Quantification of Nitrogen Fixation By Black Cottonwood's Endophytic Microbiome

Previous work has shown that symbiotic microbes live in the interstitial space between plant cells, and are thus endophytic.  It has been shown that many of these endophytes are able to mitigate plant stressors, and even degrade toxins to help plants survive in harsh environments.  These are exciting findings but the details of the relationship between plant and microbe have still to be elucidated.  Unlike the known symbiosis of leguminous plants with Rhizobia, it has been demonstrated that endophytic microbes are less host specific and do not require any specific host structure.  However, just as the relationship between plant and rhizobia is based upon the exchange of fixed nitrogen with fixed carbon, it seems likely that the symbiosis of some endophytes would be based on a similar exchange. 

Many diazotrophic endophytes have been isolated from black cottonwood, Populus trichocarpa, that were growing in a low nitrogen riparian ecosystem.  These endophytes were identified through 16s amplification and shown to have the ability to fix nitrogen by amplification of the nifH gene.  The most direct means of quantifying this nitrogen fixation is the ¹⁵N₂ incorporation assay.  Using this assay it was shown that the diazotrophic population of the poplar microbiota was fixing nitrogen in planta.  The high level of this incorporation suggests that endophytes are capable of sustaining plant growth even in low nutrient conditions.