Host-Pathogen Interactions

Abstract

The structures of the 4 wall-released elicitor fractions isolated from the Phytophthora megasperma var. sojae mycelial walls were examined. Fraction I is primarily composed of a branched .beta.-1,3-glucan. Fractions II and IV are primarily composed of a highly branched mannan-containing glycoprotein, with fraction IV richer in protein than fraction II. Fraction III contains, attached to protein, a mixture of the 2 polysaccharide types found in fraction I and in fractions II and IV. The structural data demonstrate that the only compound produced by P. megasperma var. sojae which contains elicitor activity is the glucan. The terminal glycosyl residues of the glucan are evidently required for elicitor activity. In addition, 90% of the glucan can be removed enzymically without any loss of biological activity. The active residue of the enzymic digestion is a highly branched 3- and 3,6-linked glucan containing about 4% mannosyl residues. The mannosyl residues of the glucan, which represent only about 1% of the undegraded glucan, are likely to participate in the active site of this molecule. The role of elicitors and phytoalexins in host-pathogen interactions is discussed. Evidence for the existence of and possible identify of another factor, which determines race specificity of host-pathogen interactions, is summarized.