Ethylene signaling is important for isoflavonoid-mediated resistance to rhizoctonia solani in roots of medicago truncatula

The root-infecting necrotrophic fungal pathogen Rhizoctoniasolani causes significant disease to all the world’s major food crops. As a model for pathogenesis of legumes, we have examined the interaction of R. solani AG8 with Medicago truncatula. RNAseq analysis of the moderately resistant M. truncatula accession A17 and highly susceptible sickle (skl) mutant (defective in ethylene sensing) identified major early transcriptional reprogramming in A17. Responses specific to A17 included components of ethylene signaling, reactive oxygen species metabolism, and consistent upregulation of the isoflavonoid biosynthesispathway. Mass spectrometry revealed accumulation of the isoflavonoid-related compounds liquiritigenin, formononetin, medicarpin, and biochaninA in A17. Overexpression of an isoflavonesynthase in M. truncatula roots increased isoflavonoidaccumulation and resistance to R. solani. Addition of exogenous medicarpinsuggested this phytoalexinmay be one of several isoflavonoidsrequired to contribute...