Genomic basis of broad host range and environmental adaptability of Rhizobium tropici CIAT 899 and Rhizobium sp. PRF 81 which are used in inoculants for common bean (Phaseolus vulgaris L.)

Developed Rhizobium Strains Enhance Soil Fertility and Yield of Legume Crops in Haryana, India

Three strains of Gram-negative bacterium, Rhizobium, were developed by gamma (γ)-irradiation random mutagenesis. The developed strains were evaluated for their augmented features for symbiotic association, nitrogen fixation, and crop yield of three leguminous plants-chickpea, field-pea, and lentil-in agricultural fields of the northern Indian state of Haryana. Crops treated with developed mutants exhibited significant improvement in plant features and the yield of crops when compared to the control-uninoculated crops and crops grown with indigenous or commercial crop-specific strains of Rhizobium. This improvement was attributed to generated mutants, MbPrRz1 (on chickpea), MbPrRz2 (on lentil), and MbPrRz3 (on field-pea). Additionally, the cocultured symbiotic response of MbPrRz1 and MbPrRz2 mutants was found to be more pronounced on all three crops. The statistical analysis using Pearson's correlation coefficients revealed that nodulation and plant biomass were the most related parameters of crop yield. Among the effectiveness of developed mutants, MbPrRz1 yielded the best results for all three tested crops. Moreover, the developed mutants enhanced macro- and micronutrients of the experimental fields when compared with fields harboring the indigenous rhizobial community. These developed mutants were further genetically characterized, predominantly expressing nitrogen fixation marker, nifH, and appeared to belong to Mesorhizobium ciceri (MbPrRz1) and Rhizobium leguminosarum (both MbPrRz2 and MbPrRz3). In summary, this study highlights the potential of developed Rhizobium mutants as effective biofertilizers for sustainable agriculture, showcasing their ability to enhance symbiotic relationships, crop yield, and soil fertility.

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.

Isolation of Rhizobial Extracellular Membrane Vesicles from Bacteroids

Extracellular-membrane vesicles (EMVs) are spherical buds of the extracellular membrane, commonly produced by Gram-negative bacteria, known to mediate intricate inter-kingdom communication. In this context, comprehensive research dissecting the role of EMVs in one of the most complex nature-occurring molecular dialogues, rhizobium-legume symbiosis, has been so far neglected. During the different stages of the symbiotic process, rhizobia and their host plants establish a very specific and controlled intercellular trafficking of signal molecules. Thus, as conveyors of a broad range of molecules into the target cell, EMVs are gaining weight in the field. Here, we describe a detailed protocol to isolate EMVs from bacteroids of legume nodules, opening a new door for discovering new authors of the symbiotic process.

Genomes of two type strains of the Rhizobium tropici group: R. calliandrae CCGE524T and R. mayense CCGE526T

The genome sequences of two nitrogen-fixing type strains of the Rhizobium tropici group were obtained: Rhizobium calliandrae CCGE524T and R. mayense CCGE526T. Genomic analyses confirmed their taxonomic position and identified three complete sequences of the repABC genes, indicative of three plasmids, one of them carrying symbiotic genes.

Impact of Fe + 2 ions on structural integrity of A0A6P1CI42_RHITR NifA protein from Rhizobium tropici strain CIAT 899

A0A6P1CI42_RHITR, a protein originating from Rhizobium tropici strain CIAT 899, has emerged as a key player in leguminous plant symbiosis and nitrogen fixation processes. Understanding the intricate details of its structure and function holds immense significance for unraveling the molecular mechanisms underlying its biological activities. In this study, we employed molecular modeling and molecular dynamics (MD) simulations to investigate the A0A6P1CI42_RHITR protein, with a specific emphasis on the influence of Fe-atoms, linker structural integrity, and conformational changes within the GAF domain. Our findings unveiled noteworthy conformational changes in the A0A6P1CI42_RHITR protein, particularly within the GAF domain, when Fe-atoms were present compared to its apo form. Significant conformational rearrangements after an initial 20 ns, accompanied by the opening of the ligand substrate accommodating loop in the GAF domain influenced by Fe-atoms was observed. At the residue level, the investigation revealed substantial activity variations in individual residues, particularly in those contributing to the GAF domain from positions 51 to 223. Intriguingly, the presence of Fe-atoms led to controlled movement of conserved cysteine residues at positions 467 and 472, indicating a correlation between interlinker domain motion and the activity of the GAF domain loop responsible for substrate accommodation. Moreover, in the presence of Fe-atoms, the distance between Cys467 and Cys472 residues was maintained, ensuring the overall structural integrity of the interdomain loop necessary for protein activation. Conversely, in the apo form, a sudden flip motion of cysteine residues' thiol groups was observed, leading to a loss of structural integration. Overall, our study utilizing molecular modeling and MD simulations offers valuable insights into the structural dynamics and functional implications of the A0A6P1CI42_RHITR protein.Communicated by Ramaswamy H. Sarma.

The outstanding diversity of rhizobia microsymbionts of common bean (Phaseolus vulgaris L.) in Mato Grosso do Sul, central-western Brazil, revealing new Rhizobium species

Common bean is considered a legume of great socioeconomic importance, capable of establishing symbioses with a wide variety of rhizobial species. However, the legume has also been recognized for its low efficiency in fixing atmospheric nitrogen. Brazil is a hotspot of biodiversity, and in a previous study, we identified 13 strains isolated from common bean (Phaseolus vulgaris) nodules in three biomes of Mato Grosso do Sul state, central-western Brazil, that might represent new phylogenetic groups, deserving further polyphasic characterization. The phylogenetic tree of the 16S rRNA gene split the 13 strains into two large clades, seven in the R. etli and six in the R. tropici clade. The MLSA with four housekeeping genes (glnII, gyrB, recA, and rpoA) confirmed the phylogenetic allocation. Genomic comparisons indicated eight strains in five putative new species and the remaining five as R. phaseoli. The average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) comparing the putative new species and the closest neighbors ranged from 81.84 to 92.50% and 24.0 to 50.7%, respectively. Other phenotypic, genotypic, and symbiotic features were evaluated. Interestingly, some strains of both R. etli and R. tropici clades lost their nodulation capacity. The data support the description of the new species Rhizobium cerradonense sp. nov. (CNPSo 3464T), Rhizobium atlanticum sp. nov. (CNPSo 3490T), Rhizobium aureum sp. nov. (CNPSo 3968T), Rhizobium pantanalense sp. nov. (CNPSo 4039T), and Rhizobium centroccidentale sp. nov. (CNPSo 4062T).

