HOUSTON, Aug. 29, 2005 -- New research by biologists at Rice University, Indiana University and George Mason University reveals how some non-native fescue grass gets a leg up over competing native plants: it's passed over by plant-eating insects and animals because its leaves are laced with toxic alkaloids, thanks to a symbiotic fungus that has co-evolved with the grass.
In a 54-month study conducted at Indiana University, scientists showed that 'tall fescue,' a common variety that is infected with the symbiotic fungus Neotyphodium coenophialum, tended to choke out uninfected fescue and native plant species. Tall fescue took over test plots much more quickly when herbivores had full access.
The research appears in the Aug. 30 issue of the Proceedings of the National Academy of Science.
"The practical implications of our findings are that the more herbivores there are in an area, the more likely it will be that infected tall fescue grass will spread and suppress native plants," said co-author Jennifer Rudgers, now an assistant professor of ecology and evolutionary biology at Rice.
Fescue, which is native to the Mediterranean, covers an estimated 37 million U.S. acres. It is cultivated for grazing and is often used as turf grass on lawns, golf courses and highway rights-of-way. Ranchers do not typically cultivate tall fescue because the symbiotic fungus it carries, known as an endophyte, produces alkaloids that have negative health effects for livestock. It is estimated that 80 percent of U.S. fescue is endophyte-infected, and in some applications, like turf grass, it's the preferred variety.
Prior research on hereditary plant symbionts like the fescue endophyte have tended to look at plant-fungal pairings in isolation. Rudgers said she, post-doc advisor Keith Clay of Indiana University, and co-author Jenny Holah of George Mason University sought to get a more realistic picture of the ecological effects of symbiosis.
"We wanted to find out how the surrounding community affected the relationship between its host and its symbiont," she said. "The implications of the research are broad because it's estimated that similar fungal symbionts exist in more than 20 percent of native U.S. grasses."
The tall fescue study was conducted on 60 plots near Bloomington, Ind., that measured 25 square meters apiece. At the start of the 4.5-year study, the land was plowed and planted with fescue seeds that sprouted alongside native grasses. Half of the seeds were tall fescue, which carries the fungal symbiont, and half did not carry the symbiont. The fungus is not transmitted by insects or wind and is only found in plants that sprout from infected seeds.
Half of the test plots were fenced to keep out foraging animals, and half of the unfenced and fenced plots were sprayed with an insecticide to suppress insect herbivores. Tall fescue progressed most slowly in plots that were both sprayed and fenced, constituting about 50 percent of live plant mass in the plots by study's end. In unfenced and unsprayed plots, tall fescue faired best, contributing to 75 percent of the plot by study completion.
"Importantly, we found that more fescue in the unsprayed and unfenced plots was endophyte-infected compared to the plots with herbivore-reduction treatments," said Rudgers "This is significant because it shows that the herbivores actually drive an increase in the relative abundance of infected plants."
In follow-up studies, Rudgers plans to see how tall fescue fares against competitors under drought and non-drought conditions, and she also plans to study symbiotic relationships in native grasses, including some Texas species.
The research was funded by the National Science Foundation.
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