Complete Genome Sequence of Mesorhizobium ciceri Strain CC1192, an Efficient Nitrogen-Fixing Microsymbiont of Cicer arietinum

Comparative Genome Analysis of Polar Mesorhizobium sp. PAMC28654 to Gain Insight into Tolerance to Salinity and Trace Element Stress

In this study, Mesorhizobium sp. PAMC28654 was isolated from a soil sample collected from the polar region of Uganda. Whole-genome sequencing and comparative genomics were performed to better understand the genomic features necessary for Mesorhizobium sp. PAMC28654 to survive and thrive in extreme conditions and stresses. Additionally, diverse sequence analysis tools were employed for genomic investigation. The results of the analysis were then validated using wet-lab experiments. Genome analysis showed trace elements' resistant proteins (CopC, CopD, CzcD, and Acr3), exopolysaccharide (EPS)-producing proteins (ExoF and ExoQ), and nitrogen metabolic proteins (NarG, NarH, and NarI). The strain was positive for nitrate reduction. It was tolerant to 100 mM NaCl at 15 °C and 25 °C temperatures and resistant to multiple trace elements (up to 1 mM CuSO4·5H2O, 2 mM CoCl2·6H2O, 1 mM ZnSO4·7H2O, 0.05 mM Cd(NO3)2·4H2O, and 100 mM Na2HAsO4·7H2O at 15 °C and 0.25 mM CuSO4·5H2O, 2 mM CoCl2·6H2O, 0.5 mM ZnSO4·7H2O, 0.01 mM Cd(NO3)2·4H2O, and 100 mM Na2HAsO4·7H2O at 25 °C). This research contributes to our understanding of bacteria's ability to survive abiotic stresses. The isolated strain can be a potential candidate for implementation for environmental and agricultural purposes.

Complete Genome Sequence of Mesorhizobium ciceri Strain R30, a Rhizobium Used as a Commercial Inoculant for Chickpea in Argentina

We report the complete genome sequence of Mesorhizobium ciceri strain R30, a rhizobium strain recommended and used as a commercial inoculant for chickpea in Argentina. The genome consists of almost 7 Mb, distributed into two circular replicons: a chromosome of 6.49 Mb and a plasmid of 0.46 Mb.

Year-Long Microbial Succession on Microplastics in Wastewater: Chaotic Dynamics Outweigh Preferential Growth

Microplastics are a globally-ubiquitous aquatic pollutant and have been heavily studied over the last decade. Of particular interest are the interactions between microplastics and microorganisms, especially the pursuit to discover a plastic-specific biome, the so-called plastisphere. To follow this up, a year-long microcosm experimental setup was deployed to expose five different microplastic types (and silica beads control) to activated aerobic wastewater in controlled conditions, with microbial communities being measured four times over the course of the year using 16S rDNA (bacterial) and ITS (fungal) amplicon sequencing. The biofilm community shows no evidence of a specific plastisphere, even after a year of incubation. Indeed, the microbial communities (particularly bacterial) show a clear trend of increasing dissimilarity between plastic types as time increases. Despite little evidence for a plastic-specific community, there was a slight grouping observed for polyolefins (PE and PP) in 6–12-month biofilms. Additionally, an OTU assigned to the genus Devosia was identified on many plastics, increasing over time while showing no growth on silicate (natural particle) controls, suggesting this could be either a slow-growing plastic-specific taxon or a symbiont to such. Both substrate-associated findings were only possible to observe in samples incubated for 6–12 months, which highlights the importance of studying long-term microbial community dynamics on plastic surfaces.

Chickpea shows genotype-specific nodulation responses across soil nitrogen environment and root disease resistance categories

BACKGROUND

The ability of chickpea to obtain sufficient nitrogen via its symbiotic relationship with Mesorhizobium ciceri is of critical importance in supporting growth and grain production. A number of factors can affect this symbiotic relationship including abiotic conditions, plant genotype, and disruptions to host signalling/perception networks. In order to support improved nodule formation in chickpea, we investigated how plant genotype and soil nutrient availability affect chickpea nodule formation and nitrogen fixation. Further, using transcriptomic profiling, we sought to identify gene expression patterns that characterize highly nodulated genotypes.

RESULTS

A study involving six chickpea varieties demonstrated large genotype by soil nitrogen interaction effects on nodulation and further identified agronomic traits of genotypes (such as shoot weight) associated with high nodulation. We broadened our scope to consider 29 varieties and breeding lines to examine the relationship between soilborne disease resistance and the number of nodules developed and real-time nitrogen fixation. Results of this larger study supported the earlier genotype specific findings, however, disease resistance did not explain differences in nodulation across genotypes. Transcriptional profiling of six chickpea genotypes indicates that genes associated with signalling, N transport and cellular localization, as opposed to genes associated with the classical nodulation pathway, are more likely to predict whether a given genotype will exhibit high levels of nodule formation.

