Genome-wide identification of WOX gene family in apple and a functional analysis of MdWOX4b during adventitious root formation

The plant-specific WUSCHEL-related homeobox (WOX) genes are crucial for plant growth and development. Here, we systematically identified the MdWOX gene family in apple at the genome-wide level, and analyzed the phylogenetic relationships, conserved motifs, gene structure, and syntenic relationships of the MdWOX genes. A total of 18 MdWOX genes were identified and phylogenetic analysis placed them into three clades. The phylogenetic relationships among the WOXs were further supported by the analyses of gene structure and conserved motifs. Chromosomal distribution and synteny analysis revealed that whole-genome and segmental duplications have played key roles in MdWOX gene family expansion. Moreover, the MdWOX genes exhibit tissue-specific expression patterns and MdWOX4a, MdWOX4b, MdWOX5b, MdWOX11/12a, and MdWOX11/12b may play essential roles in adventitious root development. The adventitious rooting ability was enhanced in MdWOX4b transgenic tobacco lines. The results of this study provide useful information for future functional studies on MdWOXs in the development of apple rootstocks.