Underground fungus and plants have long been known to have a mutually beneficial relationship — mycorrhizal fungi delivers nutrients to plants, and the plants give carbon back in exchange. Now, a new study published in Ecology Letters shows in addition to that tradeoff, the mycorrhizal fungus network that connects many plants actually lets those plants defend themselves against enemies like aphids. As reported by the BBC, the study focused on the chemical response many plants emit when under attack from aphids, which commonly feed on damaged plants — the response typically repels the attacks and also the aphid's natural predator. As part of the test, the researchers grew five broad bean plans — three that were connected through the mycorrhizal fungi network, and two that didn't have that network in place, and all of the plans were covered with bags to prevent chemical communication through the air.
The researchers then let a single plant be attacked with aphids, and discovered that un-infested plants began to mount the same chemical defenses, thanks to communication through the fungi network. Plants that weren't connected to the fungi, however, did not mount their defenses. Of course, the fungi have a stake in keeping the plants alive — if the plants die off from aphid attacks, the fungi won't receive its carbon transmission. Professor John Pickett of Rothamsted Research called the fungi / plant relationship "just such a fantastic signalling system." The researchers believe that this finding could help plants defend against aphid attacks by using a "sacrificial" plant that can warn the rest of the crops.