Involvement of the ABA- and H₂O₂-Mediated Ascorbate–Glutathione Cycle in the Drought Stress Responses of Wheat Roots

Abscisic acid (ABA), hydrogen peroxide (H₂O₂) and ascorbate (AsA)–glutathione (GSH) cycle are widely known for their participation in various stresses. However, the relationship between ABA and H₂O₂ levels and the AsA–GSH cycle under drought stress in wheat has not been studied. In this study, a hydroponic experiment was conducted in wheat seedlings subjected to 15% polyethylene glycol (PEG) 6000–induced dehydration. Drought stress caused the rapid accumulation of endogenous ABA and H₂O₂ and significantly decreased the number of root tips compared with the control. The application of ABA significantly increased the number of root tips, whereas the application of H₂O₂ markedly reduced the number of root tips, compared with that under 15% PEG-6000. In addition, drought stress markedly increased the DHA, GSH and GSSG levels, but decreased the AsA levels, AsA/DHA and GSH/GSSG ratios compared with those in the control. The activities of the four enzymes in the AsA–GSH cycle were also markedly increased under drought stress, including glutathione reductase (GR), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR), compared with those in the control. However, the application of an ABA inhibitor significantly inhibited GR, DHAR and APX activities, whereas the application of an H₂O₂ inhibitor significantly inhibited DHAR and MDHAR activities. Furthermore, the application of ABA inhibitor significantly promoted the increases of H₂O₂ and the application of H₂O₂ inhibitor significantly blocked the increases of ABA, compared with those under 15% PEG-6000. Taken together, the results indicated that ABA and H₂O₂ probably interact under drought stress in wheat; and both of them can mediate drought stress by modulating the enzymes in AsA–GSH cycle, where ABA acts as the main regulator of GR, DHAR, and APX activities, and H₂O₂ acts as the main regulator of DHAR and MDHAR activities.