CONCLUSIONS

This research identified a number of key abiotic and genetic factors affecting chickpea nodule development and nitrogen fixation. These findings indicate that an improved understanding of genotype-specific factors affecting chickpea nodule induction and function are key research areas necessary to improving the benefits of rhizobial symbiosis in chickpea.

Evolution of diverse effective N2-fixing microsymbionts of Cicer arietinum following horizontal transfer of the Mesorhizobium ciceri CC1192 symbiosis integrative and conjugative element

Rhizobia are soil bacteria capable of forming N₂-fixing symbioses with legumes, with highly effective strains often selected in agriculture as inoculants to maximize symbiotic N₂ fixation. When rhizobia in the genus Mesorhizobium have been introduced with exotic legumes into farming systems, horizontal transfer of symbiosis Integrative and Conjugative Elements (ICEs) from the inoculant strain to soil bacteria has resulted in the evolution of ineffective N₂-fixing rhizobia that are competitive for nodulation with the target legume. In Australia, Cicer arietinum (chickpea) has been inoculated since the 1970's with Mesorhizobium ciceri sv. ciceri CC1192, a highly effective strain from Israel. Although the full genome sequence of this organism is available, little is known about the mobility of its symbiosis genes and the diversity of cultivated C. arietinum-nodulating organisms. Here, we show the CC1192 genome harbors a 419-kb symbiosis ICE (ICEMcSym¹¹⁹²) and a 648-kb repABC-type plasmid pMC1192 carrying putative fix genes. We sequenced the genomes of 11 C. arietinum nodule isolates from a field site exclusively inoculated with CC1192 and showed they were diverse unrelated Mesorhizobium carrying ICEMcSym¹¹⁹², indicating they had acquired the ICE by environmental transfer. No exconjugants harboured pMc1192 and the plasmid was not essential for N₂ fixation in CC1192. Laboratory conjugation experiments confirmed ICEMcSym¹¹⁹² is mobile, integrating site-specifically within the 3' end of one of the four ser-tRNA genes in the R7ANS recipient genome. Strikingly, all ICEMcSym¹¹⁹² exconjugants were as efficient at fixing N₂ with C. arietinum as CC1192, demonstrating ICE transfer does not necessarily yield ineffective microsymbionts as previously observed.Importance Symbiotic N₂ fixation is a key component of sustainable agriculture and in many parts of the world legumes are inoculated with highly efficient strains of rhizobia to maximise fixed N₂ inputs into farming systems. Symbiosis genes for Mesorhizobium spp. are often encoded chromosomally within mobile gene clusters called Integrative and Conjugative Elements or ICEs. In Australia, where all agricultural legumes and their rhizobia are exotic, horizontal transfer of ICEs from inoculant Mesorhizobium strains to native rhizobia has led to the evolution of inefficient strains that outcompete the original inoculant, with the potential to render it ineffective. However, the commercial inoculant strain for Cicer arietinum (chickpea), M. ciceri CC1192, has a mobile symbiosis ICE (ICEMcSym¹¹⁹²) which can support high rates of N₂ fixation following either environmental or laboratory transfer into diverse Mesorhizobium backgrounds, demonstrating ICE transfer does not necessarily yield ineffective microsymbionts as previously observed.

Significance of Plant Growth Promoting Rhizobacteria in Grain Legumes: Growth Promotion and Crop Production

Grain legumes are an important component of sustainable agri-food systems. They establish symbiotic association with rhizobia and arbuscular mycorrhizal fungi, thus reducing the use of chemical fertilizers. Several other free-living microbial communities (PGPR—plant growth promoting rhizobacteria) residing in the soil-root interface are also known to influence biogeochemical cycles and improve legume productivity. The growth and function of these microorganisms are affected by root exudate molecules secreted in the rhizosphere region. PGPRs produce the chemicals which stimulate growth and functions of leguminous crops at different growth stages. They promote plant growth by nitrogen fixation, solubilization as well as mineralization of phosphorus, and production of phytohormone(s). The co-inoculation of PGPRs along with rhizobia has shown to enhance nodulation and symbiotic interaction. The recent molecular tools are helpful to understand and predict the establishment and function of PGPRs and plant response. In this review, we provide an overview of various growth promoting mechanisms of PGPR inoculations in the production of leguminous crops.

