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.
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