Enhanced low-cost lipopeptide biosurfactant production by Bacillus velezensis from residual glycerin.

Biosurfactants (BSFs) are molecules produced by microorganisms from various carbon sources, with applications in bioremediation and petroleum recovery. However, the production cost limits large-scale applications. This study optimized BSFs production by Bacillus velezensis (strain MO13) using residual glycerin as a substrate. The spherical quadratic central composite design (CCD) model was used to standardize carbon source concentration (30 g/L), temperature (34 °C), pH (7.2), stirring (239 rpm), and aeration (0.775 vvm) in a 5-L bioreactor. Maximum BSFs production reached 1527.6 mg/L of surfactins and 176.88 mg/L of iturins, a threefold increase through optimization. Microbial development, substrate consumption, concentration of BSFs, and surface tension were also evaluated on the bioprocess dynamics. Mass spectrometry Q-TOF-MS identified five surfactin and two iturin isoforms produced by B. velezensis MO13. This study demonstrates significant progress on BSF production using industrial waste as a microbial substrate, surpassing reported concentrations in the literature.

Genome-based analyses to learn from and about Paenibacillus sonchi genomovar Riograndensis SBR5T.

Paenibacillus sonchi genomovar Riograndensis SBR5T is a plant growth-promoting rhizobacterium (PGPR) isolated in the Brazilian state of Rio Grande do Sul from the rhizosphere of Triticum aestivum. It fixes nitrogen, produces siderophores as well as the phytohormone indole-3-acetic acid, solubilizes phosphate and displays antagonist activity against Listeria monocytogenes and Pectobacterium carotovorum. Comprehensive omics analysis and the development of genetic tools are key to characterizing and engineering such non-model microorganisms. Therefore, the complete genome of SBR5T was sequenced, and shown to encode 6,705 proteins, 87 tRNAs, and 27 rRNAs and it enabled a landscape transcriptome analysis that unveiled conserved transcriptional and translational patterns and characterized operon structures and riboswitches. The pangenome of P. sonchi species is open with a stable core pangenome. At the same time, the analysis of genes coding for nitrogenases revealed that the trait of nitrogen fixation is sparse within the Paenibacillaceae family and the presence of Fe-only nitrogenase in the P. sonchi group was exclusive to SBR5T. The development of genetic tools for SBR5T enabled genetic transformation, plasmid construction for constitutive and inducible gene expression, and gene repression using the CRISPRi system. Altogether, the work with P. sonchi can guide the study of non-model bacteria with economic potential.

Genome-based reclassification of Azospirillum brasilense Az39 as the type strain of Azospirillum argentinense sp. nov.

Strain Az39T of Azospirillum is a diazotrophic plant growth-promoting bacterium isolated in 1982 from the roots of wheat plants growing in Marcos Juárez, Córdoba, Argentina. It produces indole-3-acetic acid in the presence of l-tryptophan as a precursor, grows at 20-38 °C (optimal 38 °C), and the cells are curved or spiral-shaped, with diameters ranging from 0.5-0.9 to 1.8-2.2 µm. They contain C16 : 0, C18 : 0 and C18 : 1 ω7c/ω6c as the main fatty acids. Phylogenetic analysis of its 16S rRNA gene sequence confirmed that this strain belongs to the genus Azospirillum, showing a close relationship with Azospirillum baldaniorum Sp245T, Azospirillum brasilense Sp7T and Azospirillum formosense CC-Nfb-7T. Housekeeping gene analysis revealed that Az39T, together with five strains of the genus (Az19, REC3, BR 11975, MTCC4035 and MTCC4036), form a cluster apart from A. baldaniorum Sp245T, A. brasilense Sp7T and A. formosense CC-Nfb-7T. Average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) between Az39T and the aforementioned type strains revealed values below 96 %, the circumscription limit for the species delineation (ANI: 95.3, 94.1 and 94.0 %; dDDH: 62.9, 56.3 and 55.6 %). Furthermore, a phylogeny evaluation of the core proteome, including 809 common shared proteins, showed an independent grouping of Az39T, Az19, REC3, BR 11975, MTCC4035 and MTCC4036. The G+C content in the genomic DNA of these six strains varied from 68.3 to 68.5 %. Based on the combined phylogenetic, genomic and phenotypic characterization presented here, we consider that strain Az39T, along with strains Az19, REC3, BR 11975, MTCC4035 and MTCC4036, are members of a new Azospirillum species, for which the name Azospirillum argentinense sp. nov. is proposed. The type strain is Az39T (=LBPCV39T=BR 148428T=CCCT 22.01T).

