Plant-Microbe Symbiosis
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Mycorrhizal networks in ecosystem structure and functioning

Mycorrhizal networks in ecosystem structure and functioning | Plant-Microbe Symbiosis | Scoop.it
The vast majority of land plants (> 80%) form mutualistic symbioses with soil-dwelling fungi known as mycorrhizas. Such symbioses typically involve the reciprocal exchange of fungal-acquired nutrients for plant-fixed carbohydrates (Smith and Read 2008). As mycorrhizal fungi tend to be non-specific in their choice of hosts, many plants can be linked through fungal hyphae in a common mycelial network (CMN). These networks can be enormous, with around 200m of mycorrhizal fungal hyphae present in a single gram of typical forest soil (Dickie, 2006). The flow of nutrients between plants and mycorrhiza and the resulting redistribution of nutrients throughout a community is an area of much recent research with important contributions having been made by publications in Functional Ecology. This Virtual Issue highlights those contributions covering three major themes in mycorrhizal research, namely; the movement of plant-fixed carbon, reciprocal exchange of nutrients, and the wider impacts on biodiversity and ecosystem function.
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Plant-Microbe Symbiosis
Beneficial associations between plants and microbes
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A seed change in our understanding of legume biology from genomics to the efficient cooperation between nodulation and arbuscular mycorrhizal fungi

Grain legumes play a significant role in global food security. They have an advantage over cereals in that they can form symbiotic associations with nitrogen‐fixing bacteria, making them self‐sufficient in terms of nitrogen acquisition. In addition to this superior agronomic trait, grain legumes have excellent nutritional properties and are thus widely used as animal feed as well as in human nutrition. Current global trends towards increased legume consumption and availability of value‐added products, as well as legume production in developing countries require the provision of improved cultivars with better productivity and adaptability. Intensive efforts are thus underway to elaborate genomic resources and gain an improved knowledge base in a number of legume crops. There is also an emerging understanding of the beneficial interactions between legume‐associated organisms, particularly rhizobia and arbuscular mycorrhizal fungi, which result in improved nodulation and nutrient acquisition. The emerging focus on legume breeding for high sustainable yields as well as improved biotic and abiotic stress tolerance traits will serve to close the current gap between grain legume production and demand. With the support from policymakers, this increase in knowledge can be readily translated into increased crop production to meet the demands of an increasing global population.

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Suppression of innate immunity mediated by the CDPK‐Rboh complex is required for rhizobial colonization in Medicago truncatula nodules

Suppression of innate immunity is essential for rhizobial infection and colonization in compatible interactions with leguminous plants. In Medicago nad1 mutant plants, innate immunity is excessively activated, resulting in necrotic cell death after rhizobia are released from infection threads into symbiotic cells, suggesting that innate immunity plays a critical role in regulating bacteroid persistence.
In this study, we identified three respiratory burst oxidase homologs (Rboh) and one calcium‐dependent protein kinase (CDPK) as key factors for the activation of immunity in Medicago nodules using genetic and biochemical methods.
Knock‐out of either MtRbohB or MtRbohD in nad1‐1 mutant plants produced effective nodules with intact symbiotic cells, while knock‐out of MtRbohC decreased brown pigment deposition, leading to less necrosis in nad1‐1 mutant nodules. MtCDPK5 directly phosphorylated MtRbohB, MtRbohC and MtRbohD, which triggered immune responses in plants. Knock‐out of MtCDPK5 in nad1‐1 mutant plants partially restored nitrogen‐fixing nodules. Overexpression of the constitutively activated variant MtCDPK5VK under the control of the NAD1 promoter elicited strong immune responses, resulting in ineffective nodules in wild‐type plants.
Our data provide direct evidence that host plants utilize innate immunity to regulate rhizobial colonization in symbiotic cells in Medicago truncatula.
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Crystal structure of VnfH, the iron protein component of vanadium nitrogenase

Nitrogenases catalyze the biological fixation of inert N2 into bioavailable ammonium. They are bipartite systems consisting of the catalytic dinitrogenase and a complementary reductase, the Fe protein that is also the site where ATP is hydrolyzed to drive the reaction forward. Three different subclasses of dinitrogenases are known, employing either molybdenum, vanadium or only iron at their active site cofactor. Although in all these classes the mode and mechanism of interaction with Fe protein is conserved, each one encodes its own orthologue of the reductase in the corresponding gene cluster. Here we present the 2.2 Å resolution structure of VnfH from Azotobacter vinelandii, the Fe protein of the alternative, vanadium-dependent nitrogenase system, in its ADP-bound state. VnfH adopts the same conformation that was observed for NifH, the Fe protein of molybdenum nitrogenase, in complex with ADP, representing a state of the functional cycle that is ready for reduction and subsequent nucleotide exchange. The overall similarity of NifH and VnfH confirms the experimentally determined cross-reactivity of both ATP-hydrolyzing reductases.

