The Physio-Biochemical Changes Induced by Gibberellic Acid Seed Priming in Rice Seeds against Submergence Stress: Consequences for Starch Degradation, Energy Metabolism and Hormonal Metabolism.

Submergence stress is a major abiotic factor that reduces the seedling establishment of rice. Gibberellic acid (GA) acts as a signal regulating anaerobic tolerance in rice, while seed priming is a pre-sowing treatment widely used to improve stress tolerance. However, whether and how GA seed priming affects rice seedling establishment under submergence stress remains unknown. In the present study, seeds of Oryza sativa L. cv. Xiangzaoxian45 were subjected to three treatments: no priming (NP), hydro-priming with distilled water (HP), and GA₃ priming (0.1 mM GA₃; GAP). Subsequent germination test was conducted under submergence (5 cm of water) in growth chambers (25 °C, 12-h photoperiod). Seedling establishment attributes, α-amylase activity, soluble sugar content, respiration rate, ATP content, respiratory enzyme activities, hormone levels, and gene expression in starch degradation, energy metabolism, and hormone biosynthesis pathways were measured. Results showed that GAP significantly promoted seed germination and seedling growth under submergence compared to NP and HP treatments. This improvement was attributed to higher GA₃ levels in rice seeds, resulting from both exogenous application during priming and the upregulation of GA biosynthesis genes (OsGA3ox1 and OsGA20ox1). Elevated GA₃ subsequently induced the expression of GA-responsive α-amylase genes (OsRamy1A, OsRamy3B, and OsRamy3E), thereby enhancing starch degradation, as evidenced by significantly increased α-amylase activity and total soluble sugar content. Furthermore, GAP enhanced the energy status of rice seedlings under submergence by increasing the oxygen consumption rate and ATP content and improved anaerobic respiration by elevating lactate dehydrogenase (LDH) activity and up-regulating OsLDH expression. However, GAP and HP did not differ significantly in regulating aerobic and anaerobic respiration under submergence. Moreover, principal component analysis, correlation analysis, and hierarchical partitioning analysis suggested a higher contribution of starch degradation than energy metabolism to seedling establishment under submergence stress. In summary, these findings indicate that GA priming enhances rice seedling establishment under submergence, and this improvement is predominantly attributed to GA-activated starch degradation.