INVOLVEMENT OF REACTIVE OXYGEN SPECIES AND THE INDUCTION OF THE CELLULAR ANTIOXIDANT MACHINERY IN THE NECROTIC DEATH OF TWO CAPSICUM ANNUUM CULTIVARS WITH DIFFERENT SENSITIVITY TO PHYTOPHTHORA CAPSICI
M.E. Requena, C. Egea-Gilabert, M.E. Candela
This study analyses the defence response of two pepper cultivars, Serrano Criollo de Morelos (SCM, resistant) and California Wonder (CW, sensitive) to oomycete pathogen Phytophthora capsici. We compare the production of reactive oxygen species (O2 - and H2O2) in vitro and in vivo, as well as the activity of antioxidative enzymes in the cuts of the decapitated stems elicited with P. capsici. In all tests, following elicitation O2 - and H2O2 are produced in both cultivars, but in higher amount in SCM cells. In vivo assays show that infection by the pathogen causes progressive necrosis in the decapitated SCM and CW stems and that necrosis length is inversely proportional to resistance. The antioxidative enzymes tested (superoxide dismutase, peroxidase, catalase and glutathione reductase) increase their activity after infection, to a significant extent in SCM plants and to a lesser extent in CW plants. Therefore, we conclude that if H2O2 were to play a role in resistance, it would be due to its possible signalling function. These findings are corroborated by the results of our analysis of the spatial distribution of H2O2 production along the stem, since in the SCM cultivar H2O2 penetrates deeper, and can be detected in regions not reached by the P. capsici hyphae. The accumulative effect of oxidative damage mediated by H2O2 plus the attainment of homeostasis in SCM plants results in cells being capable of a defense response that is efficient enough to inhibit pathogen growth. Therefore, reactive oxygen species production, lipid peroxidation increase and antioxidative enzymes change play an important role in the resistance of this pepper cultivar to P. capsici.