Identification and characterization of histone modification gene family reveal their critical responses to flower induction in apple

Background: Histone methylation and acetylation regulate biological processes in plants through various histone modifications (HMs) gene families. However, knowledge of HMs genes is limited in horticultural deciduous trees, including apple (Malus domestica). Results: Here, a comprehensive study of identifying and investigating HMs genes was performed using the recently published apple genome. In total, 198 MdHMs were identified, including 71 histone methyltransferases, 44 histone demethylases, 57 histone acetylases, and 26 histone deacetylases. Detailed analysis of the MdHMs, including chromosomes locations, gene structures, protein motif and protein-protein interactions were performed, and their orthologous genes were also predicted against nine plant species. Meanwhile, a syntenic analysis revealed that tandem, segmental, and whole genome duplications were involved in the evolution and expansion of the MdHMs gene family. Most MdHMs underwent purifying selection. The expression profiles of 198 MdHMs were investigated in response to 6-BA treatment and different flowering varieties (easy-flowering 'Yanfu No. 6' and difficult-flowering 'Nagafu No.2') using transcriptome sequencing data, and most MdHMs were involved in flower induction processes. Subsequent quantitative real-time PCR was then performed to confirm the expression levels of candidate MdHMs under different flowering-related circumstances. Conclusion: MdHMs were involved in, and responsive to, flower induction in apple. This study established an MdHMs platform that provided valuable information and presented enriched biological theories on flower induction in apple. The data could also be used to study the evolutionary history and functional prospects of MdHMs genes, as well as other trees.