Nucellar callus of 'Femminello' lemon, selected for tolerance toPhoma tracheiphila toxin, shows enhanced release of chitinase and glucanase into the culture medium.

Phoma tracheiphila is the causative agent of the disease mal secco. Citrus cultivars differ substantially in respect to their sensitivity to the pathogenP. tracheiphila and its toxin. Some cultivars (e.g., 'Femminello' lemon) are inherently sensitive while others (e.g., 'Tarocco' orange) are tolerant. Cell lines derived from nucellar tissue of 'Femminello', 'Tarocco' and a cell line selected for tolerance to the fungal toxin ('Femminello-S') were used to study host-pathogen interaction. Our results showed that calli or conditioned media of 'Tarocco' and 'Femminello-S' inhibited the size of co-cultivated fungal colonies when compared to 'Femminello'. In addition, conditioned medium of 'Tarocco' as well as 'FemminelloS', but not 'Femminello', promoted bursting of hyphal tips. A ten-fold increase in chitinase and glucanase enzymatic activity, as evaluated by radiometric assay and laminarin hydrolysis respectively, was detected in 'Femminello-S' extracellular extracts as compared to 'Femminello'. An increase in chitinase was also shown by immunoblot analysis. Our findings suggest a positive correlation between the presence of chitinase and glucanase in the conditioned media of the cultured cells and the tolerance of those cells toP. tracheiphila toxin.