The Plant Microbiome
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Exposure to the leaf litter microbiome of healthy adults protects seedlings from pathogen damage

Exposure to the leaf litter microbiome of healthy adults protects seedlings from pathogen damage | The Plant Microbiome | Scoop.it
It is increasingly recognized that microbiota affect host health and physiology. However, it is unclear what factors shape microbiome community assembly in nature, and how microbiome assembly can be manipulated to improve host health. All plant leaves host foliar endophytic fungi, which make up a diverse, environmentally acquired fungal microbiota. Here, we experimentally manipulated assembly of the cacao tree ( Theobroma cacao ) fungal microbiome in nature and tested the effect of assembly outcome on host health. Using next-generation sequencing, as well as culture-based methods coupled with Sanger sequencing, we found that manipulating leaf litter exposure and location within the forest canopy significantly altered microbiome composition in cacao. Exposing cacao seedlings to leaf litter from healthy conspecific adults enriched the seedling microbiome with Colletotrichum tropicale , a fungal endophyte known to enhance pathogen resistance of cacao seedlings by upregulating host defensive pathways. As a result, seedlings exposed to healthy conspecific litter experienced reduced pathogen damage. Our results link processes that affect the assembly and composition of microbiome communities to their functional consequences for host success, and have broad implications for understanding plant–microbe interactions. Deliberate manipulation of the plant–fungal microbiome also has potentially important applications for cacao production and other agricultural systems in general.
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Facilitation of phosphorus uptake in maize plants by mycorrhizosphere bacteria

Facilitation of phosphorus uptake in maize plants by mycorrhizosphere bacteria | The Plant Microbiome | Scoop.it
A major challenge for agriculture is to provide sufficient plant nutrients such as phosphorus (P) to meet the global food demand. The sufficiency of P is a concern because of it’s essential role in plant growth, the finite availability of P-rock for fertilizer production and the poor plant availability of soil P. This study investigated whether biofertilizers and bioenhancers, such as arbuscular mycorrhizal fungi (AMF) and their associated bacteria could enhance growth and P uptake in maize. Plants were grown with or without mycorrhizas in compartmented pots with radioactive P tracers and were inoculated with each of 10 selected bacteria isolated from AMF spores. Root colonization by AMF produced large plant growth responses, while seven bacterial strains further facilitated root growth and P uptake by promoting the development of AMF extraradical mycelium. Among the tested strains, Streptomyces sp. W94 produced the largest increases in uptake and translocation of 33P, while Streptomyces sp. W77 highly enhanced hyphal length specific uptake of 33P. The positive relationship between AMF-mediated P absorption and shoot P content was significantly influenced by the bacteria inoculants and such results emphasize the potential importance of managing both AMF and their microbiota for improving P acquisition by crops.
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The Nodule Microbiome: N2-Fixing Rhizobia Do Not Live Alone 

The Nodule Microbiome: N2-Fixing Rhizobia Do Not Live Alone  | The Plant Microbiome | Scoop.it
For decades, rhizobia were thought to be the only nitrogen-fixing inhabitants of legume nodules, and biases in culture techniques prolonged this belief. However, other bacteria, which are not typical rhizobia, are often detected within nodules obtained from soil, thus revealing the existence of a phytomicrobiome where the interaction among the individuals is not only complex, but also likely to affect the behavior and fitness of the host plant. Many of these nonrhizobial bacteria are nitrogen fixers, and some also induce nitrogen-fixing nodules on legume roots. Even more striking is the incredibly diverse population of bacteria residing within nodules that elicit neither nodulation nor nitrogen fixation. Yet, this community exists within the nodule, albeit clearly out-numbered by nitrogen-fixing rhizobia. Few studies of the function of these nodule-associated bacteria in nodules have been performed, and to date, it is not known whether their presence in nodules is biologically important or not. Do they confer any benefits to the Rhizobium-legume nitrogen-fixing symbiosis, or are they parasites/saprophytes, contaminants, or commensals? In this review, we highlight the lesser-known bacteria that dwell within nitrogen-fixing nodules and discuss their possible role in this enclosed community as well as any likely benefits to the host plant or to the rhizobial inhabitants of the nodule. Although many of these nodule inhabitants are not capable of nitrogen fixation, they have the potential to enhance legume survival especially under conditions of environmental stress. This knowledge will be useful in defining strategies to employ these bacteria as bioinoculants by themselves or combined with rhizobia. Such an approach will enhance rhizobial performance or persistence as well as decrease the usage of chemical fertilizers and pesticides.


