Virus makes plants lie to insects

Deceptively delicious smell of infected squash plants tricks aphids into spreading disease

INDIANAPOLIS — A common virus turns a squash plant into just the right kind of deceptive advertiser.

SNEAK ATTACK Cotton aphids can be tricked into tasting a virus-infected plant and spreading the infection to other plants. Clemson University, USDA Cooperative Extension Slide Series, Bugwood.com

When cucumber mosaic virus, known as CMV, infects garden-variety squash plants, the plants smell more alluring to aphids than healthy plants do, reports ecologist Kerry Mauck of Pennsylvania State University in University Park. The infected plants don’t live up to their odor, though. Upon landing on a virus-ridden plant, the aphids take a taste and, yuk, move on.

That swift retreat works out well for the virus, Mauck reported December 14 at the annual meeting of the Entomological Society of America. CMV spreads when aphids pick up the virus while probing an infected plant and then pass along the hitchhiker to the next leaf they explore. Unlike some other plant viruses, though, CMV lurks only within the outer layer of plant tissue. An aphid picks up virus particles just by tasting the outer cell layers of a leaf.

And if the leaf tasted as good as it smells, the virus would be in trouble, according to Mauck. Aphids might settle down for a long feed on a tasty plant, working their needle-like mouthparts deep into the plant’s plumbing and sucking sap. There’s no CMV there, and in the course of deep feeding, the aphid would wash out any infectious material caught inside its mouthparts. So an advertising scam that collapses quickly sends the aphids off at a fortuitous time for CMV,Mauck said.

This aphid cheater is the first virus found to manipulate a plant victim into a deception, Mauck says. Researchers have proposed that some other pathogens and parasites drive their hosts to do stupid things that help infections spread.

Mauck and her colleagues at Penn State uncovered the deception by the sick squash through a series of tests on yellow crookneck squash. Aphids that were forced to feed on sick plants didn’t reproduce as well as those eating healthy plants. Yet in a special lab arena that blocks cues other than smell, the odor alone of infected plants drew more aphids than the smell of an adjacent healthy plant. In a different experiment that gave aphids the chance to move among plants, aphids started abandoning the infected plants within half an hour of arriving — rather than settling in for hours or days.

The mouthparts-watering aroma of infected squash contains the same 20 or so components as scents from healthy plants, Mauck found. The ratios of these chemicals look roughly the same, but the infected plants put out a stronger signal. Mauck speculates that the virus may be making the squash mimic a really big, healthy plant.

“Very good work,” comments Bryony Bonning of Iowa State University in Ames. What she’d like to know, she says, is how this scenario would work where lots of plants are infected. An aphid going from one infected plant to another might not do the virus much good.

Mauck points out that CMV-style fakery wouldn’t help viruses found deep within plant plumbing tissues. For example, plants infected by potato leafroll virus release odors that invite aphids to come on over for a good long drink. It’s honest advertising though, because aphids apparently like the taste of, and thrive on, those plants.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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