Unearthing Optimal Symbiotic Rhizobia Partners from the Main Production Area of Phaseolus vulgaris in Yunnan

Phaseolus vulgaris is a globally important legume cash crop, which can carry out symbiotic nitrogen fixation with rhizobia. The presence of suitable rhizobia in cultivating soils is crucial for legume cropping, especially in areas beyond the plant-host native range, where soils may lack efficient symbiotic partners. We analyzed the distribution patterns and traits of native rhizobia associated with P. vulgaris in soils of Yunnan, where the common bean experienced a recent expansion. A total of 608 rhizobial isolates were tracked from soils of fifteen sampling sites using two local varieties of P. vulgaris. The isolates were discriminated into 43 genotypes as defined by IGS PCR-RFLP. Multiple locus sequence analysis based on recA, atpD and rpoB of representative strains placed them into 11 rhizobial species of Rhizobium involving Rhizobium sophorae, Rhizobium acidisoli, Rhizobium ecuadorense, Rhizobium hidalgonense, Rhizobium vallis, Rhizobium sophoriradicis, Rhizobium croatiense, Rhizobium anhuiense, Rhizobium phaseoli, Rhizobium chutanense and Rhizobium etli, and five unknown Rhizobium species; Rhizobium genosp. I~V. R. phaseoli and R. anhuiense were the dominant species (28.0% and 28.8%) most widely distributed, followed by R. croatiense (14.8%). The other rhizobial species were less numerous or site-specific. Phylogenies of nodC and nifH markers, were divided into two specific symbiovars, sv. phaseoli regardless of the species affiliation and sv. viciae associated with R. vallis. Through symbiotic effect assessment, all the tested strains nodulated both P. vulgaris varieties, often resulting with a significant greenness index (91-98%). However, about half of them exhibited better plant biomass performance, at least on one common bean variety, and two isolates (CYAH-6 and BLYH-15) showed a better symbiotic efficiency score. Representative strains revealed diverse abiotic stress tolerance to NaCl, acidity, alkalinity, temperature, drought and glyphosate. One strain efficient on both varieties and exhibiting stress abiotic tolerance (BLYH-15) belonged to R. genosp. IV sv. phaseoli, a species first found as a legume symbiont.

Guidelines for the description of rhizobial symbiovars

Rhizobia are bacteria that form nitrogen-fixing nodules in legume plants. The sets of genes responsible for both nodulation and nitrogen fixation are carried in plasmids or genomic islands that are often mobile. Different strains within a species sometimes have different host specificities, while very similar symbiosis genes may be found in strains of different species. These specificity variants are known as symbiovars, and many of them have been given names, but there are no established guidelines for defining or naming them. Here, we discuss the requirements for guidelines to describe symbiovars, propose a set of guidelines, provide a list of all symbiovars for which descriptions have been published so far, and offer a mechanism to maintain a list in the future.

Definition of the symbiovar viciae in the species Rhizobium azibense and biogeographic implications

Vicia faba L. (faba bean) is a legume cultivated worldwide which commonly establishes effective symbiosis with the symbiovar viciae of species from the Rhizobium leguminosarum phylogenetic group. However, on the basis of the rrs, recA, and atpD gene phylogenies, in this work we identified a strain named EFBRI 42 nodulating V. faba as Rhizobium azibense. This is the first report on the nodulation of Vicia by R. azibense which commonly nodulates P. vulgaris and to date encompasses strains harboring the nodC genes typical of the symbiovars gallicum and phaseoli. However, the strain EFBRI 42 carries a nodC gene typical of the symbiovar viciae for which we report here by the first time this symbiovar in R. azibense. This finding showed the existence of symbiotic genes horizontal transfer events during the coevolution of R. azibense with P. vulgaris and V. faba in their respective distribution centers of Mesoamerica and the Middle East.

