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Cytology and Histology

Cytochemical Aspects of Chitin Breakdown During the Parasitic Action of a Trichoderma sp. on Fusarium oxysporum f. sp. radicis-lycopersici. Mohamed Ch?rif, Departement de phytologie, Facult? des sciences de l?agriculture et de l?alimentation, Universit? Laval, Sainte-Foy, Qu?bec, Canada, G1K 7P4; Nicole Benhamou, Departement de phytologie, Facult? des sciences de l?agriculture et de l?alimentation, Universit? Laval, Sainte-Foy, Qu?bec, Canada, G1K 7P4. Phytopathology 80:1406-1414. Accepted for publication 12 July 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-1406.

A strain of Trichoderma, isolated from a sample of peat collected in New Brunswick, displays the ability to produce chitinases and inhibits growth of the pathogenic fungus, Fusarium oxysporum f. sp. radicis-lycopersici, in dual culture tests. Scanning electron microscope investigations of hyphal interactions show that growth inhibition of the host mycelium does not result from hyphal penetration by Trichoderma. The noticeable wall alterations along with the rapid collapse and loss of cell turgor of F. o. radicis-lycopersici in areas where Trichoderma was not in direct contact with the host mycelium indicates that extracellular metabolites could be responsible for the observed degradation. The cytochemical localization of N-acetylglucosamine residues in cell walls of F. o. radicis-lycopersici at different times after inoculation revealed that alteration of chitin molecules is an early event preceding wall disruption and leakage of cytoplasm. The observation that N-acetylglucosamine residues are released in the growing medium supports the idea that wall-bound chitin may be rapidly hydrolyzed by an extracellular chitinase produced by Trichoderma. Although it is likely that other lytic enzymes are involved in the complete degradation of mycelial walls of F. o. radicis-lycopersici, the present cytochemical investigation together with biochemical data on chitinase activity provides evidence for a major chitinolytic activity of Trichoderma and indicates that production of this enzyme may be of great significance in the antagonistic process.