Fruit load and gibberellic acid induce changes in flowering intensity and gene expression in ‘Nagafu No. 2’ apple

In many perennial fruit trees, heavy fruit load usually reduces the ability of trees to induce flowering in the following year, resulting in a phenomenon of alternate bearing. However, the biological nature of floral repression by fruit is currently unknown. Here, we investigated the effects of fruit load on floral determination in the biennial apple cultivar, ‘Nagafu No. 2’. The presence of fruit has a negative effect on the return to flowering, while complete fruit removal significantly induces flowering in the following year. The contents of phytohormone such as GA1+3, GA4+7 and indole-3-acetic acid (IAA) were significantly accumulated in the buds of fruit-bearing trees. Moreover, heavy fruit loads are accompanied by reduced expression of flower-promoting genes (such as MdFTs, MdSOC1, MdAFL1, MdAP1, and MdSPL5) and increased expression of flower suppressor genes (MdTFL1 and MdSVP) in apical buds. In general, our results show that fruit loads influences apple flowering by regulating endogenous hormone levels and the expression of flowering-control genes. These results are expected to further help to identify the underlying physiological and molecular mechanisms of alternate bearing.