Regulation of Reactive Oxygen Species Metabolism and Glyoxalase Systems by Exogenous Osmolytes Confers Thermotolerance in Brassica napus

In the current study, the beneficial role of proline (Pro) and glycinebetaine (GB) in alleviating high temperature (HT) stress was investigated in Brassica napus L. (rapeseed) seedlings. Ten-d-old rapeseed seedlings were treated with and/or without 2 mM Pro and 2 mM GB and exposed to 38/28 °C day/night (16 and 8 h) temperature for 24 and 48 h. Heat stress induced high amount of hydrogen peroxide (H₂O₂), lipoxygenase (LOX) and malondialdehyde (MDA) level. Exogenous Pro and GB addition in HT-affected plants increased the levels of ascorbate (AsA) and reduced glutathione (GSH) and their redox pool. Exogenous Pro and GB increased functions of AsA-GSH pathways enzymes along with catalase (CAT), glutathione S-transferase (GST) and glutathione peroxidase (GPX) in HT exposed plants. The enhanced antioxidant defense system by supplementation of Pro and GB helped to reduce oxidative stress and photosynthetic pigments damage caused by HT-induced stress. High methylglyoxal (MG) content decreased upon exogenous Pro and GB application along with enhanced activities of glyoxalase I (Gly I) and Gly II enzymes in HT-stressed plants. Applied Pro and GB under HT stress enhanced endogenous Pro level further to prevent excess water loss and also improved relative water content. Thus, Pro- and/or GB-induced regulatory interactions between ROS and MG detoxification systems may be a useful approach for the reversal of HT-induced oxidative stress.