Novel symbiovars ingae, lysilomae and lysilomaefficiens in bradyrhizobia from tree-legume nodules

Inga vera and Lysiloma tree legumes form nodules with Bradyrhizobium spp. from the japonicum group that represent novel genomospecies, for which we describe here using genome data, symbiovars lysilomae, lysilomaefficiens and ingae. Genes encoding Type three secretion system (TTSS) that could affect host specificity were found in ingae but not in lysilomae nor in lysilomaefficiens symbiovars and uptake hydrogenase hup genes (that affect nitrogen fixation) were observed in bradyrhizobia from the symbiovars ingae and lysilomaefficiens. nolA gene was found in the symbiovar lysilomaefficiens but not in strains from lysilomae. We discuss that multiple genes may dictate symbiosis specificity. Besides, toxin-antitoxin genes were found in the symbiosis islands in bradyrhizobia from symbiovars ingae and lysilomaefficiens. A limit (95%) to define symbiovars with nifH gene sequences was proposed here.

Five copper homeostasis gene clusters encode the Cu-efflux resistome of the highly copper-tolerant Methylorubrum extorquens AM1

Background

In the last decade, the use of copper has reemerged as a potential strategy to limit healthcare-associated infections and to control the spread of multidrug-resistant pathogens. Numerous environmental studies have proposed that most opportunistic pathogens have acquired antimicrobial resistance in their nonclinical primary habitat. Thus, it can be presumed that copper-resistant bacteria inhabiting a primary commensal niche might potentially colonize clinical environments and negatively affect the bactericidal efficacy of Cu-based treatments. The use of copper in agricultural fields is one of the most important sources of Cu pollution that may exert selection pressure for the increase of copper resistance in soil and plant-associated bacteria. To assess the emergence of copper-resistant bacteria in natural habitats, we surveyed a laboratory collection of bacterial strains belonging to the order Rhizobiales. This study proposes that Methylorubrum extorquens AM1 is an environmental isolate well adapted to thrive in copper-rich environments that could act as a reservoir of copper resistance genes.

Methods

The minimal inhibitory concentrations (MICs) of CuCl₂ were used to estimate the copper tolerance of eight plant-associated facultative diazotrophs (PAFD) and five pink-pigmented facultative methylotrophs (PPFM) belonging to the order Rhizobiales presumed to come from nonclinical and nonmetal-polluted natural habitats based on their reported source of isolation. Their sequenced genomes were used to infer the occurrence and diversity of Cu-ATPases and the copper efflux resistome of Mr. extorquens AM1.

Results

These bacteria exhibited minimal inhibitory concentrations (MICs) of CuCl₂ ranging between 0.020 and 1.9 mM. The presence of multiple and quite divergent Cu-ATPases per genome was a prevalent characteristic. The highest copper tolerance exhibited by Mr. extorquens AM1 (highest MIC of 1.9 mM) was similar to that found in the multimetal-resistant model bacterium Cupriavidus metallidurans CH34 and in clinical isolates of Acinetobacter baumannii. The genome-predicted copper efflux resistome of Mr. extorquens AM1 consists of five large (6.7 to 25.7 kb) Cu homeostasis gene clusters, three clusters share genes encoding Cu-ATPases, CusAB transporters, numerous CopZ chaperones, and enzymes involved in DNA transfer and persistence. The high copper tolerance and the presence of a complex Cu efflux resistome suggest the presence of relatively high copper tolerance in environmental isolates of Mr. extorquens.

Adaptive Evolution of Rhizobial Symbiosis beyond Horizontal Gene Transfer: From Genome Innovation to Regulation Reconstruction

There are ubiquitous variations in symbiotic performance of different rhizobial strains associated with the same legume host in agricultural practices. This is due to polymorphisms of symbiosis genes and/or largely unexplored variations in integration efficiency of symbiotic function. Here, we reviewed cumulative evidence on integration mechanisms of symbiosis genes. Experimental evolution, in concert with reverse genetic studies based on pangenomics, suggests that gain of the same circuit of key symbiosis genes through horizontal gene transfer is necessary but sometimes insufficient for bacteria to establish an effective symbiosis with legumes. An intact genomic background of the recipient may not support the proper expression or functioning of newly acquired key symbiosis genes. Further adaptive evolution, through genome innovation and reconstruction of regulation networks, may confer the recipient of nascent nodulation and nitrogen fixation ability. Other accessory genes, either co-transferred with key symbiosis genes or stochastically transferred, may provide the recipient with additional adaptability in ever-fluctuating host and soil niches. Successful integrations of these accessory genes with the rewired core network, regarding both symbiotic and edaphic fitness, can optimize symbiotic efficiency in various natural and agricultural ecosystems. This progress also sheds light on the development of elite rhizobial inoculants using synthetic biology procedures.

Nod factor perception: an integrative view of molecular communication during legume symbiosis

Compatible interaction between rhizobial Nod factors and host receptors enables initial recognition and signaling events during legume-rhizobia symbiosis. Molecular communication is a new paradigm of information relay, which uses chemical signals or molecules as dialogues for communication and has been witnessed in prokaryotes, plants as well as in animal kingdom. Understanding this fascinating relay of signals between plants and rhizobia during the establishment of a synergistic relationship for biological nitrogen fixation represents one of the hotspots in plant biology research. Predominantly, their interaction is initiated by flavonoids exuding from plant roots, which provokes changes in the expression profile of rhizobial genes. Compatible interactions promote the secretion of Nod factors (NFs) from rhizobia, which are recognised by cognate host receptors. Perception of NFs by host receptors initiates the symbiosis and ultimately leads to the accommodation of rhizobia within root nodules via a series of mutual exchange of signals. This review elucidates the bacterial and plant perspectives during the early stages of symbiosis, explicitly emphasizing the significance of NFs and their cognate NF receptors.

Native Rhizobia Improve Plant Growth, Fix N2, and Reduce Greenhouse Emissions of Sunnhemp More than Commercial Rhizobia Inoculants in Florida Citrus Orchards

Sunnhemp (Crotalaria juncea L.) is an important legume cover crop used in tree cropping systems, where there is increased interest by growers to identify rhizobia to maximize soil nitrogen (N) inputs. We aimed to isolate and identify native rhizobia and compare their capabilities with non-native rhizobia from commercial inoculants to fix atmospheric dinitrogen (N₂), produce and reduce nitrous oxide (N₂O), and improve plant growth. Phylogenetic analyses of sequences of the 16S rRNA and recA, atpD, and glnII genes showed native rhizobial strains belonged to Rhizobium tropici and the non-native strain to Bradyrhizobium japonicum. Plant nodulation tests, sequencing of nodC and nifH genes, and the acetylene-dependent ethylene production assay confirmed the capacity of all strains to nodulate sunnhemp and fix N₂. Inoculation with native rhizobial strains resulted in significant increases in root and shoot weight and total C and N contents in the shoots, and showed greater N₂-fixation rates and lower emissions of N₂O compared to the non-native rhizobium. Our results suggest that native rhizobia improve plant growth, fix N₂, and reduce greenhouse emissions of sunnhemp more than commercial rhizobia inoculants in Florida citrus orchards.

