How many salicylic acid receptors does a plant cell need?

The small-molecule hormone salicylic acid (SA) is a plant immune signal for which the receptors have only recently been identified. Two recent studies reported that the transcriptional coactivator nonexpresser of pathogenesis-related genes 1 (NPR1) and its paralogues NPR3 and NPR4 are bona fide SA immune signal receptors in plants. Fu et al. demonstrated that because of their binding affinity for SA, NPR3 and NPR4 are SA receptors for immune responses in Arabidopsis thaliana. Both NPR3 and NPR4 function as adaptors in proteasomal degradation of NPR1 in an SA-dependent manner. By applying nonequilibrium methods, they showed very low binding affinity of NPR1 for SA, suggesting that it may not qualify as an SA immune signal transduction receptor. However, using a method of equilibrium dialysis, Wu et al. found that SA binds to NPR1 and induces a conformational change in NPR1 or introduces steric hindrance that relieves repression of the transcriptional activation domain of NPR1 by an autoinhibitory N-terminal domain. This derepression leads to the expression of SA-dependent defense genes. Here, we discuss the importance of emerging SA perception models.