A Structural Model for the Mechanisms of Elicitor Release from Fungal Cell Walls by Plant [beta]-1,3-Endoglucanase

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Abstract
The release of elicitor-active carbohydrates from fungal cell walls by [beta]-1,3-endoglucanase contained in host tissues has been implicated as one of the earliest processes in the interaction between soybean (Glycine max) and the fungal pathogen Phytophthora megasperma f. sp. glycinea leading to host defense responses such as phytoalexin production. The present study was conducted to evaluate the primary structure of the glucanase-released elicitor (RE). Gel-filtration chromatography of carbohydrates released from mycelial walls by purified soybean [beta]-1,3-endoglucanase resolved them into the four fractions (elicitor-active RE-I, -II, and -III and elicitor-inactive RE-IV). Sugar composition analysis indicated that all of the fractions were composed almost entirely of glucose. 1H- and 13C-nuclear magnetic resonance analysis indicated the presence of both [beta]-1,3- and [beta]-1,6-linkages for the elicitor-active RE-I, -II, and -III fractions and only [beta]-1,3 linkage for the elicitor-inactive RE-IV fraction. Methylation analysis and degradation studies employing [beta]-1,3-endo- and [beta]-1,3-exoglucanase further suggested that the basic structure of elicitor-active RE consists of [beta]-1,6-linked glucan backbone chains of various lengths with frequent side branches composed of [beta]-1,3-linked one or two glucose moieties. From these structural analyses of RE, a structural model of how RE is originally present in fungal cell walls and released by host [beta]-1,3-endoglucanase is also proposed.