Differential occurrence of the oxidative burst and the activity of defence-related enzymes in compatible and incompatible tomato-Oidium neolycopersici interactions

The generation and accumulation of hydrogen peroxide (H2O2) and superoxide anion (O 2 .- ), as well as guaiacol peroxidase, catalase, polyphenol oxidase, β-1,3-glucanase and chitinase activity, were studied in leaves of resistant and susceptible tomato genotypes inoculated with Oidium neolycopersici. Plants of the resistant genotype CNPH 1287 (Solanum habrochaites sin. Lycopersicon hirsutum) and susceptible genotype Santa Cruz Kada (S. lycopersicum sin. Lycopersicon esculentum), with the seven-nine and five-seven leaves completely developed, respectively, were inoculated in the second, third and fourth true leaves. Leaves were collected at the time of inoculation and at 4, 8, 12, 24, 48, 72, 96 and 120 h post inoculation (hpi). The production and accumulation of H2O2 and O 2 .- were evaluated in situ using diaminobenzidine and nitroblue tetrazolium, respectively. Starting at 24–48 hpi, high accumulation of H2O2 and O 2 .- was detected, and epidermal cells demonstrated a hypersensitive response, especially in the inoculated leaves of the resistant plant (S. habrochaites). An increase in guaiacol peroxidase, catalase, polyphenol oxidase, β-1,3-glucanase and chitinase activity was mainly detected by 24 hpi in the resistant plant. An association between the production of reactive oxygen species and the activity of enzymes related to reactive oxygen species metabolism (guaiacol peroxidase, catalase), hydrolytic enzymes (β-1,3-glucanase and chitinase) and phenol metabolism enzymes (polyphenol oxidase), as well as hypersensitive response, was evident during the defence responses of the resistant plants when inoculated with O. neolycopersici.