A specific binding site on soybean membranes for a phytoalexin elicitor released from fungal cell walls by beta-1,3-endoglucanase

The elicitor molecules that function in vivo for phytoalexin elicitation in soybean (Glycine max) infected with Phytophthora megasperma f. sp. glycinea have been identified as beta-1,6- and beta- 1,3-linked glucans that are released from fungal cell walls by beta-1,3-endoglucanase (EC 3.2.1.39) contained in host tissues. The present study was conducted to identify the putative receptor-like target sites for the glucanase-released elicitor in soybean membranes. The glucanase-released elicitor did not bind to the previously reported binding sites for mycolaminaran, a branched beta-1,3- glucan produced by the same fungus. Instead, preparations of soybean membranes contained a specific and high-affinity binding site for the glucanase-released [(14)C]-labeled elicitor. Highest binding activity was associated with a plasma membrane-enriched fraction. A Scatchard plot of the binding data revealed the presence of a single class of binding sites with a Kd value of 4.2 x 10(-7) M for the elicitor. The binding was dependent on the pH of the incubation mixture, as well as on the duration and temperature of the incubation. To assess the physiological importance of elicitor binding in elicitation of phytoalexins, competition studies were conducted with [(14)C]-labeled elicitor and a number of polysaccharides with or without phytoalexin-eliciting activity. The results indicated that the ability of different carbohydrates to displace the radiolabeled ligand from the membrane was closely correlated with their elicitor activity, except in the case of a hepta- beta-glucopyranoside elicitor which did not compete with the [(14)C]-labeled elicitor for binding. The biding of the glucanase-released elicitor to membranes was abolished by both heat and proteolytic enzymes. Therefore, the binding site is probably composed of proteinaceous molecules.