Rhizobium tibeticum sp. nov., a symbiotic bacterium isolated from Trigonella archiducis-nicolai (Sirj.) Vassilcz

Genetic diversity of rhizobia isolated from nodules of Trigonella foenum-graecum L. (fenugreek) cultivated in Northwestern Morocco

The present work aimed to characterize rhizobia nodulating Trigonella foenum-graecum L. (fenugreek) cultivated in six different geographical locations in Northwestern Morocco. Forty-seven rhizobial isolates from nodules of Trigonella foenum-graecum were grouped into thirteen clusters using the Rep-PCR technique. The phylogenetic analysis based on 16S rRNA gene sequences of 13 groups showed that all representative strains were closely related to members of the genus Ensifer (syn. Sinorhizobium) of the Alphaproteobacteria. All the representative strains shared 100% similarity with Ensifer medicae WSM419^T. NodC and nifH gene analysis revealed a close phylogenetic relationship of the representative strains with those of the strains belonging to the symbiovar meliloti. Furthermore, nodulation ability of our rhizobial strains was efficient in their host plant (Trigonella foenum-graecum L.).

Diversity and phylogeny of the bacterial strains isolated from nodules of fenugreek (Trigonella foenum-graecum L.) in Iran

The diversity of fenugreek (Trigonella foenum-graecum L.) microsymbionts has been barely studied even though it is of great interest for being a spice and a medicinal plant. Here we analyzed fifty-nine bacterial strains isolated from fenugreek nodules originating from different geographic and climatic areas of Iran. Most of these strains exhibit phenotypic characteristics compatible with rhizobia and they nodulate fenugreek. Analysis of the recA and atpD genes shows that representative strains of ERIC-BOX-PCR groups cluster with the type strains of Ensifer meliloti and Ensifer kummerowiae as well as with strains capable of nodulating different Trigonella species found in other countries. The closeness of E. meliloti and E. kummerowiae suggests there is a need to revise the taxonomic status of the latter species. The nodC gene analysis shows that most Trigonella-nodulating strains belong to the symbiovar meliloti except those nodulating Trigonella arcuata in China, which belong to the symbiovar rigiduloides. This analysis shows that the type strains of E. kummerowiae, E. meliloti and E. medicae belonged to three well-defined groups within the symbiovar meliloti, with the Iranian strains belonging to the E. kummerowiae subgroup. The small group of strains unable to nodulate fenugreek isolated in this study belong to Enterobacter cloacae, reported for the first time as being a possible endophyte of fenugreek nodules.

The Genome of the Acid Soil-Adapted Strain Rhizobium favelukesii OR191 Encodes Determinants for Effective Symbiotic Interaction With Both an Inverted Repeat Lacking Clade and a Phaseoloid Legume Host

Although Medicago sativa forms highly effective symbioses with the comparatively acid-sensitive genus Ensifer, its introduction into acid soils appears to have selected for symbiotic interactions with acid-tolerant R. favelukesii strains. Rhizobium favelukesii has the unusual ability of being able to nodulate and fix nitrogen, albeit sub-optimally, not only with M. sativa but also with the promiscuous host Phaseolus vulgaris. Here we describe the genome of R. favelukesii OR191 and genomic features important for the symbiotic interaction with both of these hosts. The OR191 draft genome contained acid adaptation loci, including the highly acid-inducible lpiA/acvB operon and olsC, required for production of lysine- and ornithine-containing membrane lipids, respectively. The olsC gene was also present in other acid-tolerant Rhizobium strains but absent from the more acid-sensitive Ensifer microsymbionts. The OR191 symbiotic genes were in general more closely related to those found in Medicago microsymbionts. OR191 contained the nodA, nodEF, nodHPQ, and nodL genes for synthesis of polyunsaturated, sulfated and acetylated Nod factors that are important for symbiosis with Medicago, but contained a truncated nodG, which may decrease nodulation efficiency with M. sativa. OR191 contained an E. meliloti type BacA, which has been shown to specifically protect Ensifer microsymbionts from Medicago nodule-specific cysteine-rich peptides. The nitrogen fixation genes nifQWZS were present in OR191 and P. vulgaris microsymbionts but absent from E. meliloti-Medicago microsymbionts. The ability of OR191 to nodulate and fix nitrogen symbiotically with P. vulgaris indicates that this host has less stringent requirements for nodulation than M. sativa but may need rhizobial strains that possess nifQWZS for N₂-fixation to occur. OR191 possessed the exo genes required for the biosynthesis of succinoglycan, which is required for the Ensifer-Medicago symbiosis. However, ¹H-NMR spectra revealed that, in the conditions tested, OR191 exopolysaccharide did not contain a succinyl substituent but instead contained a 3-hydroxybutyrate moiety, which may affect its symbiotic performance with Medicago hosts. These findings provide a foundation for the genetic basis of nodulation requirements and symbiotic effectiveness with different hosts.