Via Jean-Michel Ané
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Distance decay relationships in foliar fungal endophytes are driven by rare taxa

Distance decay relationships in foliar fungal endophytes are driven by rare taxa | The Plant Microbiome | Scoop.it

Foliar fungal endophytes represent a diverse and species-rich plant microbiome. Their biogeography provides essential clues to their cryptic relationship with hosts and the environment in which they disperse. We present species composition, diversity, and dispersal patterns of endophytic fungi associated with needles of Pinus taeda trees across regional scales in the absence of strong environmental gradients as well as within individual trees. An empirical designation of rare and abundant taxa enlightens us on the structure of endophyte communities. We report multiple distance-decay patterns consistent with effects of dispersal limitation, largely driven by community changes in rare taxa, those taxonomic units that made up less than 0.31% of reads per sample on average. Distance-decay rates and community structure also depended on specific classes of fungi and were predominantly influenced by rare members of Dothideomycetes. Communities separated by urban areas also revealed stronger effects of distance on community similarity, confirming that host density and diversity plays an important role in symbiont biogeography, which may ultimately lead to a mosaic of functional diversity as well as rare species diversity across landscapes.

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Frontiers | Root Hair Mutations Displace the Barley Rhizosphere Microbiota | Plant Science

Frontiers | Root Hair Mutations Displace the Barley Rhizosphere Microbiota | Plant Science | The Plant Microbiome | Scoop.it
The rhizosphere, the thin layer of soil surrounding and influenced by plant roots, defines a distinct and selective microbial habitat compared to unplanted soil. The microbial communities inhabiting the rhizosphere, the rhizosphere microbiota, engage in interactions with their host plants which span from parasitism to mutualism. Therefore, the rhizosphere microbiota emerges as one of the determinants of yield potential in crops. Studies conducted with different plant species have unequivocally pointed to the host plant as a driver of the microbiota thriving at the root-soil interface. Thus far, the host genetic traits shaping the rhizosphere microbiota are not completely understood. As root hairs play a critical role in resource exchanges between plants and the rhizosphere, we hypothesized that they can act as a determinant of the microbiota thriving at the root-soil interface. To test this hypothesis, we took advantage of barley (Hordeum vulgare) mutant lines contrasting for their root hair characteristics. Plants were grown in two agricultural soils, differentiating in their organic matter contents, under controlled environmental conditions. At early stem elongation rhizosphere specimens were collected and subjected to high-resolution 16S rRNA gene profiling. Our data revealed that the barley rhizosphere microbiota is largely dominated by members of the phyla Bacteroidetes and Proteobacteria, regardless of the soil type and the root hair characteristics of the host plant. Conversely, ecological indices calculated using OTUs (Operational Taxonomic Units) presence, abundance and phylogeny revealed a significant impact of root hair mutations on the composition of the rhizosphere microbiota. In particular, our data indicate that mutant plants host a reduced-complexity community compared to wild type genotypes and unplanted soil controls. Congruently, the host genotype explained up to 18% of the variation in ecological distances computed for the rhizosphere samples. Importantly, this effect is manifested in a soil-dependent manner. A closer inspection of the sequencing profiles revealed that the root hair-dependent diversification of the microbiota is supported by a taxonomically narrow group of bacteria, with a bias for members of the orders Actinomycetales, Burkholderiales, Rhizobiales, Sphingomonadales and Xanthomonadales. Taken together, our results indicate that the presence and function of root hairs are a determinant of the
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A signature of tree health? Shifts in the microbiome and the ecological drivers of horse chestnut bleeding canker disease

A signature of tree health? Shifts in the microbiome and the ecological drivers of horse chestnut bleeding canker disease | The Plant Microbiome | Scoop.it

