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Response Of Soil Processes to an Above Ground Plant-Fungal Symbiosis: From Rhizosphere to Regional Scales
D.H. McNear, R.L. McCulley, N. Fierer
Department of Plant and Soil Sciences
Tall fescue (TF) covers greater than 15 million hectares within the United States and is considered one of the most desirable forage species for grazing livestock. Because of animal toxicity issues associated with common toxic endophyte-TF varieties, novel endophyte infected TF varieties are gaining popularity. Only anecdotal evidence exists which specifically addresses the role of the endophyte in dictating root exudate composition or the role that these exudates may play in altering soil microbial composition or nutrient cycling.
The research proposed herein will provide important and novel data on how the soil chemical environment, soil microbial communities, and soil C and N dynamics are altered by shoot-specific common and novel fungal endophyte symbioses with tall fescue. The proposed study will be unique from other studies on this topic in that it will integrate advanced analytical and molecular genetics techniques with classical soil chemical methods to identify root exudates specific to the fungal-TF symbiosis and elucidate their effects on rhizosphere and bulk soil microbial composition, and carbon and nitrogen pools in field soils (specifically addressing numbers 1 and 2 of 2008 fiscal year Priorities for Research Projects).
These goals will be accomplished by combining methodical pure culture, bioactivity guided assays of root exudates from endophyte infected and endophyte free tall fescue varieties with lab and field-based observations to verify the validity of our results. Results from this research will generate basic knowledge on how an agriculturally-important, widely occurring plant-fungal symbiosis impacts soil microbial communities, chemical properties and nutrient cycling.
2009 Project Description
During the reporting period pure culture experiments were initiated to begin identifying the chemical composition of Tall fescue (Tf) root exudates as influenced by endophyte infection (Obj 1) as well as rhizobox design, construction and pilot testing in preparation for evaluating how soil physicochemical properties alter exudate persistence and their influence on microbial community structure (Obj 2).
Results from these initial experiments were presented in several University seminars (Plant Physiology, Plant Pathology), incorporated into several invited course lectures (e.g. Soil - Plant relations), and an oral presentation at the USDA NRI project director meeting in East Lansing, MI. Research highlights have appeared in several press releases by University and local press outlets.
In laboratory experiments we have obtained evidence that there are differences in the total quantity of carbon and nitrogen and in the chemical composition of root exudates that may be dependent upon the Tf-endophyte combination. Having made this discovery while examining the initially proposed Tf cultivar-endophyte combinations we moved to expand the type of Tf cultivar-endophyte combinations in our experiments with assistance from the Tf breeder at UK and an individual at the Noble Foundation.
Experiments are currently under way to test the validity of our hypothesis. Microbial bioassays using the whole root exudates from some of these plant-fungus varieties have been performed and reveal that the presence of the common toxic endophyte in Tf resulted in root exudates that were inhibitorier to the growth of Sinorhizobium meliloti, a key soil microorganism and symbiont to many legumes, than those exudates coming from endophyte free varieties. These findings have important implications for a variety of areas including soil nutrient cycling, microbial function and community composition and carbon sequestration, to name a few.