Many tree fruit crops exhibit yearly cyclical fluctuations in flowering and fruiting, including apple (Malus × domestica Borkh.). This phenomenon, known as biennial bearing, is an intriguing biological problem, as well as a production limitation of many high-value apple cultivars. The current hypotheses to explain biennial bearing focus on the repressive effects of fruit on the initiation of floral primordia, which would develop into flowers the following year. This floral repressive response could be the result of nutrient competition between the spur apex and the strong sink of the developing fruit, or flowering-inhibitory gibberellins (GAs) that are produced in the seeds of the developing fruit and exported to the spur apex. However, the molecular mechanism by which fruit load and/or GAs represses floral initiation is unknown.The first aim of my dissertation was to identify the genes involved in the floral initiation pathway. Utilizing transcripts assembled from a transcriptome of the biennial cultivar 'Honeycrisp' and the known flowering genes from Arabidopsis, I identified a comprehensive list of flowering-related homologs based on sequence homology, phylogenetic relationship, and syntenic organization. The second aim was to characterize the expression of the flowering-related homologs during the period of floral initiation under crop load conditions that either promote or repress flowering. Homologs of AGL24/SVP, AP1, FT, LFY, and SPLs were strongly expressed in apices of floral-induced trees, supporting their presumed role as floral promoters. In contrast, a homolog of the floral repressor TERMINAL FLOWER 1 (MdTFL1-2), was strongly up-regulated in apices of fruit-bearing, floral-repressed trees. Additionally, genes involved in the deactivation of bioactive GAs were strongly up-regulated in the floral-repressed trees. The timing of floral initiation is also known to vary between cultivars in apple. However, the diversity in flowering gene regulation that underlies this variation is unknown. The third aim of my dissertation was to investigate diversity of gene expression in six different apple species and cultivars. Generally, the expression profiles of the flowering-related genes were similar, suggesting a widely conserved mechanism. However, a few key genes involved in the regulation of floral initiation and development exhibited differential expression. For example, during the floral initiation period, five of the six genotypes were found to have differentially expressed MdTFL1s. This result implicates a potential role for the MdTFL1s in determining the timing of floral initiation in addition to a role in biennial bearing. Plant growth regulators (PGRs) offer the potential to control flowering and biennial bearing in established apple plantings. This final aim was to evaluate and improve the efficacy of foliar-applied GA regiments to repress flowering in apple. This study included the evaluation of application timings, GA formulations and concentrations, and cultivar-specificity. The most successful PGR trial used GA3 to inhibit flowering in 'Honeycrisp' when a strong return bloom was expected. As a result of the decreased flowering and fruit load, the harvested fruit in the year following GA application exhibited higher fruit-quality. The results from this collection of studies provide insight into the molecular control of flowering and biennial bearing while demonstrating a practical approach to managing flowering in a high-value apple cultivar.
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