Phytoalexin Elicitor Activity of Carbohydrates from Phytophthora megasperma f.sp. glycinea and Other Sources

Three unique classes of carbohydrates were isolated from the hyphal cell walls of Phytophthora megasperma f.sp. glycinea (Pmg) and compared with other substances for their activity as elicitors of the phytoalexin glyceollin in soybean tissues. Glucomannans extracted from cell walls with soybean beta-1,3-endoglucanase were purified and proved to be the most active elicitors yet reported. They were approximately 10 times more active in soybean cotyledons than the heterogeneous beta-glucan elicitor fraction extracted from Pmg walls. In addition, the glucomannan fraction gave race-specific elicitor activity in soybean hypocotyls. Pronase was found to be a suitable reagent for the mild extraction of glycopeptides from Pmg cell walls. All of the carbohydrates isolated from Pmg cell walls possessed significant elicitor activity, but other glucans, a glucomannan and mannan from other sources, were much less active. Chitin and chitosan, reported to function as elicitors in other plants, had low activity in soybean cotyledons. Arachidonic acid was inactive, despite its previously observed elicitor activity in potato tubers. The results indicated that, for Pmg, the carbohydrate elicitor most probably involved in the initiation of phytoalexinmediated defense during fungus infection of soybean plants is the glucomannan fraction liberated by endoglucanase.