Abstract
Shoot branching is regulated by phytohormones, including cytokinin (CK), strigolactone (SL), and auxin in axillary buds. The correlative importance of these phytohormones in the outgrowth of apple axillary buds remains unclear. In this study, the outgrowth dynamics of axillary buds of a more-branching mutant (MB) and its wild-type (WT) of Malus spectabilis were assessed using exogenous chemical treatments, transcriptome analysis, paraffin section, and reverse transcription-quantitative PCR analysis (RT-qPCR). High contents of CK and abscisic acid coincided in MB axillary buds. Exogenous CK promoted axillary bud outgrowth in the WT but not in MB, whereas exogenous gibberellic had no significant effect on bud outgrowth in the WT. Functional analysis of transcriptome data and RT-qPCR analysis of gene transcripts revealed that MB branching were associated with CK signaling, auxin transport, and SL signaling. Transcription of the SL-related genes MsMAX1, MsD14, and MsMAX2 in the axillary buds of MB was generally upregulated during bud outgrowth, whereas MsBRC1/2 were generally downregulated both in WT and MB. Exogenous SL inhibited outgrowth of axillary buds in the WT and the apple varieties T337, M26, and Nagafu 2, whereas axillary buds of the MB were insensitive to SL treatment. Treatment with N-1-naphthylphalamic acid (NPA; an auxin transport inhibitor) inhibited bud outgrowth in plants of the WT and MB. The transcript abundance of MsPIN1 was generally decreased in response to NPA and SL treatments, and increased in CK and decapitation treatments, whereas no consistent pattern was observed for MsD14 and MsMAX2. Collectively, the present results suggest that in apple auxin transport from the axillary bud to the stem may be essential for the outgrowth of axillary buds, and at least, is involved in the process of bud outgrowth.
Highlights
Shoot branching is important for tree structure, blossoming, and fruit yield management in apple
Shoot branching occurs throughout the growing period of a plant, with patterns of branching generally classified as monopodial, sympodial, or dichotomous, and tillering (McSteen and Leyser, 2005; Barthélémy and Caraglio, 2007)
Plant height, bud length, stem diameter, leaf size, and branch angle between shoot and branch were initially similar in the WT and more-branching mutant (MB), but significant differences were observed at 60 days after bud break (DAB) (Figures 1A,B and Supplementary Figure S1; Li et al, 2016)
Summary
Shoot branching is important for tree structure, blossoming, and fruit yield management in apple. Shoot branching is a major determinant of aboveground plant architecture and occurs through the growth of axillary shoot meristems referred to as axillary buds. In both dicots and monocots, plant architecture is generally dependent on branching characteristics that result from interactions. Monopodial branching occurs during the vegetative stage, and sympodial branching during reproductive stages of the life cycle (Costes et al, 2014)
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