Revised phylogeny of Rhizobiaceae: Proposal of the delineation of Pararhizobium gen. nov., and 13 new species combinations

Phenanthrene-Degrading and Nickel-Resistant Neorhizobium Strain Isolated from Hydrocarbon-Contaminated Rhizosphere of Medicago sativa L

Pollutant degradation and heavy-metal resistance may be important features of the rhizobia, making them promising agents for environment cleanup biotechnology. The degradation of phenanthrene, a three-ring polycyclic aromatic hydrocarbon (PAH), by the rhizobial strain Rsf11 isolated from the oil-polluted rhizosphere of alfalfa and the influence of nickel ions on this process were studied. On the basis of whole-genome and polyphasic taxonomy, the bacterium Rsf11 represent a novel species of the genus Neorhizobium, so the name Neorhizobium phenanthreniclasticum sp. nov. was proposed. Analysis of phenanthrene degradation by the Rsf1 strain revealed 1-hydroxy-2-naphthoic acid as the key intermediate and the activity of two enzymes apparently involved in PAH degradation. It was also shown that the nickel resistance of Rsf11 was connected with the extracellular adsorption of metal by EPS. The joint presence of phenanthrene and nickel in the medium reduced the degradation of PAH by the microorganism, apparently due to the inhibition of microbial growth but not due to the inhibition of the activity of the PAH degradation enzymes. Genes potentially involved in PAH catabolism and nickel resistance were discovered in the microorganism studied. N. phenanthreniclasticum strain Rsf11 can be considered as a promising candidate for use in the bioremediation of mixed PAH-heavy-metal contamination.

Characterization and Genomic Analysis of Affinirhizobium gouqiense sp. nov. Isolated from Seawater of Gouqi Island Located in the East China Sea and Reclassification of Rhizobium lemnae to the Genus Affinirhizobium as Affinirhizobium lemnae comb. nov

A novel bacterium designated as SSA5.23T was isolated from seawater. Cells of SSA5.23T are Gram-stain-negative, short, rod-shaped, and exhibit motility via numerous peritrichous flagella. The strain could grow at temperatures ranging from 15 to 35 °C (optimum at 25 °C), in a salinity range of 0-5.0% (w/v) NaCl, and within a pH range of 6.0-9.0 (optimum at pH 7.0). The predominant cellular fatty acid of SSA5.23T was C18:1 ω7c/C18:1 ω6c, and the major respiratory quinones were Q-9 and Q-10. Diphosphatidylglycerol, phosphatidylethanolamine, and phosphatidylglycerol were identified as the primary polar lipids. The complete genome (5.47 Mb) of SSA5.23T comprises of a circular chromosome of 3.64 Mb and three plasmids, specifically sized at 59.73 kb, 227.82 kb, and 1.54 Mb, respectively. Certain genes located on the plasmids play roles in denitrification, oxidative stress resistance, and osmotic tolerance, which likely contribute to the adaptability of this strain in marine conditions. Core-proteome average amino acid identity analysis effectively identified the strain's affiliation with the genus Affinirhizobium, showing the highest value (89.9%) with Affinirhizobium pseudoryzae DSM 19479T. This classification was further supported by the phylogenetic analysis of concatenated alignment of 170 single-copy orthologous proteins. When compared to related reference strains, SSA5.23T displayed an average nucleotide identity ranging from 74.9 to 80.3% and digital DNA-DNA hybridization values ranging from 19.9 to 23.9%. Our findings confirmed that strain SSA5.23T represents a novel species of the genus Affinirhizobium, for which the name Affinirhizobium gouqiense sp. nov. (type strain SSA5.23T = LMG 32560T = MCCC 1K07165T) was suggested.

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.

Defensive alteration of root exudate composition by grafting Prunus sp. onto resistant rootstock contributes to reducing crown gall disease

Grafting is a traditional and significant strategy to suppress soil-borne diseases, such as the crown gall disease caused by tumorigenic Agrobacterium and Rhizobium. Root exudates and the rhizosphere microbiome play critical roles in controlling crown gall disease, but their roles in suppressing crown gall disease in grafted plants remain unclear. Here, disease-susceptible cherry rootstock 'Gisela 6' and disease-resistant cherry rootstock 'Haiying 1' were grafted onto each other or self-grafted. The effect of their root exudates on the soil microbiome composition and the abundance of pathogenic Agrobacterium were studied. Grafting onto the disease-resistant rootstock helped to reduce the abundance of pathogenic Agrobacterium, accompanied by altering root exudation, enriching potential beneficial bacteria, and changing soil function. Then, the composition of the root exudates from grafted plants was analyzed and the potential compounds responsible for decreasing pathogenic Agrobacterium abundance were identified. Based on quantitative measurement of the concentrations of the compounds and testing the impacts of supplied pure chemicals on abundance and chemotaxis of pathogenic Agrobacterium and potential beneficial bacteria, the decreased valine in root exudates of the plant grafted onto resistant rootstock was found to contribute to decreasing Agrobacterium abundance, enriching some potential beneficial bacteria and suppressing crown gall disease. This study provides insights into the mechanism whereby grafted plants suppress soil-borne disease.

Ciceribacter sichuanensis sp. nov., a plant growth promoting rhizobacterium isolated from root nodules of soybean in Sichuan, China

The fast-growing rhizobia-like strains S101T and S153, isolated from root nodules of soybean (Glycine max) in Sichuan, People's Republic of China, underwent characterization using a polyphasic taxonomy approach. The strains exhibited growth at 20-40 °C (optimum, 28 °C), pH 4.0-10.0 (optimum, pH 7.0) and up to 2.0% (w/v) NaCl (optimum, 0.01%) on Yeast Mannitol Agar plates. The 16S rRNA gene of strain S101T showed 98.4% sequence similarity to the closest type strain, Ciceribacter daejeonense L61T. Major cellular fatty acids in strain S101T included summed feature 8 (C18:1ω7c and/or C18:1ω6c) and C19:0 cyclo ω8c. The predominant quinone was ubiquinone-10. The polar lipids of strain S101T included diphosphatidylglycerol, phosphatidylglycerol, phosphatidylmethyl ethanolamine, phosphatidyl ethanolamine, amino phospholipid, unidentified phosphoglycolipid and unidentified amino-containing lipids. The DNA G + C contents of S101T and S153 were 61.1 and 61.3 mol%, respectively. Digital DNA-DNA hybridization relatedness and average nucleotide identity values between S101T and C. daejeonense L61T were 46.2% and 91.4-92.2%, respectively. In addition, strain S101T promoted the growth of soybean and carried nitrogen fixation genes in its genome, hinting at potential applications in sustainable agriculture. We propose that strains S101T and S153 represent a novel species, named Ciceribacter sichuanensis sp. nov., with strain S101T as the type strain (= CGMCC 1.61309 T = JCM 35649 T).

Rhizoviticin is an alphaproteobacterial tailocin that mediates biocontrol of grapevine crown gall disease

Tailocins are headless phage tail structures that mediate interbacterial antagonism. Although the prototypical tailocins, R- and F-pyocins, in Pseudomonas aeruginosa, and other predominantly R-type tailocins have been studied, their presence in Alphaproteobacteria remains unexplored. Here, we report the first alphaproteobacterial F-type tailocin, named rhizoviticin, as a determinant of the biocontrol activity of Allorhizobium vitis VAR03-1 against crown gall. Rhizoviticin is encoded by a chimeric prophage genome, one providing transcriptional regulators and the other contributing to tail formation and cell lysis, but lacking head formation genes. The rhizoviticin genome retains a nearly intact early phage region containing an integrase remnant and replication-related genes critical for downstream gene transcription, suggesting an ongoing transition of this locus from a prophage to a tailocin-coding region. Rhizoviticin is responsible for the most antagonistic activity in VAR03-1 culture supernatant against pathogenic A. vitis strain, and rhizoviticin deficiency resulted in a significant reduction in the antitumorigenic activity in planta. We identified the rhizoviticin-coding locus in eight additional A. vitis strains from diverse geographical locations, highlighting a unique survival strategy of certain Rhizobiales bacteria in the rhizosphere. These findings advance our understanding of the evolutionary dynamics of tailocins and provide a scientific foundation for employing rhizoviticin-producing strains in plant disease control.

