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Hyphal Interactions Between Trichoderma harzianum and Rhizoctonia solani: Ultrastructure and Gold Cytochemistry of the Mycoparasitic Process. Nicole Benhamou, Département de phytologie, Faculté des sciences de l'agriculture et de l'alimentation, Université Laval, Sainte-Foy, Québec, Canada G1K 7P4; Ilan Chet, Department of Plant Pathology and Microbiology, Faculty of Agriculture, The Hebrew University of Jerusalem, Rehovot, Israël. Phytopathology 83:1062-1071. Accepted for publication 11 May 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/Phyto-83-1062.

The interaction between Trichoderma harzianum and the soilborne plant pathogen Rhizoctonia solani was studied by both scanning (SEM) and transmission (TEM) electron microscopy. Macroscopic observations of fungal growth in dual cultures revealed that pathogen growth inhibition occurred soon after contact with the antagonist. SEM investigations demonstrated that coiling of the antagonist (T. harzianum) around its host (R. solani) was an early event preceding hyphal damage. Ultrastructural observations of hyphal interactions showed that contact between the two fungi was mediated by a fine, extracellular matrix originating from cells of R. solani. The use of gold-complexed Ricinus communis agglutinin provided evidence that this matrix was rich in galactose residues. Attachment of hyphae of Trichoderma to cells of R. solani was followed by a series of degradation events in the host. The cytochemical localization of N-acetylglucosamine residues with wheat germ agglutinin (WGA)/ovomucoid-gold complex revealed that chitin breakdown occurred gradually, suggesting a continuous production of chitinases by the antagonist. Disorganization of the cell-wall structure of R. solani appeared to be an early event that promoted internal osmotic imbalances that, in turn, triggered intracellular disorders, such as retraction of the plasma membrane and cytoplasm aggregation.