May 29, 2018

Wild Roses: Why Do They Smell So Nice?

The wild roses have been in bloom in Denver this past week and I’ve been stopping to smell the ones on the corner of Bellaire Street and then those south of the new museum plaza in City Park. I find myself wondering why they smell so good?

So why do I find such pleasure in the odors released by roses? Presumably the flowers have evolved the chemistry to generate these odors in order to attract insects that in turn will aid in pollinating their flowers. One would think the genetics developed over many millennia as a process of co-evolution, the insects are drawn to a food source by the odor, easier able to find the nectar or pollen, and the plant gains a higher rate of fertilization and reproduction. All that makes sense.

But why is the odor of a rose so pleasing to my human sense of smell? Pleasing my sense of smell does not provide any conceivable evolutionary advantage to a wild rose. Sure there is the argument that a cultivated rose might be trading on its pleasant odor to bargain its way into cultivated rose gardens where it will be carefully tended and protected. But humans have no special quid pro quo deal with wild roses.

In fact those really pretty cultivated roses, the ones purchased at the florists no longer smell. “Breeding with a focus on cut flowers and visual attributes can leave scent traits disadvantaged. The cause for the lack of fragrance in these flowers is unknown and does not seem to be linked to increased vase life.”

About 3 years ago, a team of scientists isolated the gene and the protein it codes for that are responsible for the odor of roses; in their report they “…. propose that RhNUDX1 is a cytosolic component of a terpene synthase–independent pathway for monoterpene biosynthesis that leads to scent production in roses.”1

This may be a troubling development as now that the gene has been identified it may only be a matter of time until some enterprising researcher splices it into a lactobacilli leading the way to probiotics that make fecal material smell like roses.

Is our sense of smell so similar to an insect’s, or specifically a honeybee’s, that both our species take pleasure in the exact same odor? That strikes me as an odd coincidence, perhaps serendipitous, but really too connected to be pure coincidence.

In reading about roses I found a 2015 report stating that extracts made from white rose petals apparently delay aging in skin. At least if you are a mouse and want to stay young looking. The same extracts have also been reported to calm atopic dermatitis, again in mouse models.2 Nowak et al report in 2014 that rose petals have a cytotoxic effect against HeLa and breast cancer cells and a long list of pathogenic bacteria.3 Rose petal extracts seem to kill off a range of undesirable gut bacteria and apparently stimulate the growth of some desirable bacteria.4

But why do they smell so good? I’m still not finding a simple answer.

The Smithsonian confirms my earlier assumptions. “Unsurprisingly, many flowers emit scents to aid reproduction. Some flowering plants are generalists and use their odors to entice a host of insects and birds to fertilize their flowers. Others specialize, releasing scents that only appeal to a particular insect. The Soaptree yucca, for example, emits an aroma that attracts a single, aptly named species of yucca moth. As pollinators travel from flower to flower, they collect and deposit pollen, fertilizing the plants.”5

So we get why flowers have odors. What I don’t get is why my appreciation of certain odors is so similar to that of an insect. Why do both of us slow down to smell a rose?

Apparently the odors of some flowers mimic the breeding attractant pheromones of certain insects and that’s why the bugs have such a hard time resisting. Could it be that humans at one point in evolution employed pheromones that were close enough to insect pheromones that we still respond to their attraction? It would seem so as implausible as it may be when looking at this head on. Pheromones are apparently such a basic method of communication that the same chemicals have similar meanings across the gap that separates humans from insects. The scent of a rose needs no translation. I suppose this is not news to most people, those of a more poetic nature than writer. It inspires me to take another walk over to Bellaire and to take another whiff of this year’s wild rose.

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