Pararhizobium qamdonense sp. nov., Isolated from an Alpine Soil in Tibet and the Reclassification of Rhizobium gei Shi et al. 2016 as Pararhizobium gei comb. nov

A novel Gram-stain-negative, aerobic, rod-shaped bacterium named T808T was isolated from an alpine soil in Qamdo, Tibet, PR China. Strain T808T grew at 5-30℃, pH 5.0-9.0 (optimum, 25℃ and pH 7.0-8.0) with 0-2% (w/v) NaCl (optimum, 0%). The 16S rRNA gene sequences of strain T808T showed the highest similarity with Pararhizobium herbae CCBAU83011T (98.8%), followed by Pararhizobium polonicum F5.1T (98.7%), Pararhizobium giardinii H152T (98.5%), Rhizobium gei ZFJT-2 T (98.4%), and Pararhizobium antarcticum NAQVI59T (97.5%). The highest digital DNA-DNA hybridization (dDDH), core-proteome average amino acid identity (cpAAI) and average nucleotide identity (ANI) values between strain T808T and related strains were estimated as 28.0%, 92.1% and 84.4%, respectively. Phylogenetic analysis based on 16S rRNA, core-proteome and whole-genome indicated that strain T808T belonged to the genus Pararhizobium. The genome size was 6.24 Mbp with genomic DNA G + C content of 60.1%. The major cellular fatty acids were Summed feature 8 (C18:1 ω7c or C18:1 ω6c), C16:0 and C19:0 cyclo ω8c. The polar lipids were diphosphatidyl glycerol, phosphatidyl glycerol, phosphatidyl ethanolamine, phosphatidyl choline and unidentified aminophospholipid. The isoprenoid quinone were ubiquinone-10 and ubiquinone-9. Based on phenotypic, phylogenetic, and genotypic data, strain T808T is considered to represent a novel species of the genus Pararhizobium, for which the name Pararhizobium qamdonense sp. nov. is proposed. The type strain is T808T (= JCM 36247 T = CICC 25216 T). According to phylogenetic coherence based on 16S rRNA, core-proteome and whole-genome, it is also proposed that the type strain Rhizobium gei Shi et al. 2016 should be reclassified as Pararhizobium gei comb. nov., the type strain is ZFJT-2 T (= CCTCC AB 2013015 T = KCTC 32301 T = LMG 27603 T).

Characterization and Genomic Analysis of Fererhizobium litorale gen. nov., sp. nov., Isolated from the Sandy Sediments of the Sea of Japan Seashore

The taxonomic status of two gram-negative, whitish-pigmented motile bacteria KMM 9576T and KMM 9553 isolated from a sandy sediment sample from the Sea of Japan seashore was defined. Phylogenetic analysis revealed that strains KMM 9576T and KMM 9553 represent a distinct lineage within the family Rhizobiaceae, sharing 100% 16S rRNA sequence similarity and 99.5% average nucleotide identity (ANI) to each other. The strains showed the highest 16S rRNA sequence similarities of 97.4% to Sinorhizobium garamanticum LMG 24692T, 96.9% to Ensifer adhaerens NBRC 100388T, and 96.8% to Pararhizobium giardinii NBRC 107135T. The ANI values between strain KMM 9576T and Ensifer adhaerens NBRC 100388T, Sinorhizobium fredii USDA 205T, Pararhizobium giardinii NBRC 107135T, and Rhizobium leguminosarum NBRC 14778T were 79.9%, 79.6%, 79.4%, and 79.2%, respectively. The highest core-proteome average amino acid identity (cpAAI) values of 82.1% and 83.1% were estimated between strain KMM 9576T and Rhizobium leguminosarum NBRC 14778T and 'Rhizobium album' NS-104, respectively. The DNA GC contents were calculated from a genome sequence to be 61.5% (KMM 9576T) and 61.4% (KMM 9553). Both strains contained the major ubiquinone Q-10 and C18:1ω7c as the dominant fatty acid followed by 11-methyl C18:1ω7c and C19:0 cyclo, and polar lipids consisted of phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, an unidentified aminophospholipid, and two unidentified phospholipids. Based on phylogenetic and phylogenomic analyses, and phenotypic characterization, strains KMM 9576T and KMM 9553 are concluded to represent a novel genus and species, for which the name Fererhizobium litorale gen. nov., sp. nov. is proposed. The type strain of the type species is strain KMM 9576T (=NRIC 0957T).

Virulence and Ecology of Agrobacteria in the Context of Evolutionary Genomics

Among plant-associated bacteria, agrobacteria occupy a special place. These bacteria are feared in the field as agricultural pathogens. They cause abnormal growth deformations and significant economic damage to a broad range of plant species. However, these bacteria are revered in the laboratory as models and tools. They are studied to discover and understand basic biological phenomena and used in fundamental plant research and biotechnology. Agrobacterial pathogenicity and capability for transformation are one and the same and rely on functions encoded largely on their oncogenic plasmids. Here, we synthesize a substantial body of elegant work that elucidated agrobacterial virulence mechanisms and described their ecology. We review findings in the context of the natural diversity that has been recently unveiled for agrobacteria and emphasize their genomics and plasmids. We also identify areas of research that can capitalize on recent findings to further transform our understanding of agrobacterial virulence and ecology.

Genome-based analyses of family Oxalobacteraceae reveal the taxonomic classification

Family Oxalobacteraceae is known for the indicator of bacterial diversity in the environment and many of which are important beneficial bacteria. Previous studies on the taxonomic structure of family Oxalobacteraceae mostly relied on 16S rRNA gene analysis, or core-genome phylogeny of a limited number of species and resulted in taxonomic confusion within several genera. Developments in sequencing technologies have allowed more genome sequences to be obtained, enabling the revision of family Oxalobacteraceae. Here, we report a comprehensive analysis of phylogenomic trees, concatenated protein and up-to-date bacterial core gene phylogenetic trees, and genomic metrics for genus demarcation on 135 genomes of Oxalobacteraceae species to elucidate their interrelationships. Following this framework for classification of species in family Oxalobacteraceae, all the proposed genera formed monophyletic lineages in the phylogenomic trees and could also be clearly separated from others in the genomic similarity indexes of average amino acid identity, percentage of conserved proteins and core-proteome average amino acid identity.

Description of Flavimaribacter sediminis gen. nov., sp. nov., a New Member of the Family Rhizobiaceae Isolated from Marine Sediment

A novel Gram-staining-negative, aerobic and rod-shaped bacterium, designated WL0058T, was isolated from coastal sediment sample collected in Nantong city, Jiangsu province of China (120° 51' 13″ E, 32° 6' 26″ N) in October 2020. Strain WL0058T was found to grow at 4-37 °C (optimum, 28 °C) with 1.5-4.0% NaCl (optimum, 4.0%) and displayed alkaliphilic growth with the pH range of pH 6.0-10.0 (optimum, pH 6.0). Phylogenetic trees constructed based on 16S rRNA gene sequence indicated that strain WL0058T is a member of the family Rhizobiaceae, shared the highest similarity with "Hoeflea prorocentri" CCTCC AB 2016294T (97.7%) and constituted a sub-cluster within the family with it, while the similarity with others in the family Rhizobiaceae was lower than 97.0%. The G + C content of genomic DNA was 59.5 mol%. Polar lipids profile of strain WL0058T included phosphatidylcholine (PC), phosphatidylethanolamine (PE), and glycolipid (GL), phosphatidylmonomethylethanolamine (PME) and two unidentified polar lipids (L). The major isoprenoid quinone was determined to be Q-10 and the major fatty acids were C16:0, C18:0, summed features 4 (iso-C17:1 and/or anteiso-C17:1), and summed features 8 (C18:1ω6c and/or C18:1ω7c). As inferred from the morphology, physiology, and biochemical analysis, genotypic characteristics, and the phylogenetic trees, strain WL0058T ought to be recognized as a novel genus in the family Rhizobiaceae, for which the name Flavimaribacter sediminis gen. nov., sp. nov. The type strain of Flavimaribacter sediminis gen. nov., sp. nov. is WL0058T (= MCCC 1K06063T = JCM 34659T = GDMCC 1.2448T).

