Plant-Microbe Symbiosis
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Symbiosis: Herbivory Alters Mycorrhizal Nutrient Exchange

Symbiosis: Herbivory Alters Mycorrhizal Nutrient Exchange | Plant-Microbe Symbiosis | Scoop.it
A new study shows that a plant gives less carbon to its root-associated mycorrhizal fungus when targeted by herbivores, but the fungus doesn’t retaliate.

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Plant-Microbe Symbiosis
Beneficial associations between plants and microbes
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Gibberellin Promotes Fungal Entry and Colonization during Paris-Type Arbuscular Mycorrhizal Symbiosis in Eustoma grandiflorum

Gibberellin Promotes Fungal Entry and Colonization during Paris-Type Arbuscular Mycorrhizal Symbiosis in Eustoma grandiflorum | Plant-Microbe Symbiosis | Scoop.it
Arbuscular mycorrhizas (AMs) are divided into two types according to morphology: Arum- and Paris-type AMs. Gibberellins (GAs) mainly inhibit the establishment of Arum-type AM symbiosis in most model plants, whereas the effects of GAs on Paris-type AM symbiosis are unclear. To provide insight into the mechanism underlying this type of symbiosis, the roles of GAs were investigated in Eustoma grandiflorum when used as the host plant for Paris-type AM establishment. Eustoma grandiflorum seedlings were inoculated with the model AM fungus, Rhizophagus irregularis, and the effects of GA and the GA biosynthesis inhibitor uniconazole-P on the symbiosis were quantitatively evaluated. Exogenous GA significantly increased hyphopodium formation at the epidermis, thus leading to the promotion of fungal colonization and arbuscule formation in the root cortex. By contrast, the suppression of GA biosynthesis and signaling attenuated fungal entry to E. grandiflorum roots. Moreover, the exudates from GA-treated roots strongly induced the hyphal branching of R. irregularis. Our results show that GA has an contrasting effect on Paris-type AM symbiosis in E. grandiflorum compared with Arum-type AM symbiosis. This finding could be explained by the differential regulation of the early colonization stage, where fungal hyphae make contact with and penetrate the epidermis.

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The Penium margaritaceum Genome: Hallmarks of the Origins of Land Plants

The Penium margaritaceum Genome: Hallmarks of the Origins of Land Plants | Plant-Microbe Symbiosis | Scoop.it
The evolutionary features and molecular innovations that enabled plants to first colonize land are not well understood. Here, insights are provided through our report of the genome sequence of the unicellular alga Penium margaritaceum, a member of the Zygnematophyceae, the sister lineage to land plants. The genome has a high proportion of repeat sequences that are associated with massive segmental gene duplications, likely facilitating neofunctionalization. Compared with representatives of earlier diverging algal lineages, P. margaritaceum has expanded repertoires of gene families, signaling networks, and adaptive responses that highlight the evolutionary trajectory toward terrestrialization. These encompass a broad range of physiological processes and protective cellular features, such as flavonoid compounds and large families of modifying enzymes involved in cell wall biosynthesis, assembly, and remodeling. Transcriptome profiling further elucidated adaptations, responses, and selective pressures associated with the semi-terrestrial ecosystems of P. margaritaceum, where a simple body plan would be an advantage.

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Genome-scale metabolic reconstruction of the symbiosis between a leguminous plant and a nitrogen-fixing bacterium

Genome-scale metabolic reconstruction of the symbiosis between a leguminous plant and a nitrogen-fixing bacterium | Plant-Microbe Symbiosis | Scoop.it
The mutualistic association between leguminous plants and endosymbiotic rhizobial bacteria is a paradigmatic example of a symbiosis driven by metabolic exchanges. Here, we report the reconstruction and modelling of a genome-scale metabolic network of Medicago truncatula (plant) nodulated by Sinorhizobium meliloti (bacterium). The reconstructed nodule tissue contains five spatially distinct developmental zones and encompasses the metabolism of both the plant and the bacterium. Flux balance analysis (FBA) suggests that the metabolic costs associated with symbiotic nitrogen fixation are primarily related to supporting nitrogenase activity, and increasing N2-fixation efficiency is associated with diminishing returns in terms of plant growth. Our analyses support that differentiating bacteroids have access to sugars as major carbon sources, ammonium is the main nitrogen export product of N2-fixing bacteria, and N2 fixation depends on proton transfer from the plant cytoplasm to the bacteria through acidification of the peribacteroid space. We expect that our model, called ‘Virtual Nodule Environment’ (ViNE), will contribute to a better understanding of the functioning of legume nodules, and may guide experimental studies and engineering of symbiotic nitrogen fixation.

