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‐β‐glucoside originally isolated from the mycelial walls of the phytopathogenic oomycete Phytophthora sojae . Our research has focused on the specific recognition of the hepta‐β‐glucoside elicitor by binding proteins in soybean cells. Elicitor‐binding proteins with properties expected of physiological receptors for the hepta‐β‐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‐β‐glucoside elicitor by membrane‐localized elicitor‐binding proteins is specific, reversible, saturable, and of high affinity (K d ≅ 1 n M ). After solubilization with the nonionic detergent, n ‐dodecylsucrose, the elicitor‐binding proteins retain their high affinity (K d = 1.8 n M ) 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‐β‐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‐β‐glucoside elicitor‐binding proteins by using ligand affinity chromatography. Purification and characterization of the hepta‐β‐glucoside binding proteins are among the first steps toward elucidating how the hepta‐β‐glucoside elicitor triggers the signal transduction pathway that ultimately leads to the synthesis of phytoalexins in soybean.