Identification and expression analysis of the IPT and CKX gene families during axillary bud outgrowth in apple (Malus domestica Borkh.)

Cytokinins (CKs) play a crucial role in promoting axillary bud outgrowth and targeting the control of CK metabolism can be used to enhance branching in plants. CK levels are maintained mainly by CK biosynthesis (isopentenyl transferase, IPT) and degradation (dehydrogenase, CKX) genes in plants. A systematic study of the IPT and CKX gene families in apple, however, has not been conducted. In the present study, 12 MdIPTs and 12 MdCKXs were identified in the apple genome. Systematic phylogenetic, structural, and synteny analyses were performed. Expression analysis of these genes in different tissues was also assessed. MdIPT and MdCKX genes exhibit distinct expression patterns in different tissues. The response of MdIPT, MdCKX, and MdPIN1 genes to various treatments (6-BA, decapitation and Lovastatin, an inhibitor of CKs synthesis) that impact branching were also investigated. Results indicated that most of the MdIPT and MdCKX, and MdPIN1 genes were upregulated by 6-BA and decapitation treatment, but inhibited by Lovastatin, a compound that effectively suppresses axillary bud outgrowth induced by decapitation. These findings suggest that cytokinin biosynthesis is required for the activation of bud break and the export of auxin from buds in apple tree with intact primary shoot apex or decapitated apple tree. MdCKX8 and MdCKX10, however, exhibited little response to decapitation, but were significantly up-regulated by 6-BA and Lovastatin, a finding that warrants further investigation in order to understand their function in bud-outgrowth. •A total of 12 MdIPTs and 12 MdCKXs were identified in the apple genome and their systematic phylogenetic, structural, and synteny analyses were performed.•Analyses of the response of MdIPT, MdCKX and MdPIN1 genes to 6-BA treatment indicated that the initial up-regulation of MdIPTs may not be induced directly by 6-BA but rather due to the export of auxin from the axillary bud.•All of the detected MdIPTs were significantly up-regulated initially after decapitation, suggesting that apple buds may also be a primary source for the synthesis of CKs during the activation of bud outgrowth.•Results of the expression patterns of MdIPT, MdCKX and MdPIN1 genes after decapitation and Lovastatin treatments indicated that the absence of CK biosynthesis is not sufficient for the activation of bud break and the bud auxin export in apple tree with intact apical dominance or decapitated apple tree.•MdCKX8 and MdCKX10 exhibited little response to