The Rhizobium tropici CIAT 899 NodD2 protein promotes symbiosis and extends rhizobial nodulation range by constitutive nodulation factor synthesis

In the symbiotic associations between rhizobia and legumes, the NodD regulators orchestrate the transcription of the specific nodulation genes. This set of genes is involved in the synthesis of nodulation factors, which are responsible for initiating the nodulation process. Rhizobium tropici CIAT 899 is the most successful symbiont of Phaseolus vulgaris and can nodulate a variety of legumes. Among the five NodD regulators present in this rhizobium, only NodD1 and NodD2 seem to have a role in the symbiotic process. However, the individual role of each NodD in the absence of the other proteins has remained elusive. In this work, we show that the CIAT 899 NodD2 does not require activation by inducers to promote the synthesis of nodulation factors. A CIAT 899 strain overexpressing nodD2, but lacking all additional nodD genes, can nodulate three different legumes as efficiently as the wild type. Interestingly, CIAT 899 NodD2-mediated gain of nodulation can be extended to another rhizobial species, since its overproduction in Sinorhizobium fredii HH103 not only increases the number of nitrogen-fixing nodules in two host legumes but also results in nodule development in incompatible legumes. These findings potentially open exciting opportunities to develop rhizobial inoculants and increase legume crop production.

Identification of sigma factor 54-regulated small non-coding RNAs by employing genome-wide and transcriptome-based methods in rhizobium strains

Rhizobium-legume symbiosis is considered as the major contributor of biological nitrogen fixation. Bacterial small non-coding RNAs are crucial regulators in several cellular adaptation processes that occur due to the changes in metabolism, physiology, or the external environment. Identifying and analysing the conditional specific/sigma factor-54 regulated sRNAs provides a better understanding of sRNA regulation/mechanism in symbiotic association. In the present study, we have identified sigma factor 54-regulated sRNAs from the genome of six rhizobium strains and from the RNA-seq data of free-living and symbiotic conditions of Bradyrhizobium diazoefficiens USDA 110 to identify the novel putative sRNAs that are over expressed during the regulation of nitrogen fixation. A total of 1351 sRNAs were predicted from the genome of six rhizobium strains and 1375 sRNAs were predicted from the transcriptome data of B. diazoefficiens USDA 110. Analysis of target mRNA for these novel sRNAs was inferred to target several nodulation and nitrogen fixation genes including nodC, nodJ, nodY, nodJ, nodM, nodW, nodZ, nifD, nifN, nifQ, fixK, fixL, fdx, nolB, and several cytochrome proteins. In addition, sRNAs of B. diazoefficiens USDA 110 which targeted the regulatory genes of nitrogen fixation were confirmed by wet-lab experiments with semi-quantitative reverse transcription polymerase chain reaction. Predicted target mRNAs were functionally classified based on the COG analysis and GO annotations. The genome-wide and transcriptome-based integrated methods have led to the identification of several sRNAs involved in the nodulation and symbiosis. Further validation of the functional role of these sRNAs can help in exploring the role of sRNAs in nitrogen metabolism during free-living and symbiotic association with legumes.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03394-x.

Revealing potential functions of hypothetical proteins induced by genistein in the symbiosis island of Bradyrhizobium japonicum commercial strain SEMIA 5079 (= CPAC 15)

BACKGROUND

Bradyrhizobium japonicum strain SEMIA 5079 (= CPAC 15) is a nitrogen-fixing symbiont of soybean broadly used in commercial inoculants in Brazil. Its genome has about 50% of hypothetical (HP) protein-coding genes, many in the symbiosis island, raising questions about their putative role on the biological nitrogen fixation (BNF) process. This study aimed to infer functional roles to 15 HP genes localized in the symbiosis island of SEMIA 5079, and to analyze their expression in the presence of a nod-gene inducer.

RESULTS

A workflow of bioinformatics tools/databases was established and allowed the functional annotation of the HP genes. Most were enzymes, including transferases in the biosynthetic pathways of cobalamin, amino acids and secondary metabolites that may help in saprophytic ability and stress tolerance, and hydrolases, that may be important for competitiveness, plant infection, and stress tolerance. Putative roles for other enzymes and transporters identified are discussed. Some HP proteins were specific to the genus Bradyrhizobium, others to specific host legumes, and the analysis of orthologues helped to predict roles in BNF.

CONCLUSIONS

All 15 HP genes were induced by genistein and high induction was confirmed in five of them, suggesting major roles in the BNF process.

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.

Nodulation competitiveness and diversification of symbiosis genes in common beans from the American centers of domestication

Phaseolus vulgaris (common bean), having a proposed Mexican origin within the Americas, comprises three centers of diversification: Mesoamerica, the southern Andes, and the Amotape-Huancabamba Depression in Peru-Ecuador. Rhizobium etli is the predominant rhizobium found symbiotically associated with beans in the Americasalthough closely related Rhizobium phylotypes have also been detected. To investigate if symbiosis between bean varieties and rhizobia evolved affinity, firstly nodulation competitiveness was studied after inoculation with a mixture of sympatric and allopatric rhizobial strains isolated from the respective geographical regions. Rhizobia strains harboring nodC types α and [Formula: see text], which were found predominant in Mexico and Ecuador, were comparable in nodule occupancy at 50% of each in beans from the Mesoamerican and Andean gene pools, but it is one of those two nodC types which clearly predominated in Ecuadorian-Peruvian beans as well as in Andean beans nodC type [Formula: see text] predominated the sympatric nodC type δ. The results indicated that those beans from Ecuador-Peru and Andean region, respectively exhibited no affinity for nodulation by the sympatric rhizobial lineages that were found to be predominant in bean nodules formed in those respective areas. Unlike the strains isolated from Ecuador, Rhizobium etli isolated from Mexico as well from the southern Andes was highly competitive for nodulation in beans from Ecuador-Peru, and quite similarly competitive in Mesoamerican and Andean beans. Finally, five gene products associated with symbiosis were examined to analyze variations that could be correlated with nodulation competitiveness. A small GTPase RabA2, transcriptional factors NIN and ASTRAY, and nodulation factor receptors NFR1 and NFR5- indicated high conservation but NIN, NFR1 and NFR5 of beans representative of the Ecuador-Peru genetic pool clustered separated from the Mesoamerican and Andean showing diversification and possible different interaction. These results indicated that both host and bacterial genetics are important for mutual affinity, and that symbiosis is another trait of legumes that could be sensitive to evolutionary influences and local adaptation.

