The Analysis of the BpBZR1 Gene Family Indicates That BpBEH4-2 Positively Regulates the Cold Tolerance of Bougainvillea

Cold stress represents one of the major limiting factors restricting the large-scale cultivation and industrial application of Bougainvillea in northern cold regions. Brassinosteroids (BRs) are essential steroid phytohormones that regulate plant tolerance to diverse abiotic stresses; however, their regulatory roles and molecular mechanisms underlying cold response in Bougainvillea remain unclear. In this study, exogenous 24-epibrassinolide (EBR) application significantly enhanced the cold tolerance of Bougainvillea, while the BR biosynthesis inhibitor brassinazole (Brz) significantly reduced cold resistance. EBR treatment can reduce malondialdehyde (MDA) accumulation, increase soluble sugar content, and enhance antioxidant enzyme activity. Furthermore, we systematically identified seven members of the BpBZR1 transcription factor family. qRT-PCR results showed that BpBZR1s are responsive to EBR treatment. Notably, BpBEH4-2 can be significantly induced under cold stress. Heterologous overexpression of BpBEH4-2 in Arabidopsis thaliana improved plant chilling tolerance and activated the expression of cold-responsive genes. These results indicated that the BR signaling pathway positively regulates cold tolerance in Bougainvillea, and BpBEH4-2 serves as a critical regulatory gene in cold stress adaptation. This study provides new insights into BR-mediated cold resistance mechanisms and supplies promising gene resources for cold-tolerant Bougainvillea breeding.