The root-infecting necrotrophic fungal pathogen Rhizoctonia solani causes significant disease to all the worldâ€™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 transcriptional reprogramming early in A17. Responses specific to A17 included components of ethylene signalling, numerous class IX ERF transcription factor family members, reactive oxygen species metabolism and consistent up-regulation of the isoflavonoid biosynthesis pathway. Mass-spectrometry revealed accumulation of the isoflavonoid related compounds liquiritigenin, formononetin, medicarpin and biochanin A in A17. Over-expression of an isoflavone synthase (IFS) in M. truncatula roots increased isoflavonoid accumulation and resistance to R. solani. Addition of exogenous medicarpin suggested this phytoalexin may be one of several isoflavonoids required to contribute to resistance to R. solani. Together these results provide evidence for the role of ethylene-mediated accumulation of isoflavonoids during defence against root pathogens in legumes. Overall design: Root tissue of moderately resistant wild type and highly susceptible mutant lines of Medicago truncatula responding to infection from Rhizoctona solani at two time points. Each time point contains mock treated and R. solani inoculated samples. All samples in triplicate, sequenced with strand specific illumina paired end RNA sequencing.