Diversity, Genomics and Symbiotic Characteristics of Sinorhizobia That Nodulate Desmanthus spp. in Northwest Argentina

Desmanthus spp. are legumes with the ability to associate with diverse α-proteobacteria-a microsymbiont-in order to establish nitrogen-fixing root nodules. A previous investigation from our laboratory revealed that the main bacteria associated with Desmanthus paspalaceus in symbiosis in central Argentina (Province of Santa Fe) were quite diverse and belonged to the genera Rhizobium and Mesorhizobium. To achieve a more extensive view of the local microsymbionts associated with Desmanthus spp., we sampled three different sites in Jujuy and Salta, in northwest Argentina. Matrix-assisted Laser-Desorption-Ionization Time-of-Flight mass spectrometry (MALDI-TOF) typing, 16S-rDNA analysis, and genome sequencing demonstrated that the dominant root-nodule microsymbionts belonged to the genus Sinorhizobium, with some sequenced genomes related to Sinorhizobium mexicanum, Sinorhizobium chiapanecum, and Sinorhizobium psoraleae. An analysis of nodA and nodC markers indicated that, in some of the isolates, horizontal gene transfer appeared to be responsible for the lack of congruence between the phylogenies of the chromosome and of the symbiotic region. These results revealed diverse evolutionary strategies for reaching the current Desmanthus-microsymbiont diversity. What is remarkable beside their observed genetic diversity is that the tolerance profiles of these isolates to abiotic stresses (temperature, salt concentration, pH) were quite coincident with the separation of the sinorhizobia according to place of origin, suggesting possible ecoedaphic adaptations. This observation, together with the higher aerial dry-weight matter that some isolates generated in Desmanthus virgatus cv. Marc when compared to the biomass generated by the commercial strain Sinorhizobium terangae CB3126, distinguish the collected sinorhizobia as constituting valuable germplasm for evaluation in local fields to select for more efficient symbiotic pairs.

Insight into the population dynamics of pathogenic bacteria causing grapevine crown gall in snowfall areas: snow cover protects the proliferation of pathogenic bacteria

Grapevine crown gall (GCG) is a significant bacterial disease caused by tumorigenic Allorhizobium vitis (TAV) and is prevalent worldwide. TAV infects grapevines through wounds such as freezing injuries. Although grapevines typically avoid being wounded under snow cover, GCG occurs in many commercial vineyards in snowy regions. This study investigated the TAV population in GCG gall tissues, grapevine skins, and snow on grapevine skins from six infected vineyards located in Hokkaido, Japan, an area known for heavy snowfall. TAV was isolated not only from gall tissues but also from skins and snow on skins throughout the year. Hierarchical Bayesian model (HBM) analysis revealed that the number of TAV cells in gall tissues was affected by cultivar and low temperature, while those in skins were affected by location and low temperature. Additionally, Bayesian changepoint detection (BCD) showed that the number of TAV cells in gall and skin tissues increased during winter, including the snowfall season. Furthermore, the TAV population in grapevine skins under the snow was significantly higher than those above the snow, indicating that TAV under the snow is protected by the snow and can survive well during the snowfall season. This study highlights the ability of TAV to overwinter on/in galls and skins under the snow and act as inoculum for the next season.

The Ti Plasmid, Driver of Agrobacterium Pathogenesis

The phytopathogenic bacterium Agrobacterium tumefaciens causes crown gall disease in plants, characterized by the formation of tumor-like galls where wounds were present. Nowadays, however, the bacterium and its Ti (tumor-inducing) plasmid is better known as an effective vector for the genetic manipulation of plants and fungi. In this review, I will briefly summarize some of the major discoveries that have led to this bacterium now playing such a prominent role worldwide in plant and fungal research at universities and research institutes and in agricultural biotechnology for the production of genetically modified crops. I will then delve a little deeper into some aspects of Agrobacterium biology and discuss the diversity among agrobacteria and the taxonomic position of these bacteria, the diversity in Ti plasmids, the molecular mechanism used by the bacteria to transform plants, and the discovery of protein translocation from the bacteria to host cells as an essential feature of Agrobacterium-mediated transformation.

Agrobacterium cucumeris sp. nov. isolated from crazy roots on cucumber (Cucumis sativus)

Three plant rhizogenic strains O132^T, O115 and O34 isolated from Cucumis sp. L. were assessed for taxonomic affiliation by using polyphasic taxonomic methods. Based on the results of the sequence analysis of the 16S rRNA and multilocus sequence analysis (MLSA) of the three housekeeping genes atpD, recA and rpoB, all the strains were clustered within the genus Agrobacterium where they form a novel branch. Their closest relative was Agrobacterium tomkonis (genomospecies G3). Moreover, digital DNA-DNA hybridization (dDDH) and average nucleotide identity (ANI) comparisons between strains O132^T and O34 and their closest relatives provided evidence that they constitute a new species, because the obtained values were significantly below the threshold considered as a borderline for the species delineation. Whole-genome phylogenomic analysis also indicated that the cucumber strains are located within the separate, well-delineated biovar 1 sub-clade of the genus Agrobacterium. Furthermore, the physiological and biochemical properties of these strains allowed to distinguish them from their closest related species of the genus Agrobacterium. As a result of the performed overall characterization, we propose a new species as Agrobacterium cucumeris sp. nov., with O132^T (=CFBP 8997^T = LMG 32451^T) as the type strain.

Biological Control for Grapevine Crown Gall Evaluated by a Network Meta-Analysis

Grapevine crown gall (GCG), which is caused by Allorhizobium vitis (=Rhizobium vitis) tumorigenic strains, is the most important disease of grapevine around the world. Previously, nonpathogenic A. vitis strains VAR03-1, ARK-1, ARK-2, and ARK-3 were identified as promising biological control agents, but the control effects of each strain were not directly compared and assessed in the field because field trials were conducted in different fields and years. Thus, the results of the control effects obtained from 16 field trials in 12 years from 2006 to 2017 were analyzed and evaluated by a linear mixed model (LMM) and a network meta-analysis (NMA). The results of the LMM strongly indicate that the factor "antagonistic strain" was significantly related to the biological control activity in this study, but the other factors, "concentration of cell suspension", "field", and "year", were not. Then, the results of 16 field trials were combined in an NMA. The estimated relative risk (RR) after treatment with ARK-1, ARK-2, ARK-3, VAR03-1, and K84 were 0.16, 0.20, 0.22, 0.24, and 0.74, respectively. In conclusion, strain ARK-1 was the best antagonist regardless of the concentration of the cell suspension, field, and year differences, and it can be recommended to control GCG.

An exopolysaccharide-producing novel Agrobacterium pusense strain JAS1 isolated from snake plant enhances plant growth and soil water retention

A peculiar bacterial growth was very often noticed in leaf-initiated tissue cultures of Sansevieria trifasciata, a succulent belonging to the Asparagaceae family. The isolate left trails of some highly viscous material on the walls of the suspension vessels or developed a thick overlay on semisolid media without adversities in plant growth. FTIR identified this substance to be an extracellular polysaccharide. Various morphological, biochemical tests, and molecular analyses using 16S rRNA, atpD, and recA genes characterized this isolate JAS1 as a novel strain of Agrobacterium pusense. Its mucoidal growth over Murashige and Skoog media yielded enormous exopolysaccharide (7252 mg l^-1), while in nutrient agar it only developed fast-growing swarms. As a qualifying plant growth-promoting bacteria, it produces significant indole-3-acetic acid (86.95 mg l^-1), gibberellic acid (172.98 mg l^-1), ammonia (42.66 µmol ml^-1). Besides, it produces siderophores, 1-aminocyclopropane-1-carboxylic acid deaminase, fixes nitrogen, forms biofilms, and productively solubilizes soil inorganic phosphates, and zinc. Under various treatments with JAS1, wheat and chickpea resulted in significantly enhanced shoot and root growth parameters. PGP effects of JAS1 positively enhanced plants' physiological growth parameters reflecting significant increments in overall chlorophyll, carotenoids, proline, phenols, flavonoids, and sugar contents. In addition, the isolated strain maintained both plant and soil health under an intermittent soil drying regime, probably by both its PGP and EPS production attributes, respectively.

