Unlikely Chains of Dependency in the Plant World

by Robert Sanders

When you or I go looking for a free lunch we hit up our well-heeled friends, and apparently plants are no different.

In a paper in the Jan. 4 issue of Nature, Berkeley researchers report on a parasitic plant that seems to ignore the most abundant fungi in the soil to zero in on the richest--evidenced by prolific mushroom fruiting--to tap for nutrients.

The parasitic plant, known as pine drops and a common sight in Western forests in late summer, has no chlorophyll and relies on the fungus for essential nutrients. The soil fungus, in turn, gets its nutrients from nearby pines and firs.

The nutrients flow through mycorrhizae (my-ko-rye'-zee)--an association between fungi and plant roots found in 85 percent of all green plants--so that the parasite essentially taps into the tree's food stores.

The surprise finding was that pine drops home in on the identical species of fungus throughout the West, out of dozens of species present in the soil.

"People have assumed that in microbial systems there is a lot of re-dundancy, that one fungus is as good as another in establishing mycorrhizal systems with plants," says Thomas D. Bruns, an associate professor in the Department of Environmental Science, Policy and Management. "This shows that picture is wrong. All mycorrhizal fungi are not one big black box. They have different functions, we just don't know them."

The implication is that "single fungi can have a visible effect on plant community structure."

This unusual specificity could have its drawbacks, however. Co-author Ken Cullings, a former Berkeley graduate student and now a research professor in the biology department at San Francisco State, says that relying on only one species of fungus puts the parasite at a disadvantage if the ecosystem changes. If the fungus disappears through environmental degradation or global change, the pine drops disappear too.

"The question is, how common is this type of specificity? Is this an exception?" Cullings says. "If it's common, then in the forest this means a fungus may be a keystone species--if it goes extinct it could affect the whole ecosystem."

Bruns and Cullings are involved in on-going studies of mycorrhizal fungi in places ranging from Point Reyes, Calif., to Yellowstone National Park, in part to determine whether most specialize with specific trees and shrubs or are generalists and associate with a broad range of plants.

The two are among a growing number of scientists who specialize in the study of fungi that form a close, symbiotic relationship with the roots of plants. This encompasses a large range of fungi, since the vast majority of all terrestrial plants associate with soil fungi to obtain nutrients.


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