International Committee on Systematics of Prokaryotes, Subcommittee on the taxonomy of Rhizobia and Agrobacteria, minutes of the annual meeting by videoconference, 5 July 2021, followed by online discussion until 31 December 2021

Minutes of the closed meeting of the International Committee on Systematics of Prokaryotes Subcommittee on the Taxonomy of Rhizobia and Agrobacteria held by videoconference, 5 July 2021, followed by online discussion until 31 December 2021, and list of recent species.

Hidden diversity of double-stranded DNA phages in symbiotic Rhizobium species

In this study, we addressed the extent of diversification of phages associated with nitrogen-fixing symbiotic Rhizobium species. Despite the ecological and economic importance of the Rhizobium genus, little is known about the diversity of the associated phages. A thorough assessment of viral diversity requires investigating both lytic phages and prophages harboured in diverse Rhizobium genomes. Protein-sharing networks identified 56 viral clusters (VCs) among a set of 425 isolated phages and predicted prophages. The VCs formed by phages had more proteins in common and a higher degree of synteny, and they group together in clades in the associated phylogenetic tree. By contrast, the VCs of prophages showed significant genetic variation and gene loss, with selective pressure on the remaining genes. Some VCs were found in various Rhizobium species and geographical locations, suggesting that they have wide host ranges. Our results indicate that the VCs represent distinct taxonomic units, probably representing taxa equivalent to genera or even species. The finding of previously undescribed phage taxa indicates the need for further exploration of the diversity of phages associated with Rhizobium species. This article is part of the theme issue 'The secret lives of microbial mobile genetic elements'.

Genetic characterization at the species and symbiovar level of indigenous rhizobial isolates nodulating Phaseolus vulgaris in Greece

Phaseolus vulgaris (L.), commonly known as bean or common bean, is considered a promiscuous legume host since it forms nodules with diverse rhizobial species and symbiovars. Most of the common bean nodulating rhizobia are mainly affiliated to the genus Rhizobium, though strains belonging to Ensifer, Pararhizobium, Mesorhizobium, Bradyrhizobium, and Burkholderia have also been reported. This is the first report on the characterization of bean-nodulating rhizobia at the species and symbiovar level in Greece. The goals of this research were to isolate and characterize rhizobia nodulating local common bean genotypes grown in five different edaphoclimatic regions of Greece with no rhizobial inoculation history. The genetic diversity of the rhizobial isolates was assessed by BOX-PCR and the phylogenetic affiliation was assessed by multilocus sequence analysis (MLSA) of housekeeping and symbiosis-related genes. A total of fifty fast-growing rhizobial strains were isolated and representative isolates with distinct BOX-PCR fingerpriniting patterns were subjected to phylogenetic analysis. The strains were closely related to R. anhuiense, R. azibense, R. hidalgonense, R. sophoriradicis, and to a putative new genospecies which is provisionally named as Rhizobium sp. I. Most strains belonged to symbiovar phaseoli carrying the α-, γ-a and γ-b alleles of nodC gene, while some of them belonged to symbiovar gallicum. To the best of our knowledge, it is the first time that strains assigned to R. sophoriradicis and harbored the γ-b allele were found in European soils. All strains were able to re-nodulate their original host, indicating that they are true microsymbionts of common bean.

Genomic Metrics Applied to Rhizobiales (Hyphomicrobiales): Species Reclassification, Identification of Unauthentic Genomes and False Type Strains

