Development of a Radioimmunoassay for the Soybean Phytoalexin Glyceollin I

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Abstract
A radioimmunoassay for glyceollin I, the major phytoalexin produced by soybean, was developed. Antibodies were raised in rabbits against a glyceollin I-bovine serum albumin conjugate. The antisera were used to establish a radioimmunoassay for glyceollin I using [125I]glyceollin I as the tracer. A logit plot of a standard concentration series yielded a straight line in the range of 1-100 pmols (0.34-34 ng) of glyceollin I. The structurally related pterocarpan phytoalexins, glyceollins II and III, glyceollidin II and glycinol, which also accumulate in infected soybean tissue, show a low cross-reactivity in the radioimmunoassay (0.5-5% at 50% displacement of the tracer). Two related isoflavones present constitutively in soybean tissue, daidzein and genistein, have cross-reactivities of less than 0.84 and 1.1%, respectively. The radioimmunoassay permitted the quantitative determination of glyceollin I in 15-.mu.m microtome sections of soybean hypocotyl tissue infected with zoospores of Phytophthora megasperma f. sp. glycinea.