Host Recognition

In vitro, the first detectable interaction shows that the hyphae of the mycoparasite grow directly towards its host (Chet et al. 1981). This phenomenon appears as a chemotropic growth of Trichoderma in response to some stimuli produced by the host (Chet and Elad 1983). When the mycoparasite reaches the host, its hyphae often coil around it or attach to it by forming hooklike structures. Although not a frequent event production of appressoria at the tips of short branches has been observed. Coiling appears to be controlled by lectins present on the host hyphae. A R. solani lectin that binds to galactose and fucose residues was shown to agglutinate conidia of a mycoparasitic strain of Trichoderma harzianum but did not agglutinate two nonparasitic strains (Barak et al. 1985; Elad et al. 1983a).

d-glucose or d-mannose residues, apparently present on the cell walls of T. harzianum, inhibited the activity of a second lectin isolated from Sclerotium rolfsii. Inbar and Chet (1992; 1994) were able to mimic the fungus-fungus interaction in vitro using nylon fibers coated with either concanavalina A or the purified S. rolfsii lectin. During the interaction Trichoderma recognized and attached to the coated fibers, coiling around them and forming other mycoparasitism-related structures, such as appresoriumlike bodies and hyphal loops (Inbar and Chet 1992; 1994). Recently, using the biomimetic system we showed that different lectins induce coiling. Furthermore, coiling of Trichoderma around the fibers in the absence of lectins can be induced by applying cAMP or the heterotrimeric G protein activator mastoparan (Rocha-Ramírez et al. 2002). Transgenic lines that overexpress the Ga subunit coil at higher frequency than untransformed controls. Furthermore, transgenic lines that express an activated mutant protein with no GTPase activity coil at an even higher frequency. In addition, lines that express an antisense version of the gene do not appear to coil in the biomimetic assay (Rocha-Ramírez et al. 2002).

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