Biocontrol of Grapevine Crown Gall Performed Using Allorhizobium vitis Strain ARK-1

Grapevine crown gall (GCG), which is caused by tumorigenic Allorhizobium vitis (=Rhizobium vitis), is the most important bacterial disease in grapevine, and its economic impact on grapevine is very high. When young vines develop GCG, they often die, whereas older vines may show stress and poor growth depending on the severity of GCG, because GCG interferes with the vascular system of the grapevine trunk and prevents nutrient flow, leading to inferior growth and death. Viticultural practices and chemical control designed to inhibit GCG are only partially effective presently; thus, a biocontrol procedure could be a desirable and effective approach for GCG prevention. This article reviews the practical use of biocontrol options for GCG inhibition that involve using nonpathogenic and antagonistic A. vitis strains. In these studies, screening tests of biocontrol agents discovered nonpathogenic A. vitis strains VAR03-1, ARK-1, ARK-2, and ARK-3. After dipping grapevine roots in a suspension of candidate strains prior to planting in the field, treatment using ARK-1 was shown to significantly reduce the number of plants with GCG. A meta-analysis indicated that ARK-1 is very useful for controlling crown gall in various plant species, including grapevine. It was reported that when a mixture of ARK-1 and a tumorigenic strain was examined in grapevines, the expression levels of several virulence genes of the virulent strain were significantly lower. ARK-1 can reduce the pathogen population in grapevines and gall incidence. Moreover, ARK-1 can prime the induction of certain defense genes of grapevine. These results indicate that ARK-1 has a unique biocontrol mechanism and that it is a promising new biocontrol agent to control GCG.

The changing paradigm of rhizobial taxonomy and its systematic growth upto postgenomic technologies

Rhizobia are a diazotrophic group of bacteria that are usually isolated form the nodules in roots, stem of leguminous plants and are able to form nodules in the host plant owing to the presence of symbiotic genes. The rhizobial community is highly diverse, and therefore, the taxonomy and genera-wise classification of rhizobia has been constantly changing since the last three decades. This is mainly due to technical advancements, and shifts in definitions, resulting in a changing paradigm of rhizobia taxonomy. Initially, the taxonomic definitions at the species and sub species level were based on phylogenetic analysis of 16S rRNA sequence, followed by polyphasic approach to have phenotypic, biochemical, and genetic analysis including multilocus sequence analysis. Rhizobia mainly belonging to α- and β-proteobacteria, and recently new additions from γ-proteobacteria had been classified. Nowadays rhizobial taxonomy has been replaced by genome-based taxonomy that allows gaining more insights of genomic characteristics. These omics-technologies provide genome specific information that considers nodulation and symbiotic genes, along with molecular markers as taxonomic traits. Taxonomy based on complete genome sequence (genotaxonomy), average nucleotide identity, is now being considered as primary approach, resulting in an ongoing paradigm shift in rhizobial taxonomy. Also, pairwise whole-genome comparisons, phylogenomic analyses offer correlations between DNA and DNA re-association values that have delineated biologically important species. This review elaborates the present classification and taxonomy of rhizobia, vis-a-vis development of technical advancements, parameters and controversies associated with it, and describe the updated information on evolutionary lineages of rhizobia.

Genomic analysis provides novel insights into diversification and taxonomy of Allorhizobium vitis (i.e. Agrobacterium vitis)

BACKGROUND

Allorhizobium vitis (formerly named Agrobacterium vitis or Agrobacterium biovar 3) is the primary causative agent of crown gall disease of grapevine worldwide. We obtained and analyzed whole-genome sequences of diverse All. vitis strains to get insights into their diversification and taxonomy.

RESULTS

Pairwise genome comparisons and phylogenomic analysis of various All. vitis strains clearly indicated that All. vitis is not a single species, but represents a species complex composed of several genomic species. Thus, we emended the description of All. vitis, which now refers to a restricted group of strains within the All. vitis species complex (i.e. All. vitis sensu stricto) and proposed a description of a novel species, All. ampelinum sp. nov. The type strain of All. vitis sensu stricto remains the current type strain of All. vitis, K309T. The type strain of All. ampelinum sp. nov. is S4T. We also identified sets of gene clusters specific to the All. vitis species complex, All. vitis sensu stricto and All. ampelinum, respectively, for which we predicted the biological function and infer the role in ecological diversification of these clades, including some we could experimentally validate. All. vitis species complex-specific genes confer tolerance to different stresses, including exposure to aromatic compounds. Similarly, All. vitis sensu stricto-specific genes confer the ability to degrade 4-hydroxyphenylacetate and a putative compound related to gentisic acid. All. ampelinum-specific genes have putative functions related to polyamine metabolism and nickel assimilation. Congruently with the genome-based classification, All. vitis sensu stricto and All. ampelinum were clearly delineated by MALDI-TOF MS analysis. Moreover, our genome-based analysis indicated that Allorhizobium is clearly separated from other genera of the family Rhizobiaceae.

CONCLUSIONS

Comparative genomics and phylogenomic analysis provided novel insights into the diversification and taxonomy of Allorhizobium vitis species complex, supporting our redefinition of All. vitis sensu stricto and description of All. ampelinum. Our pan-genome analyses suggest that these species have differentiated ecologies, each relying on specialized nutrient consumption or toxic compound degradation to adapt to their respective niche.

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.

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.

Phylogenetically diverse Bradyrhizobium genospecies nodulate Bambara groundnut (Vigna subterranea L. Verdc) and soybean (Glycine max L. Merril) in the northern savanna zones of Ghana

A total of 102 bacterial strains isolated from nodules of three Bambara groundnut and one soybean cultivars grown in nineteen soil samples collected from northern Ghana were characterized using multilocus gene sequence analysis. Based on a concatenated sequence analysis (glnII-rpoB-recA-gyrB-atpD-dnaK), 54 representative strains were distributed in 12 distinct lineages, many of which were placed mainly in the Bradyrhizobium japonicum and Bradyrhizobium elkanii supergroups. Twenty-four of the 54 representative strains belonged to seven putative novel species, while 30 were conspecific with four recognized Bradyrhizobium species. The nodA phylogeny placed all the representative strains in the cosmopolitan nodA clade III. The strains were further separated in seven nodA subclusters with reference strains mainly of African origin. The nifH phylogeny was somewhat congruent with the nodA phylogeny, but both symbiotic genes were mostly incongruent with the core housekeeping gene phylogeny indicating that the strains acquired their symbiotic genes horizontally from distantly related Bradyrhizobium species. Using redundancy analysis, the distribution of genospecies was found to be influenced by the edaphic factors of the respective sampling sites. In general, these results mainly underscore the high genetic diversity of Bambara groundnut-nodulating bradyrhizobia in Ghanaian soils and suggest a possible vast resource of adapted inoculant strains.

Rhizobium setariae sp. nov., an Indole-3-Acetic Acid-Producing Bacterium Isolated from Green Foxtail, Setaria viridis

A Gram-negative, indole-3-acetic acid-producing, aerobic, motile strain, designated as KVB221^T, was isolated from a green foxtail plant, Setaria viridis, from a park near the coast of Haeundae Beach, Busan, Republic of Korea. The 16S rRNA gene analysis revealed strain KVB221^T to be a member of the genus Rhizobium, from which Rhizobium alvei TNR-22^T (97.2%), Rhizobium daejeonense L61^T (96.9%), and Rhizobium ipomoeae shin9-1^T (95.7%) were selected for comparative analysis. Growth of the strain was observed at 10-50 °C (optimum 25-30 °C), at pH 5-10 (optimum pH 7), and in the presence of 0-8% NaCl (optimum 0%). The strain was observed to produce 36.3 ± 0.8 μg/ml of indole following 5 days of incubation. The major fatty acids are comprised of C_16:0, C_19:0 cyclo ω8c, C_18:1 ω7c, and the unresolved group summed feature 8 (C_18:1 ω7c and/or C_18:1 ω6c), while major polar lipids are identified as phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylglycerol (PG), and phosphatidylmonomethylethanolamine (PME). The predominant quinone is Q-10 and the DNA G+C content of the strain is 59.3%. Based on publicly available genome data between strain KVB221^T and its closely related strains, the average nucleotide identity and in silico DNA-DNA hybridization values ranged from 72.7 to 73.1 and 19.7 to 20.4%, respectively. Based on the chemotaxonomic, phenotypic, and genomic comparisons reported here, we propose Rhizobium setariae sp. nov. as a novel species belonging to the genus Rhizobium. The type strain is KVB221^T (= KACC 21713^T = NBRC 114644^T).

