Effects of Salicylic Acid Signaling Inhibitors and Derivatives on Plant Disease Resistance

Abstract Salicylic acid (SA) signaling is central to plant immunity, and small molecules that modulate this pathway are useful tools for mechanistic studies. Here, we characterized NAS1 as an inhibitor of salicylic acid responses with weaker growth-inhibitory activity than PAMD. In Arabidopsis thaliana, NAS1 reduced SA-induced AtPR1 expression without measurably altering endogenous SA, jasmonic acid (JA), or abscisic acid (ABA) levels and without fully blocking SA-induced NPR1 nuclear localization, suggesting a mode of action distinct from that of PAMD. NAS1 also increased susceptibility to microbial infection in Arabidopsis and tomato and promoted Rhizobium radiobacter proliferation in Arabidopsis leaves, consistent with attenuation of SA-dependent immunity. Structure–activity relationship analysis further showed that the carbonyl group, the nitrile group, and a properly configured five-membered ring are important for activity. These findings identify NAS1 as a promising chemical probe for dissecting SA signaling and plant immune regulation.