptional regulation of plant defense against pathogens.