In soils across the world, fungi trade resources with the plants they colonise in a mutually beneficial relationship. But it turns out the fungi are savvy traders, taking advantage of their partners by shuttling goods to nutrient-starved areas where plants are willing to pay more than usual.
The discovery is the latest demonstration that even simple, brainless organisms are capable of sophisticated trading strategies.
The roots of most land plants are colonised by arbuscular mycorrhizal fungi, which comprise elaborate networks of fine white filaments. The fungi provide plants with phosphorus and receive carbon in return.
A single fungal network can be connected to many plants, and vice versa, meaning the two parties can switch between trading partners and there is plenty of scope of wheeling and dealing.
Toby Kiers, an evolutionary biologist at the Free University of Amsterdam in The Netherlands has previously shown that fungi tend to avoid trading with plants growing in the shade. She has even caught them hoarding phosphorus to inflate the amount of carbon they get in return.
Now, inspired by economist Thomas Piketty’s best-selling book on the subject, she was curious to see how the fungi deal with resource inequality. How do they adapt their trading strategies when phosphorus supply is patchy?
To find out, Kiers and her colleagues used differently coloured light-emitting particles called quantum dots to tag phosphorus, allowing them to track its movements through fungal networks connected to a host root in petri dishes.
They exposed the networks to unequal distributions of the mineral. In one dish, the left side received 70 per cent of the phosphorus while the right got 30. In another, the left got 90 per cent and the right 10. In a control, the two sides received 50 per cent each.
After 60 days of exposure, the researchers noticed two clear patterns in the unequal treatments. First, inequality stimulated trade. The greater the disparity across its network, the more the fungus transferred phosphorus to the root.
Second, the fungus did this in an unusual way. “We could see the fungus was moving phosphorus across the network to its lower-resource side,” says Kiers. If the resource-rich patch was on the right side of the dish, for example, most of the phosphorus ended up on the left side.
Buy low, sell high
This suggests the fungus shifts its resources to the side of the root bundle where resources are scare. “Here, demand is higher, and the plant is potentially willing to pay more,” says Kiers. It’s a classic trading strategy – buy low, sell high.
The researchers hope to conduct another experiment tracking carbon trading between the roots and fungus to firm up this conclusion. “That would allow us to study variation in local exchange rates across more complex trade networks,” says Kiers.
David Johnson, a soil ecologist at the University of Manchester, UK, was not surprised that the fungi have this ability. “The fungus is not merely a passive passenger in this symbiosis,” he says. “It may be dependent on the plant but it is there for its own benefit, and it has evolved mechanisms to look after itself, which include the ability to move resources around in response to environmental conditions.”
The big mystery is how fungi coordinate trading strategies in the absence of cognition. This new fluorescent tracking technique should help, says Johnson. “They may be able to identify hotspots of activity and look at what the molecules are doing there, compared to places where there is no activity.”
Journal reference: Current Biology, DOI: 10.1016/j.cub.2019.04.061
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