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Cytochemical Investigation of the Antagonistic Interaction Between a Microsphaeropsis sp. (Isolate P130A) and Venturia inaequalis

First and third authors: Agriculture and Agri-food Canada, Horticultural Research and Development Center, 430 Boul, Gouin, St-Jean-sur- Richelieu, Québec, Canada J3B 3E6; second author: Département de Phytologie, Recherche en sciences de la vie et de la santé, Pavillon C. E. Marchand, Université Laval, Sainte-Foy, Québec, Canada G1K 7P4

In an attempt to better understand the mode of action of the antagonistic fungus Microsphaeropsis sp., the interaction between this fungus and Venturia inaequalis was studied, using both light and electron microscopy. Cytological observations indicated that the antagonistic interaction between the two fungi likely involves a sequence of events, including (i) attachment and local penetration of Microsphaeropsis sp. into V. inaequalis hyphae; (ii) induction of host structural response at sites of potential antagonist entry; (iii) alteration of host cytoplasm; and (iv) active multiplication of antagonistic cells in pathogen hyphae, leading to host cell breakdown and release of the antagonist. The interaction was investigated further by gold cytochemistry. The use of gold-complexed β-1,4-exoglucanase and wheat germ agglutinin/ovomucoid-gold complex to localize cellulosic β-1,4-glucans and chitin monomers, respectively, resulted in regular labeling of V. inaequalis cell walls. This finding supports other studies refuting the classification of ascomycetes as only a glucan-chitin group. At an advanced state of parasitism, the labeling pattern of cellulose and chitin, which clearly showed that the level of integrity of these compounds was affected, suggested the production of cellulolytic and chitinolytic enzymes by Microsphaeropsis sp. Wall appositions formed in V. inaequalis in response to the antagonist's attack contained both cellulose and chitin. However, penetration of this newly formed material frequently succeeded. This study provides the first detailed picture of the cytological events associated with mycoparasitism in V. inaequalis.

Additional keywords: apple scab , biocontrol agent .