The antifungal glycoalkaloid alpha-tomatine of the tomato plant (Lycopersicon 
esculentum) is proposed to protect the plant against phytopathogenic fungi. 
Fusarium oxysporum f. sp. lycopersici, a vascular pathogen of tomato, produces a 
tomatinase enzyme which hydrolyses the glycoalkaloid into non-fungitoxic 
compounds. Detoxification of alpha-tomatine may be how this fungus avoids the 
plant glycoalkaloid barrier. As an initial step to evaluate this possibility we 
have studied the induction of tomatinase; (i) in fungal cultures containing 
extracts from leaf, stem or root of tomato plants; and (ii) in stem and root of 
tomato plants infected with the pathogen at different infection stages. The 
kinetics of tomatinase induction with leaf extract (0.6% dry weight) was similar 
to that observed with 20 micrograms ml-1 of alpha-tomatine. In the presence of 
stem extract, tomatinase activity was less than 50% of that induced with leaf 
extract, whereas in the presence of root extract tomatinase activity was very 
low. In the stem of infected tomato plants tomatinase activity was higher at the 
wilt stage than in previous infections stages and in root, tomatinase activity 
appeared with the first symptoms and was maintained until wilting. TLC analysis 
showed that the tomatinase induced in culture medium with plant extracts and in 
infected tomato plants had the same mode of action as the enzyme induced with 
pure alpha-tomatine, hydrolysing the glycoalkaloid into its non-fungitoxic 
forms, tomatidine and beta-lycotetraose. The antisera raised against purified 
tomatinase recognized in extracts of root and stem of infected tomato plants a 
protein of 50000 (45000 when proteins were deglycosylated), corresponding to the 
tomatinase enzyme. Therefore, it is concluded that F. oxysporum f. sp. 
lycopersici express tomatinase in vivo as a result of the infection of tomato 
plant.