Genetic characterization of soybean rhizobia in Paraguay?

The soybean is an exotic plant introduced in Paraguay in this century; comm ercial cropping expanded after the 1970s. Inoculation is practiced in just 15 to 20% of the cropping areas, but root nodulation occurs in most sites w here soybeans grow. Little is known about rhizobial diversity in South Amer ica, and no study has been performed in Paraguay until this time. Therefore , in this study, the molecular characterization of 78 rhizobial isolates fr om soybean root nodules, collected under field conditions in 16 sites locat ed in the two main producing states, Alto Parana and Itapua, was undertaken . A high level of genetic diversity was detected by an ERIC-REP-PCR analysi s, with the majority of the isolates representing unique strains. Most of t he 58 isolates characterized by slow growth and alkaline reactions in a med ium containing mannitol as a carbon source were clustered with strains repr esentative of the Bradyrhizobium japonicum and Bradyrhizobium elkanii speci es, and the 16S ribosomal DNA (rDNA) sequences of 5 of those isolates confi rmed the species identities. However, slow growers were highly polymorphic in relation to the reference strains, including five carried in commercial inoculants in neighboring countries, thus indicating that the Paraguayan is olates might represent native bradyrhizobia. Twenty isolates highly polymor phic in the ERIC-REP-PCR profiles were characterized by fast growth and aci d reactions in vitro, and two of them showed high 168 rDNA identities with Rhizobium genomic species Q. However, two other fast growers showed high 16 S rDNA identity with Agrobacterium spp., and both of these strains establis hed efficient symbioses with soybean plants.