Host susceptibility to pathogens can be shaped by genetic, ecological, and evolutionary factors. The ability to predict the spread of disease therefore requires an integrated understanding of these factors, including effects of pests on pathogen growth and competition between pathogens and commensal microbiota for host resources. We examined interactions between the leaf-mining moth Cameraria ohridella, the bacterial causal agent of bleeding canker disease Pseudomonas syringae pv aesculi, and the bark-associated microbiota of horse chestnut (Aesculus hippocastanum) trees. Through surveys of > 900 trees from 60 sites in the UK, we tested for ecological or life history predictors of leaf miner infestation, bleeding canker, or coinfection. Using culture-independent sequencing, we then compared the bark microbiomes from 46 trees to measure the association between microbiome composition and key ecological variables, including the severity of disease. Both pest and pathogen were found to respond to tree characteristics, but neither explained damage inflicted by the other. However, we found a clear loss of microbial diversity and associated shift in microbiome composition of trees as a function of disease. These results show a link between bark-associated microbiota and tree health that introduces the intriguing possibility that tree microbiota play key roles in the spread of disease.

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Understanding and exploiting plant beneficial microbes

Understanding and exploiting plant beneficial microbes | The Plant Microbiome | Scoop.it
After a century of incremental research, technological advances, coupled with a need for sustainable crop yield increases, have reinvigorated the study of beneficial plant–microbe interactions with attention focused on how microbiomes alter plant phenotypes. We review recent advances in plant microbiome research, and describe potential applications for increasing crop productivity. The phylogenetic diversity of plant microbiomes is increasingly well characterized, and their functional diversity is becoming more accessible. Large culture collections are available for controlled experimentation, with more to come. Genetic resources are being brought to bear on questions of microbiome function. We expect that microbial amendments of varying complexities will expose rules governing beneficial plant–microbe interactions contributing to plant growth promotion and disease resistance, enabling more sustainable agriculture.
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Leaf bacterial diversity mediates plant diversity and ecosystem function relationships

Leaf bacterial diversity mediates plant diversity and ecosystem function relationships | The Plant Microbiome | Scoop.it
Research on biodiversity and ecosystem functioning has demonstrated links between plant diversity and ecosystem functions such as productivity1, 2. At other trophic levels, the plant microbiome has been shown to influence host plant fitness and function3, 4, and host-associated microbes have been proposed to influence ecosystem function through their role in defining the extended phenotype of host organisms5, 6 However, the importance of the plant microbiome for ecosystem function has not been quantified in the context of the known importance of plant diversity and traits. Here, using a tree biodiversity–ecosystem functioning experiment, we provide strong support for the hypothesis that leaf bacterial diversity is positively linked to ecosystem productivity, even after accounting for the role of plant diversity. Our results also show that host species identity, functional identity and functional diversity are the main determinants of leaf bacterial community structure and diversity. Our study provides evidence of a positive correlation between plant-associated microbial diversity and terrestrial ecosystem productivity, and a new mechanism by which models of biodiversity–ecosystem functioning relationships can be improved
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Addition of plant-growth-promoting Bacillus subtilis PTS-394 on tomato rhizosphere has no durable impact on composition of root microbiome

Addition of plant-growth-promoting Bacillus subtilis PTS-394 on tomato rhizosphere has no durable impact on composition of root microbiome | The Plant Microbiome | Scoop.it
Representatives of the genus Bacillus are increasingly used in agriculture to promote plant growth and to protect against plant pathogens. Unfortunately, hitherto the impact of Bacillus inoculants on the indigenous plant microbiota has been investigated exclusively for the species Bacillus amyloliquefaciens and was limited to prokaryotes, whilst eukaryotic member of this community, e.g. fungi, were not considered. The root-colonizing Bacillus subtilis PTS-394 supported growth of tomato plants and suppressed soil-borne diseases. Roche 454 pyrosequencing revealed that PTS-394 has only a transient impact on the microbiota community of the tomato rhizosphere. The impact on eukaryota could last up to 14 days, while that on bacterial communities lasted for 3 days only. Ecological adaptation and microbial community-preserving capacity are important criteria when assessing suitability of bio-inoculants for commercial development. As shown here, B. subtilis PTS-394 is acting as an environmentally compatible plant protective agent without permanent effects on rhizosphere microbial community.
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The endobacterium of an arbuscular mycorrhizal fungus modulates the expression of its toxin-antitoxin systems during the life cycle of its host 

