Specific saccharide residues are involved in the recognition of plant root surfaces by zoospores of Pythium aphanidermatum

Abstract

The binding and encystment of zoospores of Pythium aphanidermatum to root surfaces of cress ( Lepidium sativum) was used as a quantitative model of fungal-plant recognition. Binding principally involves the root elongation zone of intact (undamaged) seedlings, and was standardized with respect to time, temperature and zoospore concentration to that the initial rate of zoospore binding could be assessed. Zoospore-root binding is abolished by trypsinization of zoospores or periodate oxidation of the root surface. Lectin and enzyme modification of the root surface mucilage polysaccharide reduced zoospore binding; treatments that block, or remove terminal fucosyl residues are especially effective. Incubation of zoospores with exogenous root mucilage polysaccharide also reduced zoospore binding but the effectiveness of the mucilage was reduced if pretreated with periodate or with exofucosidase. The results suggest that protein-containing receptors on the zoospore surface interact with fucose-containing ligands present in root surface mucilage to promote zoospore-binding and subsequent encystment. The involvement of other sugar residues cannot, however, be ruled out.