Effect of spores of saprophytic fungi on phytoalexin accumulation in seedsof frog-eye leaf spot and stem canker-resistant and -susceptible soybean (Glycine max L.) cultivars

Two saprophytic fungi (Mucor ramosissimus and Rhizopus sp.) were tested for their ability to induce phytoalexin production by seeds of frog-eye leaf s pot and stem canker-resistant and -susceptible soybean (Glycine max L.) cul tivars. Only M. ramosissimus was shown to elicit a response and qualitative differences in phytoalexin accumulation were found between the susceptible and resistant cultivars. Glyceollins I, II, and III and glycinol were isol ated from the susceptible cultivar, whereas Glyceollins I, II, and III, gly cinol, glyceocarpin, genistein, isoformononetin, and N-acetyltyramine accum ulated in the resistant cultivar in response to the same fungal elicitor. G enistein was found to be an inducibly formed isoflavonoid instead of a cons titutive metabolite in the resistant cultivar, whereas N-acetyltyramine is described for the first time as a soybean phytoalexin. All the compounds, e xcept genistein, showed fungitoxic activity against Cladosporium sphaerospe rmum. Spectral data of the pterocarpan phytoalexins, genistein, and N-acety ltyramine are also given in this work.