Modification of chalcone biosynthesis in Petunia hybrida

Yellow is a very popular flower colour that is not available in several commercially important ornamental crops. Many yellow colours are derived from the carotenoid pathway. However the flavonoid pathway is also capable of producing yellow colours through the accumulation of chalcones, aurones or certain flavonol types. Chalcones are intermediates in the biosynthesis of all flavonoids and in some species they are the major yellow flower pigments. There are two major types of chalcones, distinguished by the presence (the 6'-hydroxychalcones) or absence (6'-deoxychalcones) of a hydroxyl group at the 6' position of the A-ring. The 6'-deoxychalcones are formed when the enzyme chalcone reductase (CHR) is active in conjunction with chalcone synthase (CHS). In Petunia, only the 6'-hydroxychalcones are synthesized, and except in the pollen of some genotypes, they are ephemeral intermediates in flavonoid metabolism. By introducing a CHR cDNA from Medicago sativa under the control of the 35S CaMV promoter into the white-flowered Mitchell line of Petunia, flower colour was changed from white to pale yellow. Lines were generated that accumulated up to 50% of their petal flavonoids as 6'-deoxychalcones. Several different 6'-deoxychalcones accumulated in the petals of the CHR-transgenics. The structures of three of these were determined; one, butein 4-glucoside, is a novel plant chalcone. Another chalcone compound was identified in the pollen of the transgenic plants. The results suggest that the Petunia chalcone isomerase is unable to use 6'-deoxychalcones as a substrate so that the 6'-deoxychalcones are stable in Petunia petals, leaves and pollen, but some Petunia flavonoid enzymes can use the 6' deoxychalcones as substrates to modify their structures. The introduction of CHR provides a method to redirect the flavonoid pathway into chalcone production, in order to modify flower colour or to reduce the biosynthesis of other flavonoid types.