Diverse bacterial taxa inhabit root nodules of lucerne (Medicago sativa L.) in New Zealand pastoral soils

Pseudomonas spp. have previously been isolated from lucerne nodules. The aims of this study were to: 1) investigate the microbiome within a lucerne nodule; and 2) assess the ability of two Pseudomonas spp. isolated from lucerne nodules to form nodules. The microbial community within 27 lucerne nodules, collected from plants inoculated with Sinorhizobium meliloti as a seed coat or peat slurry and an uninoculated control, was identified using 16S rRNA based Illumina sequencing. Lucerne seedlings were inoculated with the two Pseudomonas spp. strains. The plants were grown in sterile conditions for 6 weeks and nodulation was assessed. 16S rRNA, nodC, nodA and nifH genes were amplified. Sinorhizobium was the dominant genus in nodules, comprising 90-99% of all sequences regardless of inoculation treatment. Overall, 9 other genera were identified, with each represented by < 3% of the total sequences. Both Pseudomonas strains were able to form nodules with lucerne. From one of these strains, a nodC gene was detected. Lucerne nodules contained a diverse assemblage of bacterial species, some of which were capable of forming nodules in the absence of rhizobia.