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 hepta-beta-glucoside originally isolated from the mycelial walls of the phytopathogenic oomycete Phytophthora sojae. Our research has focused on the specific recognition of the hepta-beta-glucoside elicitor by binding proteins in soybean cells. Elicitor-binding proteins with properties expected of physiological receptors for the hepta-beta-glucoside elicitor have been identified in soybean root membranes. These elicitor-binding proteins co-migrate with a plasma membrane marker (vanadate-sensitive H+-ATPase) on linear sucrose density gradients. Binding of a radio-iodinated derivative of the hepta-beta-glucoside elicitor by membrane-localized elicitor-binding proteins is specific, reversible, saturable, and of high affinity (K-d approximate to 1 nM). After solubilization with the nonionic detergent, n-dodecylsucrose, the elicitor-binding proteins retain their high affinity (K-d=1.8 nM) for the radiolabeled elicitor and their binding specificity for elicitor-active oligoglucosides. A direct correlation is observed between the ability of oligoglucosides to displace labeled elicitor from the elicitor-binding proteins and the elicitor activity of the oligosaccharides. Thus, the elicitor-binding proteins recognize the same structural elements of the hepta-beta-glucoside elicitor that are essential for its phytoalexin-inducing activity, suggesting that the binding proteins are physiological 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 characterization of the hepta-beta-glucoside binding proteins are among the first steps toward elucidating how the hepta-beta-glucoside elicitor triggers the signal transduction pathway that ultimately leads to the synthesis of phytoalexins in soybean.