Perception of the plant immune signal salicylic acid

Abstract

Salicylic acid (SA) plays a central role in plant innate immunity. The diverse functions of this simple phenolic compound suggest that plants may have multiple SA receptors. Several SA-binding proteins have been identified using biochemical approaches. However, genetic evidence supporting that they are the bona fide SA receptors has not been forthcoming. Mutant screens revealed that NPR1 is a master regulator of SA-mediated responses. Although NPR1 cannot bind SA in a conventional ligand-binding assay, its homologs NPR3 and NPR4 bind SA and function as SA receptors. During pathogen challenge, the SA gradient generated at the infection site is sensed by NPR3 and NPR4, which serve as the adaptors for the Cullin 3-based E3 ubiquitin ligase to regulate NPR1 degradation. Consequently, NPR1 is degraded at the infection site to remove its inhibition on effector-triggered cell death and defense, whereas NPR1 accumulates in neighboring cells to promote cell survival and SA-mediated resistance.

Figure 1. NPR3 and NPR4 bind salicylic acid to control NPR1 level

NPR1 is a key positive regulator of SA-mediated responses. NPR3 and NPR4 are the adaptor proteins to mediate NPR1 degradation. NPR4 is a high affinity SA receptor and NPR3 is a low affinity SA receptor. SA blocks NPR4-NPR1 interaction and facilitates NPR3-NPR1 interaction. When SA level is very low, NPR1 level is low because NPR4 mediates its degradation. When SA level is very high, NPR1 level is also low because NPR3 mediates its degradation. At the medium SA level, NPR1 level is the highest because SA is enough to disrupt NPR4-NPR1 interaction, but not enough to facilitate NPR3-NPR1 interaction.