Effects of Exogenous Spermidine on Germination and Seedling Growth of Rice Under Salt Stress: Physiological and Transcriptomic Insights

Salt stress severely impairs rice (Oryza sativa L.) germination and seedling establishment. Exogenous spermidine (Spd) has been shown to regulate stress tolerance in plants, but whether it acts during rice germination and seedling establishment under salt stress remains unclear. Here, rice seeds (cv. Jindao 919) were exposed to 75 mM NaCl with different Spd concentrations (0–1.4 mM), and physiological, biochemical, and transcriptomic responses were evaluated. The findings showed that salt stress had a pronounced inhibitory effect on both seed germination and seedling development. Exogenous Spd effectively alleviated these negative effects, with the most significant improvements observed at 1.0–1.2 mM: germination rate increased by 3.98–8.52%, seedling root length increased by 17.74–37.68%, soluble sugar content increased by 29.83–230%, and SOD and POD activities increased by 29.81–40.3% and 18.45–44.0%, respectively, while MDA content decreased by 36.28–40.3%. Further transcriptomic analysis identified a total of 1835 differentially expressed genes (DEGs). KEGG enrichment analysis revealed these genes were concentrated in key pathways including terpenoid biosynthesis, phenylpropanoid biosynthesis, and amino sugar and nucleotide sugar metabolism, thus alleviating the negative impact of salt stress on rice germination and seedling development. These pathways are closely related to gibberellin metabolism, lignin biosynthesis, and amino sugar metabolism, further revealing the regulatory role of Spd. Overall, 1.0–1.2 mM Spd was most effective in alleviating salt stress by synergistically regulating antioxidant defense, osmoregulation, and metabolic reprogramming, enhancing rice’s overall stress tolerance. This study provides theoretical guidance for precise regulation of Spd concentration to improve rice performance in saline-alkaline soils, and reveals the sustained promoting effects of Spd across various developmental stages of rice and its underlying molecular mechanisms.