Aurone

Abstract: 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.

Abstract: Tyrosinase is a copper-dependent enzyme which converts l- tyrosine to dopaquinone and is involved in different biological processes such as melanogenesis and skin hyperpigmentation. The purpose of this study was to investigate naturally occurring aurones (Z-benzylidenebenzofuran-3(2H)-one) and analogues as human tyrosinase inhibitors. Several aurones bearing hydroxyl groups on A-ring and different substituents on B-ring were synthesized and evaluated as inhibitors of human melanocyte-tyrosinase by an assay which measures tyrosinase-catalyzed l-Dopa oxidation. We found that unsubstituted aurones were weak inhibitors; however, derivatives with two or three hydroxyl groups preferably at 4,6 and 4' positions are able to induce significant tyrosinase inhibition. The most potent aurone was found to be the naturally occurring 4,6,4'-trihydroxyaurone which induces 75% inhibition at 0.1 mM concentration and is highly effective when compared to kojic acid, one of the best tyrosinase inhibitors known so far (the latter is completely inactive at such concentrations). Active aurones are devoid of toxic effects as shown by in vivo studies.