The biotic and abiotic interactions that occur between roots and the soil rhizosphere environment are easily the most complex and least understood interactions in plants. Roots of many trees, in particular pines, are usually colonized by a large number of mycorrhizal species that are generally regarded as beneficial to both the host and fungus. Mycorrhizas generally improve access to limiting soil nutrient and water resources, and offer protective benefits against root pathogens, in exchange for the tree's carbon. However, the association can become less beneficial and even parasitic when tree development or environmental conditions results in carbon costs becoming greater than benefits, or when fungal genotypes do not form beneficial associations with the host genotype. Loblolly pine (Pinus taeda L.), the most widely-planted tree species in the Atlantic Coastal Plain, has co-evolved a high dependency on ectomycorrhizal associations because its natural range includes dry soils that are P- and/or N-deficient. However, rates of colonization by individual fungal species are dynamic, mediated by a hierarchy of biotic and abiotic factors at the rhizosphere, community and ecosystem levels, with some fungal species more beneficial to the tree than others. Since roots of an individual tree can be colonized by 20-35 different ectomycorrhizal species, it is the tree genotype + fungal community interaction with its soil environment that determines the potential functioning of mycorrhizal roots along this mutualism-parasitism continuum. Elucidating the roles that mycorrhizal biodiversity plays in tree physiology and growth, and in the carbon/nutrient cycles of forested ecosystems are essential for predicting soil carbon sequestration under various global climate change scenarios. Our experiments are designed to test the hypothesis that there is a wide variation in how roots respond to their environment because tree origin (i.e. ecotype) and growth traits influence fungal diversity and mycorrhizal community structure. We are using molecular identification techniques to characterize mycorrhizal species diversity and community structure in ten families of loblolly pine, and to determine whether specific mycorrhizal communities are associated with faster tree growth. The proposed research represents a mid-career training opportunity for the PI in an exciting new methodology and area of root research, and will also train a postdoctoral associate. In addition, Dr. Topa's lab will participate in a local outreach program (New Visions: Explorations in the Biological Sciences) that is designed for college-bound high school seniors who have a strong interest in research and experimentation in the biological sciences.
