Missing enzyme to blame for scentless roses

Stopping to smell the roses might be a letdown — and now researchers know why.

The sweet-smelling flowers craft their scent using a surprising enzyme, previously thought to help prune genetic errors, researchers report July 3 in Science. That enzyme — and potent aroma — is missing in many roses bred for dazzling color and long-lasting blooms. The finding could help scientists solve the thorny problem of scentless blossoms.

“Usually, the first thing that people do when they get [a rose] is smell it,” says plant biochemist Philippe Hugueney of INRA, the National Institute for Agricultural Research in Colmar, France.  “Most of the time it’s not scented and it’s very disappointing,” he says.

When roses smell like roses, they do so by emitting a distinct assortment of monoterpene alcohols, he says. These 10-carbon molecules, found in many odorous plants, come in different shapes and scents. The ones in roses tend to have floral and citrus notes. But it was unknown how roses make —and lose —their bouquet.

In other plants, these fragrant chemicals are made using specialized enzymes. These enzymes perform the last step of the aroma-making process by snipping two phosphate groups from a pre-perfumy version of the 10-carbon molecule.

But when Hugueney and colleagues compared aromatic and scentless roses, they discovered a different enzyme, called RhNUDX1, at work. This enzyme was active in the sweet-smelling roses and mysteriously shut down in the bland blooms.

Based on its similarity to bacterial enzymes, scientists expected that RhNUDX1 removed toxic compounds from DNA. But in roses, the enzyme trims a single phosphate group from the fragrance precursor. Unidentified enzymes in rose petals finish the job by cleaving off the lingering phosphate group.

The finding raises the question of why roses use this unusual method, says plant biochemist Dorothea Tholl of Virginia Tech in Blacksburg. One possible explanation is that RhNUDX1 is more efficient than other enzymes, she says.

Hugueney hopes the finding helps future roses come up smelling like roses.