Flavonoid biosynthesis in gerbera hybrids: Genetics and enzymology of flavones

Flowers of Gerbera Hybrids (Compositae) are known to contain the two flavones apigenin and lutcolin. These compounds belong to the most abundant groups of naturally occuring flavonoid classes. Besides their occurrence in flowers, where they affect the flower colour together with other flavonoids, in particular anthocyanins, they could also be found in many other parts of higher plants. The biosynthetic pathway of flavonoids is well known. Chemogenetic investigations of Gerbera flowers indicated a block of flavone formation leading to flavone-free flowers. This step is controlled by a gene named Fns. In order confirm the proposed genetic block of flavone formation we have investigated the flavone synthase activity in chemogenetic defind lines (varieties) with dominant or recessive Fns genes, respectively. For the oxidation of flavanones to flavones two different enzyme systems are known. The responsible enzyme in flowers of Gerbera could be identified as the cytochrome P-450 dependent monooxygenase flavone synthase II (FNS II). As reported for other plants the two specific cytochrome P450 inhibitors ketokonazole and ancymidol and the exclusion of oxygen inhibited FNS II activity. In agreement with chemogenetic work, activity of FNS II was only found with enzyme preparations from flowers of flavone-containing lines with dominant allele (fns +). Flavon-free lines with rezessive alleles (fns fns) shows no enzyme activity. This is the first successful correlation between a defined gene and the activity of flavone synthase II activity.