AIG2A and AIG2B limit the activation of salicylic acid-regulated defenses by tryptophan-derived secondary metabolism in Arabidopsis

Chemical defense systems involving tryptophan-derived secondary metabolites (TDSMs) and salicylic acid (SA) are induced by general nonself signals and pathogen signals, respectively, in Arabidopsis thaliana. Whether and how these chemical defense systems are connected and balanced is largely unknown. In this study, we identified the AVRRPT2-INDUCED GENE2A (AIG2A) and AIG2B genes as gatekeepers that prevent activation of SA defense systems by TDSMs. These genes also were identified as important contributors to natural variation in disease resistance among A. thaliana natural accessions. The loss of AIG2A and AIG2B function leads to upregulation of both SA and TDSM defense systems. Suppressor screens and genetic analysis revealed that a functional TDSM system is required for the upregulation of the SA pathway in the absence of AIG2A and AIG2B, but not vice versa. Furthermore, the AIG2A and AIG2B genes are co-induced with TDSM biosynthesis genes by general pathogen elicitors and nonself signals, thereby functioning as a feedback control of the TDSM defense system, as well as limiting activation of the SA defense system by TDSMs. Thus, this study uncovers an AIG2A- and AIG2B-mediated mechanism that fine-tunes and balances SA and TDSM chemical defense systems in response to nonpathogenic and pathogenic microbes.