Cadmium-tolerant bacteria: current trends and applications in agriculture

Cadmium (Cd) is considered a toxic heavy metal; nevertheless, its toxicity fluctuates for different organisms. Cadmium-tolerant bacteria (CdtB) are diverse and non-phylogenetically related. Because of their ecological importance these bacteria become particularly relevant when pollution occurs and where human health is impacted. The aim of this review is to show the significance, culturable diversity, metabolic detoxification mechanisms of CdtB and their current uses in several bioremediation processes applied to agricultural soils. Further discussion addressed the technological devices and the possible advantages of genetically modified CdtB for diagnostic purposes in the future.

The nodD1 Gene of Sinorhizobium fredii HH103 Restores Nodulation Capacity on Bean in a Rhizobium tropici CIAT 899 nodD1/nodD2 Mutant, but the Secondary Symbiotic Regulators nolR, nodD2 or syrM Prevent HH103 to Nodulate with This Legume

Rhizobial NodD proteins and appropriate flavonoids induce rhizobial nodulation gene expression. In this study, we show that the nodD1 gene of Sinorhizobium fredii HH103, but not the nodD2 gene, can restore the nodulation capacity of a double nodD1/nodD2 mutant of Rhizobium tropici CIAT 899 in bean plants (Phaseolus vulgaris). S. fredii HH103 only induces pseudonodules in beans. We have also studied whether the mutation of different symbiotic regulatory genes may affect the symbiotic interaction of HH103 with beans: ttsI (the positive regulator of the symbiotic type 3 protein secretion system), and nodD2, nolR and syrM (all of them controlling the level of Nod factor production). Inactivation of either nodD2, nolR or syrM, but not that of ttsI, affected positively the symbiotic behavior of HH103 with beans, leading to the formation of colonized nodules. Acetylene reduction assays showed certain levels of nitrogenase activity that were higher in the case of the nodD2 and nolR mutants. Similar results have been previously obtained by our group with the model legume Lotus japonicus. Hence, the results obtained in the present work confirm that repression of Nod factor production, provided by either NodD2, NolR or SyrM, prevents HH103 to effectively nodulate several putative host plants.

Gram-Negative Bacteria Holding Together in a Biofilm: The Acinetobacter baumannii Way

Bacterial biofilms are a serious public-health problem worldwide. In recent years, the rates of antibiotic-resistant Gram-negative bacteria associated with biofilm-forming activity have increased worrisomely, particularly among healthcare-associated pathogens. Acinetobacter baumannii is a critically opportunistic pathogen, due to the high rates of antibiotic resistant strains causing healthcare-acquired infections (HAIs). The clinical isolates of A. baumannii can form biofilms on both biotic and abiotic surfaces; hospital settings and medical devices are the ideal environments for A. baumannii biofilms, thereby representing the main source of patient infections. However, the paucity of therapeutic options poses major concerns for human health infections caused by A. baumannii strains. The increasing number of multidrug-resistant A. baumannii biofilm-forming isolates in association with the limited number of biofilm-eradicating treatments intensify the need for effective antibiofilm approaches. This review discusses the mechanisms used by this opportunistic pathogen to form biofilms, describes their clinical impact, and summarizes the current and emerging treatment options available, both to prevent their formation and to disrupt preformed A. baumannii biofilms.

The diverse functional genes of maize rhizosphere microbiota assessed using shotgun metagenomics

BACKGROUND

The geographical diversification in chemical, biological and physical properties of plant biospheres instigates heterogenicity in the proliferation of important soil microbiome. Controlling functions and structure of plant rhizosphere from a better understanding and prediction of a plant's immediate environment will help assess plant-microbe interplay, improve the productivity of plant ecosystems and improve plant response to adverse soil conditions. Here we characterized functional genes of the microbial community of maize rhizosphere using a culture-independent method.

RESULTS

Our metadata showed microbial genes involved in nitrogen fixation, phosphate solubilization, quorum sensing molecules, trehalose, siderophore production, phenazine biosynthesis protein, daunorubicin resistance, acetoin, 1-aminocyclopropane-1-carboxylate deaminase, 4-hydroxybenzoate, disease control and stress-reducing genes (superoxidase dismutase, catalase, peroxidase, etc.). β-Diversity showed that there is a highly significant difference between most of the genes mined from rhizosphere soil samples and surrounding soils.

CONCLUSIONS

The high relative abundance of stress-reducing genes mined from this study showed that the sampling sites harbor not only important plant-beneficial organisms but also a hotspot for developing bio-fertilizers. Nevertheless, since most of these organisms are unculturable, mapping cultivation strategies for their growth could make them readily available as bio-inoculants and possible biotechnological applications in the future. © 2020 Society of Chemical Industry.

A Novel OmpR-Type Response Regulator Controls Multiple Stages of the Rhizobium etli - Phaseolus vulgaris N2-Fixing Symbiosis

OmpR, is one of the best characterized response regulators families, which includes transcriptional regulators with a variety of physiological roles including the control of symbiotic nitrogen fixation (SNF). The Rhizobium etli CE3 genome encodes 18 OmpR-type regulators; the function of the majority of these regulators during the SNF in common bean, remains elusive. In this work, we demonstrated that a R. etli mutant strain lacking the OmpR-type regulator RetPC57 (ΔRetPC57), formed less nodules when used as inoculum for common bean. Furthermore, we observed reduced expression level of bacterial genes involved in Nod Factors production (nodA and nodB) and of plant early-nodulation genes (NSP2, NIN, NF-YA and ENOD40), in plants inoculated with ΔRetPC57. RetPC57 also contributes to the appropriate expression of genes which products are part of the multidrug efflux pumps family (MDR). Interestingly, nodules elicited by ΔRetPC57 showed increased expression of genes relevant for Carbon/Nitrogen nodule metabolism (PEPC and GOGAT) and ΔRetPC57 bacteroids showed higher nitrogen fixation activity as well as increased expression of key genes directly involved in SNF (hfixL, fixKf, fnrN, fixN, nifA and nifH). Taken together, our data show that the previously uncharacterized regulator RetPC57 is a key player in the development of the R. etli - P. vulgaris symbiosis.

