CHARACTERIZATION OF BINDING-PROTEINS THAT RECOGNIZE OLIGOGLUCOSIDE ELICITORS OF PHYTOALEXIN SYNTHESIS IN SOYBEAN

We are studying the cellular signaling pathway leading to pterocarpan phytoalexin biosynthesis in soybean that is induced by a branched hept a-beta-glucoside originally isolated from the mycelial walls of the ph ytopathogenic oomycete Phytophthora sojae. Our research has focused on the specific recognition of the hepta-beta-glucoside elicitor by bind ing proteins in soybean cells. Elicitor-binding proteins with properti es expected of physiological receptors for the hepta-beta-glucoside el icitor have been identified in soybean root membranes. These elicitor- binding proteins co-migrate with a plasma membrane marker (vanadate-se nsitive H+-ATPase) on linear sucrose density gradients. Binding of a r adio-iodinated derivative of the hepta-beta-glucoside elicitor by memb rane-localized elicitor-binding proteins is specific, reversible, satu rable, and of high affinity (K-d approximate to 1 nM). After solubiliz ation with the nonionic detergent, n-dodecylsucrose, the elicitor-bind ing proteins retain their high affinity (K-d=1.8 nM) for the radiolabe led elicitor and their binding specificity for elicitor-active oligogl ucosides. A direct correlation is observed between the ability of olig oglucosides to displace labeled elicitor from the elicitor-binding pro teins and the elicitor activity of the oligosaccharides. Thus, the eli citor-binding proteins recognize the same structural elements of the h epta-beta-glucoside elicitor that are essential for its phytoalexin-in ducing activity, suggesting that the binding proteins are physiologica l receptors for the elicitor. Current research is directed toward the purification of the hepta-beta-glucoside elicitor-binding proteins by using ligand affinity chromatography. Purification and characterizatio n of the hepta-beta-glucoside binding proteins are among the first ste ps toward elucidating how the hepta-beta-glucoside elicitor triggers t he signal transduction pathway that ultimately leads to the synthesis of phytoalexins in soybean.