CRISPR interference-based gene repression in the plant growth promoter Paenibacillus sonchi genomovar Riograndensis SBR5.

Gene repression using the endonucleolytically deactivated dCas9 protein and sgRNAs (CRISPR interference or CRISPRi) is a useful approach to study gene functions. Here, we established CRISPRi in Paenibacillus sonchi genomovar Riograndensis SBR5, a plant growth promoting bacterium. CRISPRi system with sgRNAs targeting SBR5 endogenous genes spo0A, yaaT and ydjJ and plasmid-borne gfpUV was constructed and analyzed. Flow cytometry analysis revealed a significant decrease of reporter protein GFPUV signal in P. sonchi strains expressing gfpUV sgRNA in comparison with non-targeting controls. CRISPRi-based repression of chromosomal genes for regulation of sporulation spo0A and yaaT decreased sporulation and increased biofilm formation in SBR5. Repression of the sorbitol catabolic gene ydjJ revealed decreased specific activity of YdjJ in crude cell extracts and reduced biomass formation from sorbitol in growth experiments. Our work on CRISPRi-based gene repression serves as basis for gene function studies of the plant growth promoter P. sonchi SBR5. To our knowledge, the present study presents the first tool for gene repression established in Paenibacillus species.Key points• CRISPRi toward gene repression was applied for the first time in Paenibacillus.• CRISPRi of spo0A and yaaT depleted spores and increased biofilms in SBR5.• CRISPRi-based ydjJ repression decreased specific activity of sorbitol dehydrogenase.

Genomic metrics made easy: what to do and where to go in the new era of bacterial taxonomy.

With the advent of high-throughput DNA sequencing technologies, traditional methodologies for taxonomic classification of bacteria as DNA-DNA hybridization and 16S rRNA identity analyses are being challenged by the development of a fast-growing number of genomic metrics. The large amount of portable and digitized genome sequences available in public repositories constitutes an invaluable data for bacterial classification. Consequently, several genomic metrics and tools were developed to aid the interpretation of these massive data. Genomic metrics are based on the assumption that higher genome similarities would reflect closer phylogenetic relationships. Different metrics would vary in their methodology of analysis, resolution power, limitations and easiness of use. The aim of this review is to highlight the differences among available genome-based methods and tools to provide a guide for in silico bacterial identification and classification.

The genomes of three Bradyrhizobium sp. isolated from root nodules of Lupinus albescens grown in extremely poor soils display important genes for resistance to environmental stress.

Lupinus albescens is a resistant cover plant that establishes symbiotic relationships with bacteria belonging to the Bradyrhizobium genus. This symbiosis helps the development of these plants in adverse environmental conditions, such as the ones found in arenized areas of Southern Brazil. This work studied three Bradyrhizobium sp. (AS23, NAS80 and NAS96) isolated from L. albescens plants that grow in extremely poor soils (arenized areas and adjacent grasslands). The genomes of these three strains were sequenced in the Ion Torrent platform using the IonXpress library preparation kit, and presented a total number of bases of 1,230,460,823 for AS23, 1,320,104,022 for NAS80, and 1,236,105,093 for NAS96. The genome comparison with closest strains Bradyrhizobium japonicum USDA6 and Bradyrhizobium diazoefficiens USDA110 showed important variable regions (with less than 80% of similarity). Genes encoding for factors for resistance/tolerance to heavy metal, flagellar motility, response to osmotic and oxidative stresses, heat shock proteins (present only in the three sequenced genomes) could be responsible for the ability of these microorganisms to survive in inhospitable environments. Knowledge about these genomes will provide a foundation for future development of an inoculant bioproduct that should optimize the recovery of degraded soils using cover crops.

Bradyrhizobium elkanii nod regulon: insights through genomic analysis.

A successful symbiotic relationship between soybean [Glycine max (L.) Merr.] and Bradyrhizobium species requires expression of the bacterial structural nod genes that encode for the synthesis of lipochitooligosaccharide nodulation signal molecules, known as Nod factors (NFs). Bradyrhizobium diazoefficiens USDA 110 possesses a wide nodulation gene repertoire that allows NF assembly and modification, with transcription of the nodYABCSUIJnolMNOnodZ operon depending upon specific activators, i.e., products of regulatory nod genes that are responsive to signaling molecules such as flavonoid compounds exuded by host plant roots. Central to this regulatory circuit of nod gene expression are NodD proteins, members of the LysR-type regulator family. In this study, publicly available Bradyrhizobium elkanii sequenced genomes were compared with the closely related B. diazoefficiens USDA 110 reference genome to determine the similarities between those genomes, especially with regards to the nod operon and nod regulon. Bioinformatics analyses revealed a correlation between functional mechanisms and key elements that play an essential role in the regulation of nod gene expression. These analyses also revealed new genomic features that had not been clearly explored before, some of which were unique for some B. elkanii genomes.