The endobacterium of an arbuscular mycorrhizal fungus modulates the expression of its toxin-antitoxin systems during the life cycle of its host  | The Plant Microbiome | Scoop.it
Arbuscular mycorrhizal fungi (AMF) are widespread root symbionts that perform important ecological services, such as improving plant nutrient and water acquisition. Some AMF from the Gigasporaceae family host a population of endobacteria, Candidatus Glomeribacter gigasporarum (Cagg). The analysis of the Cagg genome identified six putative toxin–antitoxin modules (TAs), consisting of pairs of stable toxins and unstable antitoxins that affect diverse physiological functions. Sequence analysis suggested that these TA modules were acquired by horizontal transfer. Gene expression patterns of two TAs (yoeB/yefM and chpB/chpS) changed during the fungal life cycle, with the expression during the pre-symbiotic phase higher than during the symbiosis with the plant host. The heterologous expression in Escherichia coli demonstrated the functionality only for the YoeB–YefM pair. On the basis of these observations, we speculate that TA modules might help Cagg adapt to its intracellular habitat, coordinating its proliferation with the physiological state of the AMF host.

Via Francis Martin, Jean-Michel Ané
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Ancestral alliances: Plant mutualistic symbioses with fungi and bacteria

Ancestral alliances: Plant mutualistic symbioses with fungi and bacteria | The Plant Microbiome | Scoop.it
BACKGROUND
Among the extensive cortège of plant-associated microorganisms (the so-called plant microbiota), mutualistic fungal and bacterial symbionts are striking examples of soil microorganisms that have successfully coevolved with their hosts since plants adapted to terrestrial ecosystems. They promote plant growth by facilitating the acquisition of scarce nutrients. In these associations, plant root colonization requires complex molecular cross-talk between symbiotic partners to activate a variety of host developmental pathways and specialized symbiotic tissues and organs. Despite the evolutionary distances that separate mycorrhizal and nitrogen-fixing symbioses, recent research has identified certain highly conserved features associated with early stages of root colonization. We focus on recent and emerging areas of investigation concerning these major mutualistic symbioses and discuss some of the molecular pathways and cellular mechanisms involved in their evolution and development.
ADVANCES
Phylogenomic analyses and divergence time estimates based on symbiotic plant fossils are shedding light on the evolution of mutualistic symbioses. The earliest land plants [~407 million years ago (Ma)] were associated with fungi producing mycorrhiza-like intracellular structures similar to extant symbioses involving Glomeromycotina and Mucoromycotina. Arbuscular mycorrhizal endosymbioses then diversified by the Late Carboniferous. Pinaceae species from the Late Jurassic and Early Cretaceous (~180 Ma) formed the first ectomycorrhizal associations involving Dikarya. More recently, certain angiosperms evolved a “predisposition” for the evolution of nitrogen-fixing root nodule symbioses (~100 Ma) with bacteria.

A conserved core module of the “common symbiotic signaling pathway” (CSSP) is shared by all host plants that establish endosymbioses, including arbuscular mycorrhizal, rhizobial, and actinorhizal associations. This striking conservation among widely divergent host species underlines the shared evolutionary origin for this ancient symbiotic signaling pathway. Furthermore, chitin-based signaling molecules secreted by both arbuscular mycorrhizal fungi and rhizobia activate the host CSSP after perception by related receptor-like kinases. Downstream signal transduction pathways then lead to the apoplastic intracellular infection modes that characterize the majority of these associations and, finally, to the coordinated development of sophisticated bidirectional symbiotic interfaces found in both arbuscules and nitrogen-fixing nodules. A common feature of all these mutualistic associations is phytohormone-associated modifications of root development, which lead to an increase in potential colonization sites as well as major structural and functional changes to the root during the establishment of symbiotic tissues.
OUTLOOK
Although we are at last beginning to understand how mutualistic microorganisms communicate with plants, how associated root developmental pathways are modulated, and how plant immune responses are successfully circumvented, many important questions remain. For example, little is currently known about more primitive modes of intercellular apoplastic colonization, whether for ectomycorrhizal fungi or for certain nitrogen-fixing symbioses. Neither do we know whether the CSSP has a key role in ectomycorrhizal associations, nor how host plants distinguish between structurally similar chitin-based “symbiotic” and “pathogenic” microbial signals. Answering these questions should contribute to our understanding of the underlying mechanisms that govern the relationships between plants and their entire microbiota. On a broader level, improved understanding of how environmental and genetic cues, together with plant metabolism, modulate microbial colonization will be crucial for the future exploitation of the microbiota for the benefit of sustainable plant growth.

