Polyphasic evidence supporting the reclassification of Bradyrhizobium japonicum group Ia strains as Bradyrhizobium diazoefficiens sp. nov. Embrapa Meio Ambiente, C.P. 69, 13820-000 Jaguariú na, Sã o Paulo, BrazilBradyrhizobium japonicum was described from soybean root-nodule bacterial isolates. Since its description, several studies have revealed heterogeneities among rhizobia assigned to this species. Strains assigned to B. japonicum group Ia have been isolated in several countries, and many of them are outstanding soybean symbionts used in inoculants worldwide, but they have also been isolated from other legume hosts. Here, we summarize published studies that indicate that group Ia strains are different from the B. japonicum type strain USDA 6 T and closely related strains, and present new morphophysiological, genotypic and genomic evidence to support their reclassification into a novel species, for which the name Bradyrhizobium diazoefficiens sp. nov. is proposed. The type strain of the novel species is the well-studied strain USDA 110 T (5IAM Biological fixation is the main source of nitrogen for natural and agricultural ecosystems. In agriculture, the symbioses of nitrogen-fixing bacteria, collectively known as rhizobia, with crops belonging to the family Leguminosae (5Fabaceae) are the most studied. Relatively high contributions to nitrogen nutrition have been demonstrated in pulses, fodders, green manures and trees (Ormeño-Orrillo et al., 2013). Members of the genus Bradyrhizobium constitute an important group of rhizobia, some of which form symbioses with economically important crops, such as soybean [Glycine max (L.) Merr.].A first systematic classification of rhizobia was proposed by Fred et al. (1932), based on the cross-inoculation concept with respect to the host legume, resulting in six species of Rhizobium: Rhizobium meliloti, R. trifolii, R. phaseoli, R. lupini, R. leguminosarum and R. japonicum, the last of which is a symbiont of soybean. Fifty years later, the taxonomy was redefined using numerical criteria, including several morphophysiological and genetic properties. A new genus was created, Bradyrhizobium, to accommodate rhizobia with the typical property of slow growth rates in culture media, with only one defined species, Bradyrhizobium japonicum (Jordan, 1982). It is noteworthy that the division of rhizobia into fast and slow growers in vitro had been proposed earlier by Löhnis & Hansen (1921).It has been suggested that Bradyrhizobium is the ancestor of the alpha-rhizobia, probably originating in the tropics (Lloret & Martínez-Romero, 2005;Norris, 1965;Provorov & Vorob'ev, 2000). However, despite its evolutionary position, significantly fewer species have been described in Bradyrhizobium in comparison with Rhizobium. One possible explanation is that the 'backbone' of modern taxonomy relies on the analysis of 16S rRNA gene sequences (Woese, 1987), which are highly conserved inAbbreviations: DDH, DNA-DNA hybridization; ITS, intergenic transcribed spacer; MLSA, multilocus sequencing analys...

No abstract

No abstract

Reclassification of Rhizobium tropici type A strains as Rhizobium leucaenae sp. nov.

Common bean (Phaseolus vulgaris L.) can establish symbiotic associations with several Rhizobium species; however, the effectiveness of most strains at fixing nitrogen under field conditions is very low. PRF 81 T is a very effective strain, usually referred to as Rhizobium tropici and used successfully in thousands of doses of commercial inoculants for the common bean crop in Brazil; it has shown high rates of nitrogen fixation in all areas representative of the crop in the country. Here, we present results that indicate that PRF 81 T , although it belongs to the 'R. tropici group', which includes 10 Rhizobium species, R. tropici, R. leucaenae, R. lusitanum, R. multihospitium, R. miluonense, R. hainanense, R. calliandrae, R. mayense, R. jaguaris and R. rhizogenes, represents a novel species. Several morpho-physiological traits differentiated PRF 81 T from related species. Differences were also confirmed in the analysis of rep-PCR (sharing less than 45 % similarity with the other species), MLSA with recA, atpD and rpoB genes, and DNA-DNA hybridization. The novel species, for which we propose the name Rhizobium freirei sp. nov., is able to establish effective root nodule symbioses with Phaseolus vulgaris, Leucaena leucocephala, Leucaena esculenta, Crotalaria juncea and Macroptilium atropurpureum. The type strain is PRF 81 T (5CNPSo 122 T 5SEMIA 4080 T 5IPR-Pv81 T 5WDCM 440 T ).

Symbiotic association of several genera of bacteria collectively called as rhizobia and plants belonging to the family Leguminosae (=Fabaceae) results in the process of biological nitrogen fixation, playing a key role in global N cycling, and also bringing relevant contributions to the agriculture. Bradyrhizobium is considered as the ancestral of all nitrogen-fixing rhizobial species, probably originated in the tropics. The genus encompasses a variety of diverse bacteria, but the diversity captured in the analysis of the 16S rRNA is often low. In this study, we analyzed twelve Bradyrhizobium strains selected from previous studies performed by our group for showing high genetic diversity in relation to the described species. In addition to the 16S rRNA, five housekeeping genes (recA, atpD, glnII, gyrB and rpoB) were analyzed in the MLSA (multilocus sequence analysis) approach. Analysis of each gene and of the concatenated housekeeping genes captured a considerably higher level of genetic diversity, with indication of putative new species. The results highlight the high genetic variability associated with Bradyrhizobium microsymbionts of a variety of legumes. In addition, the MLSA approach has proved to represent a rapid and reliable method to be employed in phylogenetic and taxonomic studies, speeding the identification of the still poorly known diversity of nitrogen-fixing rhizobia in the tropics.

No abstract

Bradyrhizobium tropiciagri sp. nov. and Bradyrhizobium embrapense sp. nov., nitrogenfixing symbionts of tropical forage legumes