MdHY5 positively regulates cold tolerance in apple by integrating the auxin and abscisic acid pathways.

Low-temperature stress causes various types of physiological and biochemical damage to plants. The basic leucine zipper (bZIP) family transcription factor HY5 plays a significant role in multiple stress responses in plants. Here, cold stress was found to induce the upregulation of MdHY5 expression, which, in turn, positively regulates the cold tolerance of apple (Malus domestica). MdHY5 directly interacts the promoters of MdGH3-2/12 (auxin-amido synthetase) and inhibits their expression. However, low-temperature stress inhibits the regulation of MdGH3-2/12 by MdHY5, which suppresses the increase in indole-3-acetic acid (IAA) mediated by the MdHY5-MdGH3-2/12 module. Alternatively, MdHY5 directly interacts with the promoter of MdNCED2, a crucial enzyme in the biosynthesis of abscisic acid (ABA), thereby activating its expression. Additionally, cold stress enhances the regulation of MdNCED2 by MdHY5, which leads to the promotion of the increase in ABA mediated by the MdHY5-MdNCED2 module. Therefore, under low-temperature stress, MdHY5 reduces the ratio of IAA : ABA within apple plants by regulating MdGH3-2/12 and MdNCED2, thereby indirectly promoting the accumulation of anthocyanins, which further improves the cold tolerance of apple. This study establishes a theoretical framework for the multiple roles and regulatory mechanisms of HY5 in integrating the IAA and ABA pathways under cold stress.