CELL-SURFACE ANTIGENS OF PHYTOPHTHORA SPORES - BIOLOGICAL AND TAXONOMIC CHARACTERIZATION

The oomycetes are a class of protists that produce biflagellate asexua l zoospores. Members of the oomycetes have close phylogenetic affiniti es with the chromophyte algae and are widely divergent from the higher fungi. This review focuses on two genera, Phytophthora and Pythium, w hich belong to the family Pythiaceae, and the order Peronosporales. Th ese two genera contain many species that cause serious diseases in pla nts. Molecules on the surface of zoospores and cysts of these organism s are likely to play crucial roles in the infection of host plants. Kn owledge of the properties of the surface of these cells should thus he lp increase our understanding of the infection process. Recent studies of Phytophthora cinnamomi and Pythium aphanidermatum have used lectin s to analyse surface carbohydrates and have generated monoclonal antib odies (MAbs) directed towards a variety of zoospore and cysts surface components. Labelling studies with these probes have detected molecula r differences between the surface of the cell body and of the flagella of the zoospores. They have been used to follow changes in surface co mponents during encystment, including the secretion of an adhesive tha t bonds the spores to the host surface. Binding of lectin and antibody probes to the surface of living zoospores can induce encystment, givi ng evidence of cell receptors involved in this process. Freeze-substit ution and immunolabelling studies have greatly augmented our understan ding of the synthesis and assembly of the zoospore surface during zoos porogenesis. Synthesis of a variety of zoospore components begins when sporulation is induced. Cleavage of the multinucleate sporangium is a chieved through the progressive extension of partitioning membranes, a nd a number of surface antigens are assembled onto the zoospore surfac e during cleavage. Comparisons of antibody binding to many isolates an d species of Phytophthora and Pythium have revealed that surface compo nents on zoospores and cysts exhibit a range of taxonomic specificitie s. Surface antigens or epitopes may occur on only a few isolates of a species; they may be species-specific, genus-specific or occur on the spores of both genera. Spore surface antigens thus promise to be of si gnificant value for studies of the taxonomy and phylogeny of these pro tists, as well as for disease diagnosis.