PCR use of highly conserved DNA regions for identification of Sinorhizobium meliloti

Exopolysaccharide production by nitrogen-fixing bacteria within nodules of Medicago plants exposed to chronic radiation in the Chernobyl exclusion zone

Nitrogen-fixing bacteria isolated from root nodules of Medicago plants growing in the 10 km zone around the Chernobyl nuclear power plant were screened for the production of new water-soluble acidic exopolysaccharides (EPSs). The different strains belonged to the Enteriobacteriaceae family (Enterobacter ludwigii, Raoultella terrigena, Klebsiella oxytoca), except for one which belonged to the Rhizobiaceae family (Sinorhizobium meliloti). All of the bacteria produced highly viscous EPS with an average molecular weight comprised between 1 x 10(6) and 3 x 10(6) Da. Five different compositions of EPS were characterized by physico-chemical analyses and (1)H NMR spectroscopy: galactose/mannose (2/1), galactose/glucose (1/1), galactose/glucose/mannose (1/2/1), fucose/galactose/glucose (2/1/1) and fucose/galactose/glucose/mannose (2/2/1/1 or 1/1/2/4). Glucuronic acid, a charged monosaccharide, was also recovered in most of the different EPSs.

Diversity of Sinorhizobium strains nodulating Medicago sativa from different Iranian regions

Alfalfa is believed to have originated in north-western Iran and has a long history of coexistence with its bacterial symbiont Sinorhizobium in soils of Iran. However, little is known about the diversity of Sinorhizobium strains nodulating Iranian alfalfa genotypes. In this study, Sinorhizobium populations were sampled from eight different Iranian sites using three cultivars of Medicago sativa as trap host plants. A total of 982 rhizobial strains were isolated and species were identified showing a large prevalence of Sinorhizobium meliloti over Sinorhizobium medicae. Analysis of salt tolerance demonstrated a great phenotypic diversity. The genetic diversity of the Sinorhizobium isolates was analysed using BOX-PCR and enterobacterial repetitive intergenic consensus (ERIC)-PCR. Patterns ofBOX-PCR fingerprinting were statistically analysed with AMOVA to evaluate the role of plant variety and site of origin in the genetic variance observed. Results indicated that most of the total molecular variance was attributable to divergence among strains isolated from different sites and cultivars (intrapopulation, strain-by-strain variance). Moreover, the analysis showed the presence of two geographic populations (west and northwest), indicating that the effect of the site of origin could be more relevant in shaping population genetic diversity than the effect of cultivar or individual plant.

New taxonomic markers for identification of Rhizobium leguminosarum and discrimination between closely related species

Rhizobia, producing species-specific exopolysaccharides (EPSs), comprise a very diverse group of soil bacteria that are able to establish nitrogen-fixing symbioses with legumes. Based on the sequences of R. leguminosarum EPS synthesis genes, a sensitive and reliable PCR-based method for identification and subsequent discrimination between Rhizobium species has been developed and tested. For identification of R. leguminosarum, primer sets I-III complementary to sequences of rosR, pssA and pssY genes were proposed. Further sets of primers (IV-VII) were designed for discrimination between R. leguminosarum biovars. The usefulness of the method was examined using a wide range of R. leguminosarum strains isolated from different host plants nodules originating from different regions of Poland. We demonstrate a high discriminating power of primer sets I-III that allow distinguishing R. leguminosarum and two closely related species, R. etli and R. gallicum. This new approach is applicable to identification of R. leguminosarum strains, originating from nodules or soil, where many other closely related bacteria are expected to be present. Based on the nucleotide sequence of rosR and pssA genes, phylogenetic relationships of selected R. leguminosarum isolates were determined. Our results indicate that both rosR and pssA might be useful markers to differentiate and define relationships within a group of R. leguminosarum strains.

Identification of fast and slow growing rhizobia nodulating soybean (Glycine max [L.] Merr) by a multiplex PCR reaction

Two DNA fragments, a 730-bp and a 900-bp fragment, one homologous to host cultivar specificity genes nolBT of Sinorhizobium fredii and the other one homologous to RSalpha, an insertion-like sequence present in Bradyrhizobium japonicum, were generated by polymerase chain reaction (PCR) with two pairs of primers. The amount of each fragment generated by the multiplex PCR was proportional to the amount of template DNA present. The amplification of the 900-bp RSalpha fragment was more sensitive, since it was amplified from a smaller amount of template DNA than the 730-bp nolBT fragment. By running the multiplex reaction in the presence of template DNA isolated from different sources, we confirmed that the reaction can discriminate between S. fredii, Bradyrhizobium japonicum and Sinorhizobium xinjiangensis.