Zinc Finger–homeodomain Gene Family in Apple and Their Expression Analysis in Apple Rootstock Malus hupehensis Under Abiotic Stress

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Zinc finger–homeodomain (ZF-HD) proteins, a family of plant-specific transcription factors, play an important role in regulating plant growth and development, as well as responses to stress. Although ZF-HDs have been investigated in several model plants, no systematic studies have been reported in apple (Malus ×domestica). In this study, 14 putative ZF-HD genes were identified in the apple genome and characterized using bioinformatics tools. All members harbored complete canonical structures of the ZF-HD motif. Phylogenetic analysis demonstrated that ZF-HD genes in the genome of apple could be classified into four subfamilies, with high intragroup similarities. Gene-structure analysis revealed that although 11 MdZHDs had only one exon, MdZHD6 and MdZHD13 had two exons and MdZHD8 had six exons, suggesting limited variation among the apple ZHD genes. The expression profiles of MdZHD genes revealed their involvement in the growth and development of different tissues. Numerous binding sites for transcription factors, such as MYB, bZIP, and AP2, were found in the promoter region of the putative MdZHD genes. Nearly all putative MdZHDs were predicted to localize in the nucleus. Finally, the expression levels of the MdZHD genes under abiotic stress were examined in apple rootstock Malus hupehensis and the results showed that the expression of 10 MdZHD genes was induced in response to three abiotic stress factors. Exceptionally, the expression of MdZHD11 was not induced in response to any of the abiotic stress treatments, MdZHD12 was only induced in response to salt stress, and MdZHD7 and MdZHD9 were induced in response to both drought and salt stress. The present results provide valuable insights into the putative physiological and biochemical functions of MdZHDs in apple.