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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.
2011 Project Description
During the reporting period multiple pure culture plant growth experiments were performed using several combinations of tall fescue cultivar and fungal endophyte genotype in an effort to identifying the influence of tall fescue and endophyte identity on the chemical composition of tall fescue root exudates (Obj 1).
In addition to laboratory work conducted in our lab at the University of Kentucky, students, a lab analyst and PI McNear had the opportunity of working at the University of Louisville Center for Regulatory and Environmental Metabolomics (CREAM) and the Mass Spectrometry Research Center at Vanderbilt University. At these facilities they worked with and learn from experts in mass spectrometry for the analysis and identification of components in complex environmental and biological samples.
Soil sampling was performed in June of 2010 along with my PhD student, Co-PI Rebecca McCulley and a student and post doc from her lab at a site maintained by Dr. Keith Clay at Indiana University. Bulk and rhizosphere soils were collected from field plots where endophyte free, common toxic endophyte infected and novel endophyte infected tall fescue varieties were planted over 10 years ago. Soils collected from the endophyte-free tall fescue field plots in June will be used in rhizobox studies to evaluate 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), a poster presentations at the USDA NRI project director meeting in Ashville, N.C. (2011) in a presentation at the 7th International Symposium on Fungal Endophytes of Grasses and at the 73rd annual international soil science society of America meetings.
One of the major impacts from this work is the discovery that tall fescue cultivar together with endophyte genotype influence the total quantity of carbon and nitrogen and the overall chemical composition of root exudates which differentially influence key soil microbe activity.
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 tall fescue resulted in root exudates that were more inhibitory to the growth of Sinorhizobium meliloti, (a key soil microorganism and symbiont to many legumes co-established with tall fescue in pasture systems) than those exudates coming from endophyte free or novel endophyte infected 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. A presentation and proceedings paper based in part on these findings was recently published.
These findings could have an impact on ecosystem level processes and possibly explain the differences in total soil carbon and nitrogen that are observed under pastures planted with endophyte-free and endophyte infected grasses.
Validation of these findings could lead to selective breeding of grass cultivar-fungal endophyte combinations that promote carbon sequestration or favorably influence nutrient cycling in soils.
In addition, the qRT-PCR based method we are developing to detect endophyte function (not just presence) will help define the timing of endophyte influence on the plant, validate the current methods used to detect endophyte presence (e.g. immunoblot, PCR) and provide a new strategy for investigating the relationship between tall fescue and its fungal symbiont.
McNear, Jr., D.H. and R.L. McCulley. 2011. Influence of the Neotyphodium and tall fescue symbiosis on below ground processes. Proceedings of the 7th International Symposium on Fungal Endophytes of Grasses, Noble Foundation, Ardmore, OK.
Guo, Jingqi and David McNear. 2011. Influence of Tall Fescue Cultivar and Endophyte Genotype on Rhizosphere Processes. 73rd International Annual Meeting of the Soil Science Society of America, San Antonio, TX.