Induced Resistance to Powdery Mildew by 2,6-Dichloroisonicotinic Acid is Associated with Activation of Active Oxygen Species-mediated Enzymes in Cucumber Plants

Powdery mildew infection in cucumber plants was significantly reduced by foliar application of 2,6dichloroisonicotinic acid (INA). Although the overall isozyme banding patterns of superoxide dismutase (SOD) and peroxidase (POD) were very similar, there were differences in the densities of major bands between the leaves of control and INA-applied plants. When plants were subjected to foliar applications of INA, the densities of SOD and POD isoforms was increased more abruptly 3 days after foliar application compared with the control plants; the high density was maintained thereafter. The specific activities of SOD and POD were increased rapidly by foliar application of INA. Although enzyme activities were increased with the development of colonies 9 days after the inoculation of pathogen in control plants, it did not reach the level of activity in the leaves of INA-applied plants. The expression of β-1,3-glucanase was only detected in the leaves of INA-applied plants. The specific activity of β-1,3-glucanase was also significantly increased in the leaves of INA-applied plants. Activities of SOD and POD by foliar application of INA were significantly reduced by diphenylene iodinium (DPI), an inhibitor of oxidative burst. The expression of β-1,3-glucanase was also restricted by foliar application of INA with DPI. However, the β-1,3-glucanase isozyme band was detected ambiguously with increasing of SOD and POD 6 days after foliar application. Although gel and specific activity of SOD and POD were significantly increased in the leaves of H2O2-applied plants, no β-1,3-glucanase isoforms were detected. These results suggest that activation of AOS-mediated enzymes by foliar application of INA seem to take part in defense reaction to induce the resistance to powdery mildew in cucumber plants.