Phtoograph by Lisa Geoghegan, Alamy
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This little pollinator loves tomato plants, but especially those infected with the cucumber mosaic virus.

Phtoograph by Lisa Geoghegan, Alamy

The Curious Case of the Bumblebee, the Virus and the Tomato

The virus-pollinator-plant relationship is not all bad. Bees prefer virus-infected tomato plants, and those plants put out more seeds.

Cucumber mosaic virus infects nearly 1,200 species of plants, and it’s spread by more than 75 species of aphids. In tomatoes, the virus causes stunted growth, decreased fertility, and fruit that are few, small, mottled, and sometimes rotten. CMV is obviously no friend to farmers, but a surprising new study suggests that the virus may not be all bad for the plants it infects.

This is because bumblebees are more attracted to tomato plants infected with CMV. And while tomato plants can self-pollinate, they produce more seeds when a bumblebee does their dirty work.

Simon “Niels” Groen, a post-doctoral student at New York University and lead author of the new study published today in the journal PLOS Pathogens, likens the relationship to the story of Dr. Jekyll and Mr. Hyde. In other words, the more we learn about viruses, the more we see that they can provide benefits to their hosts as well as drawbacks.

These findings call for a shift in how we should view the relationships between plants, pollinators, and viruses, says Groen.

Pollination with a Buzz

Unlike most flowers, tomato plants do not give their pollen freely to any old bee or butterfly passing by. They only release it after intense stimulation, such as the kind bumblebees create by rapidly flexing their flight muscles.

“The vibrations produced by the bee can be of several hundreds of cycles per second,” says Mario Vallejo-Marin, a National Geographic Explorer and evolutionary biologist at the University of Stirling in the United Kingdom.

This is what’s known as “buzz-pollination”, and only 8 percent of plant species are thought to employ it. Vallejo-Marin has even found that bumblebees can adjust their buzzing frequencies to maximize the amount of pollen they get for the amount of energy they give.

The interaction between bee and tomato plant is so specialized, scientists have had a hard time recreating it in the lab. Groen’s team used an electric toothbrush nicknamed the “oral bee”, for instance, but plants visited by real life pollinators still out performed the artificial alternative.

Buzz-pollination helps tomato plants cut down on the amount of pollen used, since every time a deer brushes past or a stiff wind blows good pollen can go to waste. But it also limits the number of species that can help the plant reproduce.

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CMV attacks tomatoes and distorts their leaves.

Which is why the plants seem to have evolved a way to advertise themselves to the bees.

Something in the Air

Scientists have done a lot of work trying to understand how bees see the world around them, but these creatures can also sniff with the best of them. And to a bumblebee, tomato plants must smell as amazing as hot apple pie cooling on a windowsill.

What’s even more interesting is that Groen and his co-authors found that, for some reason, bumblebees like the smell of CMV-infected tomato plants even more. After a series of tests, they determined that the virus is able to alter the plant’s volatile profile, or the chemicals the plants emit into the air to attract bees.

Considering that CMV-infected tomato plants are more attractive to bumblebees, and tomato plants that are pollinated by bumblebees produce more seeds, Groen and his coauthors suggest it’s possible that this arrangement could influence the tomato plant population to become more susceptible to the disease. (But luckily, the seeds don’t carry the virus.)

The wheel, in other words, keeps on turning.

Real World Consequences

But there’s another species involved in this saga—the humans who like to eat salsa and spaghetti.

Asked whether this information could mitigate some of the losses seen from CMV in commercial growing operations, Edward Sikora, a plant pathologist at Auburn University, says probably not. Tomato farmers are interested in increasing yield and quality of the fruit, not the seeds.

And when this virus hits, it hits hard. Sikora points to an epidemic that struck Alabama in 1992. CMV ripped through 25 percent of the state’s tomato fields, destroying hundreds of acres worth of fruit. The counties affected lost everything.

By the way, there’s no way to stop the virus once it’s infected a plant, so prevention is a farmer’s best defense against CMV. This can include clearing fields and nearby areas of weeds, which can serve as reservoirs for the virus, or planting barrier crops that aren’t susceptible to CMV, such as corn.

Bringing bumblebees into those infected fields probably wouldn’t do anything to mitigate those losses, says Sikora. “Even with increased fertility levels, the plant will still be at the mercy of the virus.”

But there is one possible avenue for application. Now that the scientists have identified how the virus changes the tomato plant’s scent, Groen says we might be able to use this create crops that are more attractive to pollinators, and thus more productive in the long run.

In any event, it seems our perception that all viruses are harmful may be keeping us from understanding what’s really going on out there. In fact, perhaps it’s time to do away with black and white terms like “good” and “bad” when talking about virus-host relationships, says Groen.

A more accurate description would be, “It’s complicated.”

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