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Symbiogenesis as Evolution of Open Genetic Systems

The evolution of intracellular symbioses formed by bacteria with plants and animals is addressed as a model for reconstructing the origin of eukaryotic cells as a symbiosis between different forms of prokaryotes (symbiogenesis). In microorganisms that are in facultative or conditionally obligatory (ecologically obligatory) dependence on symbiosis, their gene networks arise on the basis of host-activated intragenomic rearrangements and horizontal gene transfer. The latter factor determines the evolution of the genomes of symbiotic bacteria as open genetic systems (OGSs), in which the ratio of accessory genome regions to its core regions is increased compared to free-living relatives. Coevolution of bacteria and eukaryotic hosts results in the formation of higher rank OGSs, symbiogenomes, the integrity of which is mediated by signaling interactions that determine cross-regulation of partner genes. Increasing the effectiveness of their cooperation is achieved with the transition of bacteria to strictly obligatory (genetically obligatory) dependence on hosts, determined by (a) the loss of considerable regions of the microbial genome encoding the functions of autonomous development and (b) adaptation of bacteria to permanent intracellular existence, endocytobiosis. At this stage, symbiogenomes acquire the status of inheritance systems, determined by vertical (as a rule, transovarial) transfer of microsymbionts through host generations. The transformation of endocytobionts into cellular organelles is associated with the loss of their genetic autonomy, i.e., the ability to maintain and express their rudimentary genomes, until their complete loss. However, organelles partially retain phenotypic identity of ancestral bacteria, which is determined by the importation from the host cell of the gene products (proteins, RNA) obtained earlier from microsymbionts, which led to the formation of structurally integrated hologenomes. The gene loss and gain strategy realized in this way led to the formation of different patterns of eukaryotic cell organization in accordance with the mosaic scenario, which includes sequential introduction of several symbionts into the host cell, or with the matryoshka doll scenario, in which new symbionts are introduced into the cells of previously acquired symbionts.

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Evidence for Phosphate Starvation of Rhizobia without Terminal Differentiation in Legume Nodules

Evidence for Phosphate Starvation of Rhizobia without Terminal Differentiation in Legume Nodules | Plant-Microbe Symbiosis | Scoop.it
Phosphate homeostasis is tightly modulated in all organisms, including bacteria, which harbor both high- and low-affinity transporters acting under conditions of fluctuating phosphate levels. It was thought that nitrogen-fixing rhizobia, named bacteroids, inhabiting root nodules of legumes are not phosphate limited. Here, we show that the high-affinity phosphate transporter PstSCAB, rather than the low-affinity phosphate transporter Pit, is essential for effective nitrogen fixation of Sinorhizobium fredii in soybean nodules. Symbiotic and growth defects of the pst mutant can be effectively restored by knocking out PhoB, the transcriptional repressor of pit. The pst homologs of representative rhizobia were actively transcribed in bacteroids without terminal differentiation in nodules of diverse legumes (soybean, pigeonpea, cowpea, common bean, and Sophora flavescens) but exhibited a basal expression level in terminally differentiated bacteroids (alfalfa, pea, and peanut). Rhizobium leguminosarum bv. viciae Rlv3841 undergoes characteristic nonterminal and terminal differentiations in nodules of S. flavescens and pea, respectively. The pst mutant of Rlv3841 showed impaired adaptation to the nodule environment of S. flavescens but was indistinguishable from the wild-type strain in pea nodules. Taken together, root nodule rhizobia can be either phosphate limited or nonlimited regarding the rhizobial differentiation fate, which is a host-dependent feature.

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Super interesting... I love MPMI.... This journal would deserve a higher impact factor.

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The Mycoheterotrophic Symbiosis Between Orchids and Mycorrhizal Fungi Possesses Major Components Shared with Mutualistic Plant-Mycorrhizal Symbioses

The Mycoheterotrophic Symbiosis Between Orchids and Mycorrhizal Fungi Possesses Major Components Shared with Mutualistic Plant-Mycorrhizal Symbioses | Plant-Microbe Symbiosis | Scoop.it
Achlorophylous and early developmental stages of chorolophylous orchids are highly dependent on carbon and other nutrients provided by mycorrhizal fungi, in a nutritional mode termed mycoheterotrophy. Previous findings have implied that some common properties at least partially underlie the mycorrhizal symbioses of mycoheterotrophic orchids and that of autotrophic arbuscular mycorrhizal (AM) plants; however, information about the molecular mechanisms of the relationship between orchids and their mycorrhizal fungi is limited. In this study, we characterized the molecular basis of an orchid-mycorrhizal (OM) symbiosis by analyzing the transcriptome of Bletilla striata at an early developmental stage associated with the mycorrhizal fungus Tulasnella sp. The essential components required for the establishment of mutual symbioses with AM fungi or rhizobia in most terrestrial plants were identified from the B. striata gene set. A cross-species gene complementation analysis showed one of the component genes, calcium and calmodulin-dependent protein kinase gene CCaMK in B. striata, retains functional characteristics of that in AM plants. The expression analysis revealed the activation of homologs of AM-related genes during the OM symbiosis. Our results suggest that orchids possess, at least partly, the molecular mechanisms common to AM plants.

