ble S2) than those of nonsymbiotic plants in soils Յ40°C. In soils above 40°C, nonsymbi- otic plants did not survive while symbioticplants thrived. The beneficial effect of fungal symbiosis increased with soil temperatures,demonstrating that Curvularia sp. providedthermal protection for D. lanuginosum. We rei- Regina S. Redman,1,2 Kathy B. Sheehan,3,5 Richard G. Stout,4,5 solated Curvularia sp. from D. lanuginosum Russell J. Rodriguez,1,2* Joan M. Henson3,5 roots at 45°C field soil temperatures, indicat-ing that thermal protection was also providedto the fungus, which corroborated our labo- All plants studied in natural ecosystems are or inoculated with Curvularia sp. by pipet- symbiotic with fungi (1), which obtain nu- ting 105 spores between the crown and first In addition to thermotolerance, the basis of trients while either positively, negatively, leaf. In the absence of thermal stress, en- mutualism in this system may involve other or neutrally affecting host fitness (2). Plant benefits (e.g., nutrient acquisition by the fun- adaptation to selective pressures is consid- gus). Several possible symbiotic mechanisms could confer thermotolerance. In planta, the (3). To test whether mutualistic fungi con- fungal endophyte produces cell wall melanin tribute to plant adaptation, we collected with thermal tape (Fig. S1), nonsymbiotic (Fig. S3) that may dissipate heat along the 200 Dichanthelium lanuginosum plants plants (45/45) became shriveled and chlo- hyphae and/or complex with oxygen radicals from geothermal soils at 10 sites in Lassen rotic at 50°C (Fig. 1A). In contrast, symbi- generated during heat stress (9). Alternatively, otic plants (45/45) tolerated constant 50°C the endophyte may act as a “biological trigger” soil temperature for 3 days and intermittent allowing symbiotic plants to activate stress- soil temperatures as high as 65°C for 10 response systems more rapidly and strongly days. All nonsymbiotic plants (45/45) died than nonsymbiotic plants (10).
Plants and their roots were removed and as- during the 65°C heat regime, whereas sym- sessed for fungal colonization (5). A fungal biotic plants (45/45) survived. The endo- References and Notes
1. O. Petrini, in Microbiology of the Phyllosphere, N. J.
Fokkema, J. van den Heuvel, Eds. (Cambridge Univ.
Press, Cambridge, 1986), pp. 175–187.
2. D. H. Lewis, in The Biology of Mutualism, D. H.
Boucher, Ed. (Croom Helm, London, 1985), pp. 29– 3. M. F. Smallwood, C. M. Calvert, D. J. Bowles, Eds., Plant Responses to Environmental Stress (BIOS Scien- 4. R. S. Redman, A. Litvintseva, K. B. Sheehan, J. M.
Henson, R. J. Rodriguez, Appl. Environ. Microbiol. 65,
5. Materials and Methods are available as supporting 6. A. Sivanesan, Mycol. Pap. 158, 104 (1987).
7. T. J. White, T. Bruns, S. Lee, J. Taylor, in PCR Protocols: A Guide to Methods and Applications, M. A. Innis, D. H. Gelfand, J. J. Sninsky, T. J. White, Eds. (Academic Press, San Diego, CA, 1990), pp.
8. R. S. Redman, D. D. Dunigan, R. J. Rodriguez, New Phytol. 151, 705 (2001).
9. J. F. Davidson, B. Whyte, P. H. Bissinger, R. H. Schiestl, Proc. Natl. Acad. Sci. U.S.A. 93, 5116(1996).
10. R. S. Redman et al., Plant Physiol. 119, 795 (1999).
11. We thank T. Al-Niemi, L. Brasche, M. Bateson, E.
Kuhn, A. Litvintseva, and J. Duda for technical and field assistance. This project was made possible by the permission, assistance, and guidelines of YNP and LVNP. This work was supported by grants from the U.S. Geological Survey, the NSF (9977922), the U.S.
Army Research Office (DAAHO4-96-1-01194), and Fig. 1. Representative symbiotic (with Curvularia sp.) and nonsymbiotic
D. lanuginosum plants with rhizosphere temperatures of 50°C for 3 days Supporting Online Material
or 65°C for 8 hours/day for 10 days under laboratory conditions (A) and in 40° or 45°C soil under field conditions (B).
void of the fungus, we conclude that this Curvu- phyte was reisolated from surface sterilized laria sp., like all known Curvularia species, is roots and leaves of all surviving plants, indicating that both the fungus and the host 1U.S. Geological Survey, WFRC, 6505 NE 65th Street, the thermotolerance of D. lanuginosum, we Seattle, WA 98115, USA. 2Department of Botany, University of Washington, Seattle, WA 98195, USA.
removed seed coats and surface sterilized symbiotic seedlings in pasteurized geother- 3Department of Microbiology, 4Department of Plant seeds (8) to generate endophyte-free plants.
mal soil collected and returned to Amphi- Sciences, and 5Thermal Biology Institute, Montana Treated seeds were planted in sterile ma- theater Springs ( YNP) in May 2001 (Fig.
State University, Bozeman, MT 59717, USA.
To whom correspondence should be addressed. E- month, plants were either mock-inoculated greener with greater root and leaf masses (Ta- SCIENCE VOL 298 22 NOVEMBER 2002



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