Functional abilities of cultivable plant growth promoting bacteria associated with wheat (Triticum aestivum L.) crops.

In the pursuit of sustainable agriculture, bioinoculants usage as providers of a crop's needs is a method to limit environmental damage. In this study, a collection of cultivable putative plant growth promoting (PGP) bacteria associated with wheat crops was obtained and this bacterial sample was characterized in relation to the functional diversity of certain PGP features. The isolates were obtained through classical cultivation methods, identified by partial 16S rRNA gene sequencing and characterized for PGP traits of interest. Functional diversity characterization was performed using Categorical Principal Component Analysis (CatPCA) and Multiple Correspondence Analysis (MCA). The most abundant genera found among the 346 isolates were Pseudomonas, Burkholderia, and Enterobacter. Occurrence of PGP traits was affected by genus, niche, and sampling site. A large number of genera grouped together with the ability to produce indolic compounds; phosphate solubilization and siderophores production formed a second group related to fewer genera, in which the genus Burkholderia has a great importance. The results obtained may help future studies aiming prospection of putative plant growth promoting bacteria regarding the desired organism and PGP trait.

Plant growth-promoting bacteria as inoculants in agricultural soils.

Plant-microbe interactions in the rhizosphere are the determinants of plant health, productivity and soil fertility. Plant growth-promoting bacteria (PGPB) are bacteria that can enhance plant growth and protect plants from disease and abiotic stresses through a wide variety of mechanisms; those that establish close associations with plants, such as the endophytes, could be more successful in plant growth promotion. Several important bacterial characteristics, such as biological nitrogen fixation, phosphate solubilization, ACC deaminase activity, and production of siderophores and phytohormones, can be assessed as plant growth promotion (PGP) traits. Bacterial inoculants can contribute to increase agronomic efficiency by reducing production costs and environmental pollution, once the use of chemical fertilizers can be reduced or eliminated if the inoculants are efficient. For bacterial inoculants to obtain success in improving plant growth and productivity, several processes involved can influence the efficiency of inoculation, as for example the exudation by plant roots, the bacterial colonization in the roots, and soil health. This review presents an overview of the importance of soil-plant-microbe interactions to the development of efficient inoculants, once PGPB are extensively studied microorganisms, representing a very diverse group of easily accessible beneficial bacteria.

Genetic diversity and symbiotic compatibility among rhizobial strains and Desmodium incanum and Lotus spp. plants.

This work aimed to evaluate the symbiotic compatibility and nodulation efficiency of rhizobia isolated from Desmodium incanum, Lotus corniculatus, L. subbiflorus, L. uliginosus and L. glaber plants by cross-inoculation. Twelve reference strains and 21 native isolates of rhizobia were genetically analyzed by the BOX-PCR technique, which showed a high genetic diversity among the rhizobia studied. The isolates were also characterized based on their production of indolic compounds and siderophores, as well as on their tolerance to salinity. Fifteen of the 33 rhizobia analyzed were able to produce indolic compounds, whereas 13 produced siderophores. All the tested rhizobia were sensitive to high salinity, although some were able to grow in solutions of up to 2% NaCl. Most of the native rhizobia isolated from L. uliginosus were able to induce nodulation in all plant species studied. In a greenhouse experiment using both D. incanum and L. corniculatus plants, the rhizobia isolate UFRGS Lu2 promoted the greatest plant growth. The results demonstrate that there are native rhizobia in the soils of southern Brazil that have low host specificity and are able to induce nodulation and form active nodules in several plant species.

Cultivable bacteria isolated from apple trees cultivated under different crop systems: Diversity and antagonistic activity against Colletotrichum gloeosporioides.

