Year of Publication

2016

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Agriculture, Food and Environment

Department

Plant and Soil Sciences

First Advisor

Dr. Rebecca L. McCulley

Abstract

Plants interact in myriad ways with microorganisms to influence ecosystem processes such as nutrient cycling, which can regulate ecosystem response to global change. One important plant-microbe symbiosis occurs between cool-season grasses and asexual fungal Epichloë endophytes, such as tall fescue (Schedonorus arundinaceus) and Epichloë coenophiala. Because the common toxic strain of the endophyte (CTE) harms grazing livestock, non-livestock toxic endophyte (NTE) strains have been developed and are increasingly deployed in pastures. Little is known about how these symbioses impact other plant-microbe interactions and microbe-mediated soil processes in grassland ecosystems. I conducted three studies to determine how E. coenophiala presence (+) or absence (−) and differences in endophyte strain affected plant-microbe-soil interactions both within tall fescue and in surrounding plants. I hypothesized that presence of CTE in tall fescue (CTE+) would suppress presence and/or activity of other microbial symbionts and related processes compared to E− tall fescue, and NTE+ tall fescue effects would be intermediate.

My first field study examined how endophyte presence and strain in tall fescue influenced symbiotic biological nitrogen fixation (BNF) in red clover, biologically-fixed N uptake in tall fescue, and non-symbiotic BNF in soils. I found that tall fescue hosting different NTE+ strains utilized different amounts of biologically-fixed N. My second field study investigated how endophyte presence and strain impacted belowground mycorrhizal colonization within the same host plant. I found no significant differences in either AMF or dark septate endophyte (DSE) colonization in tall fescue in this study. In my third field study, I investigated how these belowground symbioses were potentially altered both by tall fescue-E. coenophiala genetics and future climate change. AMF functional structures such as arbuscules in roots and extraradical hyphae in soils were significantly affected by tall fescue genotype and endophyte status. I also found that some competitive symbiont interactions were ameliorated whereas others were exacerbated by future climate change conditions such as warming and added precipitation.

Overall, the results of these studies suggest that genetically distinct E. coenophiala-tall fescue associations, through alteration of plant-microbe-soil interactions, will have divergent roles and long-term impacts on host-symbiont species interactions and nutrient cycling within pasture ecosystems.

Digital Object Identifier (DOI)

http://dx.doi.org/10.13023/ETD.2016.074