MdMYB70 transcription factor modulates malate accumulation under drought stress in apple.

Malic acid, a key component of the organoleptic quality, determines the flavor of apple (Malus domestica Borkh.) fruit. R2R3-MYB transcription factors (TFs) have important functions in stress response, but their roles in drought-mediated malate metabolism remain poorly understood. In this study, a drought-responsive gene, MdMYB70, which was relevant to the content of malate, was identified and characterized. Overexpressing MdMYB70 promoted malic acid accumulation and improved drought tolerance, while silencing MdMYB70 showed the opposite results. Moreover, MdMYB70 directly combined with the promoter sequence of MdALMT9 (aluminum-activated malate transporter 9) and transcriptionally activates MdALMT9 expression. In addition, MdMYB44, a protein negatively regulating malate accumulation, interacted with MdMYB70 and repressed the effects of MdMYB70 on the MdALMT9. Taken together, this research reveals a MdMYB70-MdMYB44-MdALMT9 molecular module that mediates malate content under drought stress in apple.