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FungalRoot: global online database of plant mycorrhizal associations

FungalRoot: global online database of plant mycorrhizal associations | Plant-Microbe Symbiosis | Scoop.it
Testing of ecological, biogeographical and phylogenetic hypotheses of mycorrhizal traits requires a comprehensive reference dataset about plant mycorrhizal associations.
Here we present a database, FungalRoot, which summarizes publicly available data about vascular plant mycorrhizal type and intensity of root colonization by mycorrhizal fungi, accompanied with rich metadata. We compiled and digitized data about plant mycorrhizal colonization in nine widespread languages.
The present version of the FungalRoot database contains 36 303 species‐by‐site observations for 14 870 plant species, tripling the previously available compiled information about plant mycorrhizal associations. Based on these data, we provide a recommended list of genus‐level plant mycorrhizal associations, based on the majority of data for species and careful analysis of conflicting data. The majority of ectomycorrhizal and ericoid mycorrhizal plants are trees (92%) and shrubs (85%), respectively. The majority of arbuscular and nonmycorrhizal plant species are herbaceous (50% and 70%, respectively).
Our publicly available database is a powerful resource for mycorrhizal scientists and ecologists. It features possibilities for dynamic updating and addition of data about plant mycorrhizal associations. The new database will promote research on plant and fungal biogeography and evolution, and on links between above‐ and belowground biodiversity and ecosystem functioning.
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A Minimal Genetic Passkey to Unlock Many Legume Doors to Root Nodulation by Rhizobia

A Minimal Genetic Passkey to Unlock Many Legume Doors to Root Nodulation by Rhizobia | Plant-Microbe Symbiosis | Scoop.it
On legume crops, formation of developmentally mature nodules is a prerequisite to efficient nitrogen fixation by populations of rhizobial bacteroids established inside nodule cells. Development of root nodules and concomitant microbial colonisation of plant cells are constrained by sets of recognition signals exchanged by infecting rhizobia and their legume hosts, with much of the specificity of symbiotic interactions being determined by the flavonoid cocktails released by legume roots and the strain-specific nodulation factors (NFs) secreted by rhizobia. Hence, much of Sinorhizobium fredii strain NGR234 symbiotic promiscuity was thought to stem from a family of >80 structurally diverse NFs and associated nodulation keys in the form of secreted effector proteins and rhamnose-rich surface polysaccharides. Here, we show instead that a mini-symbiotic plasmid (pMiniSym2) carrying only the nodABCIJ, nodS and nodD1 genes of NGR234 conferred promiscuous nodulation to ANU265, a derivative strain cured of the large symbiotic plasmid pNGR234a. The ANU265::pMiniSym2 transconjugant triggered nodulation responses on 12 of the 17 legumes we tested. On roots of Macroptilium atropurpureum, Leucaena leucocephala and Vigna unguiculata, ANU265::pMiniSym2 formed mature-like nodule and successfully infected nodule cells. While cowpea and siratro responded to nodule colonisation with defence responses that eventually eliminated bacteria, L. leucocephala formed leghemoglobin-containing mature-like nodules inside which the pMiniSym2 transconjugant established persistent intracellular colonies. This data shows seven nodulation genes of NGR234 suffice to trigger nodule formation on roots of many hosts and to establish chronic infections in Leucaena cells. View Full-Text
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Arbuscular Mycorrhiza-Associated Rhizobacteria and Biocontrol of Soilborne Phytopathogens

The mutualistic symbiosis of most land plants with arbuscular mycorrhizal (AM) fungi has been shown to favor mineral and water nutrition and to increase resistance to abiotic and biotic stresses. The main mechanisms involved in the control of the disease symptoms and intraradical proliferation of soilborne phytopathogens are due to root colonization with AM fungi. The role of the rhizobacteria is shown to be specifically associated with extraradical network of the AM and mycorrhizosphere. The mycorrhizosphere can form a favorable environment for microorganisms which have potentiality to act antagonistic to pathogen abundance. It makes an additional advantage in identifying rhizobacteria from AM fungi structures or mycorrhizosphere, which often lead to the isolation of organisms having strong properties of antagonism on various soilborne pathogens. The ability of AM fungi to control soilborne diseases is mainly related to their capacity to stimulate the establishment of rhizobacteria against the favorable environment of pathogen within the mycorrhizosphere prior to the root infection. Recent advancement in scientific research has provided more clear picture in understanding the mechanisms involved in AM fungi/rhizobacteria interactions. Herein, this chapter includes the mechanisms of the AM fungi-mediated biocontrol, interactions between AM-associated bacteria and AM fungus extraradical network, AM-associated bacteria and biocontrol activities and unfavorable zone to pathogen development: the mycorrhizosphere.