Root-specific expression of chickpea cytokinin oxidase/dehydrogenase 6 leads to enhanced root growth, drought tolerance and yield without compromising nodulation

Cytokinin group of phytohormones regulate root elongation and branching during post-embryonic development. Cytokinin-degrading enzymes cytokinin oxidases/dehydrogenases (CKXs) have been deployed to investigate biological activities of cytokinin and to engineer root growth. We expressed chickpea cytokinin oxidase 6 (CaCKX6) under the control of a chickpea root-specific promoter of CaWRKY31 in Arabidopsis thaliana and chickpea having determinate and indeterminate growth patterns, respectively, to study the effect of cytokinin depletion on root growth and drought tolerance. Root-specific expression of CaCKX6 led to a significant increase in lateral root number and root biomass in Arabidopsis and chickpea without any penalty to vegetative and reproductive growth of shoot. Transgenic chickpea lines showed increased CKX activity in root. Soil-grown advanced chickpea transgenic lines exhibited higher root-to-shoot biomass ratio and enhanced long-term drought tolerance. These chickpea lines were not compromised in root nodulation and nitrogen fixation. The seed yield in some lines was up to 25% higher with no penalty in protein content. Transgenic chickpea seeds possessed higher levels of zinc, iron, potassium and copper. Our results demonstrated the potential of cytokinin level manipulation in increasing lateral root number and root biomass for agronomic trait improvement in an edible legume crop with indeterminate growth habit.

Phenotypic and Genotypic Diversity Among Symbiotic and Non-symbiotic Bacteria Present in Chickpea Nodules in Morocco

Environmental pollution problems and increased demand for green technologies in production are forcing farmers to introduce agricultural practices with a lower impact on the environment. Chickpea (Cicer arietinum) in arid and semi-arid environments is frequently affected by harsh environmental stresses such as heat, drought and salinity, which limit its growth and productivity and affect biological nitrogen fixation ability of rhizobia. Climate change had further aggravated these stresses. Inoculation with appropriate stress tolerant rhizobia is necessary for an environmentally friendly and sustainable agricultural production. In this study, endophytic bacteria isolated from chickpea nodules from different soil types and regions in Morocco, were evaluated for their phenotypic and genotypic diversity in order to select the most tolerant ones for further inoculation of this crop. Phenotypic characterization of 135 endophytic bacteria from chickpea nodules showed a wide variability for tolerance to heavy metals and antibiotics, variable response to extreme temperatures, salinity, pH and water stress. 56% of isolates were able to nodulate chickpea. Numerical analysis of rep-PCR results showed that nodulating strains fell into 22 genotypes. Sequencing of 16S rRNA gene of endophytic bacteria from chickpea nodules revealed that 55% of isolated bacteria belong to Mesorhizobium genus. Based on MLSA of core genes (recA, atpD, glnII and dnaK), tasted strains were distributed into six clades and were closely related to Mesorhizobium ciceri, Mesorhizobium opportunistum, Mesorhizobium qingshengii, and Mesorhizobium plurifarium. Most of nodulating strains were belonging to a group genetically distinct from reference Mesorhizobium species. Three isolates belong to genus Burkholderia of the class β- proteobacteria, and 55 other strains belong to the class γ- proteobacteria. Some of the stress tolerant isolates have great potential for further inoculation of chickpea in the arid and semiarid environments to enhance biological nitrogen fixation and productivity in the context of climate change adaptation and mitigation.

A Toolbox for Nodule Development Studies in Chickpea: A Hairy-Root Transformation Protocol and an Efficient Laboratory Strain of Mesorhizobium sp

A Mesorhizobium sp. produces root nodules in chickpea. Chickpea and model legume Medicago truncatula are members of the inverted repeat-lacking clade (IRLC). The rhizobia, after internalization into the plant cell, are called bacteroids. Nodule-specific cysteine-rich peptides in IRLC legumes guide bacteroids to a terminally differentiated swollen (TDS) form. Bacteroids in chickpea are less TDS than those in Medicago spp. Nodule development in chickpea indicates recent evolutionary diversification and merits further study. A hairy-root transformation protocol and an efficient laboratory strain are prerequisites for performing any genetic study on nodulation. We have standardized a protocol for composite plant generation in chickpea with a transformation frequency above 50%, as shown by fluorescent markers. This protocol also works well in different ecotypes of chickpea. Localization of subcellular markers in these transformed roots is similar to the localization observed in transformed Medicago roots. When checked inside transformed nodules, peroxisomes were concentrated along the periphery of the nodules, while endoplasmic reticulum and Golgi bodies surrounded the symbiosomes. Different Mesorhizobium strains were evaluated for their ability to initiate nodule development and efficiency of nitrogen fixation. Inoculation with different strains resulted in different shapes of TDS bacteroids with variable nitrogen fixation. Our study provides a toolbox to study nodule development in the crop legume chickpea.

