Yellow flowers generated by expression of the aurone biosynthetic pathway

Flower color is most often conferred by colored flavonoid pigments. Aurone flavonoids confer a bright yellow color on flowers such as snapdragon (Antirrhinum majus) and dahlia (Dahlia variabilis). A. majus aureusidin synthase (AmAS1) was identified as the key enzyme that catalyzes aurone biosynthesis from chalcones, but transgenic flowers overexpressing AmAS1 gene failed to produce aurones. Here, we report that chalcone 4′-O-glucosyltransferase (4′CGT) is essential for aurone biosynthesis and yellow coloration in vivo. Coexpression of the Am4′CGT and AmAS1 genes was sufficient for the accumulation of aureusidin 6-O-glucoside in transgenic flowers (Torenia hybrida). Furthermore, their coexpression combined with down-regulation of anthocyanin biosynthesis by RNA interference (RNAi) resulted in yellow flowers. An Am4′CGT-GFP chimeric protein localized in the cytoplasm, whereas the AmAS1(N1-60)-RFP chimeric protein was localized to the vacuole. We therefore conclude that chalcones are 4′-O-glucosylated in the cytoplasm, their 4′-O-glucosides transported to the vacuole, and therein enzymatically converted to aurone 6-O-glucosides. This metabolic pathway is unique among the known examples of flavonoid, including anthocyanin biosynthesis because, for all other compounds, the carbon backbone is completed before transport to the vacuole. Our findings herein not only demonstrate the biochemical basis of aurone biosynthesis but also open the way to engineering yellow flowers for major ornamental species lacking this color variant

Flower color modification of Petunia hybrida commercial varieties by metabolic engineering

Petunia flower colors are mainly due to flavonoids. The flower color of commercial varieties of Petunia hybrida was successfully modified by the suppression of endogenous flavonoid biosynthetic genes, the expression of a hetelorogous flavonoid biosynthetic gene, and the combination of both. Flower color changed from purple to almost white or from purple to red by the suppression of the endogenous gene expression, from red to orange by the down-regulation of the flavonoid 3′-hydroxylase gene and the expression of the rose dihydroflavonol 4-reductase gene, and from violet to pale violet by the expression of the flavonol synthase or flavone synthase gene. These results clearly indicate the usefulness of metabolic engineering of the flavonoid biosynthetic pathway to modify flower color. Only a few of the transgenic petunia exhibited phenotypic stability. For commercialisation, it is necessary to generate many independent transgenic lines, select elite lines with stable phenotypes and maintain them in tissue culture