Beauty is in the nose of the beholder

It's no surprise that a flower releases scent to attract a pollinator, but why would it do it hours before the pollinator is around? New research finds scent can have more than one job.

In southwestern Utah you can find wild tobacco, Nicotiana attenuata. It’s a well-studied plant with a bag of chemical tricks for tackling predators. When a caterpillar bites the plant, it can release volatile gases, attracting carnivorous bugs. These bugs can feast on any herbivores munching the plant’s leaves. But some scents are puzzling.

Nicotiana attenuata
Image: Stan Shebs / Wikipedia

Danny Kessler and colleagues looked at when N. attenuata releases benzyl acetone. The flowers attract Manduca sexta hawkmoths. It’s no surprise then that this scent is released at night when the moths are around – but that’s not when the flower starts releasing the compound. Kessler’s team found the flowers started releasing benzyl acetone up to four hours before dusk. Producing a complex hydrocarbon is a lot of effort. Why would the flowers start so early? The authors decided to find out why N. attenuata released this chemical, by seeing what happened when it didn’t.

Kessler and colleagues used two populations of tobacco. One was typical wild tobacco. The other was almost the same, except the gene for producing benzyl acetone had been silenced. So as the sun fell in the sky, this second population would produce no scent.

Earlier experiments had shown that that cucumber beetles Diabrotica undecimpunctata would visit the tobacco flowers if they couldn’t find enough cucumber flowers to eat. So Kessler’s team found some cucumber beetles in the morning and starved them for a few hours, to make sure they were hungry. Then they released the beetles by their tobacco plants.

When evening comes, cucumber beetles look for a place to settle down for the night. The accommodation is also a snack, so hosting a cucumber beetle is usually bad news for the plant. The botanists found that when they had a choice, the beetles didn’t just look for a plant. They smelled for one too. Or rather, if they didn’t like the smell, they’d avoid a flower. The beetles did not like the smell of benzyl acetone, so the gene-silenced flowers were the favoured hosts for the beetles. When beetles did colonise the natural tobacco plants, they tended to do damage in the older, less whiffy, flowers.

Kessler and colleagues conclude that scent is doing two jobs for N. attenuata. Not only does it attract pollinators, but it also repels pests too.

Further reading

Kessler, A. (2001). Defensive Function of Herbivore-Induced Plant Volatile Emissions in Nature. Science, 291(5511), 2141–2144. https://doi.org/10.1126/science.291.5511.2141

Kessler, D., Bing, J., Haverkamp, A., & Baldwin, I. T. (2019). The defensive function of a pollinator‐attracting floral volatile. Functional Ecology. https://doi.org/10.1111/1365-2435.13332

Schuman, M. C., Barthel, K., & Baldwin, I. T. (2012). Herbivory-induced volatiles function as defenses increasing fitness of the native plant Nicotiana attenuata in nature. eLife, 1. https://doi.org/10.7554/eLife.00007