Taxonomic decisions within the order Rhizobiales have relied heavily on the interpretations of highly conserved 16S rRNA sequences and DNA-DNA hybridizations (DDH). Currently, bacterial species are defined as including strains that present 95-96% of average nucleotide identity (ANI) and 70% of digital DDH (dDDH). Thus, ANI values from 520 genome sequences of type strains from species of Rhizobiales order were computed. From the resulting 270,400 comparisons, a ≥95% cut-off was used to extract high identity genome clusters through enumerating maximal cliques. Coupling this graph-based approach with dDDH from clusters of interest, it was found that: (i) there are synonymy between Aminobacter lissarensis and Aminobacter carboxidus, Aurantimonas manganoxydans and Aurantimonas coralicida, "Bartonella mastomydis," and Bartonella elizabethae, Chelativorans oligotrophicus, and Chelativorans multitrophicus, Rhizobium azibense, and Rhizobium gallicum, Rhizobium fabae, and Rhizobium pisi, and Rhodoplanes piscinae and Rhodoplanes serenus; (ii) Chelatobacter heintzii is not a synonym of Aminobacter aminovorans; (iii) "Bartonella vinsonii" subsp. arupensis and "B. vinsonii" subsp. berkhoffii represent members of different species; (iv) the genome accessions GCF_003024615.1 ("Mesorhizobium loti LMG 6,125T"), GCF_003024595.1 ("Mesorhizobium plurifarium LMG 11,892T"), GCF_003096615.1 ("Methylobacterium organophilum DSM 760T"), and GCF_000373025.1 ("R. gallicum R-602 spT") are not from the genuine type strains used for the respective species descriptions; and v) "Xanthobacter autotrophicus" Py2 and "Aminobacter aminovorans" KCTC 2,477T represent cases of misuse of the term "type strain". Aminobacter heintzii comb. nov. and the reclassification of Aminobacter ciceronei as A. heintzii is also proposed. To facilitate the downstream analysis of large ANI matrices, we introduce here ProKlust ("Prokaryotic Clusters"), an R package that uses a graph-based approach to obtain, filter, and visualize clusters on identity/similarity matrices, with settable cut-off points and the possibility of multiple matrices entries.

The promiscuity of Phaseolus vulgaris L. (common bean) for nodulation with rhizobia: a review

Phaseolus vulgaris L. (common bean) is a legume indigenous to American countries currently cultivated in all continents, which is nodulated by different rhizobial species and symbiovars. Most of species able to nodulate this legume worldwide belong to the genus Rhizobium, followed by those belonging to the genera Ensifer (formerly Sinorhizobium) and Pararhizobium (formerly Rhizobium) and minority by species of the genus Bradyrhizobium. All these genera belong to the phylum alpha-Proteobacteria, but the nodulation of P. vulgaris has also been reported for some species belonging to Paraburkholderia and Cupriavidus from the beta-Proteobacteria. Several species nodulating P. vulgaris were originally isolated from nodules of this legume in American countries and are linked to the symbiovars phaseoli and tropici, which are currently present in other continents probably because they were spread in their soils together with the P. vulgaris seeds. In addition, this legume can be nodulated by species and symbiovars originally isolated from nodules of other legumes due its high promiscuity, a concept currently related with the ability of a legume to be nodulated by several symbiovars rather than by several species. In this article we review the species and symbiovars able to nodulate P. vulgaris in different countries and continents and the challenges on the study of the P. vulgaris endosymbionts diversity in those countries where they have not been studied yet, that will allow to select highly effective rhizobial strains in order to guarantee the success of P. vulgaris inoculation.

Rhizobium chutanense sp. nov., isolated from root nodules of Phaseolus vulgaris in China

Two Gram-stain-negative, rod-shaped bacterial strains (C5T and C16), isolated from root nodules of Phaseolus vulgaris L. in Jiangxi Province, PR China, were characterized by using a polyphasic taxonomical approach. The phylogenetic analysis of the 16S rRNA gene and three concatenated housekeeping genes (recA-glnII-atpD) revealed that C5T and C16 were members of the genus Rhizobium, yet were distinct from known species. The case for strain C5T representing a novel species was supported by genomic results. Pairwise digital DNA-DNA hybridization and average nucleotide identity values were much lower than the proposed and generally accepted species boundaries. The genome-based phylogenetic tree reconstructed by using the up-to-date bacterial core gene set consisting of 92 genes showed that the strains formed a monophyletic branch, further supporting this result. The symbiotic genes of nodC and nifH were identified in both strains; each could nodulate Phaseolus vulgaris and Glycine max but not Leucaena leucocephala, Pisum sativum or Medicago sativa plants. Major cellular fatty acids of C5T were summed feature 8 (C18 : 1 ω7c/C18 : 1 ω6c; 58.8 %), C18 : 1 ω7c 11-methyl (14.2 %) and C18 : 0 (8.1 %). The DNA G+C content of C5T was 61.4 mol%. Based on these genomic, chemotaxonomic and phenotypic characteristics, we propose a novel species: Rhizobium chutanense sp. nov. The type strain is C5T (=CCTCC AB 2018143T=LMG 30777T).

