Abstract
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.
Highlights
Flavonoids comprise a group of phenylpropanoids that, as water-soluble pigments, are stored in the vacuoles of plant cells [1]
Flavonoids can be classified into 12 subgroups—chalcones, stilbenes, aurones, flavanones, flavones, isoflavones, phlobaphenes, dihydroflavonols, flavonols, leucoanthocyanidins, proanthocyanidins, and anthocyanins (Figure 1) [3,4]—based on the degree of oxidation of the heterocyclic ring and the number of hydroxyl or methyl groups on the benzene ring
We focus on the flavonoid biosynthetic pathway, and present a model that includes eight branches—the biosynthesis of stilbenes, aurones, flavones, isoflavones, flavonols, phlobaphenes, proanthocyanidins, and anthocyanins—and four important intermediate metabolites, namely, chalcones, flavanones, dihydroflavonols, and leucoanthocyanidins (Figure 2)
Summary
Flavonoids comprise a group of phenylpropanoids that, as water-soluble pigments, are stored in the vacuoles of plant cells [1]. The second step in the general phenylpropanoid pathway involves the activity of C4H, a cytochrome P450 monooxygenase in plants, which catalyzes the hydroxylation of trans-cinnamic acid to generate p-coumaric acid. This is the first oxidation reaction in the flavonoid synthesis pathway [21]. We focus on the flavonoid biosynthetic pathway, and present a model that includes eight branches—the biosynthesis of stilbenes, aurones, flavones, isoflavones, flavonols, phlobaphenes, proanthocyanidins, and anthocyanins—and four important intermediate metabolites, namely, chalcones, flavanones, dihydroflavonols, and leucoanthocyanidins (Figure 2)
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