B096-0002
Free-living Nitrogen Fixation in the Switchgrass Rhizosphere
Tuesday, 15 December 2020
Poster
Darian Smercina1, Lisa K Tiemann2, Alan Bowsher3, Maren Friesen4,5, Elizabeth K Eder6, David W Hoyt6, William Chrisler7, John B Cliff6, Kirsten S Hofmockel8 and Sarah Evans9,10, (1)Michigan State University, Plant, Soil, and Microbial Sciences, East Lansing, MI, United States, (2)Michigan State University, Plant, Soil and Microbial Sciences, East Lansing, MI, United States, (3)Michigan State University, Microbiology and Molecular Genetics, East Lansing, MI, United States, (4)Washington State University, Plant Pathology, Pullman, WA, United States, (5)Washington State University, Crop and Soil Sciences, Pullman, WA, United States, (6)Pacific Northwest National Laboratory, Environmental Molecular Sciences Laboratory, Richland, WA, United States, (7)Pacific Northwest National Laboratory, Richland, WA, United States, (8)Pacific Northwest National Laboratory, Earth and Biological Sciences, Richland, WA, United States, (9)Michigan State University, Kellogg Biological Station, Hickory Corners, MI, United States, (10)Michigan State University, Integrative Biology, East Lansing, MI, United States
Abstract:
Free-living nitrogen fixation (FLNF) is a ubiquitous terrestrial process occurring under diverse environmental conditions, that are distinct from symbiotic N-fixation. FLNF occurs readily in the rhizosphere, where roots exude bioavailable carbon (C), and may represent an important N source for plants. Switchgrass, a promising bioenergy crop, harbors a diverse rhizosphere community of diazotrophs (N-fixers) which may help meet plant N demands. Increasingly, diazotrophs are found fixing N in the switchgrass rhizosphere, yet it is unknown if or how fixed N is exchanged between diazotrophs and switchgrass and if this is coupled with root C exudation. Our work aims to improve understanding of FLNF and its potential to contribute plant available N through systematic characterization of the switchgrass-diazotroph association.
We measured potential FLNF associated with field-grown switchgrass over two growing seasons under conditions optimized to mimic the rhizosphere. In the greenhouse, we evaluated the impact of long-term and short-term N fertilization on FLNF and the switchgrass-associated diazotroph community (nifH) using switchgrass grown in field soils. We also examined the impact of N availability on the rhizosphere metabolome, using hydroponically-grown switchgrass. Lastly, we developed a novel, sterile-growth system to assess switchgrass-diazotroph interactions and tested the “C-for-N exchange” hypothesis by pairing fluorescent in situ hybridization (FISH), NanoSIMS, and characterization of the rhizosphere metabolome.
We consistently detected FLNF in association with switchgrass and found FLNF to be driven by availability of oxygen and diverse C sources. We found a consistent rhizosphere diazotroph community, independent of long-term or short-term N fertilization, but found the rhizosphere metabolome was driven strongly by N availability. Development of diazotroph-specific FISH probes allowed us to visualize establishment of diazotroph populations on switchgrass roots in a controlled system and NanoSIMS images of those roots labeled via exposure to 13C-CO2 revealed uptake of resulting 13C-labeled root exudate by diazotrophs. Paired with this imaging, we are characterizing the switchgrass rhizosphere metabolome in response to colonization by diazotrophs and a -nifH diazotroph.
