Endophytic bacterial Nitrogen fixers associated with Pinus flexilis

Background/Question/Methods

Nitrogen (N) is the nutrient most often limiting plant growth in natural ecosystems yet the source of N to northern forests is an ecological mystery.  In these ecosystems, N accumulation in soil and plants is higher than expected, given apparent N inputs. Endophytes—bacterial symbionts inside healthy plant tissue—may be hidden sources of N in forests. The aim of our work is to determine if N₂ fixing endophytes are found inside conifer needles and if they are ecologically significant. To accomplish this goal, we used 454 pyrosequencing with primers targeting bacterial 16S rRNA genes to determine which N₂ fixing bacteria were present in Pinus flexilis and if they were dominant community members (a potential indication of their ecological significance).

Results/Conclusions

The endophytic community from 6 individuals of P. flexilis sampled at two sites were all dominated by Alphaproteobacteria (50-70%), with little variation in bacterial richness and OTU abundance among and within individuals. The dominant genera were an unclassified Acetobacteraceae, Gluconacetobacter sp., and Acetobacter sp., together making up 45-60% of total bacteria in the sample. These genera have been identified as nitrogen fixers in other plants. The presence of endophytic nitrogen fixers was confirmed with PCR amplification of the nitrogenase reductase gene, nifH in each sample. This suggests that P. flexis endophytic communities are dominated by a core community of nitrogen-fixing endophytes. Thus, although commonly attributed to soil bacteria, nitrogen-fixation in forests could also be carried out by endophytes.