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Dynamic nitrogen fixation in an aerobic endophyte of Populus

Journal Article · · The ISME Journal
Biological nitrogen fixation by microbial diazotrophs can contribute significantly to nitrogen availability in non-nodulating plant species. In this study of molecular mechanisms and gene expression relating to biological nitrogen fixation, the aerobic nitrogen-fixing endophyte Burkholderia vietnamiensis, strain WPB, isolated from Populus trichocarpa served as a model for endophyte–poplar interactions. Nitrogen-fixing activity was observed to be dynamic on nitrogen-free medium with a subset of colonies growing to form robust, raised globular like structures. Secondary ion mass spectrometry (NanoSIMS) confirmed that N-fixation was uneven within the population. A fluorescent transcriptional reporter (GFP) revealed that the nitrogenase subunit nifH is not uniformly expressed across genetically identical colonies of WPB and that only ~11% of the population was actively expressing the nifH gene. Higher nifH gene expression was observed in clustered cells through monitoring individual bacterial cells using single-molecule fluorescence in situ hybridization. Through 15N2 enrichment, we identified key nitrogenous metabolites and proteins synthesized by WPB and employed targeted metabolomics in active and inactive populations. We cocultivated WPB Pnif-GFP with poplar within a RhizoChip, a synthetic soil habitat, which enabled direct imaging of microbial nifH expression within root epidermal cells. We observed that nifH expression is localized to the root elongation zone where the strain forms a unique physical interaction with the root cells. This work employed comprehensive experimentation to identify novel mechanisms regulating both biological nitrogen fixation and beneficial plant–endophyte interactions.
Research Organization:
Pacific Northwest National Laboratory (PNNL), Richland, WA (United States); USDOE Joint Genome Institute (JGI), Berkeley, CA (United States); Univ. of Washington, Seattle, WA (United States); University of Washington, Seattle, WA (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities (SUF); USDOE Office of Science (SC), Biological and Environmental Research (BER)
Grant/Contract Number:
AC02-05CH11231; AC05-76RL01830; SC0021137
OSTI ID:
2283588
Alternate ID(s):
OSTI ID: 2336553
OSTI ID: 2507461
OSTI ID: 2337842
Report Number(s):
PNNL-SA--189611
Journal Information:
The ISME Journal, Journal Name: The ISME Journal Journal Issue: 1 Vol. 18; ISSN 1751-7362
Publisher:
Nature Publishing GroupCopyright Statement
Country of Publication:
United States
Language:
English

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