The Role of Reactive Oxygen Species and Calcium Ions in Implementing the Stress-Protective Effect of γ-Aminobutyric Acid on Wheat Seedlings Under Heat Stress Conditions

γ-aminobutyric acid (GABA) is considered a molecule that combines the properties of a stress metabolite and a signaling molecule. At the same time, the importance of its functional interaction with other signaling mediators, in particular, reactive oxygen species (ROS) and calcium ions, for the implementation of stress-protective action on plant cells remains poorly researched. The authors studied the effect of GABA on the resistance of wheat seedlings (Triticum aestivum L., cultivar Doskonala) to potentially lethal heat stress and the participation of ROS and calcium in the manifestation of the effects of GABA. Treatment of seedlings with GABA in concentrations of 0.5 and 1 mM caused a significant increase in their survival after damaging heating in a water thermostat (10 min at 45°C). Under the influence of GABA, there was a transient increase in the content of hydrogen peroxide in the roots of seedlings, followed by an increase in the activity of antioxidant enzymes: superoxide dismutase, catalase, and guaiacol peroxidase. The specified effects of GABA were eliminated by the preliminary application of the hydrogen peroxide scavenger dimethylthiourea (DMTU) to the root incubation medium and were significantly suppressed in the presence of the NADPH oxidase inhibitor imidazole. At the same time, the treatment of seedlings with the chelator of extracellular calcium EGTA only partially eliminated the increase in the content of hydrogen peroxide and hardly affected the increase in the activity of antioxidant enzymes in the roots under the influence of GABA. Treatment with neomycin, an inhibitor of calcium uptake from intracellular compartments, caused a partial reduction in the effect of GABA on indicators of the state of the pro-/antioxidant system in wheat roots but did not eliminate these effects. Under the influence of GABA, damage to root cell membranes caused by heat stress was significantly reduced, which was manifested in a decrease in the release of UV-B-absorbing compounds from the cells and a decrease in the content of lipid peroxide oxidation products. At the same time, the stress-protective effect of GABA was eliminated by DMTU treatment and changed in the presence of calcium antagonists. A conclusion was made about the important role of ROS generated with the participation of NADPH oxidase in the implementation of the protective effect of GABA on wheat seedlings under conditions of heat stress and the partial dependence of its protective effects on calcium homeostasis.