Multiple Transcriptome Comparisons Reveal the Essential Roles of in Floral Initiation and and in Floral Activation in Blueberry

The flowering mechanisms, especially chilling requirement-regulated flowering, in deciduous woody crops remain to be elucidated. Flower buds of northern highbush blueberry cultivar Aurora require approximately 1,000 chilling hours to bloom. Overexpression of a blueberry () enabled precocious flowering of transgenic "Aurora" mainly in non-terminated apical buds during flower bud formation, meanwhile, most of the mature flower buds could not break until they received enough chilling hours. In this study, we highlighted two groups of differentially expressed genes (DEGs) in flower buds caused by overexpression (VcFT-OX) and full chilling. We compared the two groups of DEGs with a focus on flowering pathway genes. We found: 1) In non-chilled flower buds, VcFT-OX drove a high expression and repressed expression of a major MADS-box gene, blueberry () resulting an increased / expression ratio; 2) In fully chilled flower buds that are ready to break, the chilling upregulated expression in non-transgenic "Aurora" and repressed expression in VcFT-OX "Aurora", and each resulted in a decreased ratio of to ; additionally, expression of a blueberry () was upregulated in chilled flower buds of both transgenic and non-transgenic' "Aurora". Together with additional analysis of and in the transcriptome data of other genotypes and tissues, we provide evidence to support that expression plays a significant role in promoting floral initiation and that expression is a key floral activator. We thus propose a new hypothesis on blueberry flowering mechanism, of which the ratios of to- at transcript levels in the flowering pathways determine flower bud formation and bud breaking. Generally, an increased ratio or increased in leaf promotes precocious flowering and flower bud formation, and a decreased ratio with increased in fully chilled flower buds contributes to flower bud breaking.