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Biochemistry and Cell Biology

Mycoparasitism of the Extramatrical Phase of Glomus intraradices by Trichoderma harzianum. Annie Rousseau, Centre de Recherche en Biologie Forestière, Pavillon C.-E. Marchand, Université Laval, Sainte-Foy, Québec, Canada G1K 7P4; Nicole Benhamou(2), Ilan Chet(3), and Yves Piché(4). (2)Recherche en sciences de la vie et de la santé, Pavillon C.-E. Marchand, Université Laval, Sainte-Foy, Québec, Canada G1K 7P4; (3)Department of Plant Pathology and Microbiology, Faculty of Agriculture, The Hebrew University of Jerusalem, Rehovot, Israël; (4)Centre de Recherche en Biologie Forestière, Pavillon C.-E. Marchand, Université Laval, Sainte-Foy, Québec, Canada G1K 7P4. Phytopathology 86:434-443. Accepted for publication 19 February 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/Phyto-86-434.

In the present study, the interaction between the saprophytic fungus Trichoderma harzianum and the arbuscular mycorrhizal (AM) fungus Glomus intraradices was studied by transmission electron microscopy (TEM) to delineate precisely the relationship established between both partners. An axenic system, divided into four compartments, proved useful for studying the interaction between T. harzianum and the extramatrical phase of G. intraradices. This experimental model, based on root-organ culture to obtain typical mycorrhizal infections, was selected as a reliable means of obtaining mycorrhizal spores and mycelium in root-free compartments. TEM observations of samples from the interaction region showed that hyphae of T. harzianum proliferated abundantly at the spore surface and penetrated the thick host wall through local hydrolysis of the wall polymers. Hyphae of the antagonist also were seen in the subtending hyphae of the AM fungus, and they grew actively in the main host hyphae. This massive colonization was associated with marked cell damage, involving partial to complete disorganization of the cytoplasm, which led in most cases to loss of the protoplasm and apparent bursting of the main hyphae of G. intraradices, resulting in the release of the actively proliferating Trichoderma hyphae. At an advanced stage of the colonization process, the main hyphae of G. intraradices were perforated in many places. The use of wheat germ agglutinin/ovo-mucoid-gold complex for the localization of chitin monomers resulted in regular labeling of the host cell walls even when spores, subtending hyphae, and main hyphae of G. intraradices were colonized massively. Chitinolytic degradation was seen only in areas adjacent to the sites of Trichoderma penetration. According to our observations, the interaction between T. harzianum and G. intraradices involves the following events: (i) recognition and local penetration of the antagonist into mycorrhizal spores; (ii) active proliferation of antagonist cells in mycorrhizal hyphae; and (iii) release of the antagonist through moribund hyphal cells.