Diversity and nodulation effectiveness of rhizobia and mycorrhizal presence in climbing dry beans grown in Prespa lakes plain, Greece

The Prespa lakes plain is an isolated area where about 1000 ha are seeded to Phaseolus vulgaris L. and Phaseolus coccineus L. Nodulation, arbuscular mycorrhizal fungal (AMF) presence and the genetic diversity of rhizobia were evaluated by 16S-ITS-23S-RFLP patterns and by sequencing. The bean rhizobial population in the region was diverse, despite its geographic isolation. No biogeographic relationships were detected, apart from a Rhizobium tropici-related strain that originated from an acidic soil. No clear pattern was detected in clustering with bean species and all isolates formed nodules with both bean species. Most strains were related to Rhizobium leguminosarum and a number of isolates were falling outside the already characterized species of genus Rhizobium. Application of heavy fertilization has resulted in high soil N and P levels, which most likely reduced nodulation and AMF spore presence. However, considerable AMF root length colonization was found in most of the fields.

Nodule bacteria from the cultured legume Phaseolus dumosus (belonging to the Phaseolus vulgaris cross-inoculation group) with common tropici phenotypic characteristics and symbiovar but distinctive phylogenomic position and chromid

Phaseolus dumosus is an endemic species from mountain tops in Mexico that was found in traditional agriculture areas in Veracruz, Mexico. P. dumosus plants were identified by ITS sequences and their nodules were collected from agricultural fields or from trap plant experiments in the laboratory. Bacteria from P. dumosus nodules were identified as belonging to the phaseoli-etli-leguminosarum (PEL) or to the tropici group by 16S rRNA gene sequences. We obtained complete closed genomes from two P. dumosus isolates CCGE531 and CCGE532 that were phylogenetically placed within the tropici group but with a distinctive phylogenomic position and low average nucleotide identity (ANI). CCGE531 and CCGE532 had common phenotypic characteristics with tropici type B rhizobial symbionts. Genome synteny analysis and ANI showed that P. dumosus isolates had different chromids and our analysis suggests that chromids have independently evolved in different lineages of the Rhizobium genus. Finally, we considered that P. dumosus and Phaseolus vulgaris plants belong to the same cross-inoculation group since they have conserved symbiotic affinites for rhizobia.

Rhizobium arenae sp. nov., isolated from the sand of Desert Mu Us, China

A Gram-strain-negative, rod-shaped, motile bacterium, designated MIM27T, was isolated from the sand of the Mu Us Desert, PR China. The strain could grow at 4-45 °C (optimum, 37 °C), at pH 6.6-9.0 (optimum, 8.0) and in the presence of 0-3 % (w/v) NaCl (optimum, 0 % in RNA liquid medium). The results of phylogenetic analysis of 16S rRNA gene sequences indicated that the strain represented a member of the genus Rhizobium, with the highest similarity (96.5 %) to Rhizobium pakistanense BN-19T. The results of analysis of the sequences of the nitrogen fixation gene nifH and three housekeeping genes, recA, atpD and glnII, also indicated that MIM27T was most closely related to the species of the genus Rhizobiumwith validly published names but the similarities were low (≤90.7 %). MIM27T did not form nodules on Pisum sativum, Vicia faba, Astragalus sinicus and Phaseolus vulgaris. The major respiratory quinone of MIM27T was Q-10. The genomic DNA G+C content was 59.8 mol%. Major fatty acids of MIM27T were summed feature 8 (C18 : 1ω7c and/or C18 : 1ω6c), C18 : 1ω7c 11-methyl, C16 : 0, summed feature 3 (C16 : 1ω7c and/or C16 : 1ω6c) and summed feature 2 (C12 : 0 aldehyde and/or unknown ECL 10.9525). On the basis of the physiological, chemotaxonomic and phenotypic data, MIM27T is suggested to represent a novel species of the genus Rhizobium, for which the name Rhizobium arenae sp. nov. is proposed. The type strain is MIM27T (=KCTC 52299T=MCCC 1K03215T).