Taxonomy of Rhizobiaceae revisited: proposal of a new framework for genus delimitation

The alphaproteobacterial family Rhizobiaceae is highly diverse, with 168 species with validly published names classified into 17 genera with validly published names. Most named genera in this family are delineated based on genomic relatedness and phylogenetic relationships, but some historically named genera show inconsistent distribution and phylogenetic breadth. The most problematic is Rhizobium , which is notorious for being highly paraphyletic, as most newly described species in the family are assigned to this genus without consideration of their proximity to existing genera, or the need to create novel genera. Moreover, many Rhizobiaceae genera lack synapomorphic traits that would give them biological and ecological significance. We propose a common framework for genus delimitation within the family Rhizobiaceae , wherein genera are defined as monophyletic groups in a core-genome gene phylogeny, that are separated from related species using a pairwise core-proteome average amino acid identity (cpAAI) threshold of approximately 86 %. We further propose that additional genomic or phenotypic evidence can justify division of species into separate genera even if they share greater than 86 % cpAAI. Applying this framework, we propose to reclassify Rhizobium rhizosphaerae and Rhizobium oryzae into Xaviernesmea gen. nov. Data is also provided to support the formation of Peteryoungia aggregata comb. nov., Endobacterium yantingense comb. nov., Neorhizobium petrolearium comb. nov., Pararhizobium arenae comb. nov., Pseudorhizobium tarimense comb. nov. and Mycoplana azooxidifex comb. nov. Lastly, we present arguments that the unification of the genera Ensifer and Sinorhizobium in Opinion 84 of the Judicial Commission is no longer justified by current genomic and phenotypic data. Despite pairwise cpAAI values for all Ensifer species and all Sinorhizobium species being >86 %, additional genomic and phenotypic data suggest that they significantly differ in their biology and ecology. We therefore propose emended descriptions of Ensifer and Sinorhizobium , which we argue should be considered as separate genera.

Modular evolution of secretion systems and virulence plasmids in a bacterial species complex

BACKGROUND

Many named species as defined in current bacterial taxonomy correspond to species complexes. Uncertainties regarding the organization of their genetic diversity challenge research efforts. We utilized the Agrobacterium tumefaciens species complex (a.k.a. Agrobacterium biovar 1), a taxon known for its phytopathogenicity and applications in transformation, as a study system and devised strategies for investigating genome diversity and evolution of species complexes.

RESULTS

We utilized 35 genome assemblies, including 14 newly generated ones, to achieve a phylogenetically balanced sampling of A. tumefaciens. Our genomic analysis suggested that the 10 genomospecies described previously are distinct biological species and supported a quantitative guideline for species delineation. Furthermore, our inference of gene content and core-genome phylogeny allowed for investigations of genes critical in fitness and ecology. For the type VI secretion system (T6SS) involved in interbacterial competition and thought to be conserved, we detected multiple losses and one horizontal gene transfer. For the tumor-inducing plasmids (pTi) and pTi-encoded type IV secretion system (T4SS) that are essential for agrobacterial phytopathogenicity, we uncovered novel diversity and hypothesized their involvement in shaping this species complex. Intriguingly, for both T6SS and T4SS, genes encoding structural components are highly conserved, whereas extensive diversity exists for genes encoding effectors and other proteins.

CONCLUSIONS

We demonstrate that the combination of a phylogeny-guided sampling scheme and an emphasis on high-quality assemblies provides a cost-effective approach for robust analysis in evolutionary genomics. We show that the T6SS VgrG proteins involved in specific effector binding and delivery can be classified into distinct types based on domain organization. The co-occurrence patterns of VgrG-associated domains and the neighboring genes that encode different chaperones/effectors can be used to infer possible interacting partners. Similarly, the associations between plant host preference and the pTi type among these strains can be used to infer phenotype-genotype correspondence. Our strategies for multi-level investigations at scales that range from whole genomes to intragenic domains and phylogenetic depths from between- to within-species are applicable to other bacteria. Furthermore, modularity observed in the molecular evolution of genes and domains is useful for inferring functional constraints and informing experimental works.

Agrobacterium leguminum sp. nov., isolated from nodules of Phaseolus vulgaris in Spain

Two endophytic strains, coded MOVP5^T and MOPV6, were isolated from nodules of Phaseolus vulgaris plants grown on agricultural soil in Southeastern Spain, and were characterized through a polyphasic taxonomy approach. Their 16S rRNA gene sequences showed 99.3 and 99.4 %, 98.9 and 99.6 %, and 99.0 and 98.7% similarity to 'A. deltaense' YIC 4121^T, A. radiobacter LGM 140^T, and A. pusense NRCPB10^T, respectively. Multilocus sequence analysis based on sequences of recA and atpD genes suggested that these two strains could represent a new Agrobacterium species with less than 96.5 % similarity to their closest relatives. PCR amplification of the telA gene, involved in synthesis of protelomerase, confirmed the affiliation of strains MOPV5^T and MOPV6 to the genus Agrobacterium. Whole genome average nucleotide identity and digital DNA-DNA hybridization average values were less than 95.1 and 66.7 %, respectively, with respect to its closest related species. Major fatty acids in strain MOPV5^T were C_18 : 1ω7c/C_18 : 1ω6c in summed feature 8, C_19 : 0 cyclo ω8c, C_16 : 0 and C_16 : 0 3-OH. Colonies were small to medium, pearl-white coloured on YMA at 28 °C and growth was observed at 10-42 °C, pH 5.0-10.0 and with 0.0-0.5 % (w/v) NaCl. The DNA G+C content was 59.9 mol%. These two strains differ from all other genomovars of Agrobacterium found so far, including those that have not yet given a Latin name. The combined genotypic, phenotypic and chemotaxonomic data support the classification of strain MOPV5^T as representing a novel species of Agrobacterium, for which the name Agrobacterium leguminum sp. nov. is proposed. The type strain is MOPV5^T (=CECT 30096^T=LMG 31779^T).

Comparative Genomics of Novel Agrobacterium G3 Strains Isolated From the International Space Station and Description of Agrobacterium tomkonis sp. nov

Strains of Agrobacterium genomospecies 3 (i.e., genomovar G3 of the Agrobacterium tumefaciens species complex) have been previously isolated from diverse environments, including in association with plant roots, with algae, as part of a lignocellulose degrading community, from a hospital environment, as a human opportunistic pathogen, or as reported in this study, from a surface within the International Space Station. Polyphasic taxonomic methods revealed the relationship of Agrobacterium G3 strains to other Agrobacterium spp., which supports the description of a novel species. The G3 strains tested (n = 9) were phenotypically distinguishable among the strains from other genomospecies of the genus Agrobacterium. Phylogenetic analyses of the 16S rRNA gene, gyrB gene, multi-locus sequence analysis, and 1,089-gene core-genome gene concatenate concur that tested G3 strains belong to the Agrobacterium genus and they form a clade distinct from other validly described Agrobacterium species. The distinctiveness of this clade was confirmed by average nucleotide identity (ANI) and in silico digital DNA-DNA hybridization (dDDH) comparisons between the G3 tested strains and all known Agrobacterium species type strains, since obtained values were considerably below the 95% (ANI) and 70% (dDDH) thresholds used for the species delineation. According to the core-genome phylogeny and ANI comparisons, the closest relatives of G3 strains were Agrobacterium sp. strains UGM030330-04 and K599, members of a novel genomospecies we propose to call genomovar G21. Using this polyphasic approach, we characterized the phenotypic and genotypic synapomorphies of Agrobacterium G3, showing it is a bona fide bacterial species, well separated from previously named Agrobacterium species or other recognized genomic species. We thus propose the name Agrobacterium tomkonis for this species previously referred to as Agrobacterium genomospecies 3. The type strain of A. tomkonis is IIF1SW-B1T (= LMG 32164 = NRRL B-65602). Comparative genomic analysis show A. tomkonis strains have species-specific genes associated with secretion of secondary metabolites, including an exopolysaccharide and putative adhesins and resistance to copper. A. tomkonis specific gene functions notably relate to surface adhesion and could be involved to colonize nutrient-poor and harsh habitats. The A. tomkonis strains from the ISS showed presence of a 40-kbp plasmid and several other potential mobile genetic elements detected that could also be part of conjugative elements or integrated prophages.

Origin of Pathogens of Grapevine Crown Gall Disease in Hokkaido in Japan as Characterized by Molecular Epidemiology of Allorhizobium vitis Strains

Crown gall is a globally distributed and economically important disease of grapevine and other important crop plants. The causal agent of grapevine crown gall is tumorigenic Allorhizobium vitis (Ti) strains that harbor a tumor-inducing plasmid (pTi). The epidemic of grapevine crown gall has not been widely elucidated. In this study, we investigated the genetic diversity of 89 strains of Ti and nonpathogenic A. vitis to clarify their molecular epidemiology. Multi-locus sequence analysis (MLSA) of the partial nucleotide sequences of pyrG, recA, and rpoD was performed for molecular typing of A. vitis strains isolated from grapevines with crown gall symptoms grown in 30 different vineyards, five different countries, mainly in Japan, and seven genomic groups A to F were obtained. The results of MLSA and logistic regression indicated that the population of genetic group A was significantly related to a range of prefectures and that the epidemic of group A strains originated mainly in Hokkaido in Japan through soil infection. Moreover, group E strains could have been transported by infected nursery stocks. In conclusion, this study indicates that both soil infection and transporting of infected nursery stocks are working as infection source in Hokkaido.