Phylogenetic diversity and plant growth-promoting activities of rhizobia nodulating fenugreek (Trigonella foenum-graecum Linn.) cultivated in different agroclimatic regions of India

Fenugreek (Trigonella foenum-graecum Linn.), is an extensively cultivated legume crop used as a herb, spice, and traditional medicine in India. The symbiotic efficiency and plant growth-promoting potential of fenugreek rhizobia depend on the symbiont strain and environmental factors. We isolated 176 root-nodulating bacteria from fenugreek cultivated in different agroclimatic regions of India. MALDI-TOF MS-based identification and phylogenetic analyses based on 16S rRNA and five housekeeping genes classified the fenugreek-rhizobia as Ensifer (Sinorhizobium) meliloti. However, the strains represent separate sub-lineages of E. meliloti, distinct from all reported sub-lineages across the globe. We also observed the spatial distribution of fenugreek rhizobia, as the three sub-lineages of E. meliloti recorded during this study were specific to their respective agroclimatic regions. According to the symbiotic gene (nodC and nifH) phylogenies, all three sub-lineages of E. meliloti harboured symbiotic genes similar to symbiovar meliloti; as with the housekeeping genes, these also revealed a spatial distribution for different clades of sv. meliloti. The strains could nodulate fenugreek plants and they showed plant growth-promoting potential. Significant differences were found in the plant growth parameters in response to inoculation with the various strains, suggesting strain-level differences. This study demonstrates that fenugreek rhizobia in India are diverse and spatially distributed in different agro-climatic regions.

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.

Agronomical parameters of host and non-host legumes inoculated with Melilotus indicus-isolated rhizobial strains in desert unreclaimed soil

In a search for identification of rhizobial strains with superior N₂-fixation efficiency and improved plant agronomic characteristics upon inoculation, four strains, 4.21, 9.17, 11.2 and 14.1, isolated from root nodules of wild-grown Melilotus indicus have been used to inoculate field-grown common bean, pea, cowpea and fenugreek plants. Uninoculated plants and those inoculated with host-specific commercial inoculants were used as a control. The root length, shoot height, shoot dry weight and root dry weight and the grain yield of the plants were determined after harvest. The content of N, organic C and carbohydrates content of the grain were also recorded. The inoculation with the strains 4.21 and 14.1 increased the grain yield of the fenugreek compared both with the uninoculated plants and those inoculated with the commercial strain ARC-1. The grain yield of the common bean treated with the strains 9.17 and 14.1 was also higher than that of the uninoculated and the commercial strains ARC-301. In contrast, none of the strains increased the grain yield of the pea and cowpea plants compared to the commercial strains ARC-201 and ARC-169, respectively. Significant increases of some agronomical parameters were observed in some plant-bacterium couples, albeit nodulation was not observed. It is possible that the positive effects of rhizobial inoculation on the agronomical parameters of the non-nodule forming legumes could be due to plant growth promotion characteristic of the strains used for inoculation. Analysis of the phylogeny of the almost complete 16S rRNA sequence of the rhizobial inoculants revealed that the strains 4.21 and 9.17 clustered together with R. skierniewicense and R. rosettiformans, respectively, and that the strains 11.2 and 14.1 grouped with E. meliloti. All the four strains produced IAA, and showed biocontrol activity against Rhizotocnia solani, Fusarium oxysporum, Pythium ultimum, Alternaria alternata and Sclerotonia rolsfi, albeit to a different extent.

