Mechanisms of Exogenous Brassinosteroids and Abscisic Acid in Regulating Maize Cold Stress Tolerance

Exogenous abscisic acid (ABA) and brassinosteroid (BR) play important roles in alleviating cold stress in maize. In this study, two maize inbred lines with differing cold tolerance were treated with exogenous ABA, BR, and their combined solution under cold stress conditions at 10 °C to investigate the effects of these treatments on the physiological characteristics of maize seedlings. The results indicated that cold stress significantly inhibited the growth of maize seedlings. Exogenous hormone treatments enhanced antioxidant enzyme activities and promoted the synthesis of osmolytes, thereby alleviating cold stress; however, the combined treatment (AR) did not significantly improve maize cold tolerance. Transcriptomic analysis revealed that pathways including plant hormone signal transduction, fatty acid elongation, and phenylpropanoid biosynthesis were involved in the interaction between ABA and BR. Weighted gene co-expression network analysis (WGCNA) identified four key candidate genes responsive to exogenous ABA and BR under cold stress, namely Zm00001eb343270, Zm00001eb401890, Zm00001eb206790, and Zm00001eb199820. Based on the gene annotation results, we speculate that ubiquitin-conjugating enzyme E2 O, tubulin–tyrosine ligase-like protein 12, the negative regulator of systemic acquired resistance SNI1, and mRNA stability regulators in response to DNA damage may be involved in regulating maize cold tolerance. These findings provide further evidence for the regulatory mechanisms by which exogenous ABA and BR affect maize cold tolerance and elucidate their interaction under cold stress.