Science 349, 81–83 (2015)

Credit: M. BENDAHMANE, ENS-LYON FRANCE

Roses have a distinctive fragrance that comprises hundreds of volatile compounds, including 2-phenylethanol, phenols and monoterpenes. The composition and abundance of these scent compound mixtures differ across rose hybrids, and breeding for features such as bloom longevity can diminish fragrance in the resulting varietals. Thus, tools for manipulating fragrant compound biosynthesis may be beneficial for the flower and fragrance industries. Magnard et al. now identify an unusual pathway used by roses to produce geraniol and other monoterpenes. Comparative amplification fragment length polymorphism–differential display and DNA microarray analysis revealed that a heavily scented rose cultivar (Papa Meilland, PM) expressed high levels of a gene with homology to the Nudix hydrolase family of pyrophosphatases relative to a lightly scented cultivar (Rouge Meilland, RM). The identified gene, which the authors termed RhNUDX1, encodes a cytosolic protein that is expressed predominantly in rose petals of PM (but not RM) during later stages of flowering. Further, RNAi knockdown experiments and quantitative trait locus (QTL) mapping revealed a functional linkage between RhNUDX1 expression and geraniol production. RhNUDX1's biosynthetic role is shared by roses, as RhNUDX1 expression correlates with monoterpene abundance in ten rose cultivars and, more generally, as heterologous expression of RhNUDX1 in tobacco plants resulted in the accumulation of geraniol and its glycosylated metabolites. Additional biochemical analysis revealed that geraniol production in roses is a two-step process: RhNUDX1 hydrolyzes geranyl diphosphate (GPP) to geranyl monophosphate, which is then converted to geraniol by a putative phosphatase in rose petals. These transformations were surprising given that GPP is the canonical substrate for terpene synthases that typically produce geraniol and other monoterpenes in plants. Though the biosynthetic source of GPP in roses remains unclear, the current RhNUDX1-dependent pathway defines an alternative mode of volatile monoterpene production in roses and offers promise for future engineering of more fragrant roses.