Bradyrhizobium frederickii sp. nov., a nitrogen-fixing lineage isolated from nodules of the caesalpinioid species Chamaecrista fasciculata and characterized by tolerance to high temperature in vitro

Bradyrhizobium xenonodulans sp. nov. isolated from nodules of Australian Acacia species invasive to South Africa

A genealogical concordance approach was used to delineate strains isolated from Acacia dealbata and Acacia mearnsii root nodules in South Africa. These isolates form part of Bradyrhizobium based on 16S rRNA sequence similarity. Phylogenetic analysis of six housekeeping genes (atpD, dnaK, glnII, gyrB, recA and rpoB) confirmed that these isolates represent a novel species, while pairwise average nucleotide identity (ANIb) calculations with the closest type strains (B. cosmicum 58S1T, B. betae PL7HG1T, B. ganzhouense CCBAU 51670 T, B. cytisi CTAW11T and B. rifense CTAW71T) resulted in values well below 95-96%. We further performed phenotypic tests which revealed that there are high levels of intraspecies variation, while an additional analysis of the nodA and nifD loci indicated that the symbiotic loci of the strains are closely related to those of Bradyrhizobium isolates with an Australian origin. Strain 14ABT (=LMG 31415 T = SARCC-753 T) is designated as the type strain of the novel species for which we propose the name Bradyrhizobium xenonodulans sp. nov.

Bradyrhizobium cenepequi sp. nov., Bradyrhizobium semiaridum sp. nov., Bradyrhizobium hereditatis sp. nov. and Bradyrhizobium australafricanum sp. nov., symbionts of different leguminous plants of Western Australia and South Africa and definition of three novel symbiovars

Bradyrhizobium is a heterogeneous bacterial genus capable of establishing symbiotic associations with a broad range of legume hosts, including species of economic and environmental importance. This study was focused on the taxonomic and symbiovar definition of four strains – CNPSo 4026T, WSM 1704T, WSM 1738T and WSM 4400T – previously isolated from nodules of legumes in Western Australia and South Africa. The 16S rRNA gene phylogenetic tree allocated the strains to the Bradyrhizobium elkanii supergroup. The multilocus sequence analysis (MLSA) with partial sequences of six housekeeping genes – atpD, dnaK, glnII, gyrB, recA and rpoB – did not cluster the strains under study as conspecific to any described Bradyrhizobium species. Average nucleotide identity and digital DNA–DNA hybridization values were calculated for the four strains of this study and the closest species according to the MLSA phylogeny with the highest values being 95.46 and 62.20 %, respectively; therefore, both being lower than the species delineation cut-off values. The nodC and nifH phylogenies included strains WSM 1738T and WSM 4400T in the symbiovars retamae and vignae respectively, and also allowed the definition of three new symbiovars, sv. cenepequi, sv. glycinis, and sv. cajani. Analysis of morphophysiological characterization reinforced the identification of four novel proposed Bradyrhizobium species that are accordingly named as follows: Bradyrhizobium cenepequi sp. nov. (CNPSo 4026T=WSM 4798T=LMG 31653T), isolated from Vigna unguiculata; Bradyrhizobium semiaridum sp. nov. (WSM 1704T=CNPSo 4028T=LMG 31654T), isolated from Tephrosia gardneri; Bradyrhizobium hereditatis sp. nov. (WSM 1738T=CNPSo 4025T=LMG 31652T), isolated from Indigofera sp.; and Bradyrhizobium australafricanum sp. nov. (WSM 4400T=CNPSo 4015T=LMG 31648T) isolated from Glycine sp.

New Insights into the Taxonomy of Bacteria in the Genomic Era and a Case Study with Rhizobia

Since early studies, the history of prokaryotes taxonomy has dealt with many changes driven by the development of new and more robust technologies. As a result, the number of new taxa descriptions is exponentially increasing, while an increasing number of others has been subject of reclassification, demanding from the taxonomists more effort to maintain an organized hierarchical system. However, expectations are that the taxonomy of prokaryotes will acquire a more stable status with the genomic era. Other analyses may continue to be necessary to determine microbial features, but the use of genomic data might be sufficient to provide reliable taxa delineation, helping taxonomy to reach the goal of correct classification and identification. Here we describe the evolution of prokaryotes' taxonomy until the genomic era, emphasizing bacteria and taking as an example the history of rhizobia taxonomy. This example was chosen because of the importance of the symbiotic nitrogen fixation of legumes with rhizobia to the nitrogen input to both natural ecosystems and agricultural crops. This case study reports the technological advances and the methodologies used to classify and identify bacterial species and indicates the actual rules required for an accurate description of new taxa.

