The distinctive scent of roses is delivered to our noses via a volatile monoterpene called geraniol. The way roses synthesize geraniol turns is equally distinctive, and scientists have tracked down the plant’s source of a precursor molecule key to geraniol’s production. (Proc. Natl. Acad. Sci. 2023, DOI: 10.1073/pnas.2221440120).
In most plants producing geraniol, the volatile is synthesized in membrane-bound organelles called plastids. But in 2015, researchers found that geraniol in roses is synthesized through a different pathway, which requires the help of an enzyme not found in plastids, but in the cell’s intracellular fluid, or cytosol. That enzyme dephosphorylates a molecule called geranyl phosphatase (GPP).
To test whether GPP was also synthesized in the cytosol or was imported from plastids, the researchers fed isotope-labeled precursor molecules to an especially fragrant rose species. Some of the precursor molecules are part of the usual terpene pathway in plastids, and others are included in a parallel terpene-precursor pathway in the cytosol. The geraniol these roses produced “was not at all labeled by the pathway happening in the plastid,” says Benoît Boachon, a plant biochemist at the National Center for Scientific Research, Jean Monnet University Saint-Étienne who led the work—suggesting a cytosolic source of GPP in roses.
The researchers searched the rose genome for genes encoding proteins that are localized to the cytosol and produce terpene precursors, including GPP. They then produced these enzymes in bacteria and tested their activity, then blocked and overexpress them in plants to home in on an enzyme in the cytosol that produced both GPP and a close cousin—farnesyl diphosphate synthase (FPP). They found an additional enzyme that synthesizes these two molecules in the cytosol.
Boachon and his colleagues are trying pin down one more part of this pathway—the phosphatase responsible for the final step in geraniol synthesis. “This enzyme is not known,” says Boachon. “It’s all part of a big project to understand how geraniol is made in the cytosol.”