Diversification of Chemical Structures of Methoxylated Flavonoids and Genes Encoding Flavonoid- O-Methyltransferases

Abstract

The O-methylation of specialized metabolites in plants is a unique decoration that provides structural and functional diversity of the metabolites with changes in chemical properties and intracellular localizations. The O-methylation of flavonoids, which is a class of plant specialized metabolites, promotes their antimicrobial activities and liposolubility. Flavonoid O-methyltransferases (FOMTs), which are responsible for the O-methylation process of the flavonoid aglycone, generally accept a broad range of substrates across flavones, flavonols and lignin precursors, with different substrate preferences. Therefore, the characterization of FOMTs with the physiology roles of methoxylated flavonoids is useful for crop improvement and metabolic engineering. In this review, we summarized the chemodiversity and physiology roles of methoxylated flavonoids, which were already reported, and we performed a cross-species comparison to illustrate an overview of diversification and conserved catalytic sites of the flavonoid O-methyltransferases.

Keywords: O-methyltransferase; methoxylated flavonoids; plant specialized metabolism.

Figure 5

Phylogenetic tree of representative OMT genes. Class I OMT genes, including caffeoyl CoA OMTs and anthocyanin OMTs; class II OMTs, including caffeic acid OMTs, flavonoid OMTs and isoflavone OMTs. The tree was built by MEGAX [97]. The neighbor-joining method was used for clustering. The percentages of replicate trees in the bootstrap test (1000 replicates) are shown next to the branches. The evolutionary distances were computed using the p-distance method and are displayed in the units of the number of amino acid differences per site. Accession numbers: CCoAOMT1, At4g34050; CCoAOMT7, At4g26220; GmSOMT9, Glyma.17g171100; GmSOMT10, Glyma.07G214700; GmIOMT1, Glyma.05g14700; GmOMT5, Glyma.05G223400; VvAOMT. NP_001290011; McPFOMT, AY145521; PsAOMT, no ID; NmAMT3, LC330945; MT2 lotus, KJ676515.1; SlAnthoOMT, Solyc09g082660; SlCCoAOMT, Solyc02g093270.2; PaF6OMT, no ID; AtCOMT1, At5g54160; MsCh2′OMT, L10211; MpOMT1A, AY337457; MpOMT2, AY337459; MpOMT3, AY337460; MpOMT4, AY337461; MsD7OMT, U97125; HI4′OMT, AB091684; HvOMT1, ABQ58825; CrOMT1. AY028439; CrOMT2, AAM97497; TaOMT1, Q84N28; TaOMT2, ABB03907; OsNOMT, BAM13734; OsROMT9, ABB90678; ShMOMT3, AGK26768; SlMOMT4, KF740343; MtIOMT1, AY942159.1; CdOMT5, LC126059; ObFOMT1, K0I977; ObFOMT3, K0I7Q2; PdPOMT7, TC29789.

Figure 6

Multiple alignment of representative OMT genes. (A) Multiple alignment of class I OMT genes, including caffeoyl-CoA OMTs and anthocyanin OMTs. (B) Multiple alignment of class II OMTs, including caffeic acid OMTs, flavonoid OMTs and isoflavone OMTs. Blue inverted triangle indicates substrate binding sites, green inverted triangle indicates SAM binding sites, orange inverted triangle indicates catalytic residues, and red box indicates mutant site mentioned in this paper. Accession numbers shown in Figure 5.

Figure 1

Schematic diagram of known methoxylated flavonoid aglycone biosynthesis. Circles indicate metabolite name, arrows indicate biosynthesis pathway, and words in italics indicate biosynthesis genes. Abbreviations: FNS: flavone synthesis; F3H: flavanone 3-hydroxylase; FLS: flavonol synthase; F3′H: flavonoid 3′-hydroxylase; DFR: dihydroflavonol 4-reductase; ANS: anthocyanin synthase; 3GT: flavonoid 3-O-glucosyltransferase; F7OMT: flavonoid 7-O-methyltransferase; F3′OMT: flavonoid 3′-O-methyltransferase; F4′OMT: flavonoid 4′-O-methyltransferase; A3′OMT: anthocyanin 3′-O-methyltransferase; N7OMT: naringenin 7-O-methyltransferase.

Figure 2

Chemical structure of major methoxylated flavonoid aglycones.

Figure 3

Methoxylated flavonoids reported in plant species. By searching the keywords “quercetin”, ”kaempferol”, “isorhamnetin”, “myricetin”, “flavonol”, “luteolin”, “tricetin”, “apigenin”, “flavone”, “daidzein”, “glycitein”, “genistein”, “isoflavone”, “petunidin”, “malvidin”, “peonidin” and “anthocyanidin” in the KNApSAcK database (http://www.knapsackfamily.com/knapsack_core/top.php, accessed date: 14 May 2021), 322 out of 1046 flavonols, 788 out of 1298 flavones, 349 out of 573 isoflavones and 120 out of 506 anthocyanidins were found to be methoxylated flavonoids.

Figure 4

Chemical diversity and species specificity of methoxylated flavonoids in Fabaceae plants.