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Cool... One more fungal symbiosis using the CSP?

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Rapid temporal changes in root colonization by arbuscular mycorrhizal fungi and fine root endophytes, not dark septate endophytes, track plant activity and environment in an alpine ecosystem

Fungal root endophytes play an important role in plant nutrition, helping plants acquire nutrients in exchange for photosynthates. We sought to characterize the progression of root colonization by arbuscular mycorrhizal fungi (AMF), dark septate endophytes (DSE), and fine root endophytes (FRE) over an alpine growing season, and to understand the role of the host plant and environment in driving colonization levels. We sampled four forbs on a regular schedule from June 26th–September 11th from a moist meadow (3535 m a.s.l) on Niwot Ridge, Rocky Mountain Front Range, CO, USA. We quantified the degree of root colonization by storage structures, exchange structures, and hyphae of all three groups of fungi. AMF and FRE percent colonization fluctuated significantly over time, while DSE did not. All AMF structures changed over time, and the degree of change in vesicles differed by plant species. FRE hyphae, AMF arbuscules and AMF vesicles peaked late in the season as plants produced seeds. AMF hyphae levels started high, decreased, and then increased within 20 days, highlighting the dynamic nature of plant-fungal interactions. Overall, our results show that AMF and FRE, not DSE, root colonization rapidly changes over the course of a growing season and these changes are driven by plant phenology and seasonal changes in the environment.

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Species-Associated Differences in the Below-Ground Microbiomes of Wild and Domesticated Setaria

Species-Associated Differences in the Below-Ground Microbiomes of Wild and Domesticated Setaria | Plant-Microbe Symbiosis | Scoop.it
The rhizosphere microbiome is known to play a crucial role in promoting plant growth, partly by countering soil-borne phytoparasites and by improving nutrient uptake. The abundance and composition of the rhizosphere and root-associated microbiota are influenced by several factors, including plant species and genotype. We hypothesize that crop domestication might influence the composition and diversity of plant-associated microbiomes. We tested the contribution of domestication to the bacterial and archaeal root and soil composition associated with six genotypes of domesticated Setaria italica and four genotypes of its wild ancestor, S. viridis. The bacterial microbiome in the rhizoplane and root endophyte compartments, and the archaea in the endophyte compartment, showed major composition differences. For instance, members of the Betaproteobacteria and Firmicutes were overrepresented in S. italica root samples compared to S. viridis. Metagenomic analysis of samples that contained both root surface-bound (rhizoplane) and inside-root (endophytic) bacteria defined two unique microbial communities only associated with S. italica roots and one only associated with S. viridis roots. Root endophytic bacteria were found in six discernible communities, of which four were primarily on S. italica and two primarily on S. viridis. Among archaea, Methanobacteria, and Methanomicrobia exhibited species-associated differences in the rhizosphere and root compartments, but most detected archaea were not classified more specifically than at the level of phylum. These results indicate a host genetic contribution to the microbial composition in Setaria, and suggest that domestication has selected for specific associations in the root and in the rhizosphere.

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International Committee on Systematics of Prokaryotes Subcommittee on the taxonomy of rhizobia and agrobacteria Minutes of the closed meeting, Granada, 4 September 2017

The Subcommittee closed meeting was called to order by Peter Young at 14:30 h on 4 September 2017 during the 20th International Conference on Nitrogen Fixation in Granada, Spain.

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Please don't change the names again :-)

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Misdiagnosis of mycorrhizas and inappropriate recycling of data can lead to false conclusions

We draw attention to a worrying trend for the uncritical use of ‘recycled’ mycorrhizal data to compile host species lists that include obvious errors or undertake risky analyses that correlate mycorrhizal colonisation levels with environmental or physiological factors despite inherent limitations in datasets. We are not suggesting that all meta‐studies are wrong, only that more care should be taken to resolve what can safely be done with recycled mycorrhizal data in the future. We also recommend that mycorrhizal species lists should be checked against standard references since the majority of EM hosts and NM plant belong to families that are well resolved. However, additional research is required in cases where plant families have multiple root types within genera or occur in habitats where mycorrhizal associations are often suppressed (see Brundrett & Tedersoo, 2018). We hope that the mycorrhizal science community will work together more closely in the future to develop and enforce standards for mycorrhizal diagnosis and to share carefully corrected datasets for realistic meta‐studies.