Horizontal Transfer of Symbiosis Genes within and Between Rhizobial Genera: Occurrence and Importance

Rhizobial symbiosis genes are often carried on symbiotic islands or plasmids that can be transferred (horizontal transfer) between different bacterial species. Symbiosis genes involved in horizontal transfer have different phylogenies with respect to the core genome of their ‘host’. Here, the literature on legume⁻rhizobium symbioses in field soils was reviewed, and cases of phylogenetic incongruence between rhizobium core and symbiosis genes were collated. The occurrence and importance of horizontal transfer of rhizobial symbiosis genes within and between bacterial genera were assessed. Horizontal transfer of symbiosis genes between rhizobial strains is of common occurrence, is widespread geographically, is not restricted to specific rhizobial genera, and occurs within and between rhizobial genera. The transfer of symbiosis genes to bacteria adapted to local soil conditions can allow these bacteria to become rhizobial symbionts of previously incompatible legumes growing in these soils. This, in turn, will have consequences for the growth, life history, and biogeography of the legume species involved, which provides a critical ecological link connecting the horizontal transfer of symbiosis genes between rhizobial bacteria in the soil to the above-ground floral biodiversity and vegetation community structure.

Sequential induction of three recombination directionality factors directs assembly of tripartite integrative and conjugative elements

Tripartite integrative and conjugative elements (ICE³) are a novel form of ICE that exist as three separate DNA regions integrated within the genomes of Mesorhizobium spp. Prior to conjugative transfer the three ICE³ regions of M. ciceri WSM1271 ICEMcSym¹²⁷¹ combine and excise to form a single circular element. This assembly requires three coordinated recombination events involving three site-specific recombinases IntS, IntG and IntM. Here, we demonstrate that three excisionases–or recombination directionality factors—RdfS, RdfG and RdfM are required for ICE³ excision. Transcriptome sequencing revealed that expression of ICE³ transfer and conjugation genes was induced by quorum sensing. Quorum sensing activated expression of rdfS, and in turn RdfS stimulated transcription of both rdfG and rdfM. Therefore, RdfS acts as a “master controller” of ICE³ assembly and excision. The dependence of all three excisive reactions on RdfS ensures that ICE³ excision occurs via a stepwise sequence of recombination events that avoids splitting the chromosome into a non-viable configuration. These discoveries expose a surprisingly simple control system guiding molecular assembly of these novel and complex mobile genetic elements and highlight the diverse and critical functions of excisionase proteins in control of horizontal gene transfer.

Bacteria evolve and adapt quickly through the horizontal transfer of DNA. A major mechanism facilitating this transfer is conjugation. Conjugative DNA elements that integrate into the chromosome are termed ‘Integrative and Conjugative Elements’ (ICE). We recently discovered a unique form of ICE that undergoes a complex series of recombination events with the host chromosome to split itself into three separate parts. This tripartite ICE must also precisely order its recombination when leaving the current host to avoid splitting the host chromosome and the ICE into non-viable parts. In this work, we show that the tripartite ICEs use chemical cell-cell communication to stimulate recombination and that recombination events are specifically ordered through cascaded transcriptional activation of small DNA-binding proteins called recombination directionality factors. Despite the inherent complexity of tripartite ICEs this work exposes a surprisingly simple system to stimulate their precise and ordered molecular assembly prior to horizontal transfer.

Rhizobia: from saprophytes to endosymbionts

Rhizobia are some of the best-studied plant microbiota. These oligotrophic Alphaproteobacteria or Betaproteobacteria form symbioses with their legume hosts. Rhizobia must exist in soil and compete with other members of the microbiota before infecting legumes and forming N₂-fixing bacteroids. These dramatic lifestyle and developmental changes are underpinned by large genomes and even more complex pan-genomes, which encompass the whole population and are subject to rapid genetic exchange. The ability to respond to plant signals and chemoattractants and to colonize nutrient-rich roots are crucial for the competitive success of these bacteria. The availability of a large body of genomic, physiological, biochemical and ecological studies makes rhizobia unique models for investigating community interactions and plant colonization.

Complete Genome Sequence of Mesorhizobium ciceri bv. biserrulae WSM1497, an Efficient Nitrogen-Fixing Microsymbiont of the Forage Legume Biserrula pelecinus

We report here the complete genome sequence of Mesorhizobium ciceri bv. biserrulae strain WSM1497, the efficient nitrogen-fixing microsymbiont and commercial inoculant in Australia of the forage legume Biserrula pelecinus. The genome consists of 7.2 Mb distributed across a single chromosome (6.67 Mb) and a single plasmid (0.53 Mb).