MdbHLH106-like transcription factor enhances apple salt tolerance by upregulating MdNHX1 expression

Soil salinization is one of the most important abiotic stresses adversely affecting plant growth and yield. In this study, we cloned and characterized a putative salt tolerance gene encoding a bHLH (basic helix-loop-helix) transcription factor, MdbHLH106L (MdbHLH106-like), in the genome of apple (Malus domestica Borkh.). The expression level and promoter activity of MdbHLH106L are increased under salt stress. Overexpressing MdbHLH106L in ‘Orin’ calli can promote the expression level of MdNHX1 (Na+/H+ exchanger 1). Further analysis showed that MdbHLH106L is able to bind the MdNHX1 promoter. Moreover, we also confirmed the interaction between MdbHLH106L and MdZAT10. In conclusion, MdbHLH106L participates in the salt-stress signaling pathway and may mediate salt tolerance by activating the MdNHX1 promoter and interacting with MdZAT10. Our findings may serve as a theoretical basis for further investigating molecular mechanisms of stress responses and for enriching genetic resources of stress-tolerant apple cultivars.