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

The sweet-smelling flowers create their scent using a surprising tool. It's an enzyme—a hardworking molecule—that was thought to help clean up DNA. This enzyme is missing in many roses. And that seems to explain why their blooms also lack a sweet floral aroma. The new finding could help scientists solve the thorny problem of why some rose varieties bred for dazzling color and long-lasting blooms have lost their scent.

When roses do smell like roses， it's because they give off a distinct mix of chemicals， says Philippe Hugueney， he studies plant biochemistry at the National Institute for Agricultural Research （INRA）. Called monoterpenes， these chemicals can be found in many odorous plants. Monoterpenes come in different shapes and scents， but all have 10 atoms of the element carbon. In roses， these chemicals usually are floral and citrusy. But it was unknown how roses make—or lose—their scent.

Other plants make fragrance chemicals using specialized chemicals. Called enzymes， these molecules speed up chemical reactions without taking part in them. In flowers， these enzymes tend to snip two pieces off an unscented monoterpene to create a scented one. But when Hugueney's team compared smelly and smell-free roses， they discovered a different enzyme at work. Called RhNUDX1， It was active in the sweet-smelling roses but mysteriously shut down in the bland blooms.

RhNUDX1 is similar to enzymes in bacteria that remove toxic compounds from DNA. But in roses， the enzyme trims a single piece from an unscented monoterpene. Other enzymes in rose petals then finish the job by chopping off the last piece.

Hugueney hopes his team's finding helps future roses come up smelling like—well， roses.