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Metabolic engineering for the production of chitooligosaccharides: advances and perspectives

Chitin oligosaccharides (CTOs) and its related compounds chitosan oligosaccharides (CSOs), collectively known as chitooligosaccharides (COs), exhibit numerous biological activities in applications in the nutraceutical, cosmetics, agriculture, and pharmaceutical industries. COs are currently produced by acid hydrolysis of chitin or chitosan, or enzymatic techniques with uncontrollable polymerization. Microbial fermentation by recombinant Escherichia coli, as an alternative method for the production of COs, shows new potential because it can produce a well-defined COs mixture and is an environmentally friendly process. In addition, Bacillus subtilis, a nonpathogenic, endotoxin-free, GRAS status bacterium, presents a new opportunity as a platform to produce COs. Here, we review the applications of COs and differences between CTOs and CSOs, summarize the current preparation approaches of COs, and discuss the future research potentials and challenges in the production of well-defined COs in B. subtilis by metabolic engineering.

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RiCRN1, a Crinkler Effector From the Arbuscular Mycorrhizal Fungus Rhizophagus irregularis, Functions in Arbuscule Development

RiCRN1, a Crinkler Effector From the Arbuscular Mycorrhizal Fungus Rhizophagus irregularis, Functions in Arbuscule Development | Plant-Microbe Symbiosis | Scoop.it
Arbuscular mycorrhizal (AM) symbiosis is one of the most prominent and beneficial plant–microbe interactions that facilitates mineral nutrition and confers tolerance to biotic and abiotic stresses. AM fungi colonize the root cortex and develop specialized structures called arbuscules where the nutrient exchange takes place. Arbuscule development is a highly controlled and coordinated process requiring the involvement of many plant proteins recruited at that interface. In contrast, much less is known about the fungal proteins involved in this process. Here, we have identified an AM fungal effector that participates in this developmental step of the symbiosis. RiCRN1 is a crinkler (CRN) effector that belongs to a subfamily of secreted CRN proteins from R. irregularis. CRNs have been so far only functionally characterized in pathogenic microbes and shown to participate in processes controlling plant cell death and immunity. RiCRN1 accumulates during symbiosis establishment parallel to MtPT4, the gene coding for an arbuscule-specific phosphate transporter. Expression in Nicotiana benthamiana leaves and in Medicago truncatula roots suggest that RiCRN1 is not involved in cell death processes. RiCRN1 dimerizes and localizes to nuclear bodies, suggesting that, similar to other CRNs, it functions in the plant nucleus. Downregulation of RiCRN1 using host-induced gene silencing led to an impairment of the symbiosis in M. truncatula and to a reduction of MtPT4, while ectopic expression of RiCRN1, surprisingly, led to a drastic reduction in arbuscule size that correlated with a decrease not only in MtPT4 but also in MtBCP1, a marker for initial stages of arbuscule development. Altogether, our results suggest that a tightly regulated expression in time and space of RiCRN1 is critical for symbiosis progression and for the proper initiation of arbuscule development.

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Why is that paper in Frontiers? Did I miss a major issue?

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Biogeography of arbuscular mycorrhizal fungi (Glomeromycota): a phylogenetic perspective on species distribution patterns

Information on the biogeography of arbuscular mycorrhizal fungi (AMF) is important because this group of obligately symbiotic soil microbes is a ubiquitous and functionally critical component of terrestrial ecosystems. In this paper, we utilize a biogeography database summarizing data on AMF species distribution linked to geographic and environmental conditions to describe global distribution patterns and interpret these patterns within a phylogenetic perspective. The data were obtained from accessions in living culture collections (INVAM, CICG), species descriptions, and other published literature from 1960 to 2012. The database contains 7105 records, 6396 of them from 768 published papers and the remaining 709 from culture accessions. Glomeromycotan species were recorded in all seven continents, 87 countries, 11 biogeographical realms, and 14 biomes. The distribution of families differed among climatic zones and continents, but they, together with all genera, appear to be cosmopolitan. Distribution of AMF species shows a slight decrease from low to high latitudes, but this decrease is steeper in the southern than in the northern hemisphere. A total of 189 species is shared between ancient supercontinents Gondwana and Laurasia and 78 species are common to all climatic zones. Ninety-five species (43% of the total) have known cosmopolitan distribution, including members of all genera except Redeckera. Some species have disjunct distribution and 26% of species have been registered from only one continent. Data on AMF distribution challenge the “Everything is everywhere” hypothesis in favor of the “moderate endemicity model” for species distribution. Data from this study provide a foundation to formulate and test hypotheses of biogeographic patterns and processes in Glomeromycota.

