Trading Roots
Cross-species mutualism
Threads of mycorrhizal fungi (white) connect to the roots of their plant hosts
ONE of the biggest underground markets on the planet nutrient trading between plant roots and fungi turns out to run on a system of reciprocal rewards for good suppliers and less business for bad ones. "It may have taken 450 million years to evolve," says Toby Kiers of VU University Amsterdam, "but unlike most human markets, here we have an example in which cheaters actually get punished and the good guys get rewarded." Most land plants participate in this exchange, as threads of specialized fungi wind into plant root tissue and form structures called arbuscular mycorrhizae. About 4 percent to 20 percent of the carbon compounds a plant produces from capturing the energy of sunlight flows into the fungus. In the other direction, minerals and other useful compounds flow from the fungus into the plant. Other cross-species mutualisms have turned out to have a lopsided power balance in which one partner, often a plant, can kill a misbehaving helper. In the arbuscular mycorrhizal system though, plant roots can detect which fungus threads are providing an abundance of a mineral and in turn reward them with extra nutrients in the form of plant-produced carbon. And the fungi also can detect and preferentially reward a good supplier and shun a slacker, Kiers and her colleagues report in the Aug. 12 Science. That's a different picture from other natural exchanges studied so far, says Jason Hoeksema of the University of Mississippi, who also studies plant-fungal interactions. "One exciting thing about these data is that they support the idea of a mycorrhizal market with competition and variation in offered prices on both sides, with reciprocal responses by choosy partners." Also, Hoeksema adds, "the authors used some really innovative techniques to get these answers." Initially, one collaborator called the project "impossible," Kiers recalls, because the plan called for tracking nutrient flows on very small scales. Fungal strands grow over roots in a spaghetti-like tangle of mingled species. Whether a plant could detect and reward an outstanding fungus while snubbing some fungal Scrooge just a thread or two away has been a matter of considerable debate.
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