Plant Networks Can Send Warnings, Spread Viruses

John Roach
for National Geographic News
October 1, 2007
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Much like how humans now send instant messages to each other on the Internet, plants such as strawberry and clover can exchange information along linked networks, ongoing research suggests.

These plants spread by sending horizontal stems known as runners along or under the ground. The runners eventually bud off new plants, which often remain connected via the stem system.

"So you end up with a network of plants," said Josef Stuefer, an ecologist at Radboud University in Nijmegen, Netherlands.

Scientists have long known that plants use these networks to share resources like food and water. (Related: "Plants Can Recognize, Communicate With Relatives, Studies Find" [June 14, 2007].)

Stuefer and his colleagues have now found the plants also use the networks to communicate, such as during an attack by leaf-eating insects.

Messaging Plants

The team's research shows that if a caterpillar attacks a strawberry leaf, for instance, the plant quickly passes along a warning of the imminent danger through the network. (Related: "Can Global Warming Cause Caterpillar Outbreaks?" [November 16, 2005].)

The message is sent through the phloem, a tube system plants use to transport organic compounds like carbohydrates, Stuefer said. This suggests the warning is also conveyed by an organic chemical.

Recipients of the message bolster their chemical and mechanical resistance so that they are able to thwart the caterpillar attack.

For example, chemical changes make the leaves less palatable and structural changes make the leaves harder to bite, Stuefer noted.

These defensive actions, in turn, significantly limit caterpillar damage to the plants.

"If the signal has been received by network members, then the damage is very much reduced compared to the control where the signal was not received," Stuefer said.

He and his colleagues first put forward the theory of communicating plants in 2004. Recent papers published in the journal Oecologia present the data on network communication.

André Kessler is an assistant professor of ecology and evolutionary biology at Cornell University in Ithaca, New York. He said this sort of signaling makes sense for networks of genetically identical plants, such as those that grow using runners.

"If one plant out of the clone is attacked, the probability that a second [connected] clone will be attacked in the future is very high," he said.

"So there must be a strong selection on actually giving the signal to the neighboring clone, because you increase your fitness that way."

Compromised Networks

The protective measures have their drawbacks, so "crying wolf" can have severe costs, study leader Stuefer pointed out.

The defensive changes, for example, limit the growth and development of the plants.

His team also found that plants allocate less energy and biomass to their root system when they are busy repelling caterpillars.

"If a plant is stressed by below-ground factors like drought or lack of nutrients, that can easily become a cost," Stuefer said.

The networks are also vulnerable to viruses, which spread through the interconnected plants with the same ease that virtual viruses multiply in a computer network, according to Stuefer.

"We don't know how much damage these viruses do, but I think that's the main drawback from having such a network," he said.

"But the fact [that the network is] there and I think it's common in these plants—this suggests the benefits over the long term are bigger than the costs."

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