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A member of the ALOG gene family has a novel role in regulating nodulation in Lotus japonicus

Legumes can control the number of symbiotic nodules that form on their roots, thus balancing nitrogen assimilation and energy consumption. Two major pathways participate in nodulation: the Nod factor (NF) signaling pathway which involves recognition of rhizobial bacteria by root cells and promotion of nodulation, and the autoregulation of nodulation (AON) pathway which involves long‐distance negative feedback between roots and shoots. Although a handful of genes have a clear role in the maintenance of nodule number, additional unknown factors may also be involved in this process. Here, we identify a novel function for a Lotus japonicus ALOG (Arabidopsis LSH1 and Oryza G1) family member, LjALOG1, involved in positively regulating nodulation. LjALOG1 expression increased substantially after inoculation with rhizobia, with high levels of expression in whole nodule primordia and in the base of developing nodules. The ljalog1 mutants, which have an insertion of the LORE1 retroelement in LjALOG1, had significantly fewer nodules compared with wild type, along with increased expression of LjCLE‐RS1 (L. japonicus CLE Root Signal 1), which encodes a nodulation suppressor in the AON pathway. In summary, our findings identified a novel factor that participates in controlling nodulation, possibly by suppressing the AON pathway.

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Local and Systemic Effect of Cytokinins on Soybean Nodulation and Regulation of Their Isopentenyl Transferase (IPT) Biosynthesis Genes Following Rhizobia Inoculation

Local and Systemic Effect of Cytokinins on Soybean Nodulation and Regulation of Their Isopentenyl Transferase (IPT) Biosynthesis Genes Following Rhizobia Inoculation | Plant-Microbe Symbiosis | Scoop.it
Cytokinins are important regulators of cell proliferation and differentiation in plant development. Here, a role for this phytohormone group in soybean nodulation is shown through the exogenous application of cytokinins (6-benzylaminopurine, N6-(Δ2-isopentenyl)-adenine and trans-zeatin) via either root drenching or a petiole feeding technique. Overall, nodule numbers were reduced by treatment with high cytokinin concentrations, but increased with lower concentrations. This was especially evident when feeding the solutions directly into the vasculature via petiole feeding. These findings highlight the importance of cytokinin in nodule development. To further investigate the role of cytokinin in controlling nodule numbers, the IPT gene family involved in cytokinin biosynthesis was characterized in soybean. Bioinformatic analyses identified 17 IPT genes in the soybean genome and homeologous duplicate gene partners were subsequently identified including GmIPT5 and GmIPT6, the orthologs of LjIPT3. Expression of GmIPT5 was upregulated in the shoot in response to nodulation, but this was independent of a functional copy of the autoregulation of nodulation (AON) receptor, GmNARK, which suggests it is unlikely to have a role in the negative feedback system called AON. Legumes also control nodule numbers in the presence of soil nitrogen through nitrate-dependent regulation of nodulation, a locally acting pathway in soybean. Upon nitrate treatment to the root, the tandem duplicates GmIPT3 and GmIPT15 were upregulated in expression indicating a role for these genes in the plant’s response to soil nitrogen, potentially including the nitrate-dependent regulation of legume nodulation pathway. Additional roles for cytokinin and their IPT biosynthetic genes in nodulation and the control of nodule numbers are discussed.

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Cell Autoaggregation, Biofilm Formation, and Plant Attachment in a Sinorhizobium meliloti lpsB Mutant

Cell Autoaggregation, Biofilm Formation, and Plant Attachment in a Sinorhizobium meliloti lpsB Mutant | Plant-Microbe Symbiosis | Scoop.it
Bacterial surface molecules are crucial for the establishment of a successful rhizobia-legume symbiosis, and, in most bacteria, are also critical for adherence properties, surface colonization, and as a barrier for defense. Rhizobial mutants defective in the production of exopolysaccharides (EPSs), lipopolysaccharides (LPSs), or capsular polysaccharides are usually affected in symbiosis with their plant hosts. In the present study, we evaluated the role of the combined effects of LPS and EPS II in cell-to-cell and cell-to-surface interactions in Sinorhizobium meliloti by studying planktonic cell autoaggregation, biofilm formation, and symbiosis with the host plant Medicago sativa. The lpsB mutant, which has a defective core portion of LPS, exhibited a reduction in biofilm formation on abiotic surfaces as well as altered biofilm architecture compared with the wild-type Rm8530 strain. Atomic force microscopy and confocal laser microscopy revealed an increase in polar cell-to-cell interactions in the lpsB mutant, which might account for the biofilm deficiency. However, a certain level of biofilm development was observed in the lpsB strain compared with the EPS II-defective mutant strains. Autoaggregation experiments carried out with LPS and EPS mutant strains showed that both polysaccharides have an impact on the cell-to-cell adhesive interactions of planktonic bacteria. Although the lpsB mutation and the loss of EPS II production strongly stimulated early attachment to alfalfa roots, the number of nodules induced in M. sativa was not increased. Taken together, this work demonstrates that S. meliloti interactions with biotic and abiotic surfaces depend on the interplay between LPS and EPS II.

