Abstract:
The isolation of rhizobial strains from the root and stem nodules remains a commonly used method
despite its limitations as it enables the identification of mainly dominant symbiotic groups within rhizobial communities. To overcome these limitations, we used genus-specific nifD primers in a cultureindependent assessment of Bradyrhizobium communities inhabiting soils in southern Brazil. The majority
of nifD sequences were generated from DNA isolated from tropical-lowland pasture soils, although some
soil samples originated from the Campos de Cima da Serra volcanic plateau. In the nifD tree, all the
bradyrhizobial sequences comprised 38 clades, including 18 new clades. The sequences generated in this
study were resolved into 22 clades and 21 singletons. The nifD bradyrhizobial assemblage contained
Azorhizobium and a-proteobacterial methylotrophic genera, suggesting that these genera may have
acquired their nif loci from Bradyrhizobium donors. The most common in the lowland pasture soils subclade III.3D branch comprises the isolates of mainly an American origin. On the other hand, subclade III.4,
which was earlier detected in Brazil among Bradyrhizobium isolates nodulating native lupins, appears
more common in the Campos de Cima da Serra soils. The second-largest group, Clade XXXVIII, has not
yet been reported in culture-dependent studies, while another common group called Clade I represents
a symbiovar predominating in Australia. The identification of the diverse nifD Clade I haplotypes in the
tropical-lowland pastures infested by Australian Acacia spp implies that the introduction of these
legumes to southern Brazil has resulted in the dissemination of their bradyrhizobial symbionts.
