The study, published this week in Ecology Letters, is the first to reveal plants' ability to communicate underground in this way.
The research changes our understanding of the ways in which living things interact with one another. If crops can be managed in a way that exploits this natural communication channel, it could provide a new weapon in the battle against insect pests.
Scientists from Aberdeen University, the James Hutton Institute and Rothamsted Research grew the bean plant in groups of five. They allowed three in each group to grow underground networks of mycelia – a thread-like fungus that grows from one set of roots to another. They kept the two remaining plants free of the fungal links.
They then infested one plant in each group with aphids, triggering the release of chemicals to repel aphids but attract wasps, one of the aphid's predators.
Remarkably, plants which were not under attack themselves, but were connected to the victim by the underground fungal network, also began to produce the defensive chemical response. Unconnected plants did not mount a chemical defence. Researchers covered the plants with bags to rule out above-ground signalling.
Dr David Johnson, of Aberdeen University, who led the study, said: "We knew that plants produce volatile chemicals when attacked, and we knew they communicate danger to each other above ground. Now we know they communicate danger through these underground fungal networks as well.
"We don't quite know the mechanism, but it's likely to be a chemical signal. Our understanding of ecological systems has not considered the fact that plants are interconnected in this way. It could have major implications for our understanding of how one organism affects another."
The roots of virtually all groups of plants, including important food crops such as wheat, rice, maize and barley, are colonised by symbiotic fungi.
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