Genome Sequence of the Symbiotic Type Strain Rhizobium tibeticum CCBAU85039T

Rhizobium tibeticum was originally isolated from root nodules of Trigonella archiducis-nicolai grown in Tibet, China. This species is also able to nodulate Medicago sativa and Phaseolus vulgaris The whole-genome sequence of the type strain, R. tibeticum CCBAU85039^T, is reported in this study.

Rhizobium populi sp. nov., an endophytic bacterium isolated from Populus euphratica

An endophytic bacterium, designated K-38T, was isolated from the storage liquid in the stems of Populus euphratica trees at the ancient Ugan River in Xinjiang, PR China. Strain K-38T was found to be rod-shaped, Gram-stain-negative, aerobic, non-motile and non-spore-forming. Strain K-38T grew at temperatures of 25–37 °C (optimum, 28 °C), at pH 6.0–9.0 (optimum, pH 7.5) and in the presence of 0–3 % (w/v) NaCl with 1 % as the optimum concentration for growth. According to phylogenetic analysis based on 16S rRNA gene sequences, strain K-38T was assigned to the genus Rhizobium with highest 16S rRNA gene sequence similarity of 97.2 % to Rhizobium rosettiformans W3T, followed by Rhizobium nepotum 39/7T (96.5 %) and Rhizobium borbori DN316T (96.2 %). Phylogenetic analysis of strain K-38T based on the protein coding genes recA, atpD and nifH confirmed (similarities were less than 90 %) it to be a representative of a distinctly delineated species of the genus Rhizobium . The DNA G+C content was determined to be 63.5 mol%. DNA–DNA relatedness between K-38T and R. rosettiformans W3T was 48.4 %, indicating genetic separation of strain K-38T from the latter strain. The major components of the cellular fatty acids in strain K-38T were revealed to be summed feature 8 (comprising C18 : 1ω7c and/or C18 : 1ω6c; 57.2 %), C16 : 0 (13.6 %) and summed feature 2 (comprising C12 : 0 aldehyde, C14 : 0 3-OH/iso-C16 : 1 I and/or unknown ECL 10.928; 11.0 %). Polar lipids of strain K-38T include phosphatidylethanolamine, phosphatidylmonomethylethanolamine, phosphatidylcholine, phosphatidylglycerol, diphosphatidylglycerol, two unidentified aminophospholipids and two unidentified phospholipids. Q-10 was the major quinone in strain K-38T. Based on phenotypic, chemotaxonomic and phylogenetic properties, strain K-38T represents a novel species of the genus Rhizobium , for which the name Rhizobium populi sp. nov. is proposed. The type strain is K-38T ( = CCTCC AB 2013068T = NRRL B-59990T = JCM 19159T).

Characterization of Rhizobium grahamii extrachromosomal replicons and their transfer among rhizobia

BACKGROUND

Rhizobium grahamii belongs to a new phylogenetic group of rhizobia together with Rhizobium mesoamericanum and other species. R. grahamii has a broad-host-range that includes Leucaena leucocephala and Phaseolus vulgaris, although it is a poor competitor for P. vulgaris nodulation in the presence of Rhizobium etli or Rhizobium phaseoli strains. This work analyzed the genome sequence and transfer properties of R. grahamii plasmids.

RESULTS

Genome sequence was obtained from R. grahamii CCGE502 type strain isolated from Dalea leporina in Mexico. The CCGE502 genome comprises one chromosome and two extrachromosomal replicons (ERs), pRgrCCGE502a and pRgrCCGE502b. Additionally, a plasmid integrated in the CCGE502 chromosome was found. The genomic comparison of ERs from this group showed that gene content is more variable than average nucleotide identity (ANI). Well conserved nod and nif genes were found in R. grahamii and R. mesoamericanum with some differences. R. phaseoli Ch24-10 genes expressed in bacterial cells in roots were found to be conserved in pRgrCCGE502b. Regarding conjugative transfer we were unable to transfer the R. grahamii CCGE502 symbiotic plasmid and its megaplasmid to other rhizobial hosts but we could transfer the symbiotic plasmid to Agrobacterium tumefaciens with transfer dependent on homoserine lactones.

