Role of Constitutive Isoflavone Conjugates in the Accumulation of Glyceollin in Soybean Infected with Phytophthora megasperma. T. L. Graham. Department of Plant Pathology, The Ohio State University, Columbus 43210 U.S.A. J. E. Kim, and M. Y. Graham. Department of Plant Pathology, The Ohio State University, Columbus 43210 U.S.A. MPMI 3:157-166. Accepted 7 December 1989. Copyright 1990 The American Phytopathological Society.

We report for the first time that the closely related isoflavones daidzein and genistein are present constitutively in large quantities as conjugates in all seedling organs of the soybean isolines Williams and Williams 79. The conjugates have been identified as the 7-O-glucosyl- and 6”-O-malonyl-7-O-glucosyl-isoflavones. Since daidzein is an immediate precursor of the pterocarpan phytoalexins, the glyceollins, we have examined the role of the daidzein conjugates in glyceollin accumulation in soybean tissues infected with Phytophthora megasperma f. sp. glycinea. The conjugates are present in both Williams isolines at levels far in excess of the amount required for glyceollin accumulation in infected tissues. During incompatible infections of cotyledon tissues (P. m. f. sp. glycinea race 1, Williams 79), the conjugates are rapidly hydrolyzed to free daidzein. High levels of glyceollin subsequently accumulate and infection is arrested within 48 hr. In compatible infections (P. m. f. sp. glycinea race 1, Williams), release of the free isoflavones is delayed; only low levels of glyceollin accumulate and then only 48 hr after the infection front has passed. These results suggest that 1) glyceollin biosynthesis may not be solely dependent on the induction of enzymes of early phenylpropanoid and flavonoid metabolism in this organ and 2) resistance to P. m. f. sp. glycinea race 1, as defined by the Rps₁^c gene in this organ, may reside partly in the rapid release of the isoflavone aglycones from their conjugates and/or in the later steps of glyceollin biosynthesis.