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Rhizobium ruizarguesonis sp. nov., isolated from nodules of Pisum sativum L

Rhizobium ruizarguesonis sp. nov., isolated from nodules of Pisum sativum L | Plant-Microbe Symbiosis | Scoop.it
Four strains, coded as UPM1132, UPM1133T, UPM1134 and UPM1135, and isolated from nodules of Pisum sativum plants grown on Ni-rich soils were characterised through a polyphasic taxonomy approach. Their 16S rRNA gene sequences were identical and showed 100% similarity with their closest phylogenetic neighbors, the species included in the ‘R. leguminosarum group’: R. laguerreae FB206T, R. leguminosarum USDA 2370T, R. anhuiense CCBAU 23252T, R. sophoreae CCBAU 03386T, R. acidisoli FH13T and R. hidalgonense FH14T, and 99.6% sequence similarity with R. esperanzae CNPSo 668T. The analysis of combined housekeeping genes recA, atpD and glnII sequences showed similarities of 92-95% with the closest relatives. Whole genome average nucleotide identity (ANI) values were 97.5-99.7% ANIb similarity among the four strains, and less than 92.4% with closely related species, while digital DNA-DNA hybridization average values (dDDH) were 82-85% within our strains and 34-52% with closely related species. Major fatty acids in strain UPM1133T were C18:1 ω7c / C18:1 ω6c in summed feature 8, C14:0 3OH/ C16:1 iso I in summed feature 2 and C18:0. Colonies were small to medium, pearl-white coloured in YMA at 28 °C and growth was observed in the ranges 8-34 °C, pH 5.5-7.5 and 0-0.7% (w/v) NaCl. The DNA G + C content was 60.8 mol %. The combined genotypic, phenotypic and chemotaxonomic data support the classification of strains UPM1132, UPM1133T, UPM1134 and UPM1135 into a novel species of Rhizobium, for which the name Rhizobium ruizarguesonis sp. nov. is proposed. The type strain is UPM1133T (=CECT 9542T = LMG 30526T).

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Metabolomic profiling of wild‐type and mutant soybean root nodules using laser‐ablation electrospray ionization mass spectrometry reveals altered metabolism

The establishment of the nitrogen‐fixing symbiosis between soybean and Bradyrhizobium japonicum is a complex process. In order to document the changes in plant metabolism due to the symbiosis, we utilized laser ablation electrospray ionization mass spectrometry (LAESI‐MS) for in situ metabolic profiling of wild‐type nodules, nodules infected with a B. japonicum nifH mutant unable to fix nitrogen, nodules doubly infected by both strains, and nodules formed on plants mutated in the stearoyl‐acyl carrier protein desaturase (sacpd‐c ) gene, which were previously shown to have altered nodule ultrastructure. Results showed that the relative abundance of fatty acids, purines, and lipids was significantly changed in response to the symbiosis. The nifH mutant nodules had elevated levels of jasmonic acid (JA), correlating with signs of nitrogen deprivation. Nodules resulting from the mixed inoculant displayed similar, overlapping metabolic distributions within the sectors of effective (fix+) and ineffective (nifH mutant, fix‐) endosymbionts. These data are inconsistent with the notion that plant sanctioning is cell autonomous. Nodules lacking sacpd‐c displayed an elevation of soyasaponins and organic acids in the central necrotic regions. This study demonstrates the utility of LAESI‐MS for high‐throughput screening of plant phenotypes. Overall, nodules disrupted in the symbiosis were elevated in metabolites related to plant defense.

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Key factors affecting ammonium production by an Azotobacter vinelandii strain deregulated for biological nitrogen fixation

Key factors affecting ammonium production by an Azotobacter vinelandii strain deregulated for biological nitrogen fixation | Plant-Microbe Symbiosis | Scoop.it
Background
The obligate aerobe Azotobacter vinelandii is a model organism for the study of biological nitrogen fixation (BNF). This bacterium regulates the process of BNF through the two component NifL and NifA system, where NifA acts as an activator, while NifL acts as an anti-activator based on various metabolic signals within the cell. Disruption of the nifL component in the nifLA operon in a precise manner results in a deregulated phenotype that produces levels of ammonium that far surpass the requirements within the cell, and results in the release of up to 30 mM of ammonium into the growth medium. While many studies have probed the factors affecting growth of A. vinelandii, the features important to maximizing this high-ammonium-releasing phenotype have not been fully investigated.

