Susceptible (cv. Sabilt) and resistant (cv. Vertus) lucerne (Medicago sativa) seedlings were infected with the stem nematode, Ditylenchus dipsaci and foliar and root tissue analysed for the presence of isoflavonoids. Within 10 days of inoculation, clear differences were seen in the responses of resistant and susceptible plants. Neither cultivar accumulated detectable concentrations of isoflavonoids or their metabolites in the foliar tissue, where all the nematodes were localized; in the roots of infected Vertus plants, the malonyl glucosides of medicarpin and formononetin and the glucosides of coumestrol and formononetin were present al two to three times the concentrations in the uninfected controls. In susceptible Sabilt, these changes were less marked and were restricted to a transient increase in the glucosides of formononetin and coumestrol. Increases in these isoflavonoid conjugates in the roots were associated with increased activities of the enzyme phenylalanine ammonia lyase, but the enzyme was not induced in infected shoots. The results suggest that although increases in isoflavonoid conjugates in the roots were not directly responsible for resistance to D. dipsaci, their accumulation was in response to a systemic signal from the infected shoots, and was correlated with resistance.
