PdbbHLH1 transcription factor improved drought tolerance of Populus davidiana × P. bolleana

The basic helix-loop-helix (bHLH) family of transcription factors (TFs) plays a critical role in regulating plant resistance to various abiotic stresses, including drought, low temperature, high salinity, and iron deficiency, as well as in overseeing key processes in plant growth and development. Despite the well-documented functions of bHLH TFs in many plant species, their roles in Populus davidiana × P. bolleana under abiotic stress conditions remain largely unexplored. This study identified a bHLH gene, designated as PdbbHLH1, whose expression is markedly upregulated in response to PEG₆₀₀₀ treatment. To investigate its function, the research generated transgenic Populus plants overexpressing (OE) PdbbHLH1 through Agrobacterium tumefaciens-mediated transformation. Stress experiments demonstrated that OE PdbbHLH1 significantly enhanced the drought resistance of these transgenic plants, as evidenced by increased fresh weight and chlorophyll content compared to wild-type (WT) plants under drought conditions. Physiological analyses further revealed that PdbbHLH1-OE plants exhibited lower levels of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA), indicating reduced cellular damage and a more robust reactive oxygen species (ROS) scavenging capability than WT plants. Furthermore, the expression of drought resistance-related genes, such as PdbAAO, PdbGST, and PdbPOD, was significantly upregulated in OE plants post-drought stress. Notably, PdbbHLH1 was shown to directly bind to the promoters of PdbPOD1 and PdbPOD4, activating their expression and thereby enhancing the drought resistance of transgenic Populus davidiana × P. bolleana. Collectively, these results suggest that PdbbHLH1 is a key regulator of drought tolerance in Populus, modulating this response through the enhancement of antioxidant enzyme activities at the physiological level.