Characterization of hepta-beta-glucoside elicitor-binding protein(s) in soybean.

We are studying the cellular signalling 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 megasperma f. sp. glycinea. Our research has focused on the first step in this signal pathway, namely the specific recognition of the hepta-beta-glucoside elicitor by plasma-membrane-localized binding protein(s) in soybean cells. Binding of a radio-iodinated derivative of the elicitor-active hepta-beta-glucoside by membrane elicitor-binding proteins is specific, reversible, saturable and of high affinity (Kd = 0.75 nM). After solubilization using the non-ionic detergent n-dodecylsucrose, the elicitor-binding proteins retain the binding affinity (Kd = 1.8 nM) for the radiolabelled elicitor and the binding specificity for elicitor-active oligoglucosides. A direct correlation is observed between the ability of elicitor-active and structurally related inactive oligoglucosides to displace labelled 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 elicitor-binding proteins are physiological receptors for the elicitor. Current research is directed toward the purification and cloning of the hepta-beta-glucoside elicitor-binding proteins. Purification and characterization of the hepta-beta-glucoside-binding protein(s) or their corresponding cDNAs is a first step toward elucidating how the hepta-beta-glucoside elicitor triggers the signal transduction pathway that ultimately leads to the synthesis of phytoalexins in soybean.