Constitutive hydrolytic enzymes are associated with polygenic resistance of tomato to Alternaria solani and may function as an elicitor release mechanism

Foliar resistance to early blight disease of tomato, caused by the necrotrophic fungus Alternaria solani, is inherited in a complex quantitative manner. Our previous studies revealed that three moderately-resistant tomato breeding lines with different sources of early blight resistance, all possessed higher constitutive and more rapid accumulation of PR proteins, including specific antifungal isozymes of chitinase and beta -1,3-glucanase than susceptible genotypes. In the present study, additional early blight resistant tomato breeding lines and susceptible genotypes were investigated for their constitutive levels of PR proteins. All resistant lines possessed higher constitutive levels of chitinase and beta -1,3-glucanase than susceptible genotypes supporting earlier reported findings. Constitutive and pathogen-induced expression of PR genes during early stages of the interaction was found to be much higher in the highly resistant breeding line NC 24-E than a susceptible line. In vitro elicitor release experiments were performed with purified ii. solani cell walls treated with constitutive total enzyme preparations isolated from resistant and susceptible plants. Enzyme preparations from resistant and susceptible genotypes differed in their ability to produce hypersensitive response (HR) elicitors from fungal cell walls: the enzyme preparations from resistant breeding lines released HR elicitors, while enzyme preparations from susceptible lines did not. Additionally, experiments with a partially purified preparation of basic tomato chitinases demonstrated that these enzymes were able to release HR elicitors from germinating spores of A. solani, but not mature, intact cell walls. The possibility that constitutively produced hydrolytic enzymes may act as an elicitor-releasing mechanism in resistance to early blight of tomato is discussed. (C) 2000 Academic Press.