Involvement of Nitrate Reductase and Nitric Oxide (NO) in Implementation of the Stress-Protective Action of a Carbon Monoxide (CO) Donor on Wheat Seedlings under Hyperthermy

The involvement of nitric oxide (NO) and its cross-talk with different signal mediators was studied upon the induction of heat-resistance of wheat (Triticum aestivum L.) seedlings afforded by a carbon monoxide (CO) donor hemin. It was found that the plant treatment with 5 µM hemin, enhancing the resistance, led to a transitory, almost twofold augmentation in the NO content and the nitrate reductase (NR) activity in the roots. These indexes attained maxima after 1 h of the seedling incubation in the hemin-containing medium. The increase in nitric oxide was eliminated by an inhibitor of nitrate reductase (sodium tungstate) but not by an inhibitor of NO synthase and diaminoxidase (aminoguanidine). The boost in NR and NO in the roots was also diminished by their treatment with a chelator of extracellular calcium (EGTA) or an inhibitor of calcium translocation from intracellular compartments into cytosol (neomycin). Meanwhile, the CO donor-induced rise in NR activity and NO content remained unaffected upon the treatment with an antioxidant dimethylthiourea (DMTU). The scavenger of nitric oxide (PTIO; 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) and an NR inhibitor sodium tungstate added to the seedling incubation medium prevented the activation of extracellular peroxidase and accumulation of hydrogen peroxide in the roots caused by hemin. If the seedlings were injured by heating at 45°C, their pretreatment with the CO donor attenuated the oxidative stress (estimated by accumulation of a POL product MDA) and potentiated the seedling survival. These effects of hemin were eliminated by the seedling treatment with the scavenger of NO (PTIO) and that of H₂O₂ (DMTU), inhibitor of NR (sodium tungstate), and antagonists of calcium (EGTA or neomycin). It is concluded that NO, produced with the help of NR, participates in the signal transduction from the exogenous CO. Rise in the NO level stimulates the formation of ROS, which also mediates the accomplishment of the stress-protective effects of carbon monoxide. The CO-induced increase in the NO generation by the root cells and the heat acclimation also depend on the calcium homeostasis.