Results
In this work, we report the effect of temperature, medium composition, and oxygen requirements on sustaining and maximizing elevated levels of ammonium production from a nitrogenase deregulated strain. We further investigated several pathways, including ammonium uptake through the transporter AmtB, which could limit yields through energy loss or futile recycling steps. Following optimization, we compared sugar consumption and ammonium production, to attain correlations and energy requirements to drive this process in vivo. Ammonium yields indicate that between 5 and 8% of cellular protein is fully active nitrogenase MoFe protein (NifDK) under these conditions.

Conclusions
These findings provide important process optimization parameters, and illustrate that further improvements to this phenotype can be accomplished by eliminating futile cycles.
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Genomic diversity of chickpea-nodulating rhizobia in Ningxia (north central China) and gene flow within symbiotic Mesorhizobium muleiense populations

Diversity and taxonomic affiliation of chickpea rhizobia were investigated from Ningxia in north central China and their genomic relationships were compared with those from northwestern adjacent regions (Gansu and Xinjiang). Rhizobia were isolated from root-nodules after trapping by chickpea grown in soils from a single site of Ningxia and typed by IGS PCR-RFLP. Representative strains were phylogenetically analyzed on the basis of the 16S rRNA, housekeeping (atpD, recA and glnII) and symbiosis (nodC and nifH) genes. Genetic differentiation and gene flow were estimated among the chickpea microsymbionts from Ningxia, Gansu and Xinjiang. Fifty chickpea rhizobial isolates were obtained and identified as Mesorhizobium muleiense. Their symbiosis genes nodC and nifH were highly similar (98.4 to 100%) to those of other chickpea microsymbionts, except for one representative strain (NG24) that showed low nifH similarities with all the defined Mesorhizobium species. The rhizobial population from Ningxia was genetically similar to that from Gansu, but different from that in Xinjiang as shown by high chromosomal gene flow/low differentiation with the Gansu population but the reverse with the Xinjiang population. This reveals a biogeographic pattern with two main populations in M. muleiense, the Xinjiang population being chromosomally differentiated from Ningxia-Gansu one. M. muleiense was found as the sole main chickpea-nodulating rhizobial symbiont of Ningxia and it was also found in Gansu sharing alkaline-saline soils with Ningxia. Introduction of chickpea in recently cultivated areas in China seems to select from alkaline-saline soils of M. muleiense that acquired symbiotic genes from symbiovar ciceri.

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Growth of chamomile (Matricaria chamomilla L.) and production of essential oil stimulated by arbuscular mycorrhizal symbiosis

Growth of chamomile (Matricaria chamomilla L.) and production of essential oil stimulated by arbuscular mycorrhizal symbiosis | Plant-Microbe Symbiosis | Scoop.it
This study aimed to evaluate the influence of arbuscular mycorrhizal fungi on the growth and production of capitula and essential oil of chamomile plants, under different growth conditions. The symbiosis between fungi of the phylum Glomeromycota and plant roots may cause changes in secondary metabolism, including the synthesis of essential oils. Increasing biomass production associated with gains in oil content promoted by mycorrhization, makes soil biota management sustainable. Two assays were carried out in a greenhouse, in plastic pots (3 L), for 150 days after emergence of the seedlings. In the first assay, plants were inoculated with soil cultivated with organic chamomile, in three levels (0, 150 and 300 g), with pH adjusted at 6.5, for evaluating the influence of the quantity of inoculum on the growth and production of plants. In the second assay (factorial design with repetition at the central point), three levels of soil pH (6.5, 7.0, 7.5) and three levels of auxin (0; 10; 20 mg L-1) were tested to identify the influence of pH and auxin on the production of chamomile capitula and essential oil. The arbuscular mycorrhizal fungi present as spores in the soil-inoculum were composed of 22 species. In the first assay, mycorrhization favored the growth and production of capitula and such benefits tended to be higher with the increase in concentration of inoculum. In the second assay, the production of capitula and essential oil was promoted by mycorrhization, with the highest content at neutral pH (0.7 mL oil in 30 g of capitula), in synergy with the highest amount of inoculum. It was concluded that the presence of arbuscular mycorrhizal fungi in the soil is the most influential variable in the production of capitula and essential oil of chamomile, but subtle differences on soil pH influenced these parameters. The levels of auxin did not influence significantly plant growth and arbuscular mycorrhizal fungi.

