Biosynthesis and regulation of flavonoids in buckwheat

Abstract

Buckwheat contains an abundance of antioxidants such as polyphenols and is considered a functional food. Among polyphenols, flavonoids have multiple functions in various aspects of plant growth and in flower and leaf colors. Flavonoids have antioxidant properties, and are thought to prevent cancer and cardiovascular disease. Here, we summarize the flavonoids present in various organs and their synthesis in buckwheat. We discuss the use of this information to breed highly functional and high value cultivars.

Keywords: Buckwheat Genome Data Base; high value cultivar; secondary metabolite; transcription factor; transport.

Fig. 1.

Distribution of flavonoids in buckwheat based on previous reports. This figure is based on Supplementary Figure S1 of Matsui et al. (2018a) with added information. References: [1] Matsui et al. (2008), [2] Li et al. (2010), [3] Matsui et al. (2016), [4] Kalinova and Vrchotova (2009), [5] Kiprovski et al. (2015), [6] Nam et al. (2015), [7] Watanabe et al. (1997), [8] Watanabe (1998), [9] Watanabe (2007), [10] Kim et al. (2007), [11] Park et al. (2017), [12] Nagatomo et al. (2014), [13] Kim et al. (2013). Plant materials, growing conditions, and methods of measurement were different in each paper.

Fig. 2.

Chemical structures of flavonoids. (A) Basic chemical structures. (B) Buckwheat flavones. Apigenin is an aglycone of vitexin and isovitexin; luteolin is an aglycone of orientin and isoorientin. However, both apigenin and luteolins themselves are not contained in buckwheat (the box with the dashed line suggests). (C) Representative flavonols and a flavonol glycoside, rutin. (D) Representative anthocyanidins. Buckwheat has only cyanidin 3-glycosides. (E) Catechins and epicatechins. Buckwheat, like most other plant species, has only (+)-catechin and (–)-epicatechin.

Fig. 3.

Putative flavonoid biosynthesis pathway in buckwheat. Enzymes are indicated in upper-case letters. Identified genes are written under the enzymes. Postulated genes are indicated with [ ]. Dashed arrows indicate uncertain steps. White arrows indicate putative transportation pathways. CHS, chalcone synthase; CHI, chalcone isomerase; F3H, flavanone 3-hydroxylase; F3′H, flavonoid 3′-hydroxylase; F3′5′H, flavonoid 3′5′-hydroxylase; FLS, flavonol synthase; FNS, flavone synthase, GT, flavonol glycosyltransferase; DFR, dihydroflavonol 4-reductase; ANS/LDOX, anthocyanidin synthase/leucoanthocyanidin dioxygenase; UFGT, UDP glucose-flavonoid 3-O-glycosyltransferase; ANR, anthocyanidin reductase; LAR, leucoanthocyanidin reductase.

Fig. 4.

A phylogenetic tree for six putative R2R3 MYB transcription factors with representatives from other plants. Bootstrap values (500 replicates) are shown next to the branches. The scale bar corresponds to 0.05 substitutions per site. The root was assumed at the midpoint of the tree. This figure was derived from Supplementary Figure S2 of Yasui et al. (2016).