Tissue necroses and resistance during the hypersensitive response (HR) of tobacco to tobacco mosaic virus (TMV) are overcome at temperatures above 28 °C and the virus multiplies to high levels in the originally resistant N-gene expressing plants. We have demonstrated that chemical compounds that generate reactive oxygen species (ROS) or directly applied hydrogen peroxide (H2O2) are able to induce HR-type necroses in TMV-inoculatad Xanthi-nc tobacco even at high temperatures (e.g. 30 °C). The amount of superoxide (O2°) decreased, while H2O2 slightly increased in TMV- and mock-inoculated leaves at 30 °C, as compared with 20 °C. Activity of NADPH oxidase and mRNA levels of genes that encode NADPH oxidase and an alternative oxidase, respectively, were significantly lower, while activity of dehydroascorbate reductase was significantly higher at 30 °C, as compared with 20 °C. It was possible to reverse or suppress the chemically induced HR-type necrotization at 30 °C by the application of antioxidants, such as superoxide dismutase and catalase, demonstrating that the development of HR-type necroses indeed depends on a certain level of superoxide and other ROS. Importantly, high TMV levels at 30 °C were similar in infected plants, whether the HR-type necrotization developed or not. Suppression of virus multiplication in resistant, HR-producing tobacco at lower temperatures seems to be independent of the appearance of necroses but is associated with temperatures below 28 °C.
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