This study evaluated the diversity of cultivable plant growth-promoting (PGP) bacteria associated with apple trees cultivated under different crop management systems and their antagonistic ability against Colletotrichum gloeosporioides. Samples of roots and rhizospheric soil from apple trees cultivated in organic and conventional orchards in southern Brazil were collected, together with soil samples from an area never used for agriculture (native field). Bacteria were identified at the genus level by PCR-RFLP and partial sequencing of the 16S rRNA, and were evaluated for some PGP abilities. The most abundant bacterial genera identified were Enterobacter (27.7%), Pseudomonas (18.7%), Burkholderia (13.7%), and Rahnella (12.3%). Sixty-nine isolates presented some antagonist activity against C. gloeosporioides. In a greenhouse experiment, five days after exposure to C. gloeosporioides, an average of 30% of the leaf area of plants inoculated with isolate 89 (identified as Burkholderia sp.) were infected, whereas 60 to 73% of the leaf area of untreated plants was affected by fungal attack. Our results allowed us to infer how anthropogenic activity is affecting the bacterial communities in soil associated with apple tree crop systems, and to obtain an isolate that was able to delay the emergence of an important disease for this culture.

Changes in root bacterial communities associated to two different development stages of canola (Brassica napus L. var oleifera) evaluated through next-generation sequencing technology.

Crop production may benefit from plant growth-promoting bacteria. The knowledge on bacterial communities is indispensable in agricultural systems that intend to apply beneficial bacteria to improve plant health and production of crops such as canola. In this work, the diversity of root bacterial communities associated to two different developmental phases of canola (Brassica napus L.) plants was assessed through the application of new generation sequencing technology. Total bacterial DNA was extracted from root samples from two different growth states of canola (rosette and flowering). It could be shown how bacterial communities inside the roots changed with the growing stage of the canola plants. There were differences in the abundance of the genera, family, and even the phyla identified for each sample. While in both root samples Proteobacteria was the most common phylum, at the rosette stage, the most common bacteria belonged to the family Pseudomonadaceae and the genus Pseudomonas, and in the flowering stage, the Xanthomonadaceae family and the genus Xanthomonas dominated the community. This implies in a switch in the predominant bacteria in the different developmental stages of the plant, suggesting that the plant itself interferes with the associated microbial community.

Plant growth-promoting rhizobacteria (PGPR): Their potential as antagonists and biocontrol agents.

Bacteria that colonize plant roots and promote plant growth are referred to as plant growth-promoting rhizobacteria (PGPR). PGPR are highly diverse and in this review we focus on rhizobacteria as biocontrol agents. Their effects can occur via local antagonism to soil-borne pathogens or by induction of systemic resistance against pathogens throughout the entire plant. Several substances produced by antagonistic rhizobacteria have been related to pathogen control and indirect promotion of growth in many plants, such as siderophores and antibiotics. Induced systemic resistance (ISR) in plants resembles pathogen-induced systemic acquired resistance (SAR) under conditions where the inducing bacteria and the challenging pathogen remain spatially separated. Both types of induced resistance render uninfected plant parts more resistant to pathogens in several plant species. Rhizobacteria induce resistance through the salicylic acid-dependent SAR pathway, or require jasmonic acid and ethylene perception from the plant for ISR. Rhizobacteria belonging to the genera Pseudomonas and Bacillus are well known for their antagonistic effects and their ability to trigger ISR. Resistance-inducing and antagonistic rhizobacteria might be useful in formulating new inoculants with combinations of different mechanisms of action, leading to a more efficient use for biocontrol strategies to improve cropping systems.

Genome sequence of the diazotrophic Gram-positive rhizobacterium Paenibacillus riograndensis SBR5(T).

Paenibacillus riograndensis SBR5(T), a nitrogen-fixing Gram-positive rhizobacterium isolated from a wheat field in the south of Brazil, has a great potential for agricultural applications due to its plant growth promotion effects. Here we present the draft genome sequence of P. riograndensis SBR5(T). Its 7.37-Mb genome encodes determinants of the diazotrophic lifestyle and plant growth promotion, such as nitrogen fixation, antibiotic resistance, nitrate utilization, and iron uptake.

Roles of flavonoids and the transcriptional regulator TtsI in the activation of the type III secretion system of Bradyrhizobium elkanii SEMIA587.