OnfD, an AraC-Type Transcriptional Regulator Encoded by Rhizobium tropici CIAT 899 and Involved in Nod Factor Synthesis and Symbiosis

Rhizobium tropici CIAT 899 is a broad-host-range rhizobial strain that establishes symbiotic interactions with legumes and tolerates different environmental stresses such as heat, acidity, or salinity. This rhizobial strain produces a wide variety of symbiotically active nodulation factors (NF) induced not only by the presence of plant-released flavonoids but also under osmotic stress conditions through the LysR-type transcriptional regulators NodD1 (flavonoids) and NodD2 (osmotic stress). However, the activation of NodD2 under high-osmotic-stress conditions remains elusive. Here, we have studied the role of a new AraC-type regulator (named as OnfD) in the symbiotic interaction of R. tropici CIAT 899 with Phaseolus vulgaris and Lotus plants. We determined that OnfD is required under salt stress conditions for the transcriptional activation of the nodulation genes and therefore the synthesis and export of NF, which are required for a successful symbiosis with P. vulgaris Moreover, using bacterial two-hybrid analysis, we demonstrated that the OnfD and NodD2 proteins form homodimers and OnfD/NodD2 form heterodimers, which could be involved in the production of NF in the presence of osmotic stress conditions since both regulators are required for NF synthesis in the presence of salt. A structural model of OnfD is presented and discussed.IMPORTANCE The synthesis and export of rhizobial NF are mediated by a conserved group of LysR-type regulators, the NodD proteins. Here, we have demonstrated that a non-LysR-type regulator, an AraC-type protein, is required for the transcriptional activation of symbiotic genes and for the synthesis of symbiotically active NF under salt stress conditions.

The invisible life inside plants: Deciphering the riddles of endophytic bacterial diversity

Endophytic bacteria often promote plant growth and protect their host plant against pathogens, herbivores, and abiotic stresses including drought, increased salinity or pollution. Current agricultural practices are being challenged in terms of climate change and the ever-increasing demand for food. Therefore, the rational exploitation of bacterial endophytes to increase the productivity and resistance of crops appears to be very promising. However, the efficient and larger-scale use of bacterial endophytes for more effective and sustainable agriculture is hindered by very little knowledge on molecular aspects of plant-endophyte interactions and mechanisms driving bacterial communities in planta. In addition, since most of the information on bacterial endophytes has been obtained through culture-dependent techniques, endophytic bacterial diversity and its full biotechnological potential still remain highly unexplored. In this study, we discuss the diversity and role of endophytic populations as well as complex interactions that the endophytes have with the plant and vice versa, including the interactions leading to plant colonization. A description of biotic and abiotic factors influencing endophytic bacterial communities is provided, along with a summary of different methodologies suitable for determining the diversity of bacterial endophytes, mechanisms governing the assembly and structure of bacterial communities in the endosphere, and potential biotechnological applications of endophytes in the future.

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.

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.

noeM, a New Nodulation Gene Involved in the Biosynthesis of Nod Factors with an Open-Chain Oxidized Terminal Residue and in the Symbiosis with Mimosa pudica

The β-rhizobium Cupriavidus taiwanensis is a nitrogen-fixing symbiont of Mimosa pudica. Nod factors produced by this species were previously found to be pentameric chitin-oligomers carrying common C18:1 or C16:0 fatty acyl chains, N-methylated and C-6 carbamoylated on the nonreducing terminal N-acetylglucosamine and sulfated on the reducing terminal residue. Here, we report that, in addition, C. taiwanensis LMG19424 produces molecules where the reducing sugar is open and oxidized. We identified a novel nodulation gene located on the symbiotic plasmid pRalta, called noeM, which is involved in this atypical Nod factor structure. noeM encodes a transmembrane protein bearing a fatty acid hydroxylase domain. This gene is expressed during symbiosis with M. pudica and requires NodD and luteolin for optimal expression. The closest noeM homologs formed a separate phylogenetic clade containing rhizobial genes only, which are located on symbiosis plasmids downstream from a nod box. Corresponding proteins, referred to as NoeM, may have specialized in symbiosis via the connection to the nodulation pathway and the spread in rhizobia. noeM was mostly found in isolates of the Mimoseae tribe, and specifically detected in all tested strains able to nodulate M. pudica. A noeM deletion mutant of C. taiwanensis was affected for the nodulation of M. pudica, confirming the role of noeM in the symbiosis with this legume.

Identification of the symbiosis island of Bradyrhizobium paxllaeri LMTR 21T

The complete symbiosis island (SI) of Bradyrhizobium paxllaeri LMTR 21^T, a mutualistic symbiont of the legume Phaseolus lunatus, was identified and analyzed. The SI was 646 kb in size, had lower G+C content than the genome average, and encoded not only nodulation and nitrogen fixation functions but also those for hydrogen uptake, vitamin and phytohormone biosynthesis, molybdenum transport, nonribosomal peptide synthesis, and type III secretion. Additionally, two divergent nodA genes were encoded in the SI.

Phylogenetic diversity of rhizobia nodulating Phaseolus vulgaris in Croatia and definition of the symbiovar phaseoli within the species Rhizobium pisi

Phaseolus vulgaris is a legume indigenous to America which is currently cultivated in Europe including countries located at the Southeast of this continent, such as Croatia, where several local landraces are cultivated, most of them of Andean origin. In this work we identify at species and symbiovar levels several fast-growing strains able to form effective symbiosis with P. vulgaris in different Croatian soils. The identification at species level based on MALDI-TOF MS and core gene sequence analysis showed that most of these strains belong to the species R. leguminosarum, R. hidalgonense and R. pisi. In addition, several strains belong to putative new species phylogenetically close to R. ecuadorense and R. sophoriradicis. All Croatian strains belong to the symbiovar phaseoli and harbour the α and γ nodC alleles typical for American strains of this symbiovar. Nevertheless, most of Croatian strains harboured the γ nodC gene allele supporting its Andean origin since it is also dominant in other European countries, where Andean cultivars of P. vulgaris are traditionally cultivated, as occurs in Spain. The only strains harbouring the α nodC allele belong to R. hidalgonense and R. pisi, this last only containing the symbiovars viciae and trifolii to date. This is the first report about the presence in Europe of the species R. hidalgonense, the nodulation of P. vulgaris by R. pisi and the existence of the symbiovar phaseoli within this species. These results significantly increase the knowledge of the biogeography of Rhizobium-P. vulgaris symbiosis.

Genomic insights into plant growth promoting rhizobia capable of enhancing soybean germination under drought stress

BACKGROUND

The role of soil microorganisms in plant growth, nutrient utilization, drought tolerance as well as biocontrol activity cannot be over-emphasized, especially in this era when food crisis is a global challenge. This research was therefore designed to gain genomic insights into plant growth promoting (PGP) Rhizobium species capable of enhancing soybean (Glycine max L.) seeds germination under drought condition.

