The bHLH Transcription Factor PdbUNE12 Functions as a Positive Regulator of the Salt Stress Response in Populus davidiana × Populus bolleana.

Basic Helix-Loop-Helix (bHLH) proteins, as one of the most common types of transcription factors in eukaryotes, play a crucial role in the stress response of plants. This study characterized the response of the bHLH transcription factor PdbUNE12 from Populus davidiana × P. bolleana to salt stress, and it revealed that this protein is localized in the nucleus and does not possess transcriptional activation activity. PdbUNE12 overexpression and knockout plants were generated to study its role in salt tolerance. The phenotypic and physiological indicators showed that the overexpression of PdbUNE12 enhanced the salt tolerance in P. davidiana × P. bolleana, while the knockout plants exhibited the opposite effect. These results showed that PdbUNE12 improves the osmotic regulation and reactive oxygen species (ROS) scavenging capacity of the plants by increasing the activity of defense enzymes such as peroxidase (POD), superoxide dismutase (SOD), and pyrroline-5-carboxylate synthetase (P5CS), contributing to the improved resilience to salt stress. In addition, PdbUNE12 can regulate the salt tolerance gene PdbbHLH1 by binding to its promoter. In summary, PdbUNE12 is a key regulator of salt stress tolerance by enhancing the expression of relevant resistance genes, thereby improving the plant's ability to tolerate salt stress.