Host-Pathogen Interactions XXXIX.: A Soybean Pathogenesis-Related Protein with Β-1,3-Glucanase Activity Releases Phytoalexin Elicitor-Active Heat-Stable Fragments from Fungal Walls. Kyung-Sik Ham. Complex Carbohydrate Research Center and Department of Biochemistry, The University of Georgia, Athens 30602 U.S.A. Serge Kauffmann, Peter Albersheim, and Alan G. Darvill. Complex Carbohydrate Research Center and Department of Biochemistry, The University of Georgia, Athens 30602 U.S.A. MPMI 4:545-552. Accepted 18 July 1991. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1991.

Evidence has been obtained that fungal cell wall oligosaccharide fragments (oligosaccharins) solubilized by a pathogenesis-related (PR) Β-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 Β-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 Β-1,3-glucanase activity, providing evidence that Β1,3-glucanase is a principal elicitor-releasing enzyme of the extracts. Antiserum raised against a tobacco PR Β-1,3-glucanase cross-reacted with the purified soybean Β-1,3-glucanase that accounted for major elicitor-releasing activity in the basic fraction of the soybean leaf extracts. Soybean Β-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 Β-1,3-glucanase is a PR protein. These results suggest that a Β-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.