The major focus of our work is on the way mycorrhizal symbioses and root systems operate in soil in the field. Plants obtain nutrients and water from soil via root systems and their associated mycorrhizal fungal symbionts. The functioning of roots and mycorrhizas therefore determines productivity in terrestrial ecosystems. However, absorption of nutrients from soil is determined as much by geometry as by physiology: severe physical and chemical constraints on rates of transport in soil result in a very heterogeneous nutrient supply. The location of roots in time and space is therefore critical to an optimal acquisition strategy, which depends on the patterns of growth of root systems in situ in soil. Because of the constraints, the root alone is often an ineffective absorbing agent; the behaviour of the symbiotic mycorrhizal fungi then determines root system function, especially for the uptake of relatively immobile phosphate ions, one of the three major plant nutrients. In acquiring water and nutrients, roots and mycorrhizas consume around half of global primary production and represent a major linkage in the global carbon cycle.
Arbuscular mycorrhizal (AM) fungi are ubiquitous symbionts with plants, occurring with over 2/3 of all plant species. They apparently exhibit low diversity and are held to be non-specific, a contention supported by the ability of cultured AM fungi to colonise host plants indiscriminately. However, field collected roots of co-existing plant species invariably support distinct assemblages of AM fungi taxa, as determined by molecular analyses, and many of the sequence types detected by this method are unknown in culture. There are several possible explanations for this paradox: (i) there are many uncultured and unculturable taxa that do exhibit specificity, whereas the easily cultured taxa are non-specific; (ii) AM fungi 'species' have great genetic diversity (perhaps because their spores may contain many hundreds of nuclei and their mycelia are coenocytic), so that cultures only represent a small selection of their actual genetic range; (iii) AM fungi taxa differentiate in terms of their ecological response to the soil environment resulting in different assemblages in the roots of plants because they too exhibit such differentiation.
Which (or which combination) of these explanations is true has large implications for the understanding of the biology and ecological role of this important symbiosis. Of the three explanations, the first two would imply that the biology of the fungi was distinctive and would require new theoretical concepts to be developed. This explanation is therefore favoured, Occam-like, since it rests on the simple assumption that AM fungi behave like other fungi. Much of our current research aims to test the hypothesis that the AM fungi in soil exhibit a distribution that is responsive to soil heterogeneity and that can explain the pattern of colonisation of plant roots. To that end we are using several systems:
HEINEMEYER A, INESON P, OSTLE N, FITTER AH (2006) Respiration of the external mycelium in the arbuscular mycorrhizal symbiosis shows strong dependence on recent photosynthates and acclimation to temperature. New Phytologist (in press)
FITTER. A.H. (2005). Darkness visible: reflections on underground ecology. Journal of Ecology 93: 231-243 http://eprints.whiterose.ac.uk/archive/00000496/
FITTER, A.H., GILLIGAN, C.A., KLECZKOWSKI, A., TWYMAN, R.W., PITCHFORD, J.W. and the Members of the Soil Biodiversity Programme (2005). Biodiversity and ecosystem function in soil. Functional Ecology 19: 369-377 http://eprints.whiterose.ac.uk/archive/00000571/
EDWARDS, E.J., BENHAM, D.G., MARLAND, L.A. & FITTER, A.H. (2004). Root production is determined by radiation flux in a temperate grassland community. Global Change Biology 10: 209-227 http://eprints.whiterose.ac.uk/archive/00000495/
HEINEMEYER, A., RIDGWAY, K.P., EDWARDS E.J., BENHAM D.G., YOUNG J.P.W & FITTER A.H. (2004). Impact of soil warming and shading on colonisation and community structure of arbuscular mycorrhizal fungi in roots of a native grassland community. Global Change Biology 10: 52-64 http://eprints.whiterose.ac.uk/archive/00000494/
HEINEMEYER, A. & FITTER, A.H. (2004). Impact of temperature on the arbuscular mycorrhizal (AM) symbiosis: growth responses of the host plant and its AM fungal partner. Journal of experimental Botany 55: 525-534
FITTER, A.H., HEINEMEYER, A., HUSBAND, R., OLSEN, E., RIDGWAY, K.P. & STADDON, P.L. (2004). Global environmental change and the biology of arbuscular mycorrhizal fungi: gaps and challenges. Canadian Journal of Botany 82: 1133-1139
RIDGWAY, K.P., MARLAND, L.A., HARRISON, A.F., WRIGHT, J., YOUNG, J.P.W., FITTER, A.H. (2004). Molecular diversity of Frankia in root nodules of Alnus incana grown with inoculum from polluted urban soils. FEMS Microbiology Ecology 50: 255-263
STADDON, P. L., THOMPSON, K., FITTER, A.H., JAKOBSEN, I., GRIME, J.P. & ASKEW, A.P. (2003). Mycorrhizal fungal abundance is affected by long-term climatic manipulations in the field. Global Change Biology 9: 186-194 http://eprints.whiterose.ac.uk/archive/00000492/
STADDON, P.L.M RAMSEY, C.B., OSTLE, N, INESON, P. & FITTER, A.H. (2003). Rapid turnover of hyphae of mycorrhizal fungi determined by AMS microanalysis of 14C. Science 300: 1138-1140 http://eprints.whiterose.ac.uk/archive/00000200/
STADDON, P.L., OSTLE N., DAWSON L.A. & FITTER A.H. (2003). The speed of soil carbon throughput in an upland grassland is increased by liming. Journal of Experimental Botany 54: 1461-1469.
