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Ultrastructural and Immunochemical Changes During the In Vitro Development of Plasmopara halstedii

First, second, fourth, sixth, and seventh authors: Groupe d'Etude des Interactions Hôte-Parasite, UPRES-EA 3142, Laboratoire de Parasitologie-Mycologie, Faculté de Pharmacie, 16 Bd. Daviers, 49100 Angers; third author: Service Commun de Microscopie Electronique, Faculté de Médecine, rue Haute de Reculée, 49045 Angers, France; and fifth author: Groupe d'Etude des Variétés et des Semences, rue Georges Morel, 49071 Beaucouzé Cedex, France

The asexual phase of the life cycle of Plasmopara halstedii, the causal agent of downy mildew of sunflower, plays a key role in the propagation of the disease. We investigated the morphological and ultrastructural changes that occur during the asexual development of the pathogen. Direct examination of infected cotyledons confirmed the presence of sporangiophores. In contact with water, important ultrastructural changes occurred, affecting the surface of zoosporangia, which became smoother, and their cytoplasm, which differentiated into flagellate zoospores. The subsequent encystment of zoospores was characterized by the synthesis of a cell wall and the loss of the flagella. In addition, two monoclonal antibodies (MAbs) specific for P. halstedii were used to analyze the immunochemical changes associated with these modifications. MAb 16A6, which bound to a 48-kDa glycoprotein, mainly labeled the surface of mobile or encysted zoospores and of mother cells of germ tubes. Conversely, MAb 2F9, which recognized highly glycosylated antigens, labeled the surface of zoosporangia and of flagellate zoospores, but not the encysted zoospores. These results provide new insights into the morphological and ultrastructural changes associated with the release and the encystment of zoospores which may be interesting targets for the development of new antimicrobial products.