Production of yellow colour in flowers: Redirection of flavonoid biosynthesis in Petunia

Abstract

Chalcones are intermediates in the biosynthesis of all flavonoids. In addition, in some species they constitute the major yellow flower pigments. There are two types of chalcones, distinguished by the presence (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 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 acyanic- or cyanic-flowered lines of Petunia, flower colour was changed from either white to pale yellow or deep purple to pale purple, respectively. Lines were generated that accumulated up to 60% 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-O-glucoside, is a novel plant chalcone. Another chalcone compound was identified in the pollen of the transgenics. The results show that the Petunia chalcone isomerase is unable to use 6'-deoxychalcones as substrates so that 6'-deoxychalcones are stable in Petunia petals, leaves and pollen, but some Petunia flavonoid enzymes can use 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.