Natural rhizobial diversity helps to reveal genes and QTLs associated with biological nitrogen fixation in common bean

Common bean is one of the most important crops for human feed, and the most important legume for direct consumption by millions of people, especially in developing countries. It is a promiscuous host legume in terms of nodulation, able to associate with a broad and diverse range of rhizobia, although the competitiveness for nodulation and the nitrogen fixation capacity of most of these strains is generally low. As a result, common bean is very inefficient for symbiotic nitrogen fixation, and nitrogen has to be supplied with chemical fertilizers. In the last years, symbiotic nitrogen fixation has received increasing attention as a sustainable alternative to nitrogen fertilizers, and also as a more economic and available one in poor countries. Therefore, optimization of nitrogen fixation of bean-rhizobia symbioses and selection of efficient rhizobial strains should be a priority, which begins with the study of the natural diversity of the symbioses and the rhizobial populations associated. Natural rhizobia biodiversity that nodulates common bean may be a source of adaptive alleles acting through phenotypic plasticity. Crosses between accessions differing for nitrogen fixation may combine alleles that never meet in nature. Another way to discover adaptive genes is to use association genetics to identify loci that common bean plants use for enhanced biological nitrogen fixation and, in consequence, for marker assisted selection for genetic improvement of symbiotic nitrogen fixation. In this review, rhizobial biodiversity resources will be discussed, together with what is known about the loci that underlie such genetic variation, and the potential candidate genes that may influence the symbiosis' fitness benefits, thus achieving an optimal nitrogen fixation capacity in order to help reduce reliance on nitrogen fertilizers in common bean.

Draft genome sequence of type strain HBR26T and description of Rhizobium aethiopicum sp. nov

Rhizobium aethiopicum sp. nov. is a newly proposed species within the genus Rhizobium. This species includes six rhizobial strains; which were isolated from root nodules of the legume plant Phaseolus vulgaris growing in soils of Ethiopia. The species fixes nitrogen effectively in symbiosis with the host plant P. vulgaris, and is composed of aerobic, Gram-negative staining, rod-shaped bacteria. The genome of type strain HBR26^T of R. aethiopicum sp. nov. was one of the rhizobial genomes sequenced as a part of the DOE JGI 2014 Genomic Encyclopedia project designed for soil and plant-associated and newly described type strains. The genome sequence is arranged in 62 scaffolds and consists of 6,557,588 bp length, with a 61% G + C content and 6221 protein-coding and 86 RNAs genes. The genome of HBR26^T contains repABC genes (plasmid replication genes) homologous to the genes found in five different Rhizobium etli CFN42^T plasmids, suggesting that HBR26^T may have five additional replicons other than the chromosome. In the genome of HBR26^T, the nodulation genes nodB, nodC, nodS, nodI, nodJ and nodD are located in the same module, and organized in a similar way as nod genes found in the genome of other known common bean-nodulating rhizobial species. nodA gene is found in a different scaffold, but it is also very similar to nodA genes of other bean-nodulating rhizobial strains. Though HBR26^T is distinct on the phylogenetic tree and based on ANI analysis (the highest value 90.2% ANI with CFN42^T) from other bean-nodulating species, these nod genes and most nitrogen-fixing genes found in the genome of HBR26^T share high identity with the corresponding genes of known bean-nodulating rhizobial species (96–100% identity). This suggests that symbiotic genes might be shared between bean-nodulating rhizobia through horizontal gene transfer. R. aethiopicum sp. nov. was grouped into the genus Rhizobium but was distinct from all recognized species of that genus by phylogenetic analyses of combined sequences of the housekeeping genes recA and glnII. The closest reference type strains for HBR26^T were R. etli CFN42^T (94% similarity of the combined recA and glnII sequences) and Rhizobium bangladeshense BLR175^T (93%). Genomic ANI calculation based on protein-coding genes also revealed that the closest reference strains were R. bangladeshense BLR175^T and R. etli CFN42^T with ANI values 91.8 and 90.2%, respectively. Nevertheless, the ANI values between HBR26^T and BLR175^T or CFN42^T are far lower than the cutoff value of ANI (> = 96%) between strains in the same species, confirming that HBR26^T belongs to a novel species. Thus, on the basis of phylogenetic, comparative genomic analyses and ANI results, we formally propose the creation of R. aethiopicum sp. nov. with strain HBR26^T (=HAMBI 3550^T=LMG 29711^T) as the type strain. The genome assembly and annotation data is deposited in the DOE JGI portal and also available at European Nucleotide Archive under accession numbers FMAJ01000001-FMAJ01000062.

