A cytochrome b(5) is required for full activity of flavonoid 3 ',5 '-hydroxylase, a cytochrome P450 involved in the formation of blue flower colors

The substitution pattern of anthocyanin pigments is a main determinant of f lower color, Flavonoid 3',5'-hydroxylase (F3'5'H) is a cytochrome P450 enzy me (Cyt P450) that catalyzes the 3',5'-hydroxylation of dihydroflavonols, t he precursors of purple anthocyanins. Species such as rose and carnation la ck F3'5'H activity and are, therefore, unable to generate purple or blue fl owers. Petunia, on the other hand, contains two loci, termed hf1 and hf2, t hat encode a Cyt P450 with F3'5'H activity, Here we report the identificati on of an additional petunia gene that is required for 3',5' substitution of anthocyanins and purple flower colors. It encodes a cytochrome b(5) and is expressed exclusively in the flower. Inactivation of the gene by targeted transposon mutagenesis reduced F3'5'H enzyme activity and the accumulation of 5'-substituted anthocyanins, resulting in an altered flower color. Howev er, no phenotypic effect on the activity of other Cyt P450s, involved in th e synthesis of hormones or general phenylpropanoids, was observed. These da ta provide in vivo evidence for the regulation of the activity of specific Cyt P450s by a cytochrome b(5).