Polyphasic evidence supporting the reclassification of Bradyrhizobium japonicum group Ia strains as Bradyrhizobium diazoefficiens sp. nov.

Abstract

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...

Genetic Divergence of Bradyrhizobium Strains Nodulating Soybeans as Revealed by Multilocus Sequence Analysis of Genes Inside and Outside the Symbiosis Island

“…Recently, Bradyrhizobium huanghuaihaiense, Bradyrhizobium daqingense, Sinorhizobium sojae, and several unnamed species were also found to be effective microsymbionts of soybeans (2,3,(6)(7)(8). Strain USDA110 represents a widely distributed type formerly known as B. japonicum Ia, but it has recently been proposed as a member of the new species Bradyrhizobium diazoefficiens (9).…”

Bradyrhizobium manausense sp. nov., isolated from effective nodules of Vigna unguiculata grown in Brazilian Amazonian rainforest soils

“…The whole-cell fatty acid methyl esters of strain BR 3351 T were extracted according to the MIDI protocol (http:// www.microbialid.com/PDF/TechNote_101.pdf; Delamuta et al, 2013). Cultures were grown for 5 days at 28 u C on YMA prior to extraction.…”

Co-inoculation with diazotrophic bacteria in soybeans associated to urea topdressing

“…(CCGL)] cultivar had an indeterminate growth type and RMG equal to 5.7, and the BMX Potência RR (Brasmax) cultivar had an indeterminate growth type and RMG equal to 6.7. The conventional inoculation contained bacteria of the genus Bradyrhizobium, B. japonicum strain CPAC 15 (SEMIA 5079) and B. diazoefficiens (Delamuta et al, 2013) strain CPAC 7 (SEMIA 5080) at 7x10 9 colony-forming units (CFU) mL -1 of a commercial product at a dose of 0.002 L kg -1 seeds. The co-inoculation contained the same strains as the conventional inoculation plus Azospirillum brasilense strains Ab-V5 and Ab-V6 at 2x10 8 CFU mL…”