White flowers of the Petunia hybrida line W43 accumulate glucosides of 4-coumaric acid and caffeic acid and are able to synthesize anthocyanins from exogeneously supplied naringenin, suggesting that W43 is blocked in a biosynthetic step preceding the formation of naringenin. The cultivar Red Star contains a similar mutation as W43; the genetic background of this cultivar, however, allows the production of considerable amounts of anthocyanins in certain areas of the flower. When grown at reduced light, flowers of Red Star are uniformly coloured, whereas under an enhanced light regime the flowers exhibit alternating white and coloured areas.
In white sectors of flowers with a colour pattern virtually no chalcone synthase (CHS) enzyme activity could be demonstrated. The enzymes chalcone isomerase (CHI), UDPG: flavonoid-3-0-glucosyltransferase (3GT) and SAM:anthocyanin methyltransferase (OMT) are present, although at a more or less reduced level. From Western blotting and in vitro translation experiments we infer that the absence of CHS enzyme activity in the white sectors of the cultivar Red Star is due to the absence of translationally active CHS mRNA. The potential use of the mutants for genetic engineering in plants is emphasized.