GARDENERS, keep an eye on your tomato plants. There's no knowing what they are plotting underground.
Some 80 per cent of plants are colonised by fungi that form the familiar network of fine white threads that hang off many roots. The threads, called mycorrhizae, take in water and minerals from the soil, and hand some over to the plant in exchange for nutrients. Now it seems plants use them to communicate too.
Ren Sen Zeng and colleagues at South China Agricultural University in Guangzhou, grew pairs of tomato plants in pots. The team allowed some pairs to form mycorrhizal networks between their roots. Plants connected this way can exchange nutrients and water, staving off the effects of drought. But Zeng wanted to know if the networks had any other function.
The team sprayed one plant in each pair with Alternaria solani, a fungus which causes early blight. Sixty-five hours later, they infected the second plant and observed how well it coped.
Plants sharing a mycorrhizal network were less likely to develop the blight, and when they did, symptoms were milder. They were also more likely to activate defensive genes and enzymes (PLoS One, DOI: 10.1371/journal.pone.0013324).
The first plant was signalling to its neighbour, Zeng says, and he has dubbed mycorrhizae "the internet of plant communities".
Although nobody knows how they pass signals, the networks could be more reliable and efficient than other plant-to-plant signalling systems. These include chemicals released into the air to warn neighbours of impending attacks - which Zeng blocked by encasing the tomato plants in airtight bags. Airborne signals are slow and depend on the weather. Roots can also release chemicals, though these do not travel far.
"The research is a milestone in our understanding of communication between plants," says Suzanne Simard of the University of British Columbia in Vancouver, Canada. She points out that intensively farmed plants don't have mycorrhizae. With access to ample fertiliser and water they do not bother to grow them. As a result, they may be missing out on health benefits.
Together with Dan Durall of the University of British Columbia in Kelowna, Simard has shown that mycorrhizal networks can be enormous. Last year they found a network weaving its way through an entire Canadian forest, with each tree connected to dozens of its neighbours over distances of 30 metres (New Phytologist, vol 185, p 543).
"It's a very robust system that could allow for the movement of signal proteins over many metres," Durall says. Mycorrhizal networks even tie together plants of different species, which means different species might be able to communicate with each other.
Durall cautions that nobody has looked for Zeng's kind of communication outside the lab. But if the signalling system works as well in the messy real world as it did in the lab then many plants could well be chatting away beneath our feet.