Complete genomic sequence and phylogenomics analysis of Agrobacterium strain AB2/73: a new Rhizobium species with a unique mega-Ti plasmid

Background

The Agrobacterium strain AB2/73 has a unique host range for the induction of crown gall tumors, and contains an exceptionally large, over 500 kbp mega Ti plasmid. We used whole genome sequencing to fully characterize and comparatively analyze the complex genome of strain AB2/73, including its Ti plasmid and virulence factors.

Results

We obtained a high-quality, full genomic sequence of AB2/73 by a combination of short-read Illumina sequencing and long-read Nanopore sequencing. The AB2/73 genome has a total size of 7,266,754 bp with 59.5% GC for which 7012 genes (6948 protein coding sequences) are predicted. Phylogenetic and comparative genomics analysis revealed that strain AB2/73 does not belong to the genus Agrobacterium, but to a new species in the genus Rhizobium, which is most related to Rhizobium tropici. In addition to the chromosome, the genome consists of 6 plasmids of which the largest two, of more than 1 Mbp, have chromid-like properties. The mega Ti plasmid is 605 kbp in size and contains two, one of which is incomplete, repABC replication units and thus appears to be a cointegrate consisting of about 175 kbp derived from an unknown Ti plasmid linked to 430 kbp from another large plasmid. In pTiAB2/73 we identified a complete set of virulence genes and two T-DNAs. Besides the previously described T-DNA we found a larger, second T-DNA containing a 6b-like onc gene and the acs gene for agrocinopine synthase. Also we identified two clusters of genes responsible for opine catabolism, including an acc-operon for agrocinopine degradation, and genes putatively involved in ridéopine catabolism. The plasmid also harbours tzs, iaaM and iaaH genes for the biosynthesis of the plant growth regulators cytokinin and auxin.

Conclusions

The comparative genomics analysis of the high quality genome of strain AB2/73 provided insight into the unusual phylogeny and genetic composition of the limited host range Agrobacterium strain AB2/73. The description of its unique genomic composition and of all the virulence determinants in pTiAB2/73 will be an invaluable tool for further studies into the special host range properties of this bacterium.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-021-02358-0.

Opine biosynthesis in naturally transgenic plants: Genes and products

The plant pathogen Agrobacterium transfers DNA into plant cells by a specific transfer mechanism. Expression of this transferred DNA or T-DNA leads to crown gall tumors or abnormal, hairy roots and the synthesis of specific compounds, called opines. Opines are produced from common plant metabolites like sugars, amino acids and α-keto acids, which are combined into different low molecular weight structures by T-DNA-encoded opine synthase enzymes. Opines can be converted back by Agrobacterium into the original metabolites and used for agrobacterial growth. Recently it has been discovered that about 7% of Angiosperms carry T-DNA-like sequences. These result from ancient Agrobacterium transformation events, followed by spontaneous regeneration of transformed cells into natural genetically transformed organisms (nGMOs). Nearly all nGMOs identified up to date carry opine synthesis genes, several of these are intact and potentially encode opine synthesis. So far, only tobacco and cuscuta have been demonstrated to contain opines. Whereas opines from crown gall and hairy root tissues have been studied for over 60 years, those from the nGMOs remain to be explored.

The Rhizobial Microbiome from the Tropical Savannah Zones in Northern Côte d'Ivoire

Over the past decade, many projects have been initiated worldwide to decipher the composition and function of the soil microbiome, including the African Soil Microbiome (AfSM) project that aims at providing new insights into the presence and distribution of key groups of soil bacteria from across the African continent. In this national study, carried out under the auspices of the AfSM project, we assessed the taxonomy, diversity and distribution of rhizobial genera in soils from the tropical savannah zones in Northern Côte d’Ivoire. Genomic DNA extracted from seven sampled soils was analyzed by sequencing the V4-V5 variable region of the 16S rDNA using Illumina’s MiSeq platform. Subsequent bioinformatic and phylogenetic analyses showed that these soils harbored 12 out of 18 genera of Proteobacteria harboring rhizobia species validly published to date and revealed for the first time that the Bradyrhizobium genus dominates in tropical savannah soils, together with Microvirga and Paraburkholderia. In silico comparisons of different 16S rRNA gene variable regions suggested that the V5-V7 region could be suitable for differentiating rhizobia at the genus level, possibly replacing the use of the V4-V5 region. These data could serve as indicators for future rhizobial microbiome explorations and for land-use decision-making.

Pararhizobium mangrovi sp. nov., Isolated From Aegiceras corniculatum Stem

A novel Gram-stain-negative, catalase- and oxidase-positive, motile, short rod-shaped bacterium designated BGMRC 6574^T was isolated from stems of Aegiceras corniculatum collected from Hainan province, China. The strain grew at 25-37 °C (optimal at 28 °C), pH 5.0-10.0 (pH 7.0), and 3-8% (w/v) NaCl (3%). Based on the 16S rRNA phylogenetic analysis, the strain was closely related to Pararhizobium haloflavum MCCC 1K03228^T (96.45% sequence similarity). The novel strain showed an average nucleotide identity value and a digital DNA-DNA hybridization of 72.62 and 27.1%, respectively, to P. haloflavum MCCC 1K03228^T based on draft genome sequences. The G+C content of the genomic DNA was 64.7 mol%. The major respiratory quinone was Q-10. The strain possessed genes putatively encoding choline uptake and conversion to betaine gene clusters. The extract significantly delayed the lifespan of Caenorhabditis elegans compared to the control (P < 0.05). The major polar lipids were phosphatidylcholine, seven unidentified phospholipids, three unidentified ninhydrin-positive phospholipids, and two unidentified lipids. The major cellular fatty acid was C_19:0 cyclo ω8c. The results of a polyphasic taxonomic study showed that strain BGMRC 6574^T represents a new species of the genus Pararhizobium, and it was named Pararhizobium mangrovi sp. nov. The type strain is BGMRC 6574^T (=KCTC 72636^T = CGMCC 1.16783).

Peteryoungia gen. nov. with four new species combinations and description of Peteryoungia desertarenae sp. nov., and taxonomic revision of the genus Ciceribacter based on phylogenomics of Rhizobiaceae

A novel bacterial strain designated as ADMK78^T was isolated from the saline desert soil. The cells were rod-shaped, Gram-stain-negative, and non-motile. The strain ADMK78^T grows best at 28 °C. Phylogeny of 16S rRNA gene placed the strain ADMK78^T with the members of genera Ciceribacter and Rhizobium, while the highest sequence similarity was with Rhizobium wuzhouense W44^T (98.7%) and Rhizobium ipomoeae shin9-1 ^T (97.9%). Phylogenetic analysis based on 92 core-genes extracted from the genome sequences and average amino acid identity (AAI) revealed that the strain ADMK78^T forms a distinct cluster including five species of Rhizobium, which is separate from the cluster of the genera Rhizobium and Ciceribacter. We propose re-classification of Rhizobium ipomoeae, R. wuzhouense, R. rosettiformans and R. rhizophilum into the novel genus Peteryoungia. The average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) values of ADMK78^T were less than 82 and 81%, respectively, among all type strains included in the genus Peteryoungia. The strain ADMK78^T showed differences in physiological, phenotypic, and protein profiles estimated by MALDI-TOF MS to its closest relatives. Based on the phenotypic, chemotaxonomic properties, and phylogenetic analyses, the strain ADMK78^T represents a novel species, Peteryoungia desertarenae sp. nov. The type strain is ADMK78^T (= MCC 3400^T; KACC 21383^T; JCM 33657^T). We also proposed the reclassification of Rhizobium daejeonense, R. naphthalenivorans and R. selenitireducens, into the genus Ciceribacter, based on core gene phylogeny and AAI values.

