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

Abstract

The release of elicitor-active carbohydrates from fungal cell walls by p-1,3-endoglucanase contained i n host tissues has been implicated as one of the earliest processes in the interaction between soybean (Glycine max) and the fungal pathogen Pbytopbtbora megasperma f. sp. glycinea leading t o host defense responses such as phytoalexin production. The present study was conducted t o evaluate the primary structure of the glucanase-released elicitor (RE).Cel-filtration chromatography of carbohydrates released from mycelial walls by purified soybean p-1,3-endoglucanase resolved them into the four fractions (elicitor-active RE-I, -11, and -111 and elicitorinactive RE-IV). Sugar composition analysis indicated that all of the fractions were composed almost entirely of glucose. 'H-and 13C-nuclear magnetic resonance analysis indicated the presence of both p-1,3-and p-1,6-linkages for the elicitor-active RE-I, -11, and -111 fractions and only p-1,3 linkage for the elicitor-inactive RE-IV fraction. Methylation analysis and degradation studies employing p-1 ,3-endo-and p-1,3-exoglucanase further suggested that the basic structure of elicitor-active RE consists of P-1,6-linked glucan backbone chains of various lengths with frequent side branches composed of p-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 p-1,3-endoglucanase is also proposed.

An Ancient Enzyme Domain Hidden in the Putative β-Glucan Elicitor Receptor of Soybean May Play an Active Part in the Perception of Pathogen-associated Molecular Patterns during Broad Host Resistance

“…This mechanism might result in an amplification of the number of elicitor molecules that have been calculated to be represented by only one of 150 to 300 different heptaglucoside structures present in the Phytophthora cell walls (53). Pioneering work on plant defense already proposed this mechanism of elicitor release by an endo-1,3-␤-glucanase from soybean (54,55). More recently, a number of pathogenesis-related proteins have been shown to be glucanases or chitinases, which may play an active role during plant defense by attacking fungal cell walls.…”

Differential Effect of Purified Spruce Chitinases and β-1,3-Glucanases on the Activity of Elicitors from Ectomycorrhizal Fungi

“…The finding that the spruce chitinases identified cannot generate elicitors from cell walls of H. crustuliniforme, A. muscaria, and S. variegatus further supports the conclusion that chitinases in ectomycorrhizal interactions have different functions compared with those in plant-pathogen interactions. Our studies on P-1,3-glucanase from spruce cells showed that the enzyme does not influence the effectiveness of fungal elicitors and does not generate elicitors, as was reported in the case of walls from Phytophthora megasperma (Yoshikawa and Sugimoto, 1993;Okinaka et al, 1995). Likewise, /3-1,3-glucanase alone and together with chitinases did not inhibit the growth of the ectomycorrhizal fungi A. muscaria, H. crustuliniforme, and S. variegatus.…”

“…Originally, these signal molecules were components of the cell walls; chitin and P-glucans constitute the cell walls of Basidiomycetes and Ascomycetes (Wessels, 1993). Elicitors derived from chitin and P-glucan were demonstrated to induce lignification (Kurosaki et al, 1988;Barber et al, 1989;Okinaka et al, 1995;Cosi0 et al, 1996) and reactions that are assumed to be part of the hypersensitive Plant Physiol. Vol.…”

The structure and function of a soybean β-glucan-elicitor-binding protein

“…glycinea was grown at 25ЊC for 7 days (22), and the mycelial wall was isolated (14). The cell wall was digested with ␤-1,3-glucanase to obtain ␤-glucan polymer mixtures (16,23). They were further fractionated and purified by gel filtration (Sephadex G-75).…”

“…The first event between the soybean and fungus is an attack of the fungal cell wall by soybean ␤-1,3-glucanase, resulting in the release of active GEs, which then initiate phytoalexin accumulation in plants (13)(14)(15). The structure of the GE was proposed to be a ␤-1,6-linked glucan backbone of varying length with frequent side branches composed of one or two ␤-1,3-linked glucose moieties, as determined from 1 H and 13 C NMR analysis, methylation analysis, and degradation studies employing endo-and exoglucanases (16). The smallest elicitor-active ␤-glucan has been determined (hepta-␤-glucoside) (17), and the chemically synthesized elicitor was reported not only to induce phytoalexin accumulation but also to have a high affinity for the plasma membrane fraction (8,18,19).…”