Regulation of the plant-microbe interactions
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AP2 transcription factor CBX1 with a specific function in symbiotic exchange of nutrients in mycorrhizal Lotus japonicus

AP2 transcription factor CBX1 with a specific function in symbiotic exchange of nutrients in mycorrhizal Lotus japonicus | Regulation of the plant-microbe interactions | Scoop.it
Arbuscular mycorrhizal (AM) fungi promote phosphorus uptake into host plants in exchange for organic carbon. Physiological tracer experiments showed that up to 100% of acquired phosphate can be delivered to plants via the mycorrhizal phosphate uptake pathway (MPU). Previous studies revealed that the CTTC cis -regulatory element (CRE) is required for promoter activation of mycorrhiza-specific phosphate transporter and H+-ATPase genes. However, the precise transcriptional mechanism directly controlling MPU is unknown. Here, we show that CBX1 binds CTTC and AW-box CREs and coregulates mycorrhizal phosphate transporter and H+-ATPase genes. Interestingly, genes involved in lipid biosynthesis are also regulated by CBX1 through binding to AW box, including RAM2 . Our work suggests a common regulatory mechanism underlying complex trait control of symbiotic exchange of nutrients.

The arbuscular mycorrhizal (AM) symbiosis, a widespread mutualistic association between land plants and fungi, depends on reciprocal exchange of phosphorus driven by proton-coupled phosphate uptake into host plants and carbon supplied to AM fungi by host-dependent sugar and lipid biosynthesis. The molecular mechanisms and cis -regulatory modules underlying the control of phosphate uptake and de novo fatty acid synthesis in AM symbiosis are poorly understood. Here, we show that the AP2 family transcription factor CTTC MOTIF-BINDING TRANSCRIPTION FACTOR1 (CBX1), a WRINKLED1 (WRI1) homolog, directly binds the evolutionary conserved CTTC motif that is enriched in mycorrhiza-regulated genes and activates Lotus japonicus phosphate transporter 4 ( LjPT4 ) in vivo and in vitro. Moreover, the mycorrhiza-inducible gene encoding H+-ATPase ( LjHA1 ), implicated in energizing nutrient uptake at the symbiotic interface across the periarbuscular membrane, is coregulated with LjPT4 by CBX1. Accordingly, CBX1 -defective mutants show reduced mycorrhizal colonization. Furthermore, genome-wide–binding profiles, DNA-binding studies, and heterologous expression reveal additional binding of CBX1 to AW box, the consensus DNA-binding motif for WRI1, that is enriched in promoters of glycolysis and fatty acid biosynthesis genes. We show that CBX1 activates expression of lipid metabolic genes including glycerol-3-phosphate acyltransferase RAM2 implicated in acylglycerol biosynthesis. Our finding defines the role of CBX1 as a regulator of host genes involved in phosphate uptake and lipid synthesis through binding to the CTTC/AW molecular module, and supports a model underlying bidirectional exchange of phosphorus and carbon, a fundamental trait in the mutualistic AM symbiosis.
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Blumenols as shoot markers for root symbiosis with arbuscular mycorrhizal fungi

High-through-put (HTP) screening for functional arbuscular mycorrhizal fungi (AMF)-associations is challenging because roots must be excavated and colonization evaluated by transcript analysis or microscopy. Here we show that specific leaf-metabolites provide broadly applicable accurate proxies of these associations, suitable for HTP-screens. With a combination of untargeted and targeted metabolomics, we show that shoot accumulations of hydroxy- and carboxyblumenol C-glucosides mirror root AMF-colonization in Nicotiana attenuata plants. Genetic/pharmacologic manipulations indicate that these AMF-indicative foliar blumenols are synthesized and transported from roots to shoots. These blumenol-derived foliar markers, found in many di- and monocotyledonous crop and model plants (Solanum lycopersicum, Solanum tuberosum, Hordeum vulgare, Triticum aestivum, Medicago truncatula and Brachypodium distachyon), are not restricted to particular plant-AMF interactions, and are shown to be applicable for field-based QTL mapping of AMF-related genes.
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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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Expression of the Arabidopsis thaliana immune receptor EFR in Medicago truncatula reduces infection by a root pathogenic bacterium, but not nitrogen‐fixing rhizobial symbiosis - Pfeilmeier - - Plan...