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Diazotrophic bacteria from maize exhibit multifaceted plant growth promotion traits in multiple hosts

Sierra Mixe maize is a geographically remote landrace variety grown on nitrogen-deficient fields in Oaxaca, Mexico that meets its nutritional requirements without synthetic fertilizer by associating with free-living diazotrophs comprising the microbiota of its aerial root mucilage. We selected nearly 500 diazotrophic bacteria isolated from Sierra Mixe maize mucilage and sequenced their genomes. Comparative genomic analysis demonstrated that isolates represented diverse genera and possessed multiple marker genes for mechanisms of direct plant growth promotion (PGP). In addition to nitrogen fixation, we examined deamination of 1-amino-1-cyclopropanecarboxylic acid, biosynthesis of indole-3-acetic acid, and phosphate solubilization. Implementing in vitro colorimetric assays revealed each isolate’s potential to confer the alternative PGP activities that corroborated genotype and pathway content. We examined the ability of mucilage diazotrophs to confer PGP by direct inoculation of clonally propagated potato plants in planta, which led to the identification of bio-stimulant candidates that were tested for PGP by inoculating a conventional maize variety. The results indicate that, while many diazotrophic isolates from Sierra Mixe maize possessed genotypes and in vitro phenotypes for targeted PGP traits, a subset of these organisms promoted the growth of potato and conventional maize using multiple promotion mechanisms.

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A Review of Studies from the Last Twenty Years on Plant–Arbuscular Mycorrhizal Fungi Associations and Their Uses for Wheat Crops

A Review of Studies from the Last Twenty Years on Plant–Arbuscular Mycorrhizal Fungi Associations and Their Uses for Wheat Crops | Plant-Microbe Symbiosis | Scoop.it
The aim of this work was to summarize the most recent research focused on the study of plant–arbuscular mycorrhizal fungi (AMF) symbiosis, both in a generic context and in the specific context of wheat cultivation. Taking into account the last 20 years, the most significant studies on the main plant advantages taken from this association are reviewed herein. Positive advances that have been reported stem from the mutualistic relationship between the plant and the mycorrhizal fungus, revealing better performance for the host in terms of nutrient uptake and protection from salinity, lack of water, and excess phytotoxic elements. Mycorrhiza studies and the recent progress in research in this sector have shown a possible solution for environmental sustainability: AMF represent a valid alternative to overcome the loss of biological fertility of soils, reduce chemical inputs, and alleviate the effects of biotic and abiotic stress.
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Investigation of Indigenous Arbuscular Mycorrhizal Performance Using a Lotus japonicus Mycorrhizal Mutant

Most plants are usually colonized with arbuscular mycorrhiza fungi (AMF) in the fields. AMF absorb mineral nutrients, especially phosphate, from the soil and transfer them to the host plants. Inoculation with exotic AMF is thought to be effective when indigenous AMF performance is low; however, there is no method for evaluating the performance of indigenous AMF. In this study, we developed a method to investigate the performance of indigenous AMF in promoting plant growth. As Lotus japonicus mutant (str) that are unable to form functional mycorrhizal roots were considered to be symbiosis negative for indigenous mycorrhizal performance, we examined the growth ratios of wild-type and str mycorrhizal mutant using 24 soils. Each soil had its own unique indigenous mycorrhizal performance, which was not directly related to the colonization level of indigenous AMF or soil phosphate level. The low indigenous mycorrhizal performance could not be compensated by the inoculation of exotic AMF. Importantly, indigenous mycorrhizal performance was never negative; however, the inoculation of exotic AMF into the same soil led to both positive and negative performances. These results suggest that indigenous mycorrhizal performance is affected by soil management history and is basically harmless to the plant.
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Hemoglobins in the legume‐rhizobium symbiosis

