PRODUCTION OF YELLOW COLOR IN FLOWERS - REDIRECTION OF FLAVONOID BIOSYNTHESIS IN PETUNIA

Chalcones are intermediates in the biosynthesis of all flavonoids. In addition, in some species they constitute the major yellow flower pigm ents. There are two types of chalcones, distinguished by the presence (6'-hydroxychalcones) or absence (6'-deoxychalcones) of a hydroxyl gro up 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 syn thesized, and except in the pollen of some genotypes, they are ephemer al intermediates in flavonoid metabolism. By introducing a CHR cDNA fr om Medicago sativa under the control of the 35S CaMV promoter into acy anic- or cyanic-flowered lines of Petunia, flower colour was changed f rom either white to pale yellow or deep purple to pale purple, respect ively. Lines were generated that accumulated up to 60% of their petal flavonoids as 6'-deoxychalcones. Several different 6'-deoxychalcones a ccumulated in the petals of the CHR transgenics. The structures of thr ee of these were determined: one, butein 4-O-glucoside, is a novel pla nt 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'-deoxychal cones are stable in Petunia petals, leaves and pollen, but some Petuni a flavonoid enzymes can use 6'-deoxychalcones as substrates to modify their structures. The introduction of CHR provides a method to redirec t the flavonoid pathway into chalcone production, in order to modify f lower colour or to reduce the biosynthesis of other flavonoid types.