RESULTS

Rhizobium sp. strain R1, Rhizobium tropici strain R2, Rhizobium cellulosilyticum strain R3, Rhizobium taibaishanense strain R4 and Ensifer meliloti strain R5 were found to possess the entire PGP traits tested. Specifically, these rhizobial strains were able to solubilize phosphate, produce exopolysaccharide (EPS), 1-aminocyclopropane-1-carboxylate (ACC), siderophore and indole-acetic-acid (IAA). These strains also survived and grew at a temperature of 45 °C and in an acidic condition with a pH 4. Consequently, all the Rhizobium strains enhanced the germination of soybean seeds (PAN 1532 R) under drought condition imposed by 4% poly-ethylene glycol (PEG); nevertheless, Rhizobium sp. strain R1 and R. cellulosilyticum strain R3 inoculations were able to improve seeds germination more than R2, R4 and R5 strains. Thus, genomic insights into Rhizobium sp. strain R1 and R. cellulosilyticum strain R3 revealed the presence of some genes with their respective proteins involved in symbiotic establishment, nitrogen fixation, drought tolerance and plant growth promotion. In particular, exoX, htrA, Nif, nodA, eptA, IAA and siderophore-producing genes were found in the two rhizobial strains.

CONCLUSIONS

Therefore, the availability of the whole genome sequences of R1 and R3 strains may further be exploited to comprehend the interaction of drought tolerant rhizobia with soybean and other legumes and the PGP ability of these rhizobial strains can also be harnessed for biotechnological application in the field especially in semiarid and arid regions of the globe.

Dynamics of oxytetracycline and resistance genes in soil under long-term intensive compost fertilization in Northern China

In the present study, we explored the dynamics of antibiotics (ciprofloxacin, norfloxacin, enrofloxacin, and oxytetracycline), tetracycline resistance genes (TRGs), and bacterial communities over 2013-2015 in soils fertilized conventionally or with two levels (82.5 and 165 t/ha) of compost for 12 years. In the soil receiving 165 t/ha of compost, only oxytetracycline was 46% higher than that in the conventionally fertilized soil. Transient enrichment of both tetM (20% to 9-fold) and tetK (25% to 67-fold) was observed in multiple instances immediately after the application of compost. The majority of genera which positively correlated with tetM or tetK were affiliated to Proteobacteria, Actinobacteria, Firmicutes, and Bacteroidetes. The structural equation model analysis indicated that fertilization regimes directly affected the bacterial composition and antibiotics and had an indirect effect on the abundance of tetK and tetM via these antibiotics. In summary, this study shed light into the complex interactions between fertilization, antibiotics, and antibiotic resistance pollution in greenhouse soil.

Regulation of hsnT, nodF and nodE genes in Rhizobium tropici CIAT 899 and their roles in the synthesis of Nod factors and in the symbiosis

Rhizobium tropici strain CIAT 899 possesses outstanding agronomic properties as it displays tolerance to environmental stresses, a broad host range and high effectiveness in fixing nitrogen with the common bean (Phaseolus vulgaris L.); in addition, it carries intriguing features such as five copies of the regulatory nodD gene, and the capacity to synthesize a variety of nodulation factors (NFs), even in a flavonoid-independent manner, when submitted to abiotic stresses. However, the roles of several nod genes of the repertoire of CIAT 899 remain to be determined. In this study, we obtained mutants for the hsnT, nodF and nodE genes of CIAT 899 and investigated their expression, NF structures and symbiotic properties. Either in the presence of the flavonoid apigenin, or of salt the expression of hsnT, nodF and nodE in wild-type CIAT 899 was highly up-regulated in comparison to the mutants of all five copies of nodD, indicating the roles that regulatory nodD genes play in the activation of hsnT, nodF and nodE; however, NodD1 was recognized as the main inducer. In total, 29 different NF structures were synthesized by wild-type CIAT 899 induced by apigenin, and 36 when induced by salt, being drastically reduced by mutations in hsnT, nodF and nodE, especially under osmotic stress, with specific changes related to each gene, indicating that the three genes participate in the synthesis of NFs. Mutations in hsnT, nodF and nodE affected differently symbiotic performance (nodule number and shoot dry weight), according to the host plant. Our results indicate that the expression of hsnT, nodF and nodE genes of CIAT 899 is mediated by nodD genes, and although these three genes do not belong to the main set of genes controlling nodulation, they contribute to the synthesis of NFs that will impact symbiotic performance and host specificity.

Rhizobium tropici CIAT 899 copA gene plays a fundamental role in copper tolerance in both free life and symbiosis with Phaseolus vulgaris

Rhizobium tropici CIAT 899 is a facultative symbiotic diazotroph able to deal with stressful concentrations of metals. Nevertheless the molecular mechanisms involved in metal tolerance have not been elucidated. Copper (Cu²⁺) is a metal component essential for the heme-copper respiratory oxidases and enzymes that catalyse redox reactions, however, it is highly toxic when intracellular trace concentrations are surpassed. In this study, we report that R. tropici CIAT 899 is more tolerant to Cu²⁺ than other Rhizobium and Sinorhizobium species. Through Tn5 random mutagenesis we identify a R. tropici mutant strain with a severe reduction in Cu²⁺ tolerance. The Tn5 insertion disrupted the gene RTCIAT899_CH17575, encoding a putative heavy metal efflux P1B-1-type ATPase designated as copA. Phaseolus vulgaris plants inoculated with the copA::Tn5 mutant in the presence of toxic Cu²⁺ concentrations showed a drastic reduction in plant and nodule dry weight, as well as nitrogenase activity. Nodules induced by the copA::Tn5 mutant present an increase in H2O2 concentration, lipoperoxidation and accumulate 40-fold more Cu²⁺ than nodules formed by the wild-type strain. The copA::Tn5 mutant complemented with the copA gene recovered the wild-type symbiotic phenotypes. Therefore, the copA gene is essential for R. tropici CIAT 899 to survive in copper-rich environments in both free life and symbiosis with P. vulgaris plants.

