Flavonols are important co-pigments in flower colour and are also esse
ntial for pollen tube growth. In petunia, flavonol synthesis is contro
lled by the FI locus. Flavonol synthase (FLS) belongs to the 2-oxoglut
arate-dependent dioxygenase family. Dioxygenase gene fragments were am
plified by PCR on cDNA made from FIFI and flfl flowers using degenerat
e primers designed from conserved dioxygenase sequences. A petunia pet
al cDNA library was screened for clones that hybridized more strongly
to the FI PCR products than the fl PCR products. A full-length cDNA cl
one identified by this screening exhibited FLS activity when expressed
in yeast. FLS gene expression is developmentally regulated during flo
wer development. Antisense expression of an FLS cDNA clone in petunia
markedly reduced flavonol synthesis in petals. RFLP mapping showed tha
t the FLS gene is linked to FI, suggesting that FI is the structural g
ene for FLS.