Diverse Roles of the Salicylic Acid Receptors NPR1 and NPR3/NPR4 in Plant Immunity

Abstract

The plant defense hormone salicylic acid (SA) is perceived by two classes of receptors, NPR1 and NPR3/NPR4. They function in two parallel pathways to regulate SA-induced defense gene expression. To better understand the roles of the SA receptors in plant defense, we systematically analyzed their contributions to different aspects of Arabidopsis (<i>Arabidopsis thaliana</i>) plant immunity using the SA-insensitive <i>npr1-1 npr4-4D</i> double mutant. We found that perception of SA by NPR1 and NPR4 is required for activation of <i>N</i>-hydroxypipecolic acid biosynthesis, which is essential for inducing systemic acquired resistance. In addition, both pattern-triggered immunity (PTI) and effector-triggered immunity (ETI) are severely compromised in the <i>npr1-1 npr4-4D</i> double mutant. Interestingly, the PTI and ETI attenuation in <i>npr1-1 npr4-4D</i> is more dramatic compared with the <i>SA-induction deficient2-1</i> (<i>sid2-1</i>) mutant, suggesting that the perception of residual levels of SA in <i>sid2-1</i> also contributes to immunity. Furthermore, NPR1 and NPR4 are involved in positive feedback amplification of SA biosynthesis and regulation of SA homeostasis through modifications including 5-hydroxylation and glycosylation. Thus, the SA receptors NPR1 and NPR4 play broad roles in plant immunity.