Natural variation in an HD-ZIP factor identifies its role in controlling apple leaf cuticular wax deposition

Natural variation is an invaluable genetic resource for plant trait improvement. Here, we performed a genome-wide association study (GWAS) analysis and identified MdHDG5, which controls apple leaf cuticular wax. An A-to-G single-nucleotide polymorphism (SNP) on the HDG5 promoter is associated with HDG5 expression and hexacosanol content (a component of leaf cuticular wax). Furthermore, the single-nucleotide variation (G/G) within a MYB cis-regulatory element (CRE) can be directly bound by MYB62, which represses HDG5 expression and leaf wax deposition. In addition, MdPIAL2, a Small Ubiquitin-like Modifier (SUMO) E3 ligase, positively controls apple leaf wax deposition by stabilizing MdHDG5, while MdMIEL1 interacts with and degrades both MdHDG5 and MdPIAL2 to negatively control leaf wax deposition. Notably, MIEL1 expression is negatively associated with leaf hexacosanol deposition. Taken together, our results provide significant genetic insights into the natural variation of leaf cuticular wax loads in apple and identify the intricate molecular regulation of MdHDG5.