Brassinosteroids Enhance Low-Temperature Resistance by Promoting the Formation of Sugars in Maize Mesocotyls

The germination and elongation of maize in the early growth stage are closely related to the elongation of the mesocotyl, which is one of the first parts to sense external temperature, aside from the coleoptile. Low-temperature (LT, 10~15 °C) stress can significantly affect the survival and growth of maize seedlings. Additionally, brassinosteroids (BRs) have been used in recent years to help alleviate damage caused by LT in various plants. However, the interaction among LT, BRs, and sugar remains unclear. Therefore, we examined the relationships among the contents of glucose, sucrose, and starch, along with the changes in differentially expressed genes (DEGs) involved in starch and sucrose metabolism and glycolysis/gluconeogenesis pathways. Compared to CK (0 μM 24-epibrassinolide (EBR) application at 25 °C), the contents of glucose and sucrose increased by 0.26, 0.47, and 0.70 mg g^-1 FW and 0.80, 0.30, and 0.61 mg g^-1 FW, respectively, under the CKE (2.0 μM 24-epibrassinolide (EBR) application at 25 °C), LT (0 μM 24-epibrassinolide (EBR) application at 10 °C), and LTE (2.0 μM 24-epibrassinolide (EBR) application at 10 °C) treatments. However, starch contents decreased under LT and LTE treatments, by -20.54% and -0.20%, respectively, compared to CK. This suggests that sugar signaling and metabolism play key roles in regulating LT tolerance, and the application of EBR may alleviate LT damage by regulating sugar accumulation levels. Furthermore, 108 DEGs were identified in the starch and sucrose metabolism pathways, along with 23 in glycolysis, with 65 DEGs at the transcriptome level. The common Zm00001d042146 (hexokinase-3) in both pathways is usually down-regulated, and the degree of down-regulation when EBR is added is less than under LT alone. Additionally, key genes such as Zm00001d021598 (glucan endo-1,3-beta-glucosidase 3), Zm00001d034017 (uncharacterized LOC541703), and Zm00001d029091 (sucrose synthase 2) were differentially expressed under LT, with their expression levels decreasing further when EBR was added. In conclusion, our results provide a new direction into the molecular mechanisms by which exogenous EBR application enhances low-temperature tolerance in maize seedlings.