Via Steve Marek, Ronny Kellner
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Global-scale structure of the eelgrass microbiome

Plant-associated microorganisms are essential for their hosts' survival and performance. Yet, most plant microbiome studies to date have focused on terrestrial species sampled across relatively small spatial scales. Here we report results of a global-scale analysis of microbial communities associated with leaf and root surfaces of the marine eelgrass Zostera marina throughout its range in the Northern Hemisphere. By contrasting host microbiomes with those of surrounding seawater and sediment, we uncovered the structure, composition and variability of microbial communities associated with eelgrass. We also investigated hypotheses about the assembly of the eelgrass microbiome using a metabolic modeling approach. Our results reveal leaf communities displaying high variability and spatial turnover, that mirror their adjacent coastal seawater microbiomes. In contrast, roots showed relatively low compositional turnover and were distinct from surrounding sediment communities — a result driven by the enrichment of predicted sulfur-oxidizing bacterial taxa on root surfaces. Predictions from metabolic modeling of enriched taxa were consistent with a habitat filtering community assembly mechanism whereby similarity in resource use drives taxonomic co-occurrence patterns on belowground, but not aboveground, host tissues. Our work provides evidence for a core eelgrass root microbiome with putative functional roles and highlights potentially disparate processes influencing microbial community assembly on different plant compartments.
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Communication in the Phytobiome

Communication in the Phytobiome | The Plant Microbiome | Scoop.it
The phytobiome is composed of plants, their environment, and diverse interacting microscopic and macroscopic organisms, which together influence plant health and productivity. These organisms form complex networks that are established and regulated through nutrient cycling, competition, antagonism, and chemical communication mediated by a diverse array of signaling molecules. Integration of knowledge of signaling mechanisms with that of phytobiome members and their networks will lead to a new understanding of the fate and significance of these signals at the ecosystem level. Such an understanding could lead to new biological, chemical, and breeding strategies to improve crop health and productivity.


Via Jean-Michel Ané
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Frontiers | The Influence of Land Use Intensity on the Plant-Associated Microbiome of Dactylis glomerata L. | Plant Science

Frontiers | The Influence of Land Use Intensity on the Plant-Associated Microbiome of Dactylis glomerata L. | Plant Science | The Plant Microbiome | Scoop.it
In this study, we investigated the impact of different land use intensities on the root-associated microbiome of Dactylis glomerata (orchardgrass). For this purpose, eight sampling sites with different land use intensity levels but comparable soil properties were selected in the southwest of Germany. Experimental plots covered land use levels from natural grassland up to intensively managed meadows. We used 16S rRNA gene based barcoding to assess the plant-associated community structure in the endosphere, rhizosphere and bulk soil of D. glomerata. Samples were taken at the reproductive stage of the plant in early summer. Our data indicated that roots harbor a distinct bacterial community, which clearly differed from the microbiome of the rhizosphere and bulk soil. Our results revealed Pseudomonadaceae, Enterobacteriaceae and Comamonadaceae as the most abundant endophytes independently of land use intensity. Rhizosphere and bulk soil were dominated also by Proteobacteria, but the most abundant families differed from those obtained from root samples. In the soil, the effect of land use intensity was more pronounced compared to root endophytes leading to a clearly distinct pattern of bacterial communities under different land use intensities from rhizosphere and bulk soil vs. endophytes. Overall, a change of community structure on the plant-soil interface was observed, as the number of shared OTUs between all three compartments investigated increased with decreasing land use intensity. Thus, our findings suggest a stronger interaction of the plant with its surrounding soil under low land use intensity. Furthermore, the amount and quality of available nitrogen was identified as a major driver for shifts in the microbiome structure in all compartments.
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Novel soil-inhabiting clades fill gaps in the fungal tree of life

Novel soil-inhabiting clades fill gaps in the fungal tree of life | The Plant Microbiome | Scoop.it