Bradyrhizobium elkanii SEMIA587 is a symbiotic nitrogen-fixing bacterium of the group commonly called rhizobia, which induce nodule formation in legumes, and is widely used in Brazilian commercial inoculants of soybean. In response to flavonoid compounds released by plant roots, besides Nod factors, other molecular signals are secreted by rhizobia, such as proteins secreted by type III secretion systems (T3SSs). Rhizobial T3SSs are activated by the transcription regulator TtsI, which binds to sequences present in the promoter regions of T3SS genes via a conserved sequence called the tts box. To study the role of the T3SS of B. elkanii SEMIA587, ttsI was mutated. Protein secretion and flavonoid induction analysis, as well as nodulation tests, were performed with the wild-type and mutant strains. The results obtained showed that B. elkanii SEMIA587 secretes at least two proteins (NopA and NopL, known rhizobial T3SS substrates) after genistein induction, whilst supernatants of the ttsI mutant did not contain these Nops. Unusually for rhizobia, the promoter region of the B. elkanii SEMIA587 ttsI gene contains a tts box, which is responsive to flavonoid induction and to which TtsI can bind. Nodulation tests performed with three different leguminous plants showed that the B. elkanii SEMIA587 ttsI mutant displays host-dependent characteristics; in particular, nodulation of two soybean cultivars, Peking and EMBRAPA 48, was more efficient when TtsI of B. elkanii was functional.

Culture-independent assessment of the diazotrophic Bradyrhizobium communities in the Pampa and Atlantic Forest Biomes localities in southern Brazil.

The isolation of rhizobial strains from the root and stem nodules remains a commonly used method despite its limitations as it enables the identification of mainly dominant symbiotic groups within rhizobial communities. To overcome these limitations, we used genus-specific nifD primers in a culture-independent assessment of Bradyrhizobium communities inhabiting soils in southern Brazil. The majority of nifD sequences were generated from DNA isolated from tropical-lowland pasture soils, although some soil samples originated from the Campos de Cima da Serra volcanic plateau. In the nifD tree, all the bradyrhizobial sequences comprised 38 clades, including 18 new clades. The sequences generated in this study were resolved into 22 clades and 21 singletons. The nifD bradyrhizobial assemblage contained Azorhizobium and α-proteobacterial methylotrophic genera, suggesting that these genera may have acquired their nif loci from Bradyrhizobium donors. The most common in the lowland pasture soils subclade III.3D branch comprises the isolates of mainly an American origin. On the other hand, subclade III.4, which was earlier detected in Brazil among Bradyrhizobium isolates nodulating native lupins, appears more common in the Campos de Cima da Serra soils. The second-largest group, Clade XXXVIII, has not yet been reported in culture-dependent studies, while another common group called Clade I represents a symbiovar predominating in Australia. The identification of the diverse nifD Clade I haplotypes in the tropical-lowland pastures infested by Australian Acacia spp implies that the introduction of these legumes to southern Brazil has resulted in the dissemination of their bradyrhizobial symbionts.

Diversity and phylogenetic affinities of Bradyrhizobium isolates from Pampa and Atlantic Forest Biomes.

In this work, we investigated Bradyrhizobium strains isolated from soils collected from the rhizosphere of native and exotic legumes species inhabiting two ecoclimatic zones - asubtropical-lowland pasture (Pampa Biome) and a volcanic plateau covered by Araucaria Moist Forests (Atlantic Forest Biome). The rhizobial strains were isolated from the nodules of seven native and one exotic legume species used as rhizobium traps. Single-gene (recA, glnII, dnaK) and combined-gene MLSA analyses (dnaK-glnII-gyrB-recA-rpoB) revealed that nearly 85% of the isolates clustered in B. elkanii supergroup, while the remaining (except for two isolates) in B. japonicum supergroup, albeit, in most cases, separately from the type strains of Bradyrhizobium species. As a symbiotic gene marker, a portion of nifD gene was sequenced for 194 strains. In the nifD-tree, an American branch III.3D (104 isolates), was the most numerous among the isolates. A significant portion of the isolates clustered in American groups; subclade III.4 (40 strains), Clade VII (3 strains), and a new Clade XX (4 strains). Most of the remaining strains belonged to a pantropical III.3C branch (39 isolates). On the other hand, identification of isolates belonging, respectively, to Clade I and Clade II may result of spreading of the Australian (Clade I) and European (Clade II) bradyrhizobia following the introduction of their legume hosts. Our study indicated that the American groups predominated in the symbiotic Bradyrhizobium communities in southern Brazil. However, there is a significant component of exotic lineages, resulting from the dispersal of pantropical Fabaceae taxa and the introduction of exotic legumes.

Inorganic Phosphate Solubilization by Rhizosphere Bacterium Paenibacillus sonchi: Gene Expression and Physiological Functions.