Genetic Diversity and Symbiotic Efficiency of Indigenous Common Bean Rhizobia in Croatia

Nodule bacteria (rhizobia) in symbiotic associations with legumes enable considerable entries of biologically fixed nitrogen into soil. Efforts are therefore made to intensify the natural process of symbiotic nitrogen fixation by legume inoculation. Studies of field populations of rhizobia open up the possibility to preserve and probably exploit some indigenous strains with hidden symbiotic or ecological potentials. The main aim of the present study is to determine genetic diversity of common bean rhizobia isolated from different field sites in central Croatia and to evaluate their symbiotic efficiency and compatibility with host plants. The isolation procedure revealed that most soil samples contained no indigenous common bean rhizobia. The results indicate that the cropping history had a significant impact on the presence of indigenous strains. Although all isolates were found to belong to species Rhizobium leguminosarum, significant genetic diversity at the strain level was determined. Application of both random amplification of polymorphic DNA (RAPD) and enterobacterial repetitive intergenic consensus-polymerase chain reaction (ERIC- -PCR) methods resulted in similar grouping of strains. Symbiotic efficiency of indigenous rhizobia as well as their compatibility with two commonly grown bean varieties were tested in field experiments. Application of indigenous rhizobial strains as inoculants resulted in significantly different values of nodulation, seed yield as well as plant nitrogen and seed protein contents. The most abundant nodulation and the highest plant nitrogen and protein contents were determined in plants inoculated with R. leguminosarum strains S_17/2 and S_21/6. Although, in general, the inoculation had a positive impact on seed yield, differences depending on the applied strain were not determined. The overall results show the high degree of symbiotic efficiency of the specific indigenous strain S_21/6. These results indicate different symbiotic potential of indigenous strains and confirmed the importance of rhizobial strain selection. These are the first studies of indigenous common bean rhizobia in Croatia that provide the basis for further characterization and selection of highly efficient indigenous strains and their potential use in agricultural practice and future research.

Rhizobium marinum sp. nov., a malachite-green-tolerant bacterium isolated from seawater

A motile, Gram-stain-negative, non-pigmented bacterial strain, designated MGL06T, was isolated from seawater of the South China Sea on selection medium containing 0.1 % (w/v) malachite green. Strain MGL06T showed highest 16S rRNA gene sequence similarity to Rhizobium vignae CCBAU 05176T (97.2 %), and shared 93.2–96.9 % with the type strains of other recognized Rhizobium species. Phylogenetic analyses based on 16S rRNA and housekeeping gene sequences showed that strain MGL06T belonged to the genus Rhizobium. Mean levels of DNA–DNA relatedness between strain MGL06T and R. vignae CCBAU 05176T, Rhizobium huautlense S02T and Rhizobium alkalisoli CCBAU 01393T were 20 ± 3, 18 ± 2 and 14 ± 3 %, respectively, indicating that strain MGL06T was distinct from them genetically. Strain MGL06T did not form nodules on three different legumes, and the nodD and nifH genes were also not detected by PCR or based on the draft genome sequence. Strain MGL06T contained Q-10 as the predominant ubiquinone. The major fatty acid was C18 : 1ω7c/C18 : 1ω6c with minor amounts of C19 : 0 cyclo ω8c, C16 : 0 and C18 : 1ω7c 11-methyl. Polar lipids of strain MGL06T included unknown glycolipids, phosphatidylcholine, aminolipid, phosphatidylglycerol, phosphatidylethanolamine, diphosphatidylglycerol, an unknown polar lipid and aminophospholipid. Based on its phenotypic and genotypic data, strain MGL06T represents a novel species of the genus Rhizobium, for which the name Rhizobium marinum sp. nov. is proposed. The type strain is MGL06T ( = MCCC 1A00836T = JCM 30155T).

