A polysaccharide from the fungal pathogen Colletotrichum lindemuthianum causes browning and phytoalexin production when applied to the cut surfaces of bean (Phaseolus vulgaris) cotyledons and hypocotyls. The application of an amount of polysaccharide equivalent to less than 100 ng of glucose will elicit this response in the bean tissues. The polysaccharide has been isolated both from culture filtrates and from the mycelial walls of the fungus. Purification of the polysaccharide involved anion and cation exchange chromatography and gel filtration. The polysaccharide has an apparent molecular weight between 1,000,000 and 5,000,000 daltons, and consists predominantly of 3-and 4-linked glucosyl residues.Beans synthesize several phytoalexins in response to attack by pathogens (5-7, 28, 33, 38, 42). Four of these phytoalexins have been isolated and characterized as phaseollin, phaseollidin, phaseollinisoflavan, and kievitone (5,6,30,31,38,42). The synthesis of these phytoalexins has been demonstrated in beans that have responded to attack by C. lindemuthianum (5,7,25). The synthesis of the four phytoalexins occurs much sooner in an incompatible reaction than in a compatible reaction. The incompatible reaction usually involves a hypersensitive response in those cells that are initially invaded by the pathogen (5, 28, 37). It has been suggested (5, 7) that the early synthesis of phytoalexins in an incompatible reaction can account for the restricted growth of the pathogen that is characteristic of the hypersensitive response.Synthesis of phytoalexins in plant tissues can be stimulated by molecules, elicitors (23), that are produced by plant pathogens (12, 23,24,34,43). However, only in one example (12) has the elicitor been purified and partially characterized. This elicitor, a small peptide isolated from the mycelia of Monilinia fructicola, is active on tissues of a nonhost of this pathogen, but the effect of the peptide on tissues of the hosts of M. fructicola has not been reported.
