Beans’ Talk

 This passage is adapted from “Beans’ Talk.” ©2013 by The

Economist Newspaper Limited.

The idea that plants have developed a

subterranean internet, which they use to raise the

alarm when danger threatens, sounds like science

fiction. But David Johnson of the University of

Aberdeen believes he has shown that just such an

internet, with fungal hyphae [the branching

filaments that make up a fungus’s body] standing

in for local Wi-Fi, alerts beanstalks to danger if one

of their neighbours is attacked by aphids.

Dr. Johnson knew from his own past work that

when broad-bean plants are attacked by aphids they

respond with volatile chemicals that both irritate the

parasites and attract aphid-hunting wasps. He did

not know, though, whether the message could spread

from plant to plant. So he set out to find out—and to

do so in a way which would show if fungi were the

messengers.

He and his colleagues set up eight “mesocosms”

[enclosed natural environments], each containing

five beanstalks. The plants were allowed to grow for

four months, and during this time every plant could

interact with symbiotic fungi in the soil.

Not all of the beanstalks, though, had the same

relationship with the fungi. In each mesocosm, one

plant was surrounded by a mesh penetrated by holes

half a micron [0.0001 centimeter] across. Gaps that

size are too small for either roots or hyphae to

penetrate, but they do permit the passage of water

and dissolved chemicals. Two plants were

surrounded with a 40-micron mesh. This can be

penetrated by hyphae but not by roots. The two

remaining plants, one of which was at the centre of

the array, were left to grow unimpeded.

Five weeks after the experiment began, all the

plants were covered by bags that allowed carbon

dioxide, oxygen and water vapor in and out, but

stopped the passage of larger molecules, of the sort a

beanstalk might use for signalling. Then, four days

from the end, one of the 40-micron meshes in each

mesocosm was rotated to sever any hyphae that had

penetrated it, and the central plant was then infested

with aphids.

At the end of the experiment Dr. Johnson and his

team collected the air inside the bags, extracted any

volatile chemicals in it by absorbing them into a

special porous polymer, and tested those chemicals

on both aphids and wasps. Each insect was placed for

five minutes in an apparatus that had two chambers,

one of which contained a sample of the volatiles and

the other an odorless control.

The researchers found that when the volatiles

came from an infested plant, wasps spent an average

of 3½ minutes in the chamber containing them and

1½ in the other chamber. Aphids, conversely, spent

1¾ minutes in the volatiles’ chamber and 3¼ in the

control. In other words, the volatiles from an infested

plant attract wasps and repel aphids.

Crucially, the team got the same result in the case

of uninfested plants that had been in uninterrupted

hyphal contact with the infested one, but had had

root contact blocked. If both hyphae and roots had

been blocked throughout the experiment, though,

the volatiles from uninfested plants actually attracted

aphids (they spent 3½ minutes in the volatiles’

chamber), while the wasps were indifferent. The

same pertained for the odor of uninfested plants

whose hyphal connections had been allowed to

develop, and then severed by the rotation of

the mesh.

Broad beans, then, really do seem to be using their

fungal symbionts as a communications network,

warning their neighbours to take evasive action. Such

a general response no doubt helps the plant first

attacked by attracting yet more wasps to the area, and

it helps the fungal messengers by preserving their

leguminous hosts.