CONCLUSION

Variable degrees of nucleotide identity and gene content conservation were found among the different R. grahamii CCGE502 replicons in comparison to R. mesoamericanum genomes. The extrachromosomal replicons from R. grahamii were more similar to those found in phylogenetically related Rhizobium species. However, limited similarities of R. grahamii CCGE502 symbiotic plasmid and megaplasmid were observed in other more distant Rhizobium species. The set of conserved genes in R. grahamii comprises some of those that are highly expressed in R. phaseoli on plant roots, suggesting that they play an important role in root colonization.

Genome sequence of Rhizobium grahamii CCGE502, a broad-host-range symbiont with low nodulation competitiveness in Phaseolus vulgaris

Here we present the genome sequence of Rhizobium grahamii CCGE502. R. grahamii groups with other newly described broad-host-range species, which are not very efficient Phaseolus vulgaris symbionts, with a wide geographic distribution and which constitutes a novel Rhizobium clade.

Rhizobium grahamii sp. nov., from nodules of Dalea leporina, Leucaena leucocephala and Clitoria ternatea, and Rhizobium mesoamericanum sp. nov., from nodules of Phaseolus vulgaris, siratro, cowpea and Mimosa pudica

Two novel related Rhizobium species, Rhizobium grahamii sp. nov. and Rhizobium mesoamericanum sp. nov., were identified by a polyphasic approach using DNA-DNA hybridization, whole-genome sequencing and phylogenetic and phenotypic characterization including nodulation of Leucaena leucocephala and Phaseolus vulgaris (bean). As similar bacteria were found in the Los Tuxtlas rainforest in Mexico and in Central America, we suggest the existence of a Mesoamerican microbiological corridor. The type strain of Rhizobium grahamii sp. nov. is CCGE 502(T) (= ATCC BAA-2124(T) = CFN 242(T) = Dal4(T) = HAMBI 3152(T)) and that of Rhizobium mesoamericanum sp. nov. is CCGE 501(T) (= ATCC BAA-2123(T) = HAMBI 3151(T) = CIP 110148(T) = 1847(T)).

MALDI-TOF mass spectrometry is a fast and reliable platform for identification and ecological studies of species from family Rhizobiaceae

Family Rhizobiaceae includes fast growing bacteria currently arranged into three genera, Rhizobium, Ensifer and Shinella, that contain pathogenic, symbiotic and saprophytic species. The identification of these species is not possible on the basis of physiological or biochemical traits and should be based on sequencing of several genes. Therefore alternative methods are necessary for rapid and reliable identification of members from family Rhizobiaceae. In this work we evaluated the suitability of Matrix-Assisted Laser Desorption Ionization-Time-of-Flight Mass Spectrometry (MALDI-TOF MS) for this purpose. Firstly, we evaluated the capability of this methodology to differentiate among species of family Rhizobiaceae including those closely related and then we extended the database of MALDI Biotyper 2.0 including the type strains of 56 species from genera Rhizobium, Ensifer and Shinella. Secondly, we evaluated the identification potential of this methodology by using several strains isolated from different sources previously identified on the basis of their rrs, recA and atpD gene sequences. The 100% of these strains were correctly identified showing that MALDI-TOF MS is an excellent tool for identification of fast growing rhizobia applicable to large populations of isolates in ecological and taxonomic studies.

Rhizobium borbori sp. nov., aniline-degrading bacteria isolated from activated sludge

Three aniline-degrading bacteria, strains DN316T, DN316-1 and DN365, were isolated from activated sludge. According to 16S rRNA gene sequence-based phylogenetic analysis, the isolates belonged to the genus Rhizobium, with Rhizobium ( = Agrobacterium) radiobacter LMG 140T as the closest relative, with 96.5 % sequence similarity. Phylogenetic analysis of the representative strain DN316T using sequences of the glnA, thrC and recA genes and the 16S–23S intergenic spacer region confirmed the phylogenetic arrangement obtained from analysis of the 16S rRNA gene. DNA–DNA relatedness between DN316T and R. radiobacter LMG 140T was 43.7 %, clearly indicating that the representative strain DN316T represents a novel species. Phenotypic and biochemical characterization of the isolates and insertion sequence-PCR fingerprinting patterns showed several distinctive features that differentiated them from closely related species. The major components of the cellular fatty acids were C18 : 1ω7c (57.10 %), C16 : 0 (11.31 %) and C19 : 0 cyclo ω8c (10.13 %). Based on our taxonomic analysis, the three isolates from activated sludge represent a novel species of the genus Rhizobium, for which the name Rhizobium borbori sp. nov. is proposed. The type strain is DN316T ( = CICC 10378T  = LMG 23925T).

