Salicylic acid inhibits transcriptional repression activities of its receptors to promote defense gene expression

Salicylic acid (SA) is a plant defense hormone required for immunity. Arabidopsis NPR1 and NPR3/NPR4 were previously shown to bind SA and proposed as SA receptors. However, unlike NPR1, loss of NPR3/NPR4 does not block SA-induced defense gene expression. Here we report that NPR3/NPR4 function as transcriptional repressors and SA inhibits their activities to promote the expression of key immune regulators. npr4-4D, a newly identified gain-of-function allele that renders NPR4 unable to bind SA, constitutively represses SA-induced immune responses. In contrast, the equivalent mutation in NPR1 abolishes its function in promoting SA-induced defense gene expression. Further analysis revealed that npr4-4D and npr1-1 have additive effect on blocking SA-induced defense gene expression, suggesting that NPR4 and NPR1 function in parallel to regulate SA-induced immune responses. Our study reveals the molecular functions of SA receptors NPR3/NPR4 and uncovers a brand new mechanism of SA perception. Overall design: RNA-seq analysis of WT, npr1-1, npr4-4D, and npr1-1 npr4-4D arabidopsis seedlings untreated or treated with SA. 4 genotypes and 2 conditions, each with 2 biological replicates resulting in a total of 16 samples.

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Ding Y, Sun T, Ao K, Peng Y, Zhang Y, Li X, Zhang Y