Rooting for swarm intelligence in plants

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Rooting for swarm intelligence in plants
Researchers argue for a type of vegetative group decision making usually associated with animals By Susan Milius Web edition : Friday, December 10th, 2010 Text Size

Three plant scientists now propose that roots growing this way and that in their dark and dangerous soil world may fit a definition for what’s called swarm intelligence. Each tip in a root system acquires information at least partly independently, says plant cell biologist František Baluška of the University of Bonn in Germany. If that information gets processed in interactions with other roots and the whole tangle then solves what might be considered a cognitive problem in a way that a lone root couldn’t, he says, then that would be swarm intelligence.

The decisions that emerge from groups of individuals have intrigued a wide range of researchers, for in some cases crowds show an eerie wisdom. Honeybees looking for a new home can collectively pick excellent nest sites even as individual scouts advocate for a variety of choices. And combining people’s estimates of how many marbles are in a jar or what an animal at a country fair would yield in pounds of butchered meat often come quite close to the correct answer.

Plant life may exhibit collective decision making too, Baluška and his colleagues propose in the December Trends in Ecology and Evolution. They urge researchers to look beyond the animal kingdom and into the behavior of plant roots for evidence of crowd wisdom. Information could pass among root tips via secreted chemicals, released gases or perhaps even electrical activity that connects “brainlike” command centers in root tips, the researchers propose. But however the information travels, the interactions could yield swarmlike decisions about where and how much to grow.

Intelligently swarming roots are plausible, responds Jens Krause of Humboldt University in Berlin, who earlier this year published a review of research on animal and human swarm intelligence. Now he says he wants to see research presenting a full case for particular examples in plants.

“Applying the notion of swarm intelligence to plants, and not just to animals,” Krause says, “is interesting in the sense that swarm intelligence might provide a drive for group living in organismal life in general.”

A plant can deploy a considerable number of roots — 13,815,672 for a barley plant according to a classic study Baluška cites. The best evidence for swarm intelligence, Baluška speculates, might be found in exploring how myriad roots grow to exploit nutrient bonanzas that they come across in the soil. Roots also must compete with the roots of other plants for food and water; news from these skirmishes apparently travels far from the front. In earlier experiments dividing a plant’s roots between two pots, the segment in a private pot still shows a response if its counterparts in another container meet some nutrient-sucking intruder. And cutting off part of a root system triggers a reaction elsewhere.

The mechanism behind this — how one root finds out what another is up to — may be the most controversial part of the smart-roots idea. In their recent commentary, Baluška and colleagues recognize a range of possibilities, but in other papers have explored the idea that news travels via nerve-like electrical signals. Hormonal signals seeping along millimeter-by-millimeter would be too slow, they reason. Contrary to the usual view of plants as living the slow life, they do need fast information transfer.

The idea that plants basically have nerves — a conclusion that grows out of hard-to-interpret observations of electrical activity in plant tissue — has ignited a thunderstorm of its own among plant scientists in recent years.

“The use of the word intelligence (with or without swarm) simply humanizes (or animalizes, since they talk about swarms) the situation,” says David Robinson or the University of Heidelberg in Germany. Such “silly” terminology, in his words, “reduces serious plant science to the level of esoterics.”

However, he’s not disputing the ability of plants to solve complicated strategic problems. “Of course,” he says, “it’s well known that roots have ‘cognitive’ abilities.”