Differential induction of pathogenesis-related proteins in tomato by Alternaria solani and the association of a basic chitinase isozyme with resistance

Accumulation of pathogenesis-related proteins is thought to play a role in pathogen-induced plant defense responses. Although early accumulation of hydrolytic enzymes such as chitinase and beta-1,3-glucanase has been associated previously with genetically-inherited and induced systemic resistance, their role in resistance in tomato (Lycopersicon esculentum) to the phytopathogenic fungus Alternaria solani is not yet understood. Here we describe the accumulation patterns of specific isozymes of pathogenesis-related proteins in the resistant tomato genotypes 71B2, NC EBR-1, NC EBR-2 and the susceptible cultivar Piedmont. Western blot analysis demonstrated that four isozymes of chitinase (26, 27, 30, and 32 kDa) were induced in all genotypes upon challenge with A. solani, but only resistant lines had significantly higher constitutive levels of the 30 kDa isozyme as well as total chitinase activity. In addition, the 30 kDa chitinase isozyme was found to accumulate to significantly higher levels in resistant lines during pathogenesis than the susceptible genotype. Two isozymes of beta-1,3-glucanase (33 and 35 kDa) were detected in ail genotypes, but a slightly higher constitutive level was detectable in all resistant lines when compared to the susceptible. Similar accumulation patterns of these isozymes were observed in all genotypes during the course of pathogenesis. Purified preparations of acidic and basic tomato chitinase and beta-1,3-glucanase isozymes were tested for their antifungal activity against A. solani in vitro. Results presented in this study indicate that only basic isozymes of chitinase and beta-1,3-glucanase were inhibitory to A. solani whereas, no inhibitory activity was observed with the acidic isozymes. The results of this study suggest that a higher constitutive level of chitinase and beta-1,3-glucanase and the induction pattern of a 30 kDa chitinase isozyme in early blight resistant breeding lines is related to genetically-inherited resistance of tomato to A. solani. (C) 1996 Academic Press Limited