Acetylcholine Alleviates Salt Stress in Zea mays L. by Promoting Seed Germination and Regulating Phytohormone Level and Antioxidant Capacity

Acetylcholine (ACh), a potential plant growth regulator, can alleviate injury caused by NaCl and cadmium stress. However, the underlying mechanisms whereby it affects seed germination remain unclear. We pre-soaked maize (Zea mays L.) in different concentrations (1–3 μm) of ACh. Seed germination and levels of β-galactosidase, α-amylase, and starch were analyzed. ACh, abscisic acid (ABA), and gibberellic acid levels were estimated via enzyme-linked immunosorbent assay. The expression of ABA and gibberellic acid signaling-related genes was analyzed. Salt stress significantly reduced seed germination and hypocotyl elongation but increased ABA and H2O2 levels and enhanced antioxidant enzyme activity. Priming with 2 μm ACh caused a significantly higher germination rate (by 13.4%) relative to that of the seeds subjected to salt stress alone. Under salt stress, ACh effectively upregulated α-amylase and β-galactosidase activity (by 0.63- and 1.04-fold) and reduced starch content (by 34.02%). ACh treatment downregulated the expression of ZmNCED3 (involved in ABA signaling) and upregulated that of ZmKAO (involved in gibberellin signaling). The leaves of the seedlings pretreated with ACh presented higher antioxidant enzyme activity and, therefore, lower H2O2 accumulation than those of seedlings subjected to salt stress alone. Priming the maize seeds with exogenous ACh maintained seedling growth under salt stress. ACh pretreatment increased antioxidant activity, reduced ABA activity, and improved salt tolerance in maize seeds exposed to salt stress. These findings suggest that pretreatment with 2 μm ACh improves seed germination, activates ROS scavenging, and regulates phytohormone levels under salt stress, thereby improving growth.