Expression of extracellular chitinolytic enzymes is highly induced by growing Trichoderma on purified chitin, fungal cell walls, or mycelia as sole carbon source. It has been proposed that chitinolytic enzymes could be induced by soluble chito-oligomers (Reyes et al. 1989; St. Leger et al. 1986). This appears to be the case for the 73 kDa N-acetyl-ß-d-glucosaminidase of T. harzianum and T. atroviride, which are induced not only by chitooligomers but also by N-acetyl-ß-d-glucosamine. The 42kDa endochitinase of T. harzianum responds similarly to these compounds but ech42 expression in T. atroviride is not induced by the products of chitin degradation (Carsolio et al. 1999; De la Cruz et al. 1993;
Mach et al. 1999; Schikler et al. 1998; Ulhoa and Peberdy 1991; Ulhoa and Peberdy 1993). Expression of ech42 in T. atroviride is strongly induced during fungus -fungus interaction. Its expression is repressed by glucose, may be affected by other environmental factors, such as light and may even be developmentally regulated (Carsolio et al. 1994). In general, formation of most chitinolytic enzymes does not occur or is inhibited in the presence of glucose, sucrose, and chitinolytic end products (Carsolio et al. 1994; 1999; De la Cruz et al. 1993; Garcia et al. 1994; Peterbauer et al. 1996; Ulhoa and Peberdy 1991). In addition, there is evidence suggesting that the expression of at least ech42 of T. atroviride and chit33 of T. harzianum is repressed by high levels of ammonium (Donzelli and Harman 2001; Mercedes de las et al. 2001). In this sense, the proteinase encoding gene prb1 responds to carbon and nitrogen limitation. It has also been suggested that the MRGs chit33, ech42, and prb1, respond to other types of physiological stress (Mach et al. 1999; de las Mercedes et al. 2001; Olmedo-Monfil et al. 2002). Recently, we found that the response of ech42 and prb1 to nutrient limitation depends on the activation of conserved mitogen activated protein kinase (MAPK) pathways (Olmedo-Monfil et al. 2002).
The level of production of ß-1,3-glucanases by T. atroviride is induced by the presence of cell walls of M. rouxii, N. crassa, S. cerevisiae, and R. solani (in ascending order of efficiency) and appears to be dependent on the amount of ß-1,3-glucan present in the cell walls of these organisms (Vazquez-Garcidueüas et al. 1998). Additional results obtained with a filtrate of autoclaved S. cerevisiae cell walls suggest that the induction observed with cell walls may be triggered by two components, one extractable and one that remains cell-wall bound (Vazquez-Garciduenas et al. 1998). In general, glucanase expression is repressed by glucose and in some cases, might be repressed by primary nitrogen sources (Donzelli and Harman 2001).
In summary, expression of all enzymes from the cell-wall degrading system of Trichoderma appears to be coordinated. Suggesting a regulatory mechanism involving substrate induction and catabolite repression. The expression of the system is controlled at the level of transcription as indicated by Northern analysis of the available genes (Carsolio et al. 1994; De la Cruz et al. 1995; Donzelli and Harman 2001; Flores et al. 1997; Geremia et al. 1993; Kim et al. 2002; Limon et al. 1995; Mercedes de las et al. 2001). An exciting finding in terms of signaling is that the induction of at least two MRGs, namely prb1 and ech42, is triggered by a diffusible factor produced by the host (Cortes et al. 1998). Recently, it has been suggested that the activation of MRGs in response to the presence of the host, through such a molecule depends on the basal expression of ech42 in T. atroviride (Kubicek et al. 2001). However, in T. virens, induction of four MRGs in response to cell walls in ech1 (the homologue of ech42) knockout mutants is still observed. Whether a key molecule produced by the host in vivo switches on the expression of MRGs remains to be proven, as well as the role of Ech42 in the production of such a molecule.
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