In this article we will discuss about the metabolic engineering of flavonoids.
Flavonoids are diverse group of plant natural product synthesised mainly from phenyl propanoid and acetate derived precursors. Flavonoids play a active role in plant defence microorganism, pest, plant microbe signalling and also in growth and development. These compounds exhibit potential antioxidant activity.
The major health benefits of fruits, green tea, and redwine might partly because of this property of flavonoids. Many flavonoids such as Quercetin and epicatecain gellat, have high levels of antioxidant activity, provide intense protection against certain cancer and cardiovascular disease. Dietary genistein reduces risk of mammary cancer in rats. Evidences from population show that it reduces risk of cancer.
Therefore considerable degree of interest in the metabolic engineering to introduce bioflavonoids from a source like soybean, chickpea, into other food crops. Although some proposal on the improvement of health benefit phytochemicals has been proposed, a major limitation has been the lack of knowledge about biosynthetic pathways involved in the synthesis of most of the plant natural products.
Fortunately, advancement in metabolic engineering strategy for modifying plant and its ability to synthesize flavonoids and iso-flavonoids has laid a key foundation in the improvement of this potential health promoting secondary metabolism and also these compounds are gold mine for metabolic engineering.
In the biosynthetic pathway of flavonoids and iso-flavonoids, all flavonoids are derived from a chalcone precursor produced due to the condensation of 4-coumaryol CoA and three molecules of melonyl CoA by the enzyme chalcone synthase (CHS).
This key enzyme (mw 42,000) acts as flavonoids synthesizing enzyme 19.9.1. The iso-flavonoids branch pathway has been well characterised at molecular level, and genetic manupulation of flavonoids, iso-flavonoid pathway can be engineering at many different biosynthetic stages.
An early objective for the metabolic engineering of the flavonoid pathway was to introduce blue (delphinidin) pigments into certain ornamentals like chrysanthemum and roses. These plants generally lack the pathway for anthocyanin biosynthesis. Transformation with F3‘5H gene did not produce complete deep blue colouration due to the lack of accessory pigments and correct vacuolar pH.
In one of the earliest attempt a new flower colour changes has been successfully attempted in petunia. In petunia, cyaniding and delphinine derivatives are produced as pigments. This is due to the substrate specificities of the dihydroflavanol 4-reductase of petunia, which cannot reduce dihydrokaempferol, but only reduce dihydromyricetin. This special substrate specificity explains the absence of anthocyanins in petunia.
However, introduction of dihydroflavonol reductase gene A, encodes dihydro quercetin 4-reductase (DQR) created a pathway new to petunia hybride and these enzyme converts dehydrokaempferol into leucopelargonidin which is then produce pelargonidin-3-glycosides.
Finally, red flower pigments are produced in petunia. Similarly, introducing alfa alfa chalcone reductase (CHR) into petunia produced yellow flowers in white flowers and verities and reduces purple pigmentation in deep-purple flowered varieties.
The forage lemon alfa alfa contains condensed tannins in seed coat. Which is important agronomic trait, but they are not expressed in leaf tissues. To overcome this problem a cloning of gene under the control of leaf specific promoter and genetic manipulation of condensed tannin level by altering the expression of CHS and DFR genes using antisense strategy improves agronomic qualities of legume forage.