Molecular Basis of Flower Formation in Apple Caused by Defoliation and Gibberellins

Adequate flower formation limits dependable apple (Malus xdomestica) production and is a major challenge for apple industries around the world. 'Honeycrisp' is a high value apple cultivar, but consistent flowering is difficult to achieve. Apple flower formation is affected by factors including defoliation, girdling, and gibberellin (GA(4 + 7)) and 1-naphthaleneacetic acid (NAA) applications. However, the molecular mechanisms that regulate the effects of these factors are not well understood. We studied the effect of local spur defoliation, GA(4 + 7) and NAA applications on `Honeycrisp' flower formation. Furthermore, we investigated the effect of local defoliation and local GA(4 + 7) application on the transcript levels of two major flower formation genes in the meristems of apple spurs. The floral inhibition gene terminal flowerl-1 (MdTFL1-1) and floral promoting genes flowering locus T (MdFT1 and MdFT 2) of apple. Local application of GA(4 +) (7) and defoliation treatments inhibited flower formation, but NAA applications were without effect. Defoliation treatments were accompanied by a significant reduction in MdFT1, 2 transcript levels compared with controls early in the growing season. Conversely, GA(4 + 7) application was accompanied by a significant increase in MdTFL1-1 transcripts compared with controls throughout the growing season. These results indicate that GA(4 + 7) inhibits flower formation by upregulating the inhibitory MdTFL1-1, and defoliation acts by downregulating transcript levels of MdFT1, 2 early in the growing season. We also provide evidence that defoliated bourse buds may receive flowering promotion signals from other parts of the tree in the absence of their local leaves.