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Below-ground-above-ground Plant-microbial Interactions: Focusing on Soybean, Rhizobacteria and Mycorrhizal Fungi

Below-ground-above-ground Plant-microbial Interactions: Focusing on Soybean, Rhizobacteria and Mycorrhizal Fungi | Plant-Microbe Symbiosis | Scoop.it
Introduction:
Organisms seldom exist in isolation and are usually involved in interactions with several hosts and these interactions in conjunction with the physicochemical parameters of the soil affect plant growth and development. Researches into below and aboveground microbial community are unveiling a myriad of intriguing interactions within the rhizosphere, and many of the interactions are facilitated by exudates that are secreted by plants roots. These interactions can be harnessed for beneficial use in agriculture to enhance crop productivity especially in semi-arid and arid environments.

The Rhizosphere:
The rhizosphere is the region of soil close to plants roots that contain large number of diverse organisms. Examples of microbial candidates that are found in the rhizosphere include the Arbuscular Mycorrhizal Fungi (AMF) and rhizobacteria. These rhizosphere microorganisms use plant root secretions such as mucilage and flavonoids which are able to influence their diversity and function and also enhance their potential to colonize plants root.

Natural Interactions between Microorganisms and Plant:
In the natural environments, plants live in interactions with different microorganisms, which thrive belowground in the rhizosphere and aboveground in the phyllosphere. Some of the plant-microbial interactions (which can be in the form of antagonism, amensalism, parasitism and symbiosis) protect the host plants against detrimental microbial and non-microbial invaders and provide nutrients for plants while others negatively affect plants. These interactions can influence below-ground-above-ground plants’ biomass development thereby playing significant role in sustaining plants. Therefore, understanding microbial interactions within the rhizosphere and phyllosphere is urgent towards farming practices that are less dependent on conventional chemical fertilizers, which have known negative impacts on the environments.

Below Ground Rhizobacteria Interactions Alleviate Drought Stress:
Drought stress is one of the major factors militating against agricultural productivity globally and is likely to further increase. Belowground rhizobacteria interactions could play important role in alleviating drought stress in plants. These beneficial rhizobacterial colonize the rhizosphere of plants and impart drought tolerance by up regulation or down regulation of drought responsive genes such as ascorbate peroxidase, S-adenosyl-methionine synthetase, and heat shock protein.

Insights into Below and above the Ground Microbial Interactions via Omic Studies:
Investigating complex microbial community in the environment is a big challenge. Therefore, omic studies of microorganisms that inhabit the rhizosphere are important since this is where most plant-microbial interactions occur. One of the aims of this review is not to give detailed account of all the present omic techniques, but instead to highlight the current omic techniques that can possibly lead to detection of novel genes and their respective proteins within the rhizosphere which may be of significance in enhancing crop plants (such as soybean) productivity especially in semi-arid and arid environments.

Future Prospects and Conclusions:
Plant-microbial interactions are not totally understood, and there is, therefore, the need for further studies on these interactions in order to get more insights that may be useful in sustainable agricultural development. With the emergence of omic techniques, it is now possible to effectively monitor transformations in rhizosphere microbial community together with their effects on plant development. This may pave way for scientists to discover new microbial species that will interact effectively with plants. Such microbial species can be used as biofertilizers and/or bio-pesticides to increase crop yield and enhance global food security.
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Role of Phytohormones in Piriformospora indica-Induced Growth Promotion and Stress Tolerance in Plants: More Questions Than Answers

