The involvement of reactive oxygen species in the resistance response of wheat to the Russia wheat aphid

The effect of Russian wheat aphid (RWA), (Diuraphis noxia, Mordvilko), infestation on the activities of the reactive oxygen species (ROS) generating and scavenging enzymes as well as the hydrogen peroxide content were studied in resistant (cv., Tugela DN) and the near isogenie susceptible (cv., Tugela) wheat (Triticum aestivum L.) cultivars. Infestation resulted in an early induction (6-9h after infestation) of the activities of the ROS generating enzymes, NADPH oxidase and superoxide dismutase (SOD), in resistant plants only. These increased enzyme activities correlated with the increased H202 content in infested resistant plants. The activities of the ROS scavenging enzymes, glutathione reductase and ascorbate peroxidase were selectively induced somewhat later (12h after infestation) in resistant plants. Results suggest art early involvement of the ROS, and relevant ROS generating and scavenging enzymes in the resistance response of wheat to the RWA. Inhibitory studies were performed in vivo using diphenylene iodonium (DPI), an inhibitor of NADPH oxidase, to shed light on the possible upstream role ofROS in defence against the RWA in infested resistant plants. DPI substantially inhibited the RWA induced NADPH oxidase activity and the concomitant accumulation of H202. This resulted in the inhibition of activities of the secondary defence related enzymes, β-1,3-glucanase and peroxidase, suggesting a signaling role for H202 in RWA resistance through activation of defence genes. Accordingly, treatment of plants with the H202 generating mixture of glucose and glucose oxidase resulted in induction of the H202 production as well as induction of the β-1 ,3-glucanase and peroxidase activities in resistant plants. In vitro inhibition studies confirmed that DPI is a specific inhibitor of NADPH oxidase and did not have any direct inhibitory effect on the β-1 ,3-g1ucanase and peroxidase activities. This finding further substantiated that H202 generated during RWA infestation was involved in the signaling events leading to induction of the secondary defence responses in resistant plants.