Background
Fungi are a diverse eukaryotic group of degraders, pathogens, and symbionts, with many lineages known only from DNA sequences in soil, sediments, air, and water.
Results
We provide rough phylogenetic placement and principal niche analysis for >40 previously unrecognized fungal groups at the order and class level from global soil samples based on combined 18S (nSSU) and 28S (nLSU) rRNA gene sequences. Especially, Rozellomycota (Cryptomycota), Zygomycota s.lat, Ascomycota, and Basidiomycota are rich in novel fungal lineages, most of which exhibit distinct preferences for climate and soil pH.
Conclusions
This study uncovers the great phylogenetic richness of previously unrecognized order- to phylum-level fungal lineages. Most of these rare groups are distributed in different ecosystems of the world but exhibit distinct ecological preferences for climate or soil pH. Across the fungal kingdom, tropical and non-tropical habitats are equally likely to harbor novel groups. We advocate that a combination of traditional and high-throughput sequencing methods enable efficient recovery and phylogenetic placement of such unknown taxonomic groups.


Via Steve Marek, Francis Martin
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Frontiers | A Small Number of Low-abundance Bacteria Dominate Plant Species-specific Responses during Rhizosphere Colonization | Microbiology

Frontiers | A Small Number of Low-abundance Bacteria Dominate Plant Species-specific Responses during Rhizosphere Colonization | Microbiology | The Plant Microbiome | Scoop.it
Plant growth can be affected by soil bacteria. In turn, plants are known to influence soil bacteria through rhizodeposits and changes in abiotic conditions. We aimed to quantify the phylotype richness and relative abundance of rhizosphere bacteria that are actually influenced in a plant species-specific manner and to determine the role of the disproportionately large diversity of low-abundance bacteria belonging to the rare biosphere (<0.1 relative abundance) in this process. In addition, we aimed to determine whether plant phylogeny has an influence on the plant species-specific rhizosphere bacterial community. For this purpose, 19 herbaceous plant species from five different plant orders were grown in a common soil substrate. Bacterial communities in the initial soil substrate and the established rhizosphere soils were compared by 16S rRNA gene amplicon sequencing. Only a small number of bacterial operational taxonomic units (OTUs, 97% sequence identity) responded either positively (ca. 1%) or negatively (ca. 1%) to a specific plant species. On average, 91% of plant-specific positive response OTUs comprised bacteria belonging to the rare biosphere, highlighting that low-abundance populations are metabolically active in the rhizosphere. In addition, low-abundance OTUs were in terms of their summed relative abundance major drivers of the bacterial phyla composition across the rhizosphere of all tested plant species. However, no effect of plant phylogeny could be observed on the established rhizosphere bacterial communities, neither when considering differences in the overall established rhizosphere communities nor when considering plant species-specific responders only. Our study provides a quantitative assessment of the effect of plants on their rhizosphere bacteria across multiple plant orders. Plant species-specific effects on soil bacterial communities involved only 18-111 bacterial OTUs out of several thousands; this minority may potentially impact plant growth in plant-bacteria interactions.
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Diurnal cycling of rhizosphere bacterial communities is associated with shifts in carbon metabolism

Diurnal cycling of rhizosphere bacterial communities is associated with shifts in carbon metabolism | The Plant Microbiome | Scoop.it
The circadian clock regulates plant metabolic functions and is an important component in plant health and productivity. Rhizosphere bacteria play critical roles in plant growth, health, and development and are shaped primarily by soil communities. Using Illumina next-generation sequencing and high-resolution mass spectrometry, we characterized bacterial communities of wild-type (Col-0) Arabidopsis thaliana and an acyclic line (OX34) ectopically expressing the circadian clock-associated cca1 transcription factor, relative to a soil control, to determine how cycling dynamics affected the microbial community. Microbial communities associated with Brachypodium distachyon (BD21) were also evaluated. Significantly different bacterial community structures (P = 0.031) were observed in the rhizosphere of wild-type plants between light and dark cycle samples. Furthermore, 13% of the community showed cycling, with abundances of several families, including Burkholderiaceae, Rhodospirillaceae, Planctomycetaceae, and Gaiellaceae, exhibiting fluctuation in abundances relative to the light cycle. However, limited-to-no cycling was observed in the acyclic CCAox34 line or in soil controls. Significant cycling was also observed, to a lesser extent, in Brachypodium. Functional gene inference revealed that genes involved in carbohydrate metabolism were likely more abundant in near-dawn, dark samples. Additionally, the composition of organic matter in the rhizosphere showed a significant variation between dark and light cycles. The results of this study suggest that the rhizosphere bacterial community is regulated, to some extent, by the circadian clock and is likely influenced by, and exerts influences, on plant metabolism and productivity. The timing of bacterial cycling in relation to that of Arabidopsis further suggests that diurnal dynamics influence plant-microbe carbon metabolism and exchange. Equally important, our results suggest that previous studies done without relevance to time of day may need to be reevaluated with regard to the impact of diurnal cycles on the rhizosphere microbial community.
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Interplay Between Innate Immunity and the Plant Microbiota | Annual Review of Phytopathology

