Modification of the B-ring during flavonoid synthesis in Petunia hybrida: Introduction of the 3'-hydroxyl group regulated by the gene Ht1.

The flower-color mutants of Petunia hybrida W37 and W18, which are homozygous recessive for the anthocyanin gene An3, accumulate flavanone glycosides in the flowers. It is concluded that the gene An3 is not directly involved in the synthesis of the C15 skeleton, but that it probably takes part in modifying the skeleton. Complementation experiments with the mutants W18 and M5 show that the hydroxylating gene Ht1, which is reponsible for the introduction of the second hydroxyl group in the B-ring at position 3', is expressed after gene An3. In P. hybrida introduction of the 3'-hydroxyl group is therefore not achieved by specific incorporation of caffeic acid during synthesis of the C15 skeleton, but by hydroxylation of a C15 skeleton. When anthocyanin synthesis is blocked by homozygous recessive hydroxylating genes Ht1 and Hf1, as in the mutant M5, dihydrokaempferol-7-glucoside is accumulated. This intermediate is discussed as a possible substrate for B-ring hydroxylation.