Exogenous 24-Epibrassinolide Improves Low-Temperature Tolerance of Maize Seedlings by Influencing Sugar Signaling and...

Low-temperature (LT) stress seriously affects the distribution, seedling survival, and grain yield of maize. At the seedling emergence stage, maize's coleoptile is one of the most sensitive organs in sensing LT signaling and, in general, it can envelop young leaves to protect them from LT damage. In addition, brassinolides (BRs) have been shown to enhance LT tolerance from various species, but the effects of BRs on coleoptiles in maize seedlings under LT stress are unclear. Therefore, in this study, the pre-cultured coleoptiles of Zheng58 seedlings were treated with or without 2.0 μM 24-epibrassinolide (EBR) at 25 °C and 10 °C environments for five days to analyze their physiological and transcriptomic changes. Physiological analysis showed that a 10°C LT stress increased the content of glucose (0.43 mg g^-1 FW), sucrose (0.45 mg g^-1 FW), and starch (0.76 mg g^-1 FW) of Zheng58 coleoptiles compared to a 25°C environment. After the coleoptiles were exposed to a 2.0 μM EBR application under 10°C temperature for five days, the contents of these three sugars continued to increase, and reached 2.68 mg g^-1 FW, 4.64 mg g^-1 FW, and 9.27 mg g^-1 FW, respectively, indicating that sugar signaling and metabolism played key roles in regulating LT tolerance in the coleoptiles of maize seedlings. Meanwhile, a transcriptome analysis showed that 84 and 15 differentially expressed genes (DEGs) were enriched in the sucrose and starch metabolism and photosynthesis pathways, respectively, and multiple DEGs involved in these pathways were significantly up-regulated under LT stress and EBR stimulation. Further analysis speculated that the four DEGs responsible for sucrose-phosphate synthetase (SPS, i.e., Zm00001d048979, probable sucrose-phosphate synthase 5 and Zm00001d012036, sucrose-phosphate synthase 1), sucrose synthase (SUS, Zm00001d029091, sucrose synthase 2 and Zm00001d029087, sucrose synthase 4) were crucial nodes that could potentially link photosynthesis and other unknown pathways to form the complex interaction networks of maize LT tolerance. In conclusion, our findings provide new insights into the molecular mechanisms of exogenous EBR in enhancing LT tolerance of maize seedlings and identified potential candidate genes to be used for LT tolerance breeding in maize.