Tomatinase induction in formae speciales ofFusarium oxysporumnon-pathogenic of tomato plants

The antifungal compound α-tomatine has been reported to provide a preformed chemical barrier in tomato plants against phytopathogenic fungi.Fusarium oxysporumf.sp.lycopersiciand other tomato pathogens produce extracellular enzymes induced by α-tomatine. These enzymes, known as tomatinases, provoke deglycosylation of α-tomatine yielding less toxic derivatives. To date, tomatinase activities have been associated exclusively with fungi which are able to infect tomato plants. Here, we have established that another tomato pathogen,F. oxysporumf.sp.radicis-lycopersici,also produces an inducible α-tomatine detoxifying enzyme. In addition, we found that other formae speciales which are not pathogens of tomato such astuberosi, melonis, niveumandgladiolialso have inducible tomatinases. These formae speciales are natural pathogens of other hosts, some of which are phylogenetically related to tomato but others are not. The tomatinases found in these formae speciales have the same molecular mass and act mechanistically in the same way as the tomatinase fromF. oxysporumf.sp.lycopersici,i.e. catalysing the hydrolysis of α-tomatine into the less fungitoxic molecules, tomatidine and β-lycotetraose. These enzymes were recognized by polyclonal antibodies raised against tomatinase fromF. oxysporumf.sp.lycopersici.These observations suggest that tomatinase enzymes are analogous in the different formae speciales studied. The possible role of tomatinases in fungi which are not tomato pathogens and the phylogenetic relationship between the different formae speciales ofF. oxysporumare discussed.