Due to the importance of phosphorus (P) in agriculture, crop inoculation with phosphate-solubilizing bacteria is a relevant subject of study. Paenibacillus sonchi genomovar Riograndensis SBR5 is a promising candidate for crop inoculation, as it can fix nitrogen and excrete ammonium at a remarkably high rate. However, its trait of phosphate solubilization (PS) has not yet been studied in detail. Here, differential gene expression and functional analyses were performed to characterize PS in this bacterium. SBR5 was cultivated with two distinct P sources: NaH2PO4 as soluble phosphate source (SPi) and hydroxyapatite as insoluble phosphate source (IPi). Total RNA of SBR5 cultivated in those two conditions was isolated and sequenced, and bacterial growth and product formation were monitored. In the IPi medium, the expression of 68 genes was upregulated, whereas 100 genes were downregulated. Among those, genes involved in carbon metabolism, including those coding for subunits of 2-oxoglutarate dehydrogenase, were identified. Quantitation of organic acids showed that the production of tricarboxylic acid cycle-derived organic acids was reduced in IPi condition, whereas acetate and gluconate were overproduced. Increased concentrations of proline, trehalose, and glycine betaine revealed active osmoprotection during growth in IPi. The cultivation with hydroxyapatite also caused the reduction in the motility of SBR5 cells as a response to Pi depletion at the beginning of its growth. SBR5 was able to solubilize hydroxyapatite, which suggests that this organism is a promising phosphate-solubilizing bacterium. Our findings are the initial step in the elucidation of the PS process in P. sonchi SBR5 and will be a valuable groundwork for further studies of this organism as a plant growth-promoting rhizobacterium.

Bacterial and Archaeal Communities Change With Intensity of Vegetation Coverage in Arenized Soils From the Pampa Biome.

Arenization occurs in regions that present sandy soils with normal rainfall levels. Predatory use of environmental sources, the dissolution of arenitic rocks and reworking of non-consolidated surface sands intensify this degradation scenario. Thus, this work aimed to evaluate the impact of the arenization process in the Brazilian Pampa Biome and how this phenomenon affects the soil microbial and plant communities. For this purpose, three arenized areas in Southern Brazil (Pampa Biome) were selected and, in each one, three sampling points were studied: arenized (ARA), arenized to grassland transition (AGT), and grassland (GRA) areas. In the three sampling points, soils presented low levels of nutrients, organic matter, mud and pH acidic in all regions but, the presence of vegetation coverage in AGT and GRA areas preserved the topsoil structure. Our study related ARA with bacterial families Alcaligenaceae, Pseudomonadaceae, and Xanthomonadaceae. AGT with bacterial families Bacillaceae and Burkholderiaceae, and plant species Melinis repens (Willd.) Zizka and Paspalum stellatum Humb. and Bonpl. ex Flüggé, and GRA with bacterial families Koribacteraceae, Hyphomicrobiaceae, and Chthoniobacteraceae, and plant species Croton subpannosus Müll.Arg. ex Griseb., Piptochaetium montevidense (Spreng.) Parodi and Elyonurus sp. The three studied areas (as well as sampling points) present soils extremely poor in nutrients with sandy texture, and the bacterial and plant composition well known to be resistant to environmental stresses were dominant. The vulnerability of these areas causes a degradation scenario, which is worsened by agricultural activities. However, in general, this phenomenon is a natural process that occurs mainly due to soil characteristics (poor soils) and climatic variations.

Phylogeny and Phylogeography of Rhizobial Symbionts Nodulating Legumes of the Tribe Genisteae.

The legume tribe Genisteae comprises 618, predominantly temperate species, showing an amphi-Atlantic distribution that was caused by several long-distance dispersal events. Seven out of the 16 authenticated rhizobial genera can nodulate particular Genisteae species. Bradyrhizobium predominates among rhizobia nodulating Genisteae legumes. Bradyrhizobium strains that infect Genisteae species belong to both the Bradyrhizobium japonicum and Bradyrhizobium elkanii superclades. In symbiotic gene phylogenies, Genisteae bradyrhizobia are scattered among several distinct clades, comprising strains that originate from phylogenetically distant legumes. This indicates that the capacity for nodulation of Genisteae spp. has evolved independently in various symbiotic gene clades, and that it has not been a long-multi-step process. The exception is Bradyrhizobium Clade II, which unlike other clades comprises strains that are specialized in nodulation of Genisteae, but also Loteae spp. Presumably, Clade II represents an example of long-lasting co-evolution of bradyrhizobial symbionts with their legume hosts.