Exogenous application of GA(3) inactively regulates axillary bud outgrowth by influencing of branching-inhibitors and bud-regulating hormones in apple (Malus domestica Borkh.)

摘要

Although gibberellin (GA) has been reported to control branching, little is known about how GA mediates signals regulating the outgrowth of axillary buds (ABs). In the current study, the effect of the exogenous application of 5.0mM GA(3) on ABs outgrowth on 1-year-old Nagafu No. 2'/T337/M. robusta Rehd. apple trees was investigated and compared to the bud-activating treatments, 5mM BA or decapitation. Additionally, the expression of genes related to bud-regulating signals and sucrose levels in ABs was examined. Results indicated that GA(3) did not promote ABs' outgrowth, nor down-regulate the expression of branching repressors [MdTCP40, MdTCP33, and MdTCP16 (homologs of BRANCHED1 and BRC2)], which were significantly inhibited by the BA and decapitation treatments. MdSBP12 and MdSBP18, the putative transcriptional activators of these genes, which are expressed at lower levels in BA-treated and decapitated buds, were up-regulated in the GA(3) treatment in comparison to the BA treatment. Additionally, GA(3) did not up-regulate the expression of CK response- and auxin transport-related genes, which were immediately induced by the BA treatment. In addition, GA(3) also up-regulated the expression of several Tre6P biosynthesis genes and reduced sucrose levels in ABs. Sucrose levels, however, were still higher than what was observed in BA-treated buds, indicating that sucrose may not be limiting in GA(3)-controlled AB outgrowth. Although GA(3) promoted cell division, it was not sufficient to induce AB outgrowth. Conclusively, some branching-inhibiting genes and bud-regulating hormones are associated with the inability of GA(3) to activate AB outgrowth.