The bHLH Transcription Factor IbbHLH129 Positively Regulates the Cold Tolerance of Sweetpotato Seedlings by Modulating Auxin and Gibberellin Pathways

Low-temperature stress severely inhibits the growth and development of sweetpotato, restricts its planting geographical range, and frequently causes seedling death and yield reduction, posing a major threat to sweetpotato sustainable production. Nevertheless, the molecular regulatory mechanisms by which sweetpotato perceives and adapts to low-temperature stress have not been fully elucidated and deserve further in-depth investigation. In this study, a basic helix–loop–helix (bHLH) transcription factor IbbHLH129 was cloned from a cold-tolerant sweetpotato variety Xs33. The IbbHLH129 protein is localized in the nucleus. IbbHLH129 is most highly expressed in sweetpotato leaves and upregulated by low temperature, GA3, MeJA, and IAA, while repressed by ABA. Overexpressing IbbHLH129 enhanced cold tolerance in sweetpotato seedlings under short-term cold stress through enhancing antioxidant capability and regulating hormone-related pathways. Dual-luciferase and electrophoretic mobility shift assays demonstrated that IbbHLH129 binds to the IbYUCCA2 and IbGID1 promoters to activate their expression. These findings suggest that IbbHLH129 enhances cold tolerance in sweetpotato seedlings under short-term cold stress by activating IAA and GA signaling pathways. Our study provides a novel gene resource with promising applications for reducing low-temperature injury to sweetpotato seedlings cultivated in cold growing areas.