Identification of HD-Zip gene family members in Xanthoceras sorbifolium Bunge and their response to salt stress

【Objective】 The members of homedomain-leucine zipper（HD-Zip） gene family in Xanthoceras sorbifolium Bunge were identified, and their expression under salt stress conditions was analyzed to provide theoretical basis for mining salt tolerant genes in X. sorbifolium and for breeding new salt tolerant varieties. 【Method】Based on the wholegenome data of X. sorbifolium, the members of the HD-Zip gene family were identified. The physicochemical properties, conserved motifs, gene structures, cis-acting elements, and phylogenetic evolutionary relationships of the HD-Zip gene family were predicted and analyzed using bioinformatics tools. Subcellular localization was performed by double enzyme digestion. Two-month-old X. sorbifolium seedlings were subjected to salt stress treatments with NaCl concentrations of 50, 100, and 150 mmol/L, with a control group（CK） treated with 0 mmol/L NaCl. The tissue expression patterns of HDZip family genes under salt stress in roots and leaves of X. sorbifolium were analyzed using real-time fluorescence quantitative PCR. 【Result】 A total of 29 HD-Zip gene family members were identified from the full genome of X. sorbifolium, named XsHB1-XsHB29. Except for XsHB29, XsHB1-XsHB28 genes were all located on the 12 chromosomes of X. sorbifolium. The number of amino acids encoded by HD-Zip family members ranged from 202 to 838, with molecular weights ranging from 23567.07 to 92421.90 kD, and theoretical isoelectric points（pI） ranging from 4.75 to 8.98. Subcellular localization results showed that proteins of the family were located in the nucleus. Phylogenetic analysis divided the 29 HD-Zip gene family members into 4 subfamilies（I, II, III, IV）, and the genes and proteins of each subfamily member had similarities in structure. Through the prediction of cis-acting elements in the promoter, it was found that the promoters of HD-Zip genes family had 22 types of cis-acting elements, such as abiotic stress and plant hormone response elements. Real-time fluorescence quantitative PCR results showed that the relative expression levels of XsHB1, XsHB12, XsHB13 and XsHB29 genes in leaves treated with 50 mmol/L NaCl were significantly up-regulated（P＜0.05,the same below）; while the relative expression levels of XsHB5, XsHB12, XsHB13, XsHB24 and XsHB29 genes were significantly down-regulated after treatment with 150 mmol/L NaCl. Subcellular localization and transient transformation results showed that XsHB5 was a nuclear-localized protein and could negatively regulate the salt tolerance of X. sorbifolium. 【Conclusion】 A total of 29 members of XsHD-Zip gene family can be screened from X. sorbifolium, they are divided into 4 subfamilies, and the members of each subfamily have similar gene and protein structure. Among them, genes such as XsHB1, XsHB5, XsHB12, XsHB13, XsHB24 and XsHB29 may play important roles in the salt stress response of X. sorbifolium. The HD-Zip gene family is involved in regulating the growth, development, metabolism, and other emergency response processes of X. sorbifolia.