Underground Hijinks: Thieving plants hack into biggest fungal network

Some sneak-thief plants have tapped into the most widespread network of soil fungi, and they’re using it to steal food from respectable green plants, according to a new study of underground connections.

Roots of three kinds of parasitic plants with no chlorophyll of their own have intertwining connections with fungi called arbuscular mycorrhizae, says Martin Bidartondo of University of California, Berkeley. Hundreds of thousands of green-plant species also have such a connection, and they too might be sneaking food from each other, speculate Bidartondo and his colleagues in the Sept. 26 Nature.

The study “opens the door to that possibility,” comments David S. Hibbet of Clark University in Worcester, Mass. “That’s very exciting.”

Several groups of fungi grow into roots and then sprawl outwards through soil, connecting trees, herbs, or other plants underground. The arbuscular mycorrhizae, a group of the soil fungi in the order Glomales, poke into roots of some 300,000 plants species worldwide. Among networking fungi, these form connections with the greatest number of plant species and their networks cover the largest areas.

Researchers have regarded most partnerships between arbuscular mycorrhizae and green plants as mutually beneficial. The fungi draw out some of the plants’ carbohydrates and in turn give up minerals collected by their vast network threading through soil.

Scientists suspect that 400 or so parasitic plants also tap into fungi for food collected from other plants, but it’s been hard to determine which fungi. Before the new study, the few fungal associates known came from a group called ectomycorrhizal fungi. These form connections with only about 3 percent of the world’s plant species and are less widespread.

Bidartondo and his colleagues examined DNA sequences of fungi taken from the roots of three groups of parasitic plants: the lily-related Arachnitis uniflora from Argentina and several Voyriella and Voyria species in French Guyana from the gentian family. All the associated fungi turned out to be arbuscular mycorrhizae.

Roots of Arachnitis and several green plants growing nearby showed the same type of fungi, opening the possibility of plant-to-plant food transfers.

Another surprise was that each parasitic plant species had only a few fungal species on it. “It has always been assumed that arbuscular mycorrhizae fungi are rather nonspecific in the plants they colonize,” says Larry Peterson of the University of Guelph in Canada. “This study shows a clear exception.”

Hibbet adds that the new study fits with an emerging view of mutualistic partnerships. The parasite-fungus relationship has probably derived from a plant-fungus relationship based on an exchange of nutrients. This notion of a partnership gone sour “reinforces the view that these mutualisms are not stable endpoints,” Hibbet says.

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