Osmotic stress activates nif and fix genes and induces the Rhizobium tropici CIAT 899 Nod factor production via NodD2 by up-regulation of the nodA2 operon and the nodA3 gene

The symbiosis between rhizobia and legumes is characterized by a complex molecular dialogue in which the bacterial NodD protein plays a major role due to its capacity to activate the expression of the nodulation genes in the presence of appropiate flavonoids. These genes are involved in the synthesis of molecules, the nodulation factors (NF), responsible for launching the nodulation process. Rhizobium tropici CIAT 899, a rhizobial strain that nodulates Phaseolus vulgaris, is characterized by its tolerance to multiple environmental stresses such as high temperatures, acidity or elevated osmolarity. This strain produces nodulation factors under saline stress and the same set of CIAT 899 nodulation genes activated by inducing flavonoids are also up-regulated in a process controlled by the NodD2 protein. In this paper, we have studied the effect of osmotic stress (high mannitol concentrations) on the R. tropici CIAT 899 transcriptomic response. In the same manner as with saline stress, the osmotic stress mediated NF production and export was controlled directly by NodD2. In contrast to previous reports, the nodA2FE operon and the nodA3 and nodD1 genes were up-regulated with mannitol, which correlated with an increase in the production of biologically active NF. Interestingly, in these conditions, this regulatory protein controlled not only the expression of nodulation genes but also the expression of other genes involved in protein folding and synthesis, motility, synthesis of polysaccharides and, surprinsingly, nitrogen fixation. Moreover, the non-metabolizable sugar dulcitol was also able to induce the NF production and the activation of nod genes in CIAT 899.

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

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

Complete Genome Sequence of Rhizobium sp. Strain 11515TR, Isolated from Tomato Rhizosphere in the Philippines

Rhizobium sp. strain 11515TR was isolated from the rhizosphere of tomato in Laguna, Philippines.

Rhizobium sp. strain 11515TR was isolated from the rhizosphere of tomato in Laguna, Philippines. The 7.07-Mb complete genome comprises three replicons, one chromosome, and two plasmids, with a G+C content of 59.4% and 6,720 protein-coding genes. The genome encodes gene clusters supporting rhizosphere processes, plant symbiosis, and secondary bioactive metabolites.

Revealing the roles of y4wF and tidC genes in Rhizobium tropici CIAT 899: biosynthesis of indolic compounds and impact on symbiotic properties

Rhizobium tropici CIAT 899 is a strain known by its ability to nodulate a broad range of legume species, to synthesize a variety of Nod factors, its tolerance of abiotic stresses, and its high capacity to fix atmospheric N₂, especially in symbiosis with common bean (Phaseolus vulgaris L.). Genes putatively related to the synthesis of indole acetic acid (IAA) have been found in the symbiotic plasmid of CIAT 899, in the vicinity of the regulatory nodulation gene nodD5, and, in this study, we obtained mutants for two of these genes, y4wF and tidC (R. tropiciindole-3-pyruvic acid decarboxylase), and investigated their expression in the absence and presence of tryptophan (TRP) and apigenin (API). In general, mutations of both genes increased exopolysaccharide (EPS) synthesis and did not affect swimming or surface motility; mutations also delayed nodule formation, but increased competitiveness. We found that the indole-3-acetamide (IAM) pathway was active in CIAT 899 and not affected by the mutations, and-noteworthy-that API was required to activate the tryptamine (TAM) and the indol-3-pyruvic acid (IPyA) pathways in all strains, particularly in the mutants. High up-regulation of y4wF and tidC genes was observed in both the wild-type and the mutant strains in the presence of API. The results obtained revealed an intriguing relationship between IAA metabolism and nod-gene-inducing activity in R. tropici CIAT 899. We discuss the IAA pathways, and, based on our results, we attribute functions to the y4wF and tidC genes of R. tropici.

Dissecting the Acid Stress Response of Rhizobium tropici CIAT 899

Rhizobium tropici CIAT899 is a nodule-forming α-proteobacterium displaying intrinsic resistance to several abiotic stress conditions such as low pH and high temperatures, which are common in tropical environments. It is a good competitor for Phaseolus vulgaris (common bean) nodule occupancy at low pH values, however little is known about the genetic and physiological basis of the tolerance to acidic conditions. To identify genes in R. tropici involved in pH stress response we combined two different approaches: (1) A Tn5 mutant library of R. tropici CIAT899 was screened and 26 acid-sensitive mutants were identified. For 17 of these mutants, the transposon insertion sites could be identified. (2) We also studied the transcriptomes of cells grown under different pH conditions using RNA-Seq. RNA was extracted from cells grown for several generations in minimal medium at 6.8 or 4.5 (adapted cells). In addition, we acid-shocked cells pre-grown at pH 6.8 for 45 min at pH 4.5. Of the 6,289 protein-coding genes annotated in the genome of R. tropici CIAT 899, 383 were differentially expressed under acidic conditions (pH 4.5) vs. control condition (pH 6.8). Three hundred and fifty one genes were induced and 32 genes were repressed; only 11 genes were induced upon acid shock. The acid stress response of R. tropici CIAT899 is versatile: we found genes encoding response regulators and membrane transporters, enzymes involved in amino acid and carbohydrate metabolism and proton extrusion, in addition to several hypothetical genes. Our findings enhance our understanding of the core genes that are important during the acid stress response in R. tropici.

The first Taxus rhizosphere microbiome revealed by shotgun metagenomic sequencing

In the present study, the shotgun high throughput metagenomic sequencing was implemented to globally capture the features of Taxus rhizosphere microbiome. Total reads could be assigned to 6925 species belonging to 113 bacteria phyla and 301 species of nine fungi phyla. For archaea and virus, 263 and 134 species were for the first time identified, respectively. More than 720,000 Unigenes were identified by clean reads assembly. The top five assigned phyla were Actinobacteria (363,941 Unigenes), Proteobacteria (182,053), Acidobacteria (44,527), Ascomycota (fungi; 18,267), and Chloroflexi (15,539). KEGG analysis predicted numerous functional genes; 7101 Unigenes belong to "Xenobiotics biodegradation and metabolism." A total of 12,040 Unigenes involved in defense mechanisms (e.g., xenobiotic metabolism) were annotated by eggNOG. Talaromyces addition could influence not only the diversity and structure of microbial communities of Taxus rhizosphere, but also the relative abundance of functional genes, including metabolic genes, antibiotic resistant genes, and genes involved in pathogen-host interaction, bacterial virulence, and bacterial secretion system. The structure and function of rhizosphere microbiome could be sensitive to non-native microbe addition, which could impact on the pollutant degradation. This study, complementary to the amplicon sequencing, more objectively reflects the native microbiome of Taxus rhizosphere and its response to environmental pressure, and lays a foundation for potential combination of phytoremediation and bioaugmentation.

