LAUSR

Abstract Respiratory burst oxidase homologue-dependent H2O2 (NADPH-H2O2) acts as a signal under many biotic and abiotic stress conditions, but how NADPH-H2O2 regulates thermotolerance at different high temperatures in tomatoes is not clear. Similar to the control, the application of the NADPH oxidase inhibitor, diphenylene iodonium (DPI), did not affect either the angle between the stem and leaves (ASL) or the expression of heat-responsive genes; however, the ASL decreased, and the total-H2O2 content and expression of heat-response genes increased under the 38°C treatment. When the inhibition of NADPH-H2O2 occurred before the 38°C treatment, the ASL increased, and the total-H2O2 content and expression of heat-response genes decreased at 38°C, resulting in the decline of the thermotolerance. These results suggest that NADPH-H2O2 acted as a signal regulating the ASL and the expression of heat-response genes during exposure to 38°C. Compared with the 38°C treatment, the ASL increased, the total-H2O2 further increased and most of the expression of heat-response genes declined at 45°C. Whether the inhibition of NADPH-H2O2 occurred or not, the ASL and most of the expression of heat-response genes showed no difference at 45°C; however, when NADPH-H2O2 was inhibited before the 45°C treatment, the total-H2O2 decreased significantly at 45°C, thereby alleviating oxidative stress and enhancing thermotolerance. Interestingly, whether the inhibition of NADPH-H2O2 occurred or not, the expression of HsfA1a and HsfB1 remained at a high level at 45°C, suggesting that HsfA1a and HsfB1 were not controlled by NADPH-H2O2 under extremely high temperatures. It was concluded that tomato seedlings increased their thermotolerance at moderately high temperatures by increasing NADPH-H2O2, which decreased the ASL and the enhanced expression of heat-response genes; however, under extreme temperatures, the inhibition of NADPH-H2O2 increased thermotolerance, which remitted the excessive oxidative stress. These findings will help cultivators improve the thermotolerance of tomato plants in the future by adopting different strategies according to the high temperature levels in the environment.