Legume nodules have two types of hemoglobins: symbiotic or leghemoglobins (Lbs) and nonsymbiotic or phytoglobins (Glbs). The latter are categorized into three phylogenetic classes differing in heme coordination and O2 affinity. This review is focused on the roles of Lbs and Glbs in the symbiosis of rhizobia with crop legumes and the model legumes for indeterminate (Medicago truncatula ) and determinate (Lotus japonicus ) nodulation. Only two hemoglobin functions are well established in nodules: Lbs deliver O2 to the bacteroids and act as O2 buffers, preventing nitrogenase inactivation; and Glb1‐1 modulates nitric oxide concentration during symbiosis, from the early stage, avoiding the plant's defense response, to nodule senescence. Here, we critically examine early and recent results, update and correct the information on Lbs and Glbs with the latest genome versions, provide novel expression data, and identify targets for future research. Crucial unresolved questions include the expression of multiple Lbs in nodules, their presence in the nuclei and in uninfected nodule cells, and, intriguingly, their expression in nonsymbiotic tissues. RNA‐sequencing data analysis show that Lbs are expressed as early as a few hours after inoculation and that their mRNAs are also detectable in roots and pods, which clearly suggests that these heme proteins play additional roles unrelated to nitrogen fixation. Likewise, issues awaiting investigation are the functions of other Glbs in nodules, the spatiotemporal expression profiles of Lbs and Glbs at the mRNA and protein levels, and the molecular mechanisms underlying their regulation during nodule development and in response to stress and hormones.

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Everything you must know about Azospirillum and its impact on agriculture and beyond

Everything you must know about Azospirillum and its impact on agriculture and beyond | Plant-Microbe Symbiosis | Scoop.it
Azospirillum is one of the most studied plant growth-promoting bacteria (PGPB); it represents a common model for plant-bacterial interactions. While Azospirillum brasilense is the species that is most widely known, at least 22 species, including 17 firmly validated species, have been identified, isolated from agricultural soils as well as habitats as diverse as contaminated soils, fermented products, sulfide springs, and microbial fuel cells. Over the last 40 years, studies on Azospirillum-plant interactions have introduced a wide array of mechanisms to demonstrate the beneficial impacts of this bacterium on plant growth. Multiple phytohormones, plant regulators, nitrogen fixation, phosphate solubilization, a variety of small-sized molecules and enzymes, enhanced membrane activity, proliferation of the root system, enhanced water and mineral uptake, mitigation of environmental stressors, and competition against pathogens have been studied, leading to the concept of the Multiple Mechanisms Hypothesis. This hypothesis is based on the assumption that no single mechanism is involved in the promotion of plant growth; it posits that each case of inoculation entails a combination of a few or many mechanisms. Looking specifically at the vast amount of information about the stimulatory effect of phytohormones on root development and biological nitrogen fixation, the Efficient Nutrients Acquisition Hypothesis model is proposed. Due to the existence of extensive agriculture that covers an area of more than 60 million hectares of crops, such as soybeans, corn, and wheat, for which the bacterium has proven to have some agronomic efficiency, the commercial use of Azospirillum is widespread in South America, with over 100 products already in the market in Argentina, Brazil, and Uruguay. Studies on Azospirillum inoculation in several crops have shown positive and variable results, due in part to crop management practices and environmental conditions. The combined inoculation of legumes with rhizobia and Azospirillum (co-inoculation) has become an emerging agriculture practice in the last several years, mainly for soybeans, showing high reproducibility and efficiency under field conditions. This review also addresses the use of Azospirillum for purposes other than agriculture, such as the recovery of eroded soils or the bioremediation of contaminated soils. Furthermore, the synthetic mutualistic interaction of Azospirillum with green microalgae has been developed as a new and promising biotechnological application, extending its use beyond agriculture.

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Prevalence and phenology of mycorrhizal colonization across populations of Lycopodiella inundata

Mycorrhizal fungi are critical components of terrestrial habitats and agroecosystems. Recently, Mucoromycotina fine root endophyte (MucFRE) fungi were found to engage in nutritional mutualism with the rare plant Lycopodiella inundata (‘marsh clubmoss’), one of the earliest vascular plant lineages known to associate with MucFRE. The extent to which this mutualism plays a role in resilient plant populations can only be understood by examining its occurrence rate and phenological patterns.

To test for prevalence and seasonality in colonization, we examined 1,297 individual L. inundata roots collected during spring and autumn 2019 from 11 semi-natural heathlands in Britain and the Netherlands. We quantified presence/absence of MucFRE-like hyphae and vesicles and explored possible relationships between temperature and precipitation in the months immediately before sampling.

MucFRE-like hyphae were the dominant mycorrhizal fungi observed in all of the examined heathlands. However, we found significant differences in colonization between the two seasons at every site. Overall, 14% of L. inundata roots were colonised in spring (2.4% with vesicles) compared with 86% in autumn (7.6% with vesicles). Colonization levels between populations were also significantly different, and correlated with temperature and precipitation, suggesting some local environments may be more conducive to hyphal growth.