Paraburkholderia spp. are the main rhizobial microsymbionts of Mimosa tenuiflora (Willd.) Poir. in soils of the Brazilian tropical dry forests (Caatinga biome)

Mimosa tenuiflora (Willd.) Poir. is widespread in southern and central American drylands, but little information is available concerning its associated rhizobia. Therefore, this study aimed to characterize M. tenuiflora rhizobia from soils of the tropical dry forests (Caatinga) in Pernambuco State, Brazil, at the molecular and symbiotic levels. Soil samples of pristine Caatinga areas in four municipalities were used to grow M. tenuiflora. First, the bacteria from root nodules were subjected to nodC/nifH gene amplification, and the bacteria positive for both genes had the 16S rRNA gene sequenced. Then, ten strains were evaluated using recA, gyrB, and nodC gene sequences, and seven of them had their symbiotic efficiency assessed. Thirty-two strains were obtained and 22 of them were nodC/nifH positive. Twenty strains clustered within Paraburkholderia and two within Rhizobium by 16S rRNA gene sequencing. The beta-rhizobia were similar to P. phenoliruptrix (12) and P. diazotrophica (8). Both alpha-rhizobia were closely related to R. miluonense. The recA + gyrB phylogenetic analysis clustered four and five strains within the P. phenoliruptrix and P. diazotrophica branches, respectively, but they were somewhat divergent to the 16S rRNA phylogeny. For Rhizobium sp. ESA 637, the recA + gyrB phylogeny clustered the strain with R. jaguaris. The nodC phylogeny indicated that ESA 626, ESA 629, and ESA 630 probably represented a new symbiovar branch. The inoculation assay showed high symbiotic efficiency for all tested strains. The results indicated high genetic diversity and efficiency of M. tenuiflora rhizobia in Brazilian drylands and included P. phenoliruptrix-like bacteria in the list of efficient beta-rhizobia in the Caatinga biome.

Diversity of Culturable Bacteria Isolated from Highland Barley Cultivation Soil in Qamdo, Tibet Autonomous Region

The soil bacterial communities have been widely investigated. However, there has been little study of the bacteria in Qinghai-Tibet Plateau, especially about the culturable bacteria in highland barley cultivation soil. Here, a total of 830 individual strains were obtained at 4°C and 25°C from a highland barley cultivation soil in Qamdo, Tibet Autonomous Region, using fifteen kinds of media. Seventy-seven species were obtained, which belonged to 42 genera and four phyla; the predominant phylum was Actinobacteria (68.82%), followed by Proteobacteria (15.59%), Firmicutes (14.29%), and Bacteroidetes (1.30%). The predominant genus was Streptomyces (22.08%, 17 species), followed by Bacillus (6.49%, five species), Micromonospora (5.19%, four species), Microbacterium (5.19%, four species), and Kribbella (3.90%, three species). The most diverse isolates belonged to a high G+C Gram-positive group; in particular, the Streptomyces genus is a dominant genus in the high G+C Gram-positive group. There were 62 species and 33 genera bacteria isolated at 25°C (80.52%), 23 species, and 18 genera bacteria isolated at 4°C (29.87%). Meanwhile, only eight species and six genera bacteria could be isolated at 25°C and 4°C. Of the 77 species, six isolates related to six genera might be novel taxa. The results showed abundant bacterial species diversity in the soil sample from the Qamdo, Tibet Autonomous Region.

Development of a DNA-Based Real-Time PCR Assay To Quantify Allorhizobium vitis Over Time in Grapevine (Vitis vinifera L.) Plantlets

Allorhizobium vitis is the primary causal pathogen of grapevine crown gall disease. Because this endophytic bacterium can survive as a systemic latent (symptomless) infection in grapevine, detecting and monitoring its development in planta is of great importance. In plant bacteria studies, plate counting is routinely used as a simple and reliable method to evaluate the bacterial population level in planta. However, isolation techniques are time-consuming and present some disadvantages such as the risk of contamination and the need for fresh samples for research. In this study, we developed a DNA-based real-time PCR assay that can replace the classical method to monitor the development of Allorhizobium vitis in grapevine plantlets. Primers targeting Allorhizobium vitis chromosomic genes and the virulent tumor-inducing plasmid were validated. The proposed quantitative real-time PCR technique is highly reliable and reproducible to assess Allorhizobium vitis numeration at the earliest stage of infection until tumor development in grapevine plantlets. Moreover, this low-cost technique provides rapid and robust in planta quantification of the pathogen and is suitable for fundamental research to monitor bacterial development over time.

Defining the Rhizobium leguminosarum Species Complex

Bacteria currently included in Rhizobium leguminosarum are too diverse to be considered a single species, so we can refer to this as a species complex (the Rlc). We have found 429 publicly available genome sequences that fall within the Rlc and these show that the Rlc is a distinct entity, well separated from other species in the genus. Its sister taxon is R. anhuiense. We constructed a phylogeny based on concatenated sequences of 120 universal (core) genes, and calculated pairwise average nucleotide identity (ANI) between all genomes. From these analyses, we concluded that the Rlc includes 18 distinct genospecies, plus 7 unique strains that are not placed in these genospecies. Each genospecies is separated by a distinct gap in ANI values, usually at approximately 96% ANI, implying that it is a 'natural' unit. Five of the genospecies include the type strains of named species: R. laguerreae, R. sophorae, R. ruizarguesonis, "R. indicum" and R. leguminosarum itself. The 16S ribosomal RNA sequence is remarkably diverse within the Rlc, but does not distinguish the genospecies. Partial sequences of housekeeping genes, which have frequently been used to characterize isolate collections, can mostly be assigned unambiguously to a genospecies, but alleles within a genospecies do not always form a clade, so single genes are not a reliable guide to the true phylogeny of the strains. We conclude that access to a large number of genome sequences is a powerful tool for characterizing the diversity of bacteria, and that taxonomic conclusions should be based on all available genome sequences, not just those of type strains.

Phosphorus Nutrition and Growth of Cotton Plants Inoculated With Growth-Promoting Bacteria Under Low Phosphate Availability

The low availability of phosphorus (P) in the soil drastically limits the world productivity of crops such as cotton. In order to contribute sustainably to the solution of this problem, the current study aimed to evaluate the capacity of phosphate-solubilising bacteria to improve plant growth and its relationship with physiological parameters, as well as the shoot P content in cotton plants in a soil with low P availability amended with rock phosphate. The results showed that, of the six plant growth-promoting bacteria strains evaluated under greenhouse conditions, the Rhizobium strain B02 significantly promoted growth, shoot P content and photosynthetic rate. This strain also improved the transpiration rate and the relative content of chlorophyll but without significant differences. Remarkably, Rhizobium sp. B02 had a more significant effect on plant growth compared to the P nutrition. Furthermore, the effect of its inoculation was more pronounced on the roots' growth compared to the shoot. Finally, application of Rhizobium strain B02 showed the capacity to optimize the use of low-solubility fertilizer as the rock phosphate. These findings could be associated with the metabolic activities of plant growth promotion exhibited by phosphate-solubilising strains, such as phosphate solubilisation, production of indole compounds and siderophores synthesis. In conclusion, this research provides evidence of the biotechnological potential of the Rhizobium genus as phosphate-solubilising bacteria with multiple plant growth-promoting activities capable of improving the plant growth and phosphate nutrition of non-leguminous crops such as cotton in soil with low P availability amended with rock phosphate.

Development and validation of a UHPLC-ESI-QTOF mass spectrometry method to analyze opines, plant biomarkers of crown gall or hairy root diseases

Opines are low-molecular-weight metabolites specifically biosynthesized by agrobacteria-transformed plant cells when plants are struck by crown gall and hairy root diseases, which cause uncontrolled tissue overgrowth. Transferred DNA is sustainably incorporated into the genomes of the transformed plant cells, so that opines constitute a persistent biomarker of plant infection by pathogenic agrobacteria and can be targeted for crown gall/hairy root disease diagnosis. We developed a general, rapid, specific and sensitive analytical method for overall opine detection using ultra-high-performance liquid chromatography-electrospray ionization quadrupole time-of-flight mass spectrometry (UHPLC-ESI-MS-QTOF), with easy preparation of samples. Based on MS, MS/MS and chromatography data, the detection selectivity of a wide range of standard opines was validated in pure solution and in different plant extracts. The method was successfully used to detect different structural types of opines, including opines for which standard compounds are unavailable, in tumors or hairy roots induced by pathogenic strains. As the method can detect a wide range of opines in a single run, it represents a powerful tool for plant gall analysis and crown gall/hairy root disease diagnosis. Using an appropriate dilution of plant extract and a matrix-based calibration curve, the quantification ability of the method was validated for three opines belonging to different families (nopaline, octopine, mannopine), which were accurately quantified in plant tissue extracts.