Bradyrhizobium altum sp. nov., Bradyrhizobium oropedii sp. nov. and Bradyrhizobium acaciae sp. nov. from South Africa show locally restricted and pantropical nodA phylogeographic patterns

Africa is known for its rich legume diversity with a significant number of endemic species originating in South Africa. Many of these legumes associate with rhizobial symbionts of the genus Bradyrhizobium, of which most represent new species. Yet, none of the Bradyrhizobium species from South Africa have been described. In this study, phylogenetic analysis of 16S rRNA gene sequences of fourteen strains isolated in southern Africa from root nodules of diverse legumes (i.e., from the tribes Crotalarieae, Acacieae, Genisteae, Phaseoleae and Cassieae) revealed that they belong to the Bradyrhizobium elkanii supergroup. The taxonomic position and possible novelty of these strains were further interrogated using genealogical concordance of five housekeeping genes (atpD, dnaK, glnII, gyrB and rpoB). These phylogenies consistently recovered four monophyletic groups and one singleton within Bradyrhizobium. Of these groups, two were conspecific with Bradyrhizobium brasilense UFLA 03-321^T and Bradyrhizobium ivorense CI-1B^T, while the remaining three represented novel taxa. Their existence was further supported with genome data, as well as metabolic and physiological traits. Analysis of nodA gene sequences further showed that the evolution of these bacteria likely involved adapting to local legume hosts and environmental conditions through the acquisition, via horizontal gene transfer, of optimal symbiotic loci. We accordingly propose the following names Bradyrhizobium acaciae sp. nov. 10BB^T (SARCC 730^T = LMG 31409^T), Bradyrhizobium oropedii sp. nov. Pear76^T (SARCC 731^T = LMG 31408^T), and Bradyrhizobium altum sp. nov. Pear77^T (SARCC 754^T = LMG 31407^T) to accommodate three novel species, all of which are symbionts of legumes in South Africa.

Bradyrhizobium sp. sv. retamae nodulates Retama monosperma grown in a lead and zinc mine tailings in Eastern Morocco

The aim of this work was to characterize and identify some bacteria isolated from the root nodules of Retama monosperma grown in Sidi Boubker lead and zinc mine tailings. Very few root nodules were obtained on the root nodules of R. monosperma grown in these soils. The three bacteria isolated from the root nodules were tolerant in vitro to different concentrations of heavy metals, including lead and zinc. The rep-PCR experiments showed that the three isolates have different molecular fingerprints and were considered as three different strains. The analysis of their 16S rRNA gene sequences proved their affiliation to the genus Bradyrhizobium. The analysis and phylogeny of the housekeeping genes atpD, glnII, gyrB, recA, and rpoB confirmed that the closest species was B. valentinum with similarity percentages of 95.61 to 95.82%. The three isolates recovered from the root nodules were slow-growing rhizobia capable to renodulate their original host plant in the presence of Pb-acetate. They were able to nodulate R. sphaerocarpa and Lupinus luteus also but not Glycine max or Phaseolus vulgaris. The phylogeny of the nodA and nodC nodulation genes as well as the nifH gene of the three strains showed that they belong to the symbiovar retamae of the genus Bradyrhizobium. The three strains isolated could be considered for use as inoculum for Retama plants before use in phytoremediation experiments.

Bradyrhizobium hipponense sp. nov., isolated from Lupinus angustifolius growing in the northern region of Tunisia

Strain aSej3^T was isolated from a root nodule of a Lupinus angustifolius plant growing in Bizerte, Tunisia. 16S rRNA gene analysis placed this strain within the genus Bradyrhizobium. Multilocus sequence analysis (MLSA) including three housekeeping genes (glnII, gyrB and recA) grouped aSej3^T together with Bradyrhizobium rifense CTAW71^T, Bradyrhizobium cytisi CTAW11^T, Bradyrhizobium ganzhouense RITF806^T, Bradyrhizobium lupini USDA 3051^T and Bradyrhizobium canariense BTA-1^T. MLSA with five housekeeping genes (dnaK, glnII, gyrB, recA and rpoB) revealed that this strain shares less than 93.5 % nucleotide identity with other type strains. Genome sequencing and inspection revealed a genome size of 8.83 Mbp with a G+C content of 62.8 mol%. Genome-wide average nucleotide identity and digital DNA-DNA hybridization values were below 87.5 and 36.2 %, respectively, when compared to described Bradyrhizobium species. Strain aSej3^T nodulated L. angustifolius plants under axenic conditions and its nodC gene clustered within the genistearum symbiovar. Altogether, the phylogenetic data and the chemotaxonomic characteristics of this strain support that aSej3^T represents a new species for which we propose the name Bradyrhizobium hipponense sp. nov. with the type strain aSej3^T (=DSM 108913^T=LMG 31020^T).

