|Riassunto in Inglese:|
Many pathogenic filamentous fungi produce a battery of extracellular cell wall degrading enzymes (CWDEs) capable of degrading the polymers of plant cell walls. This CWDEs mixture is thought to facilitate the fungus colonization and/or to obtain nutrients from wall polymers due to the breakdown of the host cell wall. Cellulose is a major polysaccharide constituent of plant cell walls and one of the most abundant organic compounds in the biosphere. Biological degradation of cellulose involves the synergistic action of three enzymes: endoglucanase or carboxymethyl cellulase (endo ?-1,4-glucanase), exoglucanase or cellobiohydrolase (exo-?-1,4-glucanase), and ?-glucosidase (?-Dglucoside glucohydrolase). Among these enzymes, the endoglucanases have been well studied and are produced by various microbes (bacteria, yeast, and fungi), plants, and protozoans. In some cases it was shown that specific genes in the battery of genes encoding for extracellular enzymes are expressed under specific environmental conditions, which might determine their role in pathogenesis. In several systems conclusive evidence has been obtained for the role of these enzymes in the infection process, however, targeted mutational studies have found that individual enzyme genes could play only a partial role in pathogenesis. Recently, a gene coding for an endo-1,4-?-glucanase belonging to family 61 glycoside hydrolases (GH61) has been cloned from Pyrenochaeta lycopersici, the causal agent of the Corky Root Rot (CRR) of tomato, and named plegl1. The GH61 class of proteins, produced exclusively by fungi, acts synergistically with cellulases. The expression level of this endoglucanase (PlEGL1) is positively correlated, in planta, with the progress of the disease: an initial, biotrophic phase without major fungal proliferation and a following necrotrophic phase, with pronounced fungal development, correlates with plegl1 expression profile: this is undetectable during the initial infection of tomato, in which cellulose hydrolytic activity is not started, while it is strongly expressed in the later phase, corresponding with root collapse. In vitro, plegl1 gene transcription is correlated to a cellulolytic activity of the fungus, regulated, in its turn, by the presence of sugar and/or cellulose in the culture medium (catabolite repression). The recombinant PLEGL1 protein, analysed by polysaccharide affinity precipitation assays, shows specificity for cellulose-based substrates.