Role of Phytohormones in Piriformospora indica-Induced Growth Promotion and Stress Tolerance in Plants: More Questions Than Answers | Plant-Microbe Symbiosis | Scoop.it
Phytohormones play vital roles in the growth and development of plants as well as in interactions of plants with microbes such as endophytic fungi. The endophytic root-colonizing fungus Piriformospora indica promotes plant growth and performance, increases resistance of colonized plants to pathogens, insects and abiotic stress. Here, we discuss the roles of the phytohormones (auxins, cytokinin, gibberellins, abscisic acid, ethylene, salicylic acid, jasmonates, and brassinosteroids) in the interaction of P. indica with higher plant species, and compare available data with those from other (beneficial) microorganisms interacting with roots. Crosstalks between different hormones in balancing the plant responses to microbial signals is an emerging topic in current research. Furthermore, phytohormones play crucial roles in systemic signal propagation as well as interplant communication. P. indica interferes with plant hormone synthesis and signaling to stimulate growth, flowering time, differentiation and local and systemic immune responses. Plants adjust their hormone levels in the roots in response to the microbes to control colonization and fungal propagation. The available information on the roles of phytohormones in beneficial root–microbe interactions opens new questions of how P. indica manipulates the plant hormone metabolism to promote the benefits for both partners in the symbiosis.

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Plant functional group influences arbuscular mycorrhizal fungal abundance and hyphal contribution to soil CO2 efflux in temperate grasslands

Background and aims
Arbuscular mycorrhizal (AM) fungi are abundant in grassland ecosystem. We assessed AM hyphal contributions to soil CO2 efflux across plant functional groups to better quantify AM fungal influences on soil carbon dynamics.

Methods
We conducted a field experiment using in-growth mesocosms to partition soil CO2 efflux from roots, AM hyphae, and free-living soil microbes associated with C3 grasses, C4 grasses, forbs, and diverse plant communities from May to August in 2017.

Results
AM hyphae contributed <10% to total soil respiration in forb communities and diverse plant communities but accounted for as much as 32% in C3 grasses. Plant functional groups differed in hyphal production efficiencies (the ratio of AM hyphal length to aboveground biomass), with the lowest in C3 grasses (0.47 ± 0.15 m g−1) and the greatest in forbs (3.27 ± 0.55 m g−1). Mowing reduced hyphal production efficiency of C4 grasses and forbs but did not affect total soil respiration. AM hyphal and microbial respiration peaked at the middle of the growing season, however there was no significant seasonal variation in root respiration.

Conclusion
AM hyphal respiration is an important pathway of carbon flux from plants to atmosphere. Shifts in plant community composition can influence soil carbon processes by regulating hyphal production and respiration.
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Tracing My Roots: How I Became a Plant Biologist - Frederick M. Ausubel

My trajectory to becoming a plant biologist was shaped by a complex mix of scientific, political, sociological, and personal factors. I was trained as a microbiologist and molecular biologist in the late 1960s and early 1970s, a time of political upheaval surrounding the Vietnam War. My political activism taught me to be wary of the potential misuses of scientific knowledge and to promote the positive applications of science for the benefit of society. I chose agricultural science for my postdoctoral work. Because I was not trained as a plant biologist, I devised a postdoctoral project that took advantage of my microbiological training, and I explored using genetic technologies to transfer the ability to fix nitrogen from prokaryotic nitrogen-fixing species to the model plant Arabidopsis thaliana with the ultimate goal of engineering crop plants. The invention of recombinant DNA technology greatly facilitated the cloning and manipulation of bacterial nitrogen-fixation (nif) genes, but it also forced me to consider how much genetic engineering of organisms, including human beings, is acceptable. My laboratory has additionally studied host–pathogen interactions using Arabidopsis and the nematode Caenorhabditis elegans as model hosts.

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A great scientist

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Isolation, characterization, and selection of heavy metal-resistant and plant growth-promoting endophytic bacteria from root nodules of Robinia pseudoacacia in a Pb/Zn mining area - ScienceDirect

Multiple heavy metals (HMs) commonly coexist in mining areas, which highlights the necessity to select multiple HM-resistant plant growth-promoting bacteria for improving phytoremediation efficiency. In this study, we isolated and characterized 82 endophytic bacteria from the root nodules of black locust (Robinia pseudoacacia) grown in a Pb-Zn mining area. There were 80 isolates showing resistance to four HMs, 0.01–18.0 mM/L for Cd, 0.2–40.0 mM/L for Zn, 0.3–2.2 mM/L for Pb, and 0.2–1.4 mM/L for Cu. Indole-3-acetic acid production, siderophore production, and 1-aminocyclopropane-1-carboxylate deaminase activity were detected in 43, 50, and 17 isolates, respectively. Two symbiotic isolates selected with the highest potential for HM resistance and PGP traits, designated Mesorhizobium loti HZ76 and Agrobacterium radiobacter HZ6, were evaluated for promotion of plant growth and metal uptake by R. pseudoacacia seedlings grown in pots containing different levels of Cd, Zn, Pb, or Cu. HZ76 significantly increased plant shoot biomass, while HZ6 did not, compared with non-inoculated controls. The results indicate that inoculation with HZ76 or HZ6 relieved HM stress in the plants, depending on the type and concentration of HM in the treatment. Mesorhizobium loti HZ76 may be a better candidate for application in phytoremediation than A. radiobacter HZ6. The microsymbiosis between HM-resistant rhizobia and R. pseudoacacia is an interesting mutualistic system for phytoremediation in mining areas contaminated with multiple HMs.

