Genome-Wide Characterization of the Apple HD-Zip IV Gene Family and Functional Validation of MdHDZIV3 Under...

The homeodomain-leucine zipper IV (HD-Zip IV) transcription factor subfamily plays essential roles in epidermal development, cuticle formation, lipid metabolism, and environmental adaptation in plants. Despite its biological importance, the HD-Zip IV family has not been systematically characterized in apple (Malus domestica). Here, we identified 17 apple HD-Zip IV genes and named them MdHDZIV1-MdHDZIV17 based on their locations on the chromosomes. The 17 genes showed a nonuniform distribution on eight chromosomes, while the occurrence of both tandem and segmental duplications indicated that family expansion involved more than one duplication mechanism. All MdHDZIV proteins contained the conserved HD, LZ, START, and SAD domains but lacked the MEKHLA domain, consistent with typical HD-Zip IV structural features. Phylogenetic analysis classified MdHDZIV proteins into five groups together with HD-Zip IV members from Arabidopsis thaliana and rice, indicating evolutionary conservation of this subfamily. Collinearity and Ka/Ks analyses revealed that duplicated MdHDZIV gene pairs were mainly subjected to purifying selection. Promoter scanning revealed diverse cis-regulatory motifs associated with hormonal signaling, environmental stress, light response, and epidermal regulation, including ABRE, ARE, W-box, MYC, G-box, and L1-box motifs. Integration of transcriptomic profiling with qRT-PCR validation revealed pronounced tissue-dependent differences in the expression of MdHDZIV genes in leaf, fruit skin, and branch bark. Under PEG₆₀₀₀-induced osmotic stress and NaCl-induced salt stress, 10 candidate MdHDZIV genes displayed gene-specific and stress type-specific expression patterns, with MdHDZIV3 showing strong induction under PEG₆₀₀₀ treatment. Functional validation in apple calli showed that MdHDZIV3 overexpression enhanced PEG tolerance, increased fresh weight, elevated SOD and POD activities, and reduced MDA accumulation under osmotic stress. These findings provide a genome-wide framework for understanding the apple HD-Zip IV gene family.