Leucoanthocyanidins as intermediates in anthocyanidin biosynthesis in flowers of Matthiola incana R. Br

(+)Leucopelargonidin [(2R,3S,4R)-3,4,5,7,4'-pentahydroxyflavan] and (+)leucocyanidin [(2R,3S,4R)-3,4,5,7,3',4'-hexahydroxyflavan] were synthesized from (+)dihydrokaempferol and (+)dihydroquercetin, respectively, by sodium-borohydride reduction. The chemical and optical purity of these compounds was established by ultraviolet, proton-nuclear-magnetic-resonance, and optical-rotatory-dispersion spectroscopy. Supplementation experiments with these leucoanthocyanidins were carried out with genetically defined acyanic flowers of Matthiola incana. Feeding of leucopelargonidin and leucocyanidin to line 17 (blocked between dihydroflavonols and anthocyanins) and line 18 (blocked in the chalcone-synthase gene) led to formation of the corresponding anthocyanidin 3-O-glucosides, whereas supplementation of line 19 (blocked in a locus other than line 17 between dihydroflavonols and anthocyanins) did not result in anthocyanin synthesis. The conversion of leucopelargonidin into pelargonidin 3-O-glucoside was further confirmed by incorporation of [4-(3)H]leucopelargonidin into pelargonidin derivatives. The results are strong indications for the role of leucoanthocyanidins (flavan-3,4-diols) as intermediates in anthocyanin biosynthesis.