Salicylic acid response in rice: influence of salicylic acid on H(2)O(2) accumulation and oxidative stress.

We have studied the accumulation of hydrogen peroxide in rice leaves in response to salicylic acid (SA) treatment. H(2)O(2) accumulation was localized in the veins. While the activity of the H(2)O(2) degrading enzyme peroxidase was inhibited in response to SA, superoxide dismutase activity, which generates H(2)O(2), did not show any change. An increase in the activity of glutathione reductase, a peroxide degrading enzyme, was observed upon SA treatment, similar to the response reported by accumulation of active oxygen species. In rice leaves transformed with CaMV-GUS fusions, we found that SA treatment induced transcriptional activation through as-1 element. As-1 has been implicated in protective mechanisms that operate during some types of oxidative stress and the hypersensitive response (HR). Expression of rice pathogenesis-related protein 5 (PR5), one of the PR genes associated with systemic acquired resistance, was induced by SA. This induction seems to be independent of the H(2)O(2) accumulation induced by SA. Inoculation of rice leaves with the rice blast fungus, Magneporthe grisea, also led to the accumulation of H(2)O(2) in the veins, suggesting the presence of common signals between SA and pathogen responses. Thus SA application seems to induce oxidative stress in rice through H(2)O(2); a signal molecule implicated in biotic and abiotic stress responses.