Exogenous NAA and IBA Alleviate the Adverse Effects of Gamma Radiation Stress and Increase Rice Seedling Survival Rates by Modulating Redox Balance and Metabolomic Regulation

To investigate the effects of exogenous growth regulators on the growth of rice seedlings from irradiated seeds and to provide a reference for mutation breeding, Zhejing 100 rice seeds were irradiated with gamma rays at the median lethal dose and then soaked in solutions containing naphthalene acetic acid (NAA), indolebutyric acid (IBA), or NAA and IBA. Soaking seeds in a solution containing NAA alone or combined with IBA significantly decreased superoxide anion (O2•−), hydrogen peroxide (H2O2), and malondialdehyde contents in rice seedlings. Treatments with exogenous growth regulators at appropriate concentrations significantly enhanced antioxidant enzyme activities (e.g., superoxide dismutase, peroxidase, and ascorbate peroxidase) as well as proline, ascorbic acid, and glutathione contents, thereby increasing seedling survival rates after radiation-induced mutagenesis. Non-targeted metabolomic analyses revealed that in seedlings from irradiated seeds treated with NAA, IBA, or NAA and IBA, the two most abundant metabolite categories were lipids/lipid-like molecules and phenylpropanoids/polyketides, both of which are closely associated with antioxidant metabolism in plants. NAA and IBA also participated in the regulation of flavone and flavonol biosynthesis, flavonoid biosynthesis, purine metabolism, and zeatin biosynthesis, with positive effects on the ability of seedlings to repair damage due to irradiation. Soaking irradiated seeds in an NAA solution (0.2–0.4 mg/L) or a solution containing both NAA (0.1–0.4 mg/L) and IBA (0.25–1.0 mg/L) yielded the best results.