Abstract
BackgroundFloral induction is an important stage in the apple tree life cycle. In ‘Nagafu No. 2’, which was derived from a ‘Fuji’ bud sport, flower bud formation is associated with serious problems, such as fewer and inferior flower buds, a long juvenile phase, and an alternate bearing phenotype. Moreover, the molecular regulatory mechanisms underlying apple floral induction remain unknown. To characterize these mechanisms, we compared the RNA-sequencing-based transcriptome profiles of buds during floral induction in profusely flowering ‘Qinguan’ and weakly flowering ‘Nagafu No. 2’ apple varieties.ResultsGenes differentially expressed between the buds of the two varieties were mainly related to carbohydrate, fatty acid, and lipid pathways. Additionally, the steady up-regulated expression of genes related to the fatty acid and lipid pathways and the down-regulated expression of starch synthesis-related genes in the carbon metabolic pathway of ‘Qinguan’ relative to ‘Nagafu No. 2’ were observed to contribute to the higher flowering rate of ‘Qinguan’. Additionally, global gene expression profiling revealed that genes related to cytokinin, indole-3-acetic acid, and gibberellin synthesis, signalling, and responses (i.e., factors contributing to cell division and differentiation and bud growth) were significantly differentially expressed between the two varieties. The up-regulated expression of genes involved in abscisic acid and salicylic acid biosynthesis via shikimate pathways as well as jasmonic acid production through fatty acid pathways in ‘Qinguan’ buds were also revealed to contribute to the floral induction and relatively high flowering rate of this variety. The differential expression of transcription factor genes (i.e., SPL, bZIP, IDD, and MYB genes) involved in multiple biological processes was also observed to play key roles in floral induction. Finally, important flowering genes (i.e., FT, FD, and AFL) were significantly more highly expressed in ‘Qinguan’ buds than in ‘Nagafu No. 2’ buds during floral induction.ConclusionsA complex genetic network of regulatory mechanisms involving carbohydrate, fatty acid, lipid, and hormone pathways may mediate the induction of apple tree flowering.
Highlights
Floral induction is an important stage in the apple tree life cycle
Floral induction is an important stage in the plant life cycle that is regulated by complex networks involving multiple environmental and internal signals to ensure the appropriate timing of flowering [5]
In ‘Qinguan’ trees, the peak shoot length increase occurred at 7 days after full bloom (DAFB), and shoot lengths were significantly lower than those of ‘Nagafu No 2’ from 14 to 42 DAFB (Additional file 1: Figure S1)
Summary
Floral induction is an important stage in the apple tree life cycle. In ‘Nagafu No 2’, which was derived from a ‘Fuji’ bud sport, flower bud formation is associated with serious problems, such as fewer and inferior flower buds, a long juvenile phase, and an alternate bearing phenotype. The molecular regulatory mechanisms underlying apple floral induction remain unknown. To characterize these mechanisms, we compared the RNA-sequencing-based transcriptome profiles of buds during floral induction in profusely flowering ‘Qinguan’ and weakly flowering ‘Nagafu No 2’ apple varieties. ‘Nagafu No 2’, a dominant variety representing 65% of the total cultivated apple area in China [2, 3], exhibits serious problems regarding floral induction and formation, such as a long juvenile phase, alternate bearing phenotype, and low flower and fruit production. A comprehensive characterization of the physiological and molecular regulatory mechanisms underlying floral induction and bud formation in different apple varieties is very important for solving problems with flowering. Studies have revealed that key flowering genes (FT, SOC1, LEAF, SPLs, and AP1) involved in multiple flowering pathways play an important role in floral induction [6, 8, 9]
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