MhNF-YC6 and MhCOL4 synergistically promote abscisic acid biosynthesis to combat drought stress in apple

Drought is a major environmental stress that severely affects plant growth and poses a serious threat to crop productivity. Abscisic acid is a key plant hormone that plays a critical role in regulating plant responses to drought stress. However, the regulatory pathways controlling abscisic acid for drought tolerance in apple (Malus domestica Borkh) remain largely unclear. This study identified an NF-Y transcription factor, MhNF-YC6, from the drought-tolerant wild apple species (Malus hupehensis). Functional analysis revealed that MhNF-YC6 is a positive regulator of apple drought tolerance. Further research demonstrated that MhNF-YC6 directly binds to the promoters of the ABA biosynthesis genes MhNCED3 and MhAAO3 and activates their transcription. Silencing of MhNCED3 and MhAAO3 resulted in significantly reduced abscisic acid contents and drought tolerance. Furthermore, CONSTANS-like 4 (COL4) is identified as an MhNF-YC6-interacting protein that acts as a positive regulator of drought tolerance in apple. Notably, we discovered that the interaction between MhNF-YC6 and MhCOL4 substantially enhances the regulatory effect of MhNF-YC6 on MhNCED3 and MhAAO3, thereby promoting abscisic acid accumulation, inducing stomatal closure, and ultimately improving drought tolerance. Our findings offer valuable insights into the transcriptional regulation of abscisic acid biosynthesis and provide promising targets for the molecular breeding of drought-tolerant crops.