Plants talk to each other. They don’t use their words, like our moms and dads taught us to do instead of making faces and grumping around. But when they need to — particularly when they’re under threat — they let each other know. Scientists have known for a while that plants will send out chemical signals in the air as a warning system, but now they’ve discovered that plants have a secret underground network of communication, too.

Many plants grow in partnership with mycorrhizal fungi, and, as the BBC reports, a new study found for the first time that those fungal systems transmit messages for the plants whose roots they grow on. When aphids attack one plant in the network, the fungi let the other plants know, and those plants start mounting their defenses.

It works just like any alliance, explains the BBC. Each party gets something out of it:

“Mycorrhizal fungi need to get [products of photosynthesis] from the plant, and they have to do something for the plant,” explained John Pickett of Rothamsted Research.

“In the past, we thought of them making nutrients available from the [roots and soil], but now we see another evolutionary role for them in which they pay the plant back by transmitting the signal efficiently,” he told BBC News.

No freeloading fungi here. Only sneaky spy fungi.