Interplay Between Innate Immunity and the Plant Microbiota | Annual Review of Phytopathology | The Plant Microbiome | Scoop.it
The innate immune system of plants recognizes microbial pathogens and terminates their growth. However, recent findings suggest that at least one layer of this system is also engaged in cooperative plant-microbe interactions and influences host colonization by beneficial microbial communities. This immune layer involves sensing of microbe-associated molecular patterns (MAMPs) by pattern recognition receptors (PRRs) that initiate quantitative immune responses to control host-microbial load, whereas diversification of MAMPs and PRRs emerges as a mechanism that locally sculpts microbial assemblages in plant populations. This suggests a more complex microbial management role of the innate immune system for controlled accommodation of beneficial microbes and in pathogen elimination. The finding that similar molecular strategies are deployed by symbionts and pathogens to dampen immune responses is consistent with this hypothesis but implies different selective pressures on the immune system due to contrasting outcomes on plant fitness. The reciprocal interplay between microbiota and the immune system likely plays a critical role in shaping beneficial plant-microbiota combinations and maintaining microbial homeostasis.
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Ecophylogeny of the endospheric root fungal microbiome of co-occurring Agrostis stolonifera

Ecophylogeny of the endospheric root fungal microbiome of co-occurring Agrostis stolonifera | The Plant Microbiome | Scoop.it
Background Within the root endosphere, fungi are known to be important for plant nutrition and resistance to stresses. However, description and understanding of the rules governing community assembly in the fungal fraction of the plant microbiome remains scarce. Methods We used an innovative DNA- and RNA-based analysis of co-extracted nucleic acids to reveal the complexity of the fungal community colonizing the roots of an Agrostis stolonifera population. The normalized RNA/DNA ratio, designated the ‘mean expression ratio’, was used as a functional trait proxy. The link between this trait and phylogenetic relatedness was measured using the Blomberg’s K statistic. Results Fungal communities were highly diverse. Only ∼1.5% of the 635 OTUs detected were shared by all individuals, however these accounted for 33% of the sequence number. The endophytic fungal communities in plant roots exhibit phylogenetic clustering that can be explained by a plant host effect acting as environmental filter. The ‘mean expression ratio’ displayed significant but divergent phylogenetic signals between fungal phyla. Discussion These results suggest that environmental filtering by the host plant favours the co-existence of related and similar OTUs within the Basidiomycota community assembly, whereas the Ascomycota and Glomeromycota communities seem to be impacted by competitive interactions which promote the co-existence of phylogenetically related but ecologically dissimilar OTUs.
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Iron and Immunity | Annual Review of Phytopathology

Iron and Immunity | Annual Review of Phytopathology | The Plant Microbiome | Scoop.it
Iron is an essential nutrient for most life on Earth because it functions as a crucial redox catalyst in many cellular processes. However, when present in excess iron can lead to the formation of harmful hydroxyl radicals. Hence, the cellular iron balance must be tightly controlled. Perturbation of iron homeostasis is a major strategy in host-pathogen interactions. Plants use iron-withholding strategies to reduce pathogen virulence or to locally increase iron levels to activate a toxic oxidative burst. Some plant pathogens counteract such defenses by secreting iron-scavenging siderophores that promote iron uptake and alleviate iron-regulated host immune responses. Mutualistic root microbiota can also influence plant disease via iron. They compete for iron with soil-borne pathogens or induce a systemic resistance that shares early signaling components with the root iron-uptake machinery. This review describes the progress in our understanding of the role of iron homeostasis in both pathogenic and beneficial plant-microbe interactions.
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Microbiome-on-a-Chip: New Frontiers in Plant–Microbiota Research

Microbiome-on-a-Chip: New Frontiers in Plant–Microbiota Research | The Plant Microbiome | Scoop.it
An enigmatic concoction of interactions between microbes and hosts takes place below ground, yet the function(s) of the individual components in this complex playground are far from understood. This Forum article highlights how microfluidic – or ‘Microbiome-on-a-Chip’ – technology could help to shed light on such relationships, opening new frontiers in plant–microbiota research.

