Metabolic Analysis of the Regulatory Mechanism of Sugars on Secondary Flowering in Magnolia

Background: Magnolia is a traditional and important ornamental plant in urban greening, which has been cultivated for about 2,000 years in China for its elegant flower shape and gorgeous flower color. Most varieties of Magnolia bloom once a year in the spring, whereas a few others, such as Magnolia liliiflora Desr. ‘Hongyuanbao’, also bloom for the second time in summer or a little late. Such a twice flowering trait is desirable because of its high ornamental values, but its underlying mechanism remains unclear. Results: : To explore the peculiar metabolism twice-flowering in ‘Hongyuanbao’, the chemical metabolites and the relevant genes encoding them in the flower buds during the entire flowering period were analyzed. It appears that there was a significant variation between the metabolic profiles of flower differentiation between the first and secondary flower buds. The sucrose and trehalose metabolic pathways were significantly upregulated during both flowering periods as shown by a Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis. This was further verified by quantitative real-time polymerase chain reaction analysis which showed that the expression levels of a number of trehalose-6-phosphate synthase ( TPS ) genes including MlTPS1 , MlTPS5 , MlTPS6 , MlTPS7 , and MlTPS9 were substantial increased. To further investigate the potential functional role of sucrose and trehalose in flowering regulation, the leaves of ‘Hongyuanbao’ were sprayed with the sucrose or trehalose solution with a gradient of concentrations including 30 mM, 60 mM, and 90 mM. While both trehalose and sucrose of 60 mM in concentration were able to accelerate the process of flower bud differentiation, their molecular pathways appeared to be different. Sucrose treatment increased the transcription levels of MlTPS5 and MlTPS6 , whereas trehalose treatment increased MlTPS1 . The expression of a number of flowering-related genes, such as MlFT, MlLFY , and MlSPL was also increased in response to the sprays of sucrose and trehalose. Conclusions: : Overall, the findings on the functional roles of sucrose/trehalose and their underlying molecular mechanism in flowering of ‘Hongyuanbao’ improves our current understanding on of the biochemical triggers for flowering in Magnolia and provides a useful tool in Magnolia cultivation and flowering management.