Genetic diversity and symbiotic effectiveness of Phaseolus vulgaris-nodulating rhizobia in Kenya

Phaseolus vulgaris (common bean) was introduced to Kenya several centuries ago but the rhizobia that nodulate it in the country remain poorly characterised. To address this gap in knowledge, 178 isolates recovered from the root nodules of P. vulgaris cultivated in Kenya were genotyped stepwise by the analysis of genomic DNA fingerprints, PCR-RFLP and 16S rRNA, atpD, recA and nodC gene sequences. Results indicated that P. vulgaris in Kenya is nodulated by at least six Rhizobium genospecies, with most of the isolates belonging to Rhizobium phaseoli and a possibly novel Rhizobium species. Infrequently, isolates belonged to Rhizobium paranaense, Rhizobium leucaenae, Rhizobium sophoriradicis and Rhizobium aegyptiacum. Despite considerable core-gene heterogeneity among the isolates, only four nodC gene alleles were observed indicating conservation within this gene. Testing of the capacity of the isolates to fix nitrogen (N₂) in symbiosis with P. vulgaris revealed wide variations in effectiveness, with ten isolates comparable to Rhizobium tropici CIAT 899, a commercial inoculant strain for P. vulgaris. In addition to unveiling effective native rhizobial strains with potential as inoculants in Kenya, this study demonstrated that Kenyan soils harbour diverse P. vulgaris-nodulating rhizobia, some of which formed phylogenetic clusters distinct from known lineages. The native rhizobia differed by site, suggesting that field inoculation of P. vulgaris may need to be locally optimised.

Co-inoculation of maize with Azospirillum brasilense and Rhizobium tropici as a strategy to mitigate salinity stress

Plants are highly affected by salinity, but some plant growth-promoting bacteria (PGPB) may trigger induced systemic tolerance (IST), conferring protection against abiotic stresses. We investigated plant mechanisms under saline stress (170mM NaCl) when maize was singly or co-inoculated with Azospirillum brasilense strains Ab-V5 and Ab-V6 and Rhizobium tropici strain CIAT 899. Under greenhouse conditions, plants responded positively to inoculation and co-inoculation, but with differences between strains. Inoculation affected antioxidant enzymes that detoxify reactive oxygen species (ROS) - ascorbate peroxidase (APX), catalase (CAT) and superoxide dismutase (SOD) - mainly in leaves. Proline contents in leaves and roots and malondialdehyde (MDA) in leaves - plant-stress-marker molecules - were significantly reduced due to the inoculation, indicating reduced need for the synthesis of these molecules. Significant differences were attributed to inoculation in the expression of genes related to antioxidant activity, in general with upregulation of APX1, CAT1, SOD2 and SOD4 in leaves, and APX2 in roots. Pathogenesis-related genes PR1, prp2, prp4 and heat-shock protein hsp70 were downregulated in leaves and roots, indicating that inoculation with PGPB might reduce the need for this protection. Together the results indicate that inoculation with PGPB might provide protection from the negative effects of saline stress. However, differences were observed between strains, as A. brasilense Ab-V5 did not show salt tolerance, while the best inoculation treatments to mitigate saline stress were with Ab-V6 and co-inoculation with Ab-V6+CIAT 899. Inoculation with these strains may represent an effective strategy to mitigate salinity stress.

Genomic Diversity in the Endosymbiotic Bacterium Rhizobium leguminosarum

Rhizobium leguminosarum bv. viciae is a soil α-proteobacterium that establishes a diazotrophic symbiosis with different legumes of the Fabeae tribe. The number of genome sequences from rhizobial strains available in public databases is constantly increasing, although complete, fully annotated genome structures from rhizobial genomes are scarce. In this work, we report and analyse the complete genome of R. leguminosarum bv. viciae UPM791. Whole genome sequencing can provide new insights into the genetic features contributing to symbiotically relevant processes such as bacterial adaptation to the rhizosphere, mechanisms for efficient competition with other bacteria, and the ability to establish a complex signalling dialogue with legumes, to enter the root without triggering plant defenses, and, ultimately, to fix nitrogen within the host. Comparison of the complete genome sequences of two strains of R. leguminosarum bv. viciae, 3841 and UPM791, highlights the existence of different symbiotic plasmids and a common core chromosome. Specific genomic traits, such as plasmid content or a distinctive regulation, define differential physiological capabilities of these endosymbionts. Among them, strain UPM791 presents unique adaptations for recycling the hydrogen generated in the nitrogen fixation process.

Transcriptomic Studies of the Effect of nod Gene-Inducing Molecules in Rhizobia: Different Weapons, One Purpose

Simultaneous quantification of transcripts of the whole bacterial genome allows the analysis of the global transcriptional response under changing conditions. RNA-seq and microarrays are the most used techniques to measure these transcriptomic changes, and both complement each other in transcriptome profiling. In this review, we exhaustively compiled the symbiosis-related transcriptomic reports (microarrays and RNA sequencing) carried out hitherto in rhizobia. This review is specially focused on transcriptomic changes that takes place when five rhizobial species, Bradyrhizobium japonicum (=diazoefficiens) USDA 110, Rhizobium leguminosarum biovar viciae 3841, Rhizobium tropici CIAT 899, Sinorhizobium (=Ensifer) meliloti 1021 and S. fredii HH103, recognize inducing flavonoids, plant-exuded phenolic compounds that activate the biosynthesis and export of Nod factors (NF) in all analysed rhizobia. Interestingly, our global transcriptomic comparison also indicates that each rhizobial species possesses its own arsenal of molecular weapons accompanying the set of NF in order to establish a successful interaction with host legumes.

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

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

The Rhizobium tropici CIAT 899 NodD2 protein regulates the production of Nod factors under salt stress in a flavonoid-independent manner

In the symbiotic associations between rhizobia and legumes, NodD promotes the expression of the nodulation genes in the presence of appropriate flavonoids. This set of genes is implied in the synthesis of Nodulation factors, which are responsible for launching the nodulation process. Rhizobium tropici CIAT 899 is the most successful symbiont of Phaseolus vulgaris and can nodulate a variety of legumes. This strain produces Nodulation factors under abiotic stress such as acidity or high concentration of salt. Genome sequencing of CIAT 899 allowed the identification of five nodD genes. Whereas NodD1 is essential to nodulate Leucaena leucocephala, Lotus japonicus and Macroptilium atropurpureum, symbiosis with P. vulgaris and Lotus burtii decreased the nodule number but did not abolish the symbiotic process when NodD1 is absent. Nodulation factor synthesis under salt stress is not regulated by NodD1. Here we confirmed that NodD2 is responsible for the activation of the CIAT 899 symbiotic genes under salt stress. We have demonstrated that NodD1 and NodD2 control the synthesis of the Nod factor necessary for a successful symbiosis with P. vulgaris and L. burtii. This is the first time that NodD is directly implied in the activation of the symbiotic genes under an abiotic stress.