Induction of some defense-related genes and oxidative burst is required for the establishment of systemic acquired resistance in Capsicum annuum

The inoculation of primary pepper leaves with an avirulent strain of Xanthomonas campestris pv. vesicatoria induced systemic acquired resistance (SAR) in the non-inoculated, secondary leaves. This SAR response was accompanied by the systemic expression of the defense-related genes, a systemic microoxidative burst generating H2O2, and the systemic induction of both ion-leakage and callose deposition in the non-inoculated, secondary leaves. Some defense-related genes including those encoding PR-1, chitinase, osmotin, peroxidase, PR10, thionin, and SAR8.2 were markedly induced in the systemic leaves. The conspicuous systemic accumulation of H2O2 and the strong increase in peroxidase activity in the pepper leaves was suggested to play a role in the cell death process in the systemic micro-hypersensitive responses (HR), leading to the induction of the SAR. Treatment of the primary leaves with diphenylene iodinium (DPI), an inhibitor of oxidative burst, substantially reduced the induction of some of the defense-related genes, and lowered the activation of the oxidative bursts in the systemic leaves distant from the site of the avirulent pathogen inoculation and subsequently SAR. Overall, these results suggest that the induction of some defense-related genes as well as a rapid increase in oxidative burst is essential for establishing SAR in pepper plants.