HOST-PATHOGEN INTERACTIONS .39. A SOYBEAN PATHOGENESIS-RELATED PROTEIN WITH BETA-1,3-GLUCANASE ACTIVITY RELEASES PHYTOALEXIN ELICITOR-ACTIVE HEAT-STABLE FRAGMENTS FROM FUNGAL WALLS

Evidence has been obtained that fungal cell wall oligosaccharide fragments (oligosaccharins) solubilized by a pathogenesis-related (PR) beta-1,3-glucanase elicit phytoalexin accumulation in soybean. Soybean leaves were treated with salicylic acid, polyacrylic acid, or mercuric chloride, or they were infected with Phytophthora megasperma H20 (a fungal pathogen of Douglas fir) to which soybean responds with nonhost resistance. Only mercuric chloride and the fungus induced the leaves to synthesize PR proteins. Both beta-1,3-glucanase (EC 3.2.1.39) and chitinase (EC 3.2.1.14) activities were induced by treatment with mercuric chloride and by infection with the fungus. During purification of elicitor-releasing activity to homogeneity those fractions containing elicitor-releasing activity also contained beta-1,3-glucanase activity, providing evidence that beta-1,3-glucanase is a principal elicitor-releasing enzyme of the extracts. Antiserum raised against a tobacco PR beta-1,3-glucanase cross-reacted with the purified soybean beta-1,3-glucanase that accounted for major elicitor-releasing activity in the basic fraction of the soybean leaf extracts. Soybean, beta-1,3-glucanase, induced by mercuric chloride treatment or pathogenic infection with P. m. f. sp. glycinea race 1 (incompatible to the soybean cultivar used), could not be detected by immunoblots of extracts of control plants, indicating that the beta-1,3-glucanase is a PR protein. These results suggest that a beta-1,3-glucanase, induced in soybean seedlings by pathogenic infection or by chemical stress, functions in defense by releasing a phytoalexin elicitor from the mycelial walls of a pathogenic fungus.