Biosynthesis of prenylated flavonoids by two membrane-bound prenyltransferases in glandular trichomes of Macaranga tanarius.

Abstract

Macaranga tanarius (Euphorbiaceae), a myrmecophilic plant grown in subtropical countries, was identified as a source of Okinawan propolis, a honeybee product, which is glandular trichomes developed on the surface of the fruits. This tissue contains a high amount of characteristic geranylated flavonoids known as nymphaeols (nymphaeol A - C and isonymphaeol B), which exhibit strong antioxidant activity. In this study, we characterized the structure of glandular trichomes of M. tanarius, showing that they are sac-like structures containing drupelet-like aggregates of secretary cells, and a subcuticular cavity was also observed. From a cDNA library, we identified two prenyltransferase (PT) genes involved in biosynthesis of nymphaeols, and a PT gene grouped in a primary metabolic PT. Our biochemical experiments demonstrated that one of the former secondary metabolic PTs (MtPT1) is a B-ring-specific geranyltransferase for eriodictyol to yield nymphaeols B and isonymphaeol B. MtPT1 was unusual for a plant PT, an enzyme that yielded multiple reaction products. The other PT (MtPT3) transferred a geranyl moiety to the A-ring of eriodictyol to form nymphaeol A. Although dimethylallyl diphosphate (DMAPP) was not recognized as its prenyl donor substrate of eriodictyol, giving a B-ring geranylated eriodyctiol as the prenyl acceptor substrate, this enzyme used DMAPP to attach the prenyl moiety to the A-ring and produced nymphaeol C. These data suggest that M. tanarius has evolved its PT function to produce a diverse range of prenylated flavonoids using a limited set of genes.