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Simple colony PCR procedure for the filamentous actinobacteria Frankia

Molecular analysis of the filamentous actinobacteria Frankia is laborious because of the slow growth rate and required biomass needed for these techniques. An efficient and simple colony PCR protocol for Frankia was developed that saved time for analysis of any Frankia strains growing on a plate. Previously, it took 5–6 weeks to get the correct size Frankia colonies on plates and then a minimum of 5 weeks of growth in liquid culture for DNA extraction. With this technique, these colonies could be screened after 5–6 weeks of growth by colony PCR. The procedure used a combination of mechanical and heat treatments and required no added buffers or chemicals. Our results demonstrate rapid and efficient PCR.

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Building plant microbiome vault: a future biotechnological resource

The plant-microbiome symbiotic association will need to be taken advantage of for feeding the burgeoning millions in the face of climatic perturbations and environmental deterioration. Since the plants select their microbiome from the soils on which they grow, soils, therefore, remain the key source of microbiome for sustainable food production. Building a reliable and reproducible plant microbiome vault of key crops growing with desirable traits such as high yielders under low input conditions, drought tolerant plots, disease suppressive soils, etc. can become an important and irreparable biotechnological resource for future agriculture. Based on the available literature, a complementary approach is discussed wherein i) rhizosphere and bulk soils are preserved with the best available protocols in such a way that their biological components remain undisturbed for long periods and the viable microbiome can be accessed; supplemented side-by-side with ii) systematic isolation, screening and preservation of the ‘Minimal Effective Microbiome Set’ (‘MEMS’) for building the plant microbiome vault.

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The plant-growth-promoting actinobacteria of the genus Nocardia induces root nodule formation in Casuarina glauca

Actinorhizal plants form a symbiotic association with the nitrogen-fixing actinobacteria Frankia. These plants have important economic and ecological benefits including land reclamation, soil stabilization, and reforestation. Recently, many non-Frankia actinobacteria have been isolated from actinorhizal root nodules suggesting that they might contribute to nodulation. Two Nocardia strains, BMG51109 and BMG111209, were isolated from Casuarina glauca nodules, and they induced root nodule-like structures in original host plant promoting seedling growth. The formed root nodule-like structures lacked a nodular root at the apex, were not capable of reducing nitrogen and had their cortical cells occupied with rod-shaped Nocardiae cells. Both Nocardia strains induced root hair deformation on the host plant. BMG111209 strain induced the expression of the ProCgNin:Gus gene, a plant gene involved in the early steps of the infection process and nodulation development. Nocardia strain BMG51109 produced three types of auxins (Indole-3-acetic acid [IAA], Indole-3-Byturic Acid [IBA] and Phenyl Acetic Acid [PAA]), while Nocardia BMG111209 only produced IAA. Analysis of the Nocardia genomes identified several important predicted biosynthetic gene clusters for plant phytohormones, secondary metabolites, and novel natural products. Co-infection studies showed that Nocardia strain BMG51109 plays a role as a “helper bacteria” promoting an earlier onset of nodulation. This study raises many questions on the ecological significance and functionality of Nocardia bacteria in actinorhizal symbioses.

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Phosphate Deficiency Negatively Affects Early Steps of the Symbiosis between Common Bean and Rhizobia

Phosphate (Pi) deficiency reduces nodule formation and development in different legume species including common bean. Despite the significant progress in the understanding of the genetic responses underlying the adaptation of nodules to Pi deficiency, it is still unclear whether this nutritional deficiency interferes with the molecular dialog between legumes and rhizobia, if so, what part of the molecular dialog is impaired? In this study, we provide evidence demonstrating that Pi deficiency negatively affects critical early molecular and physiological responses required for a successful symbiosis between common bean and rhizobia. We demonstrated that the infection thread formation and the expression of PvNSP2, PvNIN, and PvFLOT2, genes controlling the nodulation process, were significantly reduced in Pi-deficient common bean seedlings. Further transcriptional analysis revealed that the expression of hormones-related genes is compromised in Pi-deficient seedlings inoculated with rhizobia. Additionally, we showed that regardless of the presence or absence of rhizobia, the expression of PvRIC1 and PvRIC2, two genes participating in the autoregulation of nodule number, was higher in Pi-deficient seedlings than in control seedlings. The data presented in this study shed light on the understanding of how Pi deficiency impacts the early steps of the symbiosis between common bean and rhizobia.
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