Abstract
Semi-selective enrichment, followed by PCR screening, resulted in the successful direct isolation of fast-growing Rhizobia from a dryland agricultural soil. Over 50% of these isolates belong to the genus Neorhizobium, as concluded from partial rpoB and near-complete 16S rDNA sequence analysis. Further genotypic and genomic analysis of five representative isolates confirmed that they form a coherent group within Neorhizobium, closer to N. galegae than to the remaining Neorhizobium species, but clearly differentiated from the former, and constituting at least one new genomospecies within Neorhizobium. All the isolates lacked nod and nif symbiotic genes but contained a repABC replication/maintenance region, characteristic of rhizobial plasmids, within large contigs from their draft genome sequences. These repABC sequences were related, but not identical, to repABC sequences found in symbiotic plasmids from N. galegae, suggesting that the non-symbiotic isolates have the potential to harbor symbiotic plasmids. This is the first report of non-symbiotic members of Neorhizobium from soil.
Highlights
A group of α-proteobacteria from the Rhizobiales order, especially within the Rhizobiaceae and Bradyrhizobiaceae families, are collectively known as Rhizobia because they have the ability to establish root-nodule symbioses with legumes
Rhizobial strains were grown in Yeast Mannitol Broth (YMB; Vincent, 1970) at 28 ◦C, either in liquid culture or on solid media supplemented with 1.5% agar
RpoB sequences from the same fifty-three strains clustered with sequences from genus Neorhizobium (Fig. 2B)
Summary
A group of α-proteobacteria from the Rhizobiales order, especially within the Rhizobiaceae and Bradyrhizobiaceae families, are collectively known as Rhizobia because they have the ability to establish root-nodule symbioses with legumes Within these nodules, Rhizobia fix atmospheric nitrogen, and this fixed nitrogen is assimilated by the plant. This makes most legumes uniquely independent from the need of any exogenous nitrogen fertilizer, an important ecological and agricultural trait that is at the basis of any effort aimed at sustainable agriculture Since their first isolation from legume root nodules (Beijerinck, 1888), it has been known that Rhizobia are present in the soil, wherefrom they can colonize emerging roots of their legume host. This is very convenient, especially because rhizobial soil populations have been often estimated as ranging between 102–105 cells per gram of soil, depending on soil type
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