STADDON, P.L., OSTLE, N., FITTER, A.H. (2003). Earthworm extraction by electroshocking does not affect canopy CO2 exchange, root respiration, mycorrhizal fungal abundance or mycorrhizal fungal vitality. Soil Biology and Biochemistry 35: 421-426
LOVEYS, B.R., ATKINSON, L.J., SHERLOCK, D.J., ROBERTS, R.L., FITTER, A.H., ATKIN, O.K. (2003). Thermal acclimation of leaf and root respiration: an investigation comparing inherently fast- and slow-growing plant species. Global Change Biology 9: 895-910 http://eprints.whiterose.ac.uk/archive/00000493/
VANDENKOORNHUYSE P, RIDGWAY KP, WATSON IJ, FITTER AH, YOUNG JPW (2003). Co-existing grass species have distinctive arbuscular mycorrhizal communities. Molecular Ecology 12: 3085-3095 http://eprints.whiterose.ac.uk/archive/00000414/
BAILEY, P.H.J., CURREY, J.D. & FITTER, A.H. (2002). The role of root system architecture and root hairs in promoting anchorage against uprooting forces in Allium cepa and root mutants of Arabidopsis thaliana. Journal of experimental Botany 53: 333-340.
LINKOHR, B.I., WILLIAMSON, L.C., FITTER, A.H. & LEYSER, H.M.O. (2002). Nitrate and phosphate availability and distribution have different effects on root system architecture of Arabidopsis. Plant Journal 29: 751-760. http://eprints.whiterose.ac.uk/archive/00000489/
HELGASON, T., MERRYWEATHER, J.W., DENISON, J., WILSON, P., YOUNG, J.P.W & FITTER, A.H. (2002). Selectivity and functional diversity in arbuscular mycorrhizas of co-occurring fungi and plants from a temperate deciduous woodland. Journal of Ecology 90: 371-384 http://eprints.whiterose.ac.uk/archive/00000490/
FITTER, A.H. & FITTER, R.S.R. (2002). Rapid changes in flowering time in British plants. Science 296: 1689-1691 http://eprints.whiterose.ac.uk/archive/00000201/
LOVEYS, B.R., SCHEURWATER, I.S., PONS, T.L., FITTER, A.H. AND ATKIN, O.K. (2002). Growth temperature influences the underlying components of relative growth rate: an investigation using inherently fast- and slow-growing plant species. Plant Cell and Environment. 25: 975-988.
VANDENKOORHUYSE, P., HUSBAND, R., DANIELL, T.J., WATSON, I.J., DUCK, M., FITTER, A.H. & YOUNG, J.P.W. (2002). Arbuscular mycorrhizal community composition associated with two plant species in a grassland ecosystem. Molecular Ecology 11: 1555-1564 http://eprints.whiterose.ac.uk/archive/00000412/
STADDON P.L., HEINEMEYER A. & FITTER A.H. (2002). Mycorrhizas and global environmental change: research at different scales. Plant and Soil 244: 253-261
FITTER, A.H., WILLIAMSON, L., LINKOHR, B & LEYSER H.M.0. (2002). Root system architecture determines fitness in an Arabidopsis mutant in competition for immobile phosphate ions but not for nitrate. Proc. R. Soc. Lond. B 269: 2017-2022. http://eprints.whiterose.ac.uk/archive/00000186/