International Committee on Systematics of Prokaryotes Subcommittee on the Taxonomy of Rhizobia and Agrobacteria Minutes of the closed meeting by videoconference, 17 July 2019

Minutes of the closed meeting of the ICSP Subcommittee on the Taxonomy of Rhizobia and Agrobacteria held by videoconference on 17 July 2019, and list of recent species.

Molecular Basis of Root Nodule Symbiosis between Bradyrhizobium and 'Crack-Entry' Legume Groundnut (Arachis hypogaea L.)

Nitrogen is one of the essential plant nutrients and a major factor limiting crop productivity. To meet the requirements of sustainable agriculture, there is a need to maximize biological nitrogen fixation in different crop species. Legumes are able to establish root nodule symbiosis (RNS) with nitrogen-fixing soil bacteria which are collectively called rhizobia. This mutualistic association is highly specific, and each rhizobia species/strain interacts with only a specific group of legumes, and vice versa. Nodulation involves multiple phases of interactions ranging from initial bacterial attachment and infection establishment to late nodule development, characterized by a complex molecular signalling between plants and rhizobia. Characteristically, legumes like groundnut display a bacterial invasion strategy popularly known as "crack-entry'' mechanism, which is reported approximately in 25% of all legumes. This article accommodates critical discussions on the bacterial infection mode, dynamics of nodulation, components of symbiotic signalling pathway, and also the effects of abiotic stresses and phytohormone homeostasis related to the root nodule symbiosis of groundnut and Bradyrhizobium. These parameters can help to understand how groundnut RNS is programmed to recognize and establish symbiotic relationships with rhizobia, adjusting gene expression in response to various regulations. This review further attempts to emphasize the current understanding of advancements regarding RNS research in the groundnut and speculates on prospective improvement possibilities in addition to ways for expanding it to other crops towards achieving sustainable agriculture and overcoming environmental challenges.

Bradyrhizobium agreste sp. nov., Bradyrhizobium glycinis sp. nov. and Bradyrhizobium diversitatis sp. nov., isolated from a biodiversity hotspot of the genus Glycine in Western Australia

Strains of the genus Bradyrhizobium associated with agronomically important crops such as soybean (Glycine max) are increasingly studied; however, information about symbionts of wild Glycine species is scarce. Australia is a genetic centre of wild Glycine species and we performed a polyphasic analysis of three Bradyrhizobium strains-CNPSo 4010T, CNPSo 4016T, and CNPSo 4019T-trapped from Western Australian soils with Glycine clandestina, Glycine tabacina and Glycine max, respectively. The phylogenetic tree of the 16S rRNA gene clustered all strains into the Bradyrhizobium japonicum superclade; strains CNPSo 4010T and CNPSo 4016T had Bradyrhizobium yuanmingense CCBAU 10071T as the closest species, whereas strain CNPSo 4019T was closer to Bradyrhizobium liaoningense LMG 18230T. The multilocus sequence analysis (MLSA) with five housekeeping genes-dnaK, glnII, gyrB, recA and rpoB-confirmed the same clusters as the 16S rRNA phylogeny, but indicated low similarity to described species, with nucleotide identities ranging from 93.6 to 97.6% of similarity. Considering the genomes of the three strains, the average nucleotide identity and digital DNA-DNA hybridization values were lower than 94.97 and 59.80 %, respectively, with the closest species. In the nodC phylogeny, strains CNPSo 4010T and CNPSo 4019T grouped with Bradyrhizobium zhanjiangense and Bradyrhizobium ganzhouense, respectively, while strain CNPSo 4016T was positioned separately from the all symbiotic Bradyrhizobium species. Other genomic (BOX-PCR), phenotypic and symbiotic properties were evaluated and corroborated with the description of three new lineages of Bradyrhizobium. We propose the names of Bradyrhizobium agreste sp. nov. for CNPSo 4010T (=WSM 4802T=LMG 31645T) isolated from Glycine clandestina, Bradyrhizobium glycinis sp. nov. for CNPSo 4016T (=WSM 4801T=LMG 31649T) isolated from Glycine tabacina and Bradyrhizobium diversitatis sp. nov. for CNPSo 4019T (=WSM 4799T=LMG 31650T) isolated from G. max.