Flavone biosynthesis

Zhang, P., Wang, Y., Zhang, J., Maddock, S., Snook, M., and Peterson, T., 2003, A maize QTL for silk may sin levels contains duplicated Myb-homologous genes which jointly regulate flavone biosynthesis, Plant Mol. Biol. 52 1-15. 

Leonard E, Yan Y, Lim KH, Koffas MAG. 2005. Investigation of two distinct flavone synthases for plant-specific flavones biosynthesis in Saccharomyces cerevisiae. Appl Environ Microbiol 71 8241-8248. 

Figure 5.4. Abbreviated scheme for biosynthesis of major flavonoid subclasses, showing the primary enzymes and substrates leading to different subclasses. Bold-faced, uppercase abbreviations refer to enzyme names, whereas substrate names are presented in lowercase letters. PAL, phenylalanine ammonia lyase C4H, cinnamate 4-hydroxylase 4CL, 4-coumarate CoA ligase CHS, chalcone synthase CHI, chalcone isomerase CHR, chalcone reductase IPS, isoflavone synthase F3H, flavonone 3-hydroxylase F3 H, flavonoid 3 -hydroxylase F3 5 H, flavonoid 3 5 -hydroxylase FNSI/II, flavone synthase DFR, dihydroflavonol 4-reductase FLS, flavonol synthase ANS, anthocyanidin synthase LAR, leucoanthocyanidin reductase ANR, anthocyanidin reductase UFGT, UDP-glucose flavonoid 3-O-glucosyltransferase. R3 = H or OH. R5 = H or OH. Glc = glucose. Please refer to text for more information.

The key enzymes involved in the formation of the hydroxycinnamic acids (HCAs) from phenylalanine and malonyl-CoA are now discussed in detail, while later sections address the branches of the flavonoid pathway leading to anthocyanins, aurones, flavones, flavonols, PAs, and isotlavonoids. This is followed by brief reviews of the regulation of flavonoid biosynthesis and the use of flavonoid genes in plant biotechnology. To assist the reader. Figure 3.1 presents the carbon numbering for the various flavonoid types discussed. 

Kitada, C. et al.. Differential expression of two cytochrome P450s involved in the biosynthesis of flavones and anthocyanins in chemo-varietal forms of Perilla frutescens. Plant Cell Physiol, 42, 1338, 2001. 

Diets rich in millet have been associated with endemic goiter in parts of West Africa where millet is a staple. The damage has been attributed to vitexin, a C-glycosyl flavone, that in rats has antithyroid activity and that in vitro inhibits thyroid peroxidase and the free radical iodination step in thyroid hormone biosynthesis. Isoflavones have produced similar antithyroid effects in rats, but clinical studies in adults have not. " However, this remains a possible concern in infants fed soya-based milk-replacers, especially if iodine supply is compromised. 

Szalma, S.J. et al., Duplicate loci as QTL the role of chalcone synthase loci in flavone and phenylpropanoid biosynthesis in maize, Crop ScL, 42, 1679, 2002. 

Biogenetically, chalcones are the immediate precursors of flavanones, and some flavanones isomerize by ring opening into chalcones during isolation from plants or after chemical treatment with alkali. In turn, flavanones are intermediates in the biosynthesis of most other flavonoid groups, including flavones, flavonols, and isoflavonoids. For more information on the biosynthesis of flavonoids and flavanones in particular, the reader is referred to Chapter 3 and reviews by Heller and Forkmann. ... 

Flavonoids can be classified according to their biosynthetic origins. Some flavonoids are both intermediates in biosynthesis and end-products, e.g. chalcones, flavanones, flavanon-3-ols and flavan-3,4-diols. Other classes are only known as the end-products of biosynthesis, e.g. anthocyanins, flavones and flavonols. Two further classes of flavonoids are those in which the 2-phenyl side-chain of flavonoid isomerizes to the 3-position (giving rise to isoflavones and related isoflavonoids) and then to the 4-position (giving rise to the neoflavonoids). The major classes of flavonoids, with specific examples, are summarized helow. 

Similarly to L. densiflora, L. heteromorpha is unusual for its biosynthesis of methoxylated flavonoids glycosides (with the methoxyl groups at C-4 of ring B and C-7 of ring A). The production of such flavonoids could be a response to local environmental factors. In comparison to the other Licania species, L. heteromorpha predominantly yields flavonol glycosides instead of flavones or flavanones. 

Figure 3-7. Flavonoid biosynthesis (this page and next page). The enzymes involved in this pathway are (a) chalcone synthase (E.C. 2.3.1.73), (b) aureusidin synthase (E.C. 1.21.3.6), (c) chalcone isomerase (E.C. 5.5.1.6), (d) flavanone 3-hydroxylase (E.C. 1.14.11.9), (e) flavone synthase (E.C. 1.14.11.22), (f) flavonoid 3 -hydroxylase (E.C. 1.14.13.21),...

Figure 6-2. Biosynthesis of maysin proposed by McMullen et al. (2004) based on the analysis of flavones in the silks of maize salmon silk mutants, a. flavone 3 hydroxylase (encoded by the maize Prl gene), b. flavone synthase, c. C-glucosyltransferase, d. putative rhamnosyl transferase (encoded by the Salmon silk2 gene), e. the step(s) controlled by the Salmon silkl gene.

Leonard E, Chemler J, Lim KH, Koffas MA. 2006a. Expression of a soluble flavone synthase allows the biosynthesis of phytoestrogen derivatives in Escherichia coli. Appl Microbiol Biotechnol 70 85-91. 

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