These marked seasonal differences in host-plant colonization suggest that results about mycorrhizal status - typically drawn from single time point collections - should be carefully interpreted. Our findings are directly relevant to habitat restoration, species conservation plans, agricultural bio-inoculation nutrient enhancement treatments, microbial diversity and functional studies of host plants and symbionts.
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Editorial: Evolution of Signaling in Plant Symbioses

Plants are surrounded by microbes, but only a small number of microbes have evolved an intimate, endosymbiotic association, in which they live inside host cells. Root symbioses are important sources of nutrition for plants and microbes alike, with over 80% of all terrestrial plants forming intracellular symbioses with arbuscular mycorrhizal fungi. This Research Topic explores the evolutionary links between different plant root endosymbioses, focusing specifically on the evolution of signaling in four: the very ancient arbuscular mycorrhizae, the more recent ericoid mycorrhizae, and root nodules formed with nitrogen-fixing soil bacteria, either with Frankia (in actinorhizal nodulation) or rhizobia (in rhizobium-legume nodulation) (Genre and Russo, 2016). In each case, symbiosis is initiated after signal exchange between the two partners that ultimately leads to the plant hosting the microbes inside plant cells, requiring changes in plant host development and physiology (Geurts et al., 2016; MacLean et al., 2017). During establishment of the endosymbiosis, continued signal exchange between host and microbe functions to fine-tune the interaction.

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Silicon Nutrition and Arbuscular Mycorrhizal Fungi - Metalloids in Plants

Abiotic stresses, like salinity and heavy metal toxicity, pose a serious threat to plant growth and productivity. Agricultural soils become unsuitable for cultivation, owing to deleterious effects of these stresses on soil fertility. In recent years, supplementation of soils with silicon (Si) as a nutrient has been shown to exhibit beneficial effects on plants' health, especially under stressed conditions. The protective roles of Si could be due to precipitation of toxic ions with silica, formation of mechanical barrier through deposition of amorphous Si in the subcuticular layer of leaf, maintenance of leaf structures, and light receptiveness so as to increase photosynthetic efficiency of the plants. In addition to this, Si has also been reported to protect plants either by sequestration of toxic ions or by increasing the protein activity to counteract salt and heavy metals' mediated toxic effects. Plant species vary in their capacity to accumulate Si in their tissues, with monocots having higher tendency to take up Si than dicots. Among dicots, legumes are considered moderate to low accumulators and therefore unable to get Si‐induced benefits. Arbuscular mycorrhizal fungi (AMF) have also been recognized as a safe, cost‐effective, and environment‐friendly approach in alleviating these stresses in crop plants. The mechanisms employed by AMF to enhance stress tolerance include reduced toxic ion uptake, increased root rhizosphere, nutrient uptake, and upregulation of plant metabolic processes. Interestingly, AMF have been reported to acquire Si in their spores and hyphae. Therefore, recent studies have been directed toward use of AMF as an external agent to enhance Si uptake and the resultant stress tolerance in crop plants. This article deals with the relative roles of AMF and Si in mitigating salinity and heavy metal stresses and the impact of their dual applications in stress management in crop plants.

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The effect of soil tillage on symbiotic activity of soybean crops

This study is devoted to consideration of the practical aspect of atmospheric nitrogen biological fixation by soybean plants. This fixation is considered in terms of physiological-biochemical processes taking place at binding the inert nitrogen molecule in nitrogenous compounds accessible to plants. Searching the ways of intensifying the process of nitrogen fixation to obtain maximal yields of high-quality protein products appears to be an important practical task. It has been demonstrated that the role of soil tillage practices connected with the possibilities of soybean symbiotic nitrogen fixation, being among many factors capable of affecting this process, has not been sufficiently studied. To study effect of soil tillage and seed inoculation on soybean productivity field trials during the period from 2016 to 2018 were conducted. The following main soil treatment techniques were used: deep tillage (20–22 cm), shallow tillage of two types: up to 14–16 cm and to 12–14 cm depth. Various types of cultivators forming different degree of soil dispersity by its crumbling, fluffing, partial mixing, and also flattening the field surface were used for soil shallow tillage. It was established that inoculated soybean plants are able to uptake the maximal quantity of fixed nitrogen181.2 kg/ha under shallow tillage (12–14 cm), flattened and moderately compacted soil surface. Obtaining the highest soybean yield of 2.42 t/ha as compared with other soil tillage practices as shallow tillage (14–16 cm) and deep tillage under which the yields were 2.24 and 2.01 t/ha, respectively, was the result of such farm practices.

