Agenda - Planning Commission - 03/02/1999

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Research updates From the annual meeting in MontrdaI, Quebec, of the American Institute of Biological Sciences NOTES FROM UNDERGROUND Much of the secret of plant success is hidden below ground, where their roots establish essential associations with mycorrhizal fungi. One class of fungi, arbuscular mycorrhizal (AM) fungi, take up residence within the roots of the vast majority of plants, where they enhance nutrient uptake and phosphorous cycling, increase pathogen resistance, improve yield, and promote fitness and competitive ability. It now appears that AM fungi may influence not just individual plants but the structure of the plant community as a whole: According to James Bever, of the University of Chicago, interactions between plants and AM fungi may promote the di- versity of plant communities. This view of the ecological role of AM fungi invokes a new view of the dynamics in the interaction between plants and AM fungi. Most AM fungi can grow on many different plants, but they often sporulate at different rates on different hosts, and ecolo- gists often assume that a plant ben- efits more from the AM fungus that grows best on it than from any other AM fungus in the community. Bever noted that theoretical considerations (which he outlines in an article in the October 1997 issue of The Journal of Ecology) indicate that such posi- tive feedbacks could eventually re- sult in the homogenization of the plant and fungal populations in a community, with one plant-fungus pair ultimately taking over. But Bever pointed out that if the interactions were to involve negative by Rebecca Chasan and Deborah Schoen feedbacks rather than positive feed- backs, they would act instead to maintain diversity. Negative' feed- backs would result if a plant species were to benefit more from a fungus for which it is a relatively poor host than from.a fungus for which it is the best'host'. (They could also result if a fungus were to have a stronger growth-promoting effect on a plant on which it grows relatively poorly than on one on which it grows well.) The prediction that negative in- teractions would promote diversity makes intuitive sense. For example, if fungus X were to accumulate to highest levels under plant A but to promote the growth of plant B more than plant A, and if fungus Y were to preferentially accumulate under plant B but to better promote the growth of plant A, then both plants and both fungi would be maintained. Evidence of such negative feed- back interactions comes from Bever's studies of the dynamics of plants and AM fungi in an old field in Durham, North Carolina. This field supports at least 30-40 plant species and a remarkable 37 AM fungal species (15 of which had never before b~en described). Many of the fungal spe- cies turn out to grow better in asso- ciation with some hosts than others. After growing four perennial plants that are abundant in the field (Anthoxanthum, AlBum, Panicum, and Plantago) individually for five months in soil that had been inocu- lated with a sample of the entire mycorrhizal fungal community, Bever found that nine of the 16 most common fungal species had sporu- fated at higher rates on one of the plants than on the others. Each plant had "cultured" its own soil, promot- ing the growth of some AM fungi (and other soil microorganisms) more than others. Bever next carried out transplan- tation experiments to test whether each plant grows best in its own soil (that is, whether positive feedback occurs) or in soil cultured by another plant (.that is, whether feedback is negative). In eight of 11 cases, Bever found that a plant type (either a species or a genotype) grew better in soil in which another plant type had grown than in its own soil. At least one of these instances of negative feedback--that in which Plantago grows better in soil in which Panicum has been grown than in its "own" soil--turned out to be due to the accumuIation of an AM fungus. Other negative feedbacks were due to the accumulation of pathogenic microorganisms in the soil in which a plant was grown for five months. Soil in which another plant spec~'~" . was grown would not have promoted .~ the growth of these microorganisms?' and that soil would therefore be a better environment for the first spe- cies. (By controlling for differences in minerals, Bever ruled out the pos- sibility that a plant may have de- pleted the soil of a nutrient that it needs but another plant does not.) Bever says that it is not yet clear whether negative feedbacks medi- . ated by AM fungi are a common, mechanism for maintaining diver- sity. If they do turn out to be com- mon, such feedbacks may have im- portant practical implications for issues as diverse as agriculture and ecological succession. Crop rota- tions, for example, have been found to alter populations of AM fungi; learning more about the dynamics between AM fungi and crops re. ay lead to improvements in sustainable BioScience Vol. 48 No. 1