Israeli scientists have uncovered messages transmitted underground - not by enemy agents, but by garden pea plants.

The Ben-Gurion University team discovered that plants can transmit distress signals to each other through their roots. An injured plant "communicates" to a healthy one, which in turn relays the signal to neighboring plants, possibly enhancing the other plants' ability to deal with stress in the future, according to the study, recently published in the periodical PLoS (Public Library of Science One ).

Open gallery view Professor Ariel Novoplansky of Ben-Gurion University. Credit: Ilan Assayag

The researchers, headed by plant biologist Ariel Novoplansky of the Mitrani Department of Desert Ecology, exposed five garden pea plants to drought conditions. They found that the stressed plant closes its leaves to prevent water loss. Meanwhile its roots release signals that caused neighboring plants, which were not exposed to drought conditions, to react as if they had been. The study, "Rumor Has It ...: Relay Communication of Stress Cues in Plants," shows the unstressed plants transmitted the information on to other healthy plants.

Preliminary results indicate that plants that receive the distress signals will survive better if exposed to drought at a later stage in their life.

"The results demonstrate the ability of plants and other 'simple' organisms to learn, remember and respond to environmental challenges in ways so far known in complex creatures with a central nervous system," says Novoplansky of the Blaustein Institute for Desert Research.

In some cases the immediate response helps healthy plants to deal with distress that has not yet affected them directly, he says.

Communication among plants has been the focus of study for decades. In 1983 Jack Schultz and Ian Baldwin concluded in an essay published in Science that injured poplar and maple trees release chemical signals that are picked up by healthy neighboring trees. The latter then activate defense mechanisms as though they themselves were hurt. The two scientists were roundly criticized at the time by the scientific community for what later became known as the "talking trees" notion.

In 1988 Dutch scientists showed that plants attacked by insects release chemical substances that summon help from other insects, who prey on the plant-eating ones. Certain plants activate a chemical alert when they are bitten by caterpillars, for example, attracting caterpillar-eating wasps to the endangered plant.

Today scientists accept that this communication also takes place among the plants themselves, as Baldwin and Schultz discovered.

Novoplansky and his team found that the distress signals are passed on not only from the injured plant to the adjacent healthy one, but also from the healthy one to its neighbors, which transmit it onwards, all through the plants' roots. Previous studies have shown that plants communicate through their leaves or stems, but the Israeli team revealed "underground" communication through roots.

Novoplansky believes the signals released by plants are generic and capable of passing from one plant species to another.

"We had an accident in which one plant got into an experiment of another species, and responded to its neighbors the same way the others did. It seemed as though the signals were understood by different plants, as though they were speaking Esperanto to each other. But at this stage this is a hypothesis and we are conducting other experiments to check it," says Novoplansky.

The BGU researcher is also studying why plants developed defense mechanisms that apparently help others that compete with them over limited resources.

"There are cases in which a plant would have clear motivation to inform its neighbors. We expect such mechanisms to work especially in a large plant, where it is likely the neighboring plants' roots belong to the same large plant," he says.

"The communication has an advantage that balances the competitive cost of transmitting the information to rival plants," he says.

Novoplansky, whose team included Dr. Omer Falik and other scientists, said that underground communication probably takes place in many plant species. "But we still don't know what the communication mechanism is exactly and are focusing efforts to decode it, together with other scientists in Israel and the world," he says.