NEWS RELEASE, 1/9/96 Plants mirror people when looking for a free lunch, but they may not get the bargain they thought

by Robert Sanders

Berkeley -- 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, UC 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 redundancy, that one fungus is as good as another in establishing mycorrhizal systems with plants," says Thomas D. Bruns, an associate professor in UC Berkeley's 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 UC Berkeley graduate student and now a research professor in the biology department at San Francisco State University, 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 essential nutrients such as nitrogen. In fact most plants would die or be stunted without these fungi. "Fungi are the basis of all terrestrial ecosystems," says Bruns, who is sponsoring the First International Conference on Mycorrhizae at UC Berkeley this summer, Aug. 4-9, which could attract as many as 400 scientists. "All nutrient gathering of terrestrial plants is mediated through soil fungi." The fungi, in turn, get nutrients they need from the green plant. The most important is fixed carbon in the form of sugars, which they cannot produce on their own because they have no chlorophyll for photosynthesis. The nutrient exchange occurs through mycorrhizae, an intimate association between the root hairs of a plant and the fungus. Fungal threads (hyphae) form an interlacing network with the cells in the root tip, and completely wrap the outside of the tips so that they never directly touch the soil. Instead the fungal cells bring in nitrogen, phosphorus and other nutrients from the soil and transfer them to the root cells directly. Until recently the study of mycorrhizal fungi has consisted of counting mushrooms under trees, because many of the mycorrhizal fungi produce large, edible mushrooms, such as Boletus edulis, chanterelles and matsutakis. With the advent of molecular identification techniques such as the polymerase chain reaction, scientists such as Bruns and Cullings can now identify the precise species in the soil by sampling the fungal mycorrhizal cells and comparing mitochondrial RNA sequences. Thanks to these techniques, Bruns and his colleagues have been able to show, for example, that the most abundant mycorrhizal fungi are not the dominate fruiters. "Prior to molecular methods there was no way to pick apart the communities and thus tell who's there and who dominates," Bruns says. "It turns out that the above-ground fruiting pattern is almost completely opposite of what you find when you look at the root mycorrhizae. We find a common pattern: what dominates are the species that don't seem to spend a lot of effort on fruiting." As often happens in nature, though, some plants try to get a free lunch by tapping into the mycorrhizae without contributing anything of their own. Pine drops, or Pterospora andromedea, is one such parasitic species in the group known as the monotropes. They are common throughout mountain pine and fir forests from the Rocky Mountains westward to the Pacific, and often grow to three feet in height. Of four parasitic montropes the group looked at, pine drops were unique in that throughout their western range they tap into only one fungus, Rhizopogon subcaerulescens. They do this by forming mycorrhizae of their own with the fungus. Since the soil is permeated by a network of fungal threads (mycelia) that link all mycorrhizae involving a specific fungus, nutrients easily flow through these threads from the tree's mycorrhizae to the pine drops' mycorrhizae. On the one hand such plants would seem to suffer by limiting their association to only a few fungi. Most plants -- pines, for example -- associate with a couple dozen. The benefit, the researchers suggest, is that pine drops have evolved to sponge off the fungus that has the best source of carbon of any fungus in the soil. Bruns suspects, though, that Rhizopogon subcaerulescens -- which no doubt fruits heavily, as do other members of its subfamily, the Suilloidae -- fruits less abundantly when parasitized by pine drops. The work was supported by the National Science Foundation. Tom Bruns can be reached at (510) 642-7987 or boletus@garnet.berkeley.edu. Ken Cullings is at (415) 338-7818 (wk) or (510) 680-0856 (hm), or can be reached via email at kculling@mendel.berkeley.edu. Periodic updates on the First International Conference on Mycorrhizae can be found at http://mendel.berkeley.edu/boletus/icom.html. A line drawing depicting the interrelationships among pine tree, fungus and pine drops is available at http://pio06.urel.berkeley.edu/documentation/photo.html.