Phylogenomics reveals the basis of adaptation of Pseudorhizobium species to extreme environments and supports a taxonomic revision of the genus

The family Rhizobiaceae includes many genera of soil bacteria, often isolated for their association with plants. Herein, we investigate the genomic diversity of a group of Rhizobium species and unclassified strains isolated from atypical environments, including seawater, rock matrix or polluted soil. Based on whole-genome similarity and core genome phylogeny, we show that this group corresponds to the genus Pseudorhizobium. We thus reclassify Rhizobium halotolerans, R. marinum, R. flavum and R. endolithicum as P. halotolerans sp. nov., P. marinum comb. nov., P. flavum comb. nov. and P. endolithicum comb. nov., respectively, and show that P. pelagicum is a synonym of P. marinum. We also delineate a new chemolithoautotroph species, P. banfieldiae sp. nov., whose type strain is NT-26^T (=DSM 106348^T=CFBP 8663^T). This genome-based classification was supported by a chemotaxonomic comparison, with increasing taxonomic resolution provided by fatty acid, protein and metabolic profiles. In addition, we used a phylogenetic approach to infer scenarios of duplication, horizontal transfer and loss for all genes in the Pseudorhizobium pangenome. We thus identify the key functions associated with the diversification of each species and higher clades, shedding light on the mechanisms of adaptation to their respective ecological niches. Respiratory proteins acquired at the origin of Pseudorhizobium were combined with clade-specific genes to enable different strategies for detoxification and nutrition in harsh, nutrient-poor environments.

Strain ATCC 4720T is the authentic type strain of Agrobacterium tumefaciens, which is not a later heterotypic synonym of Agrobacterium radiobacter

The original type strains of Agrobacterium radiobacter and Agrobacterium tumefaciens recorded in the eighth edition of Bergey's Manual of Determinative Bacteriology published in 1974 were NCIB 9042^T and ATCC 4720^T, respectively. However, in the list of the valid names of bacteria compiled in 1980, both strains were changed, A. radiobacter NCIB 9042^T to ATCC 19358^T and A. tumefaciens ATCC 4720^T to ATCC 23308^T. These changes were unjustified, particularly in the case of A. tumefaciens whose type strain was replaced by another strain from the same collection, although the original type strain ATCC 4720^T was never lost and it is currently available in several culture collections. Therefore, we request that the type strain of A. tumefaciens be corrected from ATCC 23308^T to ATCC 4720^T.

Dynamic Changes in the Microbiome of Rice During Shoot and Root Growth Derived From Seeds

Microbes form close associations with host plants including rice as both surface (epiphytes) and internal (endophytes) inhabitants. Yet despite rice being one of the most important cereal crops agriculturally and economically, knowledge of its microbiome, particularly core inhabitants and any functional properties bestowed is limited. In this study, the microbiome in rice seedlings derived directly from seeds was identified, characterized and compared to the microbiome of the seed. Rice seeds were sourced from two different locations in Arkansas, USA of two different rice genotypes (Katy, M202) from two different harvest years (2013, 2014). Seeds were planted in sterile media and bacterial as well as fungal communities were identified through 16S and ITS sequencing, respectively, for four seedling compartments (root surface, root endosphere, shoot surface, shoot endosphere). Overall, 966 bacterial and 280 fungal ASVs were found in seedlings. Greater abundance and diversity were detected for the microbiome associated with roots compared to shoots and with more epiphytes than endophytes. The seedling compartments were the driving factor for microbial community composition rather than other factors such as rice genotype, location and harvest year. Comparison with datasets from seeds revealed that 91 (out of 296) bacterial and 11 (out of 341) fungal ASVs were shared with seedlings with the majority being retained within root tissues. Core bacterial and fungal microbiome shared across seedling samples were identified. Core bacteria genera identified in this study such as Rhizobium, Pantoea, Sphingomonas, and Paenibacillus have been reported as plant growth promoting bacteria while core fungi such as Pleosporales, Alternaria and Occultifur have potential as biocontrol agents.

When a Root Is the Cause of Infection

BACKGROUND

Sinorhizobium meliloti is a phytobacterium found in the root nodules of plants, where it is involved in fixing nitrogen for delivery to the roots in exchange for a photosynthate carbon source. There have been no reported cases of S. meliloti infection in humans. We conducted a retrospective review of clinical records and diagnostic tests.

CASE DESCRIPTION

An 81-year-old woman who presented to the emergency department with a 1-day history of progressive decline in her level of consciousness following a head injury and deep scalp laceration. Her medical history was significant for a ventriculoperitoneal shunt due to normal pressure hydrocephalus. Imaging studies revealed hydrocephalus and a tear in the shunt catheter. Cerebrospinal fluid analysis was not suggestive for meningitis. Cerebrospinal fluid culture revealed an unfamiliar organism, identified as S. meliloti following sequencing of its entire genome, which was considered a contaminant. The patient subsequently developed peritonitis, and the same pathogen was detected in the peritoneal fluid, suggesting distal shunt infection. Symptoms resolved after shunt removal and antibiotic treatment. Thorough history taking revealed that the patient had fallen and struck her head against a flowerpot.

CONCLUSIONS

S. meliloti is a phytopathogen that should not be easily disregarded as a contaminant when isolated from human sterile fluids or tissues. Aggressive management including removal of infected hardware, if present, is required to ensure resolution of infection. It emphasizes the importance of thorough history taking.

Genetic diversity of Agrobacterium species isolated from nodules of common bean and soybean in Brazil, Mexico, Ecuador and Mozambique, and description of the new species Agrobacterium fabacearum sp. nov

Agrobacterium strains are associated with soil, plants and animals, and known mainly by their pathogenicity. We studied 14 strains isolated from nodules of healthy soybean and common bean plants in Brazil, Mexico, Ecuador and Mozambique. Sequence analysis of the 16S rRNA gene positioned the strains as Agrobacterium, but with low phylogenetic resolution. Multilocus sequence analysis (MLSA) of three partial housekeeping genes (glnII, gyrB and recA) positioned the strains in four distinct clades, with Agrobacterium pusense, Agrobacterium deltaense, Agrobacterium radiobacter and Agrobacterium sp. genomospecies G1. Analysis by BOX-PCR revealed high intraspecies diversity. Genomic analysis of representative strains of the three clades indicated that they carry the protelomerase telA gene, and MLSA analysis with six complete housekeeping genes (atpD, glnII, gyrB, recA, rpoB and thrC), as well as average nucleotide identity (less than 90 % with closest species) and digital DNA-DNA hybridization (less than 41 % with closest species) revealed that strain CNPSo 675T and Agrobacterium sp. genomospecies G1 compose a new species. Other phenotypic and genotypic characteristics were determined for the new clade. Although not able to re-nodulate the host, we hypothesize that several strains of Agrobacterium are endophytes in legume nodules, where they might contribute to plant growth. Our data support the description of the CNPSo 675T and Agrobacterium sp. genomospecies G1 strains as a new species, for which the name Agrobacterium fabacearum is proposed. The type strain is CNPSo 675T (=UMR 1457T=LMG 31642T) and is also deposited in other culture collections.

Genome Analysis of Endobacterium cerealis, a Novel Genus and Species Isolated from Zea mays Roots in North Spain

In the present work, we analyse the genomic and phenotypic characteristics of a strain named RZME27^T isolated from roots of a Zea mays plant grown in Spain. The phylogenetic analyses of 16S rRNA gene and whole genome sequences showed that the strain RZME27^T clustered with the type strains of Neorhizobium galegae and Pseudorhizobium pelagicum from the family Rhizobiaceae. This family encompasses several genera establishing symbiosis with legumes, but the genes involved in nodulation and nitrogen fixation are absent in its genome. Nevertheless, genes related to plant colonization, such as those involved in motility, chemotaxis, quorum sensing, exopolysaccharide biosynthesis and hydrolytic enzymes production were found. The comparative pangenomic analyses showed that 78 protein clusters present in the strain RZME27^T were not found in the type strains of its closest relatives N. galegae and P. pelagicum. The calculated average nucleotide identity (ANI) values between the strain RZME27^T and the type strains of N. galegae and P. pelagicum were 75.61% and 75.1%, respectively, similar or lower than those found for other genera from family Rhizobiaceae. Several phenotypic differences were also found, highlighting the absence of the fatty acid C_19:0 cyclo ω8c and propionate assimilation. These results support the definition of a novel genus and species named Endobacterium cerealis gen. nov. sp. nov. whose type strain is RZME27^T.

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.