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Native legumes of the Farrapos protected area in Uruguay establish selective associations with rhizobia in their natural habitat

The area known as Farrapos wetlands is a Protected Natural Area of Uruguay and is a RAMSAR site. It is characterized for being one of the most extensive fluvial wetlands of the country, with an important heterogeneity of environments. The aim of this work was to survey native legumes and the nodulating rhizobial symbionts present in this protected area in order to contribute to the conservation of native biological resources. Nodules of legumes growing in their natural habitat were collected for the isolation of rhizobia. Interestingly, each legume species presented nodules with distinct morphologies reinforcing the idea that nodule phenotype can be included as additional information in legume taxonomy studies. Nodulation ability of selected isolates was assessed in gnotobiotic conditions. Isolates were identified by analysing 16S rRNA sequences and characterized by phylogenetic analysis. Papilionoideae legumes harbour rhizobia from the Bradyrhizobium, Rhizobium, Azorhizobium and Ensifer genera, while nodules from the Caesalpinoideae, including the mimosoid clade, harbour Alpha-rhizobia belonging to the Rhizobium, Bradyrhizobium and Mesorhizobium genera, as well as Beta-rhizobia from the Cupriavidus and Paraburkholderia genera. We found that the Farrapos protected area possess a great diversity of rhizobia, with high host specificity in their natural environment.

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Novel putative rhizobial species with different symbiovars nodulate Lotus creticus and their differential preference to distinctive soil properties

Phylogenetically diverse rhizobial strains endemic to Tunisia were isolated from symbiotic nodules of Lotus creticus, growing on different arid extremophile geographical regions of Tunisia, and speciated using multiloci-phylogenetic analysis as Neorhizobium huautlense (LCK33, LCK35, LCO42 and LCO49), Ensifer numidicus (LCD22, LCD25, LCK22 and LCK25), Ensifer meliloti (LCK8, LCK9 and LCK12), and Mesorhizobium camelthorni (LCD11, LCD13, LCD31 and LCD33). In addition, phylogenetic analyses revealed eight additional strains with previously undescribed chromosomal lineages within the genera Ensifer (LCF5, LCF6 and LCF8), Rhizobium (LCF11, LCF12 and LCF14) and Mesorhizobium (LCF16 and LCF19). Analysis using nodC gene identified five symbiovar groups, four of which were already known. The remaining group composed of two strains (LCD11 and LCD33) represented a new symbiovar of Mesorhizobium camelthorni, which we propose designating as sv. hedysari. Interestingly, we report that soil properties drive and structure the symbiosis of L. creticus and its rhizobia.

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Do shared traits create the same fates? Examining the link between morphological type and the biogeography of fungal and bacterial communities

Do shared traits create the same fates? Examining the link between morphological type and the biogeography of fungal and bacterial communities | Plant-Microbe Symbiosis | Scoop.it
Although it has been commonly observed that fungi and bacteria differ in their regional biogeographic patterns, it is not well understood what traits contribute to these different distributions. Here, we evaluate how morphological type (i.e. unicellular or filamentous growth form) influences the biogeography of soil fungal and bacterial communities across not only Euclidean (i.e. geographic) distances, but also across gradients of climate and edaphic factors and plant community composition. Specifically, we assessed the decay in community similarity over distance (distance-decay relationship) for microbes with unicellular and filamentous morphology in both fungi and bacteria across 40 ecologically diverse sampling sites in Minnesota, USA. Overall, we found that while distance-decay relationships were similar in fungal and bacterial communities over Euclidean distances, there were important differences among morphological groups of fungi and bacteria across gradients of environmental and plant community similarity. Specifically, the distance-decay relationship of unicellular fungi and unicellular bacteria were indistinguishable across environmental similarity. However, as plant community similarity decreased, only filamentous fungi and unicellular bacteria differed significantly in the strength of their distance-decay relationships. Like analyses of other study systems, we also found that pH explained much of the variance in community composition across microbial domains and morphological types and that plant community diversity was more closely correlated with fungal diversity than with bacterial diversity. Collectively, our results suggest that specific ecological traits such as morphological type along with microbial domain are key factors shaping the biogeography of microbial communities.

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Importance of Root Symbiomes for Plant Nutrition: New Insights, Perspectives and Future Challenges

Beneficial microbes are a particularly important component of the plant microbiome, comprising (i) mycorrhizal fungi; (ii) symbiotic nitrogen-fixing bacteria, and (iii) bacterial or fungal endophytes that promote plant growth. The roots of the majority of land plants are colonized by mycorrhizal fungi and ...

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