Abstract
The yellow color of snapdragon (Antirrhinum majus) flowers is mainly provided by the 6-glucosides of aureusidin and bracteatin. Aureusidin synthase (AmAS1), a polyphenol oxidase, catalyzes the biosynthesis of aurone glucosides from chalcones glucosides. We recently proposed that 2',4',6',3,4-pentahydroxychalcone 4'-glucoside (PHC4'G), but not 2',4',6',4-Tetrahydroxychalcone 4'-glucoside, serves as the exclusive substrate of AmAS1 in A. majus flowers. PHC is synthesized from malonyl-CoA and caffeoyl-CoA by the action of the A. majus chalcone synthase, which is followed by glucosylation. However, biosynthetic route of caffeoyl-CoA in A. majus remains to be established. In lignin biosynthesis, caffeoyl-CoA is produced from p-coumaroyl-CoA by the combined actions of hydroxycinnamoy CoA shikimate/quinate hydroxycinnamoyltransferase (HCT) and p-coumarate 3-hydroxylase (C3H). In an attempt to examine whether the HCT/C3H system is also involved in caffeoyl-CoA biosynthesis in yellow A. majus petals, we isolated a cDNA encoding a C3H from yellow A. majus petals and characterized it.