Rhizobium vignae sp. nov., a symbiotic bacterium isolated from multiple legume species

A group of rhizobial strains isolated from nodules of multiple legume species grown in different geographical regions of China had identical 16S rRNA genes. Phylogenetic analysis based on the 16S rRNA gene sequences showed that the novel strains formed a subclade in the genus Rhizobium together with Rhizobium galegae, Rhizobium huautlense and Rhizobium alkalisoli, with 99.8 % gene sequence similarity between the strains. The DNA–DNA relatedness values between the representative strain CCBAU 05176T and R. galegae ATCC 43677T, R. huautlense S02T and R. alkalisoli CCBAU 01393T were 22.6 %, 8.9 % and 15.9 %, respectively. The novel strains were distinguished from recognized species of the genus Rhizobium by using a polyphasic approach, including PCR-based restriction fragment length polymorphism analysis (RFLP) of the 16S–23S intergenic spacer (IGS), phenotypic and physiological tests, sequence comparisons of housekeeping genes and cellular fatty acid profiles. Therefore, it is suggested that this group of strains represents a novel species for which the name Rhizobium vignae sp. nov. is proposed. The type strain is CCBAU 05176T (=HAMBI 3039T=LMG 25447T).

Rhizobium sphaerophysae sp. nov., a novel species isolated from root nodules of Sphaerophysa salsula in China

Four gram-negative, aerobic, motile, non-spore, forming rods with a wide pH and temperature range for growth (pH 7.0-11.0, optimum pH 8.0; 20-45°C, optimum 28°C) strains were isolated from root nodules of Sphaerophysa salsula and characterized by means of a polyphasic approach. Phylogenetic analysis based on 16S rRNA gene sequences revealed that the four strains formed a new lineage related to the genus Rhizobium and the sequence similarities between the isolate and the most related type strain Rhizobium giardinii was 96.5%. These strains also formed a distinctive group from the reference strains for defined Rhizobium species based on housekeeping gene sequences (atpD and recA), BOX-PCR fingerprinting, phenotypic features and symbiotic properties. The representative strain CCNWGS0238(T) has DNA-DNA relatedness of less than 33.4% with the most closely related species R. giardinii. It is therefore proposed as a new species, Rhizobium sphaerophysae sp. nov., with isolate CCNWGS0238(T) (=ACCC17498(T) = HAMBI3074(T)) as the type strain.

Rhizobium vallis sp. nov., isolated from nodules of three leguminous species

Four bacterial strains isolated from root nodules of Phaseolus vulgaris, Mimosa pudica and Indigofera spicata plants grown in the Yunnan province of China were identified as a lineage within the genus Rhizobium according to the analysis of 16S rRNA gene sequences, sharing most similarity with Rhizobium lusitanum P1-7(T) (99.1 % sequence similarity) and Rhizobium rhizogenes IAM 13570(T) (99.0 %). These strains also formed a distinctive group from the reference strains for defined species of the genus Rhizobium in a polyphasic approach, including the phylogenetic analyses of the 16S rRNA gene and housekeeping genes (recA, atpD, glnII), DNA-DNA hybridization, BOX-PCR fingerprinting, phenotypic characterization, SDS-PAGE of whole-cell proteins, and cellular fatty acid profiles. All the data obtained in this study suggested that these strains represent a novel species of the genus Rhizobium, for which the name Rhizobium vallis sp. nov. is proposed. The DNA G+C content (mol%) of this species varied between 60.9 and 61.2 (T(m)). The type strain of R. vallis sp. nov. is CCBAU 65647(T) ( = LMG 25295(T) =HAMBI 3073(T)), which has a DNA G+C content of 60.9 mol% and forms effective nodules on Phaseolus vulgaris.