AUTHOR=Lu Junkun , Yang Fucheng , Wang Shengkun , Ma Haibin , Liang Junfeng , Chen Yinglong 

TITLE=Co-existence of Rhizobia and Diverse Non-rhizobial Bacteria in the Rhizosphere and Nodules of Dalbergia odorifera Seedlings Inoculated with Bradyrhizobium elkanii, Rhizobium multihospitium–Like and Burkholderia pyrrocinia–Like Strains

JOURNAL=Frontiers in Microbiology

VOLUME=8

YEAR=2017

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2017.02255

DOI=10.3389/fmicb.2017.02255

ISSN=1664-302X

ABSTRACT=<p>Rhizobia induce root nodules and fix atmospheric N<sub>2</sub> for most legume species in exchange for carbon. However, the diverse endophytic non-rhizobial bacteria in legume nodules that co-exist with rhizobia are often ignored because they are difficult to cultivate using routine cultivation approaches. To enhance our understanding of the incidence and diversity of legume–bacteria associations, a high-throughput sequencing analysis of bacterial 16S rRNA genes was used to examine the bacterial community in the rhizospheres and root nodules of <italic>Dalbergia odorifera</italic> seedlings that were uninoculated or inoculated with <italic>Bradyrhizobium elkanii</italic> H255, <italic>Rhizobium multihospitium–</italic>like HT221, or <italic>Burkholderia pyrrocinia–</italic>like H022238, in two growth media (nitrogen [N]-supplied soil or N-omitted potting mix). Seedlings inoculated with <italic>Bradyrhizobium</italic> had significantly more nodules than seedlings in the other inoculation conditions, regardless of growth media. Using the <sup>15</sup>N natural abundance method, it was shown that the inoculated plants had significantly higher N<sub>2</sub> fixation efficiency (48–57%) and specific nodule activity [269–313 μg N mg<sup>−1</sup> of dry weight (dwt) nodule] compared to the uninoculated plants (203 μg N mg<sup>−1</sup> dwt nodule). The 16S rRNA gene analysis showed that there was generally a higher bacterial diversity in the rhizosphere than in the nodules in the corresponding condition. Both rhizobial inoculation and media status significantly altered the bacterial communities in the rhizospheres and nodules (<italic>P</italic> &lt; 0.05), with the exception of the inoculated soil rhizospheres. Regarding non-rhizobial bacteria, three genera, i.e., <italic>Lactococcus, Bacillus</italic>, and <italic>Pseudomonas</italic>, were consistently enriched in the rhizosphere and <italic>Bradyrhizobium, Chloroplast</italic> norank (which belongs to <italic>Cyanobacteria</italic>), and <italic>Lactococcus</italic> were commonly found in the nodules. In contrast, common rhizobial genera (including <italic>Rhizobium, Mesorhizobium</italic>, and <italic>Burkholderia</italic>) were only present in the nodules at low relative abundances (0.01–3.41%). Regarding non-rhizobial bacteria, 32 genera were found in the nodules, with non-rhizobial bacteria being predominant in the N omitted potting mix (with a relative abundance of 56–87%). This study suggests that legume nodules are inhabited by a high diversity of non-rhizobial bacteria, which may play a vital role in nodulation and N<sub>2</sub> fixation in the host plants.</p>