The Transcription Factor BpBZR1-6 Positively Regulates Salt Stress Response in Birch via BR Signaling and Oxidative Homeostasis

Saline-alkali stress represents a major environmental constraint that severely inhibits plant growth and development. Brassinosteroids (BRs) are important phytohormones governing plant growth and abiotic stress adaptation, a process mediated by the BZR transcription factor family. However, their specific regulatory role in coordinating the response to salt stress remains unclear in Betula platyphylla (birch). This study demonstrates that overexpression (OE) lines of BpBZR1-6 exhibited enhanced salt tolerance, and BR application further alleviated damage under salt stress. In contrast, RNAi-silenced (SE) lines exhibited increased salt sensitivity, indicating that BpBZR1-6 positively regulates salt stress response via BR signaling. Mechanistically, BpBZR1-6 induced P5CSs expression, resulting in the proline content in OE lines was significantly higher than that in WT and SE lines. Furthermore, BpBZR1-6 promotes the expression of SOD and POD genes, thereby increasing SOD and POD activities, consequently, OE lines reduced H2O2 and malondialdehyde (MDA) levels. Meanwhile, exogenous BR application, the optimal BR concentration was 0.20 mg/L, enhanced antioxidant enzyme activities and decreased oxidative damage. Additionally, ChIP assays, dual-luciferase assays and RT-qPCR confirmed that BpBZR1-6 binds to the BpDWARF promoter truncation containing E-box and regulate its expression, exhibiting distinct expression patterns across different genetic backgrounds and in response to BR treatments. In summary, BpBZR1-6 is involved in regulating the salt stress response in birch, and application of an appropriate concentration of BR can effectively mitigate salt-induced damage. These findings provide a theoretical foundation for breeding salt-tolerant birch varieties and their practical application.