Some fungi trade phosphorus with plants like savvy stockbrokers

Some stringy fungi are tough negotiators, trading nutrients shrewdly with plants.

An advance in tracking the nutrient phosphorus has revealed new details of ancient trading networks between fungi and plants. Some fungal species grow what are called arbuscular mycorrhizal connections underground, reaching intimately into plant roots. These fungi pull phosphorus from the soil and trade it for carbon from a wide range of plants.

Marking phosphorus with glowing dots shows the fungi hoarding the nutrient in parts of their elaborate networks of filaments when there’s a glut of it and plants wouldn’t be likely to trade much carbon. Phosphorus also gets shipped over the fungal networks to areas where it’s scarce and thus more valuable to trade, an international research team reports June 6 in Current Biology. 

These fungal-plant trades have been frustrating to study as biological markets because, until now, researchers could see snapshots, but not details, of the negotiations, says study coauthor Toby Kiers, an evolutionary biologist at Vrije Universiteit Amsterdam. It was “like a really good poker game” where the lights go out between dealing and winning, she says. For a better view, the researchers devised a way to watch the process in action by tagging phosphorus with nanoparticles called quantum dots that glow red or blue in ultraviolet light.

Arbuscular mycorrhizal fungi have no ability to capture carbon themselves, though they need it to live. Instead, they have traded with plants for the resource for some 450 million years (SN: 9/10/11, p. 15). Today, the fungi can connect with at least 70 percent of all plant species, including most crops.  Unlike other nutrient-trading fungi that sheath a plant root, these fungi work their way inside plant cells and grow “beautiful treelike structures” with plenty of surface area that can help with swapping sustenance, Kiers says.

In the new study, the researchers allowed the fungus Rhizophagus irregularis to tangle with carrot roots growing in part of a lab dish. The fungus also grew filaments away from the carrot roots into two other compartments. To challenge the fungus with a discouraging trade market, the team added equal amounts of phosphorus to the two compartments where the fungus grazed alone. In one compartment, the phosphorus was tagged with dots that glowed red, and in the other with blue.

As if riding out a period of oversupply, the fungus took up phosphorus and stored a sizable share of it. Researchers can’t yet track the carbon that the carrot provided in return for what phosphorus was traded, but overall, the filaments didn’t grow much, suggesting the carbon payoff was ho-hum.

To see how the fungi reacted to a hot market, the scientists applied the same total amount of phosphorus but put 10 percent into one compartment and 90 percent in the other. The color-coded dots let researchers see a share of the phosphorus moving through the fungal filaments toward the undersupplied compartment. Judging by how much fungus filaments grew, the carrot trading with the merchants in the scarcity zone essentially panicked, and the fungus made a market killing. The price, the ratio of carbon gained per phosphorus traded, was around 3.8 times higher on the nutrient-poor side versus the well-supplied.

The team thinks that these fungi are somehow managing the phosphorus flow, rather than simply letting it diffuse from an area of abundance to one of scarcity. For instance, flows of material that would carry phosphorus through the network move and switch directions too fast for simple diffusion, Kiers says.

The quantum dot technique lets researchers track phosphorus flows on a scale “that was difficult, if not impossible” until now, says Ylva Lekberg, a mycorrhizal ecologist at MPG Ranch, a conservation and research group in Missoula, Mont., not involved in the work. If the researchers manage to develop dots for carbon, that view could answer many outstanding questions, such as where plants hand over their carbon payments to the fungi and — a big one — how prices change.