Flavonoid and flavonol glycoside metabolism in Arabidopsis

Plant Physiol. Biochem., 1998,36 (l-2).

135-144

Flavonoid and flavonol glycoside metabolism in ArabidopsisTerrence L. Graham

Department of Plant Pathology, Ohio State University, 201 Kottman Hall, 2021 Coffey Road, Columbus, OH 43210, USA. (Fax l-(614) 292 4455; E-mail graham.l@osu.edu)Abstract

The molecular genetic analysis of Arabidopsis promises to add greatly to our understanding of secondary product pathways and their biological roles. This review focuses on flavonoid metabolism in Arabidopsis, a particularly relevant pathway for such analysis because it is so highly conserved in plants. Several of the biosynthetic enzymes of the phenylpropanoid and flavonoid pathways have been cloned and/or mapped to specific loci. Recent advances concerning the regulation of anthocyanin metabolism by light and by possible transcriptional activators (Myc- and Myb-like elements) are discussedas well as the possible roles of flavonols in cold acclimation, ultraviolet light protection and male fertility. The use of HPLC profiling for the discovery of additional metabolic and regulatory mutants is described. The identification of the major flavonol glycosides in wild-type Arabidopsis is given and the partial characterization of two new flavonol glycoside mutants discussed.The biochemical genetics for flavonol glycoside formation in soybean is presented as an example, and the high degree of specificity of the glycosyl transferases and the interesting diversity of the resulting flavonol glycosides are discussed.Hypotheses regarding the potential biological activities of the flavonoid conjugates and the possible regulatory roles of highly aglycone-specific glucosidasesare presented. 0 Elsevier, Paris. Flavonoid, flavonol, anthocyanin, glycosides, mutant, HPLC, Arabidopsis. ACC, acetyl CoA carboxylase; 4CL, 4-coumaroyl-CoA ligase; CHI, chalcone isomerase; CHS, chalcone synthase; DFR, dihydroflavonol reductase; FST, flavonol sulphotransferase; F5H, ferulate 5hydroxylase; F3H, flavanone 3-hydroxylase; F3’H, flavonoid 3’-hydroxylase; Glc, glucose; PAL, phenylalanine ammonia-lyase; Rha, rhamnose.

Key words Abbreviations

IntroductionPlant secondary metabolites are both extremely numerous and diverse. Although the general pathways of secondary product metabolism are conserved over a wide range of plants, the great diversity is the result of specialized biosynthetic pathways. The predominant secondary metabolites so far identified in Arubidopsis are the flavonoids, the hydroxycinnamic acid esters, the glucosinolates, the brassinosteroids and the indole phytoalexins. Given the diversity of secondary metabolism, what can study of these various secondary metabolites in Arubidopsis provide us with? First of all, it can provide invaluable information on the metabolic pathways, developmental regulation and biological functions of highly conserved secondary products, such as the flavonoids. Secondly, investigation ofPlant Physiol. Biochem., 0981-9428/98/l-2/0 Elsev

ier, Paris

somewhat less common metabolites, such as the glucosinolates and indoles will still have relatively wide relevance and application, especially within the brassica. Finally, since it is currently impossible to exhaustively study the molecular genetics (let alone the biochemistry) of all plants, Arubidopsis can provide potentially valuable paradigms which will facilitate research on the regulation of secondary metabolism in even the most specialized pathways in other plants.

Arabidopsis

An excellent review of secondary metabolism in was recently published (Chapple et al., 1994) as well as a review on the characterization of the transparent testa (tt) mutants (Shirley et al., 1995). A review on the brassinosteroids appears in this volume (Szekeres and Koncz, 1998). Because of their general interest in a wide range of plants, I have cho-

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sen to focus on recent developments in the flavonoid area. In addition to providing a short general update on flavonoid work in Arubidopsis, I will highlight recent collaborative work in our lab and others on the characterization of some of the flavonoid mutants derived from HPLC screens. In addition, the biochemical genetics of flavonol glycoside formation in soybean is described as an illustration of the complexity of these metabolites and the possible biological and regulatory implications of conjugation are discussed. The biological roles of flavonoids in plants has been the subject of a very recent review by Shirley (1996) and will be only briefly alluded to as relevant to the text below. Characterization mes and mutants of flavonoid biosynthetic enzy-

The general ring structure and numbering scheme for the flavonoids is shown in figure 1. The B ring of 3’

5Figurevonoids.

4structure and numbering system

1. A general

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the flavonoids is derived from phenylpropanoid metabolism, while the A ring is the result of the headto-tail condensation of 3 acetate molecules derived from malonyl-CoA. In isoflavonoids, the B ring is covalently linked at the 3 rather than the 2 position. Of the mutants in Arubidopsis that potentially affect flavonoid metabolism, those of particular importance are the transparent testa (tt) mutants. These mutants are linked to flavonoid metabolism due to the fact that the flavan-3,4-diols are precursors of the conden …… 此处隐藏：45027字，全部文档内容请下载后查看。喜欢就下载吧 ……