Flavonoids are an important class of secondary metabolites widely found in plants, contributing to plant growth and development and having prominent applications in food and medicine. The biosynthesis of flavonoids has long been the focus of intense research in plant biology. Flavonoids are derived from the phenylpropanoid metabolic pathway, and have a basic structure that comprises a C15 benzene ring structure of C6-C3-C6. Over recent decades, a considerable number of studies have been directed at elucidating the mechanisms involved in flavonoid biosynthesis in plants. In this review, we systematically summarize the flavonoid biosynthetic pathway. We further assemble an exhaustive map of flavonoid biosynthesis in plants comprising eight branches (stilbene, aurone, flavone, isoflavone, flavonol, phlobaphene, proanthocyanidin, and anthocyanin biosynthesis) and four important intermediate metabolites (chalcone, flavanone, dihydroflavonol, and leucoanthocyanidin). This review affords a comprehensive overview of the current knowledge regarding flavonoid biosynthesis, and provides the theoretical basis for further elucidating the pathways involved in the biosynthesis of flavonoids, which will aid in better understanding their functions and potential uses.

The timely transition from vegetative to reproductive development is coordinated through the quantitative regulation of floral pathway genes in response to physiological and environmental cues. The function of ethylene-responsive elementbinding protein (ERF) transcription factors in the regulation of flowering in chrysanthemum (Chrysanthemum morifolium Ramat.) is not well understood. Here, chrysanthemum overexpressing CmERF110 flowered earlier than the wild-type plants, while those in which CmERF110 was suppressed flowered later. RNA-seq results revealed that several genes involved in the circadian rhythm were transcribed differently in CmERF110 transgenic plants from that of the wild-type plants. The rhythm peak of the circadian clock genes in transgenic plants was delayed. Yeast two-hybrid screening of CmERF110 interactors identified a chrysanthemum FLOWERING LOCUS KH DOMAIN (FLK) homologue CmFLK, which was further confirmed with both in vitro and in vivo assays. KEGG pathway enrichment also revealed that CmFLK is involved in the regulation of circadian rhythm-related genes.CmFLK transgenic plants showed a change in flowering time and delayed rhythm peak of the circadian rhythm genes. Taken together, the present data not only suggest that CmERF110 interacts with CmFLK to promote floral transition by tuning the circadian clock, but also provides evidence for the evolutionary conservation of the components in the autonomous pathway in chrysanthemum.

We previously demonstrated that 20 mM sucrose promotes the upper axillary bud outgrowth in two-node stems of Chrysanthemum morifolium. In this study, we aimed to screen for potential genes involved in this process. Quantitative reverse transcription (qRT)-PCR analysis of sugar-related genes in the upper axillary bud of plants treated with 20 mM sucrose revealed the specific expression of the gene CmSWEET17. Expression of this gene was increased in the bud, as well as the leaves of C. morifolium, following exogenous sucrose treatment. CmSWEET17 was isolated from C. morifolium and a subcellular localization assay confirmed that the protein product was localized in the cell membrane. Overexpression of CmSWEET17 promoted upper axillary bud growth in the two-node stems treatment as compared with the wild-type. In addition, the expression of auxin transporter genes CmAUX1, CmLAX2, CmPIN1, CmPIN2, and CmPIN4 was upregulated in the upper axillary bud of CmSWEET17 overexpression lines, while indole-3-acetic acid content decreased. The results suggest that CmSWEET17 could be involved in the process of sucrose-induced axillary bud outgrowth in C. morifolium, possibly via the auxin transport pathway.