Via Ryohei Thomas Nakano
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Linking rhizosphere microbiome composition of wild and domesticated Phaseolus vulgaris to genotypic and root phenotypic traits

Linking rhizosphere microbiome composition of wild and domesticated Phaseolus vulgaris to genotypic and root phenotypic traits | The Plant Microbiome | Scoop.it
Plant domestication was a pivotal accomplishment in human history, but also led to a reduction in genetic diversity of crop species compared to their wild ancestors. How this reduced genetic diversity affected plant–microbe interactions belowground is largely unknown. Here, we investigated the genetic relatedness, root phenotypic traits and rhizobacterial community composition of modern and wild accessions of common bean (Phaseolus vulgaris) grown in agricultural soil from the highlands of Colombia, one of the centers of common bean diversification. Diversity Array Technology-based genotyping and phenotyping of local common bean accessions showed significant genetic and root architectural differences between wild and modern accessions, with a higher specific root length for the wild accessions. Canonical Correspondence Analysis indicated that the divergence in rhizobacterial community composition between wild and modern bean accessions is associated with differences in specific root length. Along the bean genotypic trajectory, going from wild to modern, we observed a gradual decrease in relative abundance of Bacteroidetes, mainly Chitinophagaceae and Cytophagaceae, and an increase in relative abundance of Actinobacteria and Proteobacteria, in particular Nocardioidaceae and Rhizobiaceae, respectively. Collectively, these results establish a link between common bean domestication, specific root morphological traits and rhizobacterial community assembly.
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Let the Core Microbiota Be Functional

Let the Core Microbiota Be Functional | The Plant Microbiome | Scoop.it
The microbial community that is systematically associated with a given host plant is called the core microbiota. The definition of the core microbiota was so far based on its taxonomic composition, but we argue that it should also be based on its functions. This so-called functional core microbiota encompasses microbial vehicles carrying replicators (genes) with essential functions for holobiont (i.e., plant plus microbiota) fitness. It builds up from enhanced horizontal transfers of replicators as well as from ecological enrichment of their vehicles. The transmission pathways of this functional core microbiota vary over plant generations according to environmental constraints and its added value for holobiont fitness.
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Ecology and Genomic Insights on Plant-Pathogenic and -Nonpathogenic Endophytes | Annual Review of Phytopathology

Ecology and Genomic Insights on Plant-Pathogenic and -Nonpathogenic Endophytes | Annual Review of Phytopathology | The Plant Microbiome | Scoop.it
Plants are colonized on their surfaces and in the rhizosphere and phyllosphere by a multitude of different microorganisms and are inhabited internally by endophytes. Most endophytes act as commensals without any known effect on their plant host, but multiple bacteria and fungi establish a mutualistic relationship with plants, and some act as pathogens. The outcome of these plant-microbe interactions depends on biotic and abiotic environmental factors and on the genotype of the host and the interacting microorganism. In addition, endophytic microbiota and the manifold interactions between members, including pathogens, have a profound influence on the function of the system plant and the development of pathobiomes. In this review, we elaborate on the differences and similarities between nonpathogenic and pathogenic endophytes in terms of host plant response, colonization strategy, and genome content. We furthermore discuss environmental effects and biotic interactions within plant microbiota that influence pathogenesis and the pathobiome.

Via Ryohei Thomas Nakano
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Ecological patterns of seed microbiome diversity, transmission, and assembly

Seeds are involved in the transmission of microorganisms from one plant generation to another and consequently act as the initial inoculum for the plant microbiota. The purpose of this mini-review is to provide an overview of current knowledge on the diversity, structure and role of the seed microbiota. The relative importance of the mode of transmission (vertical vs horizontal) of the microbial entities composing the seed microbiota as well as the potential connections existing between seed and other plant habitats such as the anthosphere and the spermosphere is discussed. Finally the governing processes (niche vs neutral) involved in the assembly and the dynamics of the seed microbiota are examined.
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