Effect of exogenous 6-benzylaminopurine (6-BA) on branch type, floral induction and initiation, and related gene expression in ‘Fuji’ apple (Malus domestica Borkh)

Abstract

To investigate the mechanism by which 6-BA promotes flower bud formation, ‘Fuji’ apple trees were sprayed with 300 mg L−1 6-BA before the onset of floral induction. The differential transcription responses of flowering genes (MdFT, AFL1 and MdTFL1) and key cytokinin biosynthesis genes (MdIPT5a and MdIPT3a) in terminal buds were studied during flower development by real-time quantitative PCR. In addition, endogenous cytokinin and auxin levels in buds, shoot growth and flowering rate were surveyed. 6-BA treatment increased flowering rate, constrained shoot growth and changed the shoot components. Most shoots started to stop growing at 32 days after flowering (DAF), which marked the beginning of floral induction. The cytokinin accumulation was elevated at 30 DAF while MdTFL1 expression was down-regulated. Subsequently, there was a large increase in the transcription of AFL1 at 50 DAF. Thus, we propose that 300 mg L−1 6-BA spraying may indirectly break the cytokinin/IAA ratio suppressing MdTFL1 expression, resulting in higher transcription levels of AFL1 at flower initiation and alter shoots component and growth characteristics, which ultimately promoted flower development.