Rhizobium pakistanensis sp. nov., isolated from groundnut (Arachis hypogaea) nodules grown in rainfed Pothwar, Pakistan

A Gram-negative, white, non-motile, rod shaped bacterial strain BN-19(T) was isolated from a root nodule of groundnut (Arachis hypogaea) in Pakistan. Phylogenetic analysis based on 16S rRNA gene sequence revealed that strain BN-19(T) formed a subclade in the genus Rhizobium together with Rhizobium alkalisoli CCBAU 01393(T), Rhizobium vignae CCBAU 05176(T), Rhizobium huautlense SO2(T) and Rhizobium tarimense PL-41(T) with sequence similarities of 97.5, 97.3, 97.2 and 97.1 % respectively. Sequence analysis of housekeeping genes atpD, glnII and recA (with sequence similarities of ≤92 %) confirmed the unique position of BN-19(T) in the genus Rhizobium. DNA-DNA relatedness between the strain BN-19(T) and R. alkalisoli CCBAU 01393(T), R. vignae CCBAU 05176(T), R. huautlense SO2(T) and R. tarimense PL-41(T) were 20.6, 22.5, 15.9 and 20.5 % respectively, further confirming that BN-19(T) represents a novel species in the genus Rhizobium. The DNA G + C content was 60.1 mol%. The dominant fatty acids of strain BN-19(T) were C19:0 cyclo ω8c, summed feature 2 (C14:0 3OH and/or C16:1 iso I) and summed feature 8 (C18:1 ω7c). Some phenotypic features also differentiate the strain BN-19(T) from the related species. On the basis of these results, strain BN-19(T) is considered to represent a novel species in the genus Rhizobium, for which the name Rhizobium pakistanensis sp. nov. is proposed. The type strain is BN-19(T) (=LMG 27895(T) = CCBAU 101086(T)).

Microsymbionts of Phaseolus vulgaris in acid and alkaline soils of Mexico

In order to investigate bean-nodulating rhizobia in different types of soil, 41 nodule isolates from acid and alkaline soils in Mexico were characterized. Based upon the phylogenetic studies of 16S rRNA, atpD, glnII, recA, rpoB, gyrB, nifH and nodC genes, the isolates originating from acid soils were identified as the phaseoli symbiovar of the Rhizobium leguminosarum-like group and Rhizobium grahamii, whereas the isolates from alkaline soils were defined as Ensifer americanum sv. mediterranense and Rhizobium radiobacter. The isolates of "R. leguminosarum" and E. americanum harbored nodC and nifH genes, but the symbiotic genes were not detected in the four isolates of the other two species. It was the first time that "R. leguminosarum" and E. americanum have been reported as bean-nodulating bacteria in Mexico. The high similarity of symbiotic genes in the Rhizobium and Ensifer populations showed that these genes had the same origin and have diversified recently in different rhizobial species. Phenotypic characterization revealed that the "R. leguminosarum" population was more adapted to the acid and low salinity conditions, while the E. americanum population preferred alkaline conditions. The findings of this study have improved the knowledge of the diversity, geographic distribution and evolution of bean-nodulating rhizobia in Mexico.