Encystment ofPythium aphanidermatumZoospores is Induced by Root Mucilage Polysaccharides, Pectin and a Monoclonal Antibody to a Surface Antigen

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Abstract
The infection of roots by the pathogenic Oomycete Pythium aphanidermatum involves interactions between the fungal zoospores and root surface mucilage polysaccharides. After initial recognition at the root surface the zoospores are triggered to encyst during which adhesive glycoproteins are secreted followed by a fibrillar cyst wall. In this paper a simple in vitro assay has been used to assess the ability of a variety of macromolecules to induce zoospore encystment. Mucilage polysaccharides of the cress root surface trigger encystment. Whole mucilage was fractionated by gel filtration and a fraction low in uronic acid, containing 5% fucose, was shown to be more effective in triggering encystment than a uronic acid-rich fraction. Encystment can also be induced by commercial pectin. The lectin Con A, and PA1, one of a range of monoclonal antibodies specific for zoospore surface antigens, also triggered encystment. In Western blotting experiments PA1 recognizes protein epitopes of a 75 kDa surface antigen. The results suggest that at least one mechanism of zoospore triggering may involve a specific zoospore surface receptor.