Expression of the Arabidopsis thaliana immune receptor EFR in Medicago truncatula reduces infection by a root pathogenic bacterium, but not nitrogen‐fixing rhizobial symbiosis - Pfeilmeier - - Plan... | Regulation of the plant-microbe interactions | Scoop.it
Interfamily transfer of plant pattern recognition receptors (PRRs) represents a promising biotechnological approach to engineer broad‐spectrum, and potentially durable, disease resistance in crops. It is however unclear whether new recognition specificities to given pathogen‐associated molecular patterns (PAMPs) affect the interaction of the recipient plant with beneficial microbes. To test this in a direct reductionist approach, we transferred the Brassicaceae‐specific PRR ELONGATION FACTOR‐THERMO UNSTABLE RECEPTOR (EFR), conferring recognition of the bacterial EF‐Tu protein, from Arabidopsis thaliana to the legume Medicago truncatula. Constitutive EFR expression led to EFR accumulation and activation of immune responses upon treatment with the EF‐Tu‐derived elf18 peptide in leaves and roots. The interaction of M. truncatula with the bacterial symbiont Sinorhizobium meliloti is characterized by the formation of root nodules that fix atmospheric nitrogen. Although nodule numbers were slightly reduced at an early stage of the infection in EFR‐Medicago when compared to control lines, nodulation was similar in all lines at later stages. Furthermore, nodule colonization by rhizobia, and nitrogen fixation were not compromised by EFR expression. Importantly, the M. truncatula lines expressing EFR were substantially more resistant to the root bacterial pathogen Ralstonia solanacearum. Our data suggest that the transfer of EFR to M. truncatula does not impede root nodule symbiosis, but has a positive impact on disease resistance against a bacterial pathogen. In addition, our results indicate that Rhizobium can either avoid PAMP recognition during the infection process, or is able to actively suppress immune signaling.

Via Christophe Jacquet
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Variation in bradyrhizobial NopP effector determines symbiotic incompatibility with Rj2- soybeans via effector-triggered immunity

Variation in bradyrhizobial NopP effector determines symbiotic incompatibility with Rj2- soybeans via effector-triggered immunity | Regulation of the plant-microbe interactions | Scoop.it
Genotype-specific incompatibility in legume–rhizobium symbiosis has been suggested to be controlled by effector-triggered immunity underlying pathogenic host-bacteria interactions. However, the rhizobial determinant interacting with the host resistance protein (e.g., Rj2) and the molecular mechanism of symbiotic incompatibility remain unclear. Using natural mutants of Bradyrhizobium diazoefficiens USDA 122, we identified a type III-secretory protein NopP as the determinant of symbiotic incompatibility with Rj2-soybean. The analysis of nopP mutations and variants in a culture collection reveal that three amino acid residues (R60, R67, and H173) in NopP are required for Rj2-mediated incompatibility. Complementation of rj2-soybean by the Rj2 allele confers the incompatibility induced by USDA 122-type NopP. In response to incompatible strains, Rj2-soybean plants activate defense marker gene PR-2 and suppress infection thread number at 2 days after inoculation. These results suggest that Rj2-soybeans monitor the specific variants of NopP and reject bradyrhizobial infection via effector-triggered immunity mediated by Rj2 protein.
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Legumes versus rhizobia: a model for ongoing conflict in symbiosis - Sachs - 2018 - New Phytologist -

Legumes versus rhizobia: a model for ongoing conflict in symbiosis - Sachs - 2018 - New Phytologist - | Regulation of the plant-microbe interactions | Scoop.it
The legume–rhizobia association is a powerful model of the limits of host control over microbes. Legumes regulate the formation of root nodules that house nitrogen‐fixing rhizobia and adjust investment into nodule development and growth. However, the range of fitness outcomes in these traits reveals intense conflicts of interest between the partners. New work that we review and synthesize here shows that legumes have evolved varied mechanisms of control over symbionts, but that host control is often subverted by rhizobia. An outcome of this conflict is that both legumes and rhizobia have evolved numerous traits that can improve their own short‐term fitness in this interaction, but little evidence exists for any net improvement in the joint trait of nitrogen fixation.

Via Christophe Jacquet
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Nitrogen fixation in a landrace of maize is supported by a mucilage-associated diazotrophic microbiota

Nitrogen fixation in a landrace of maize is supported by a mucilage-associated diazotrophic microbiota | Regulation of the plant-microbe interactions | Scoop.it
Author summary Nitrogen is an essential nutrient for plants, and for many nonlegume crops, the requirement for nitrogen is primarily met by the use of inorganic fertilizers. These fertilizers are produced from fossil fuel by energy-intensive processes that are estimated to use 1% to 2% of the total global energy supply and produce an equivalent share of greenhouse gases. Because maize (Zea mays L.) is a significant recipient of nitrogen fertilization, a research goal for decades has been to identify or engineer mechanisms for biological fixation of atmospheric nitrogen in association with this crop. We hypothesized that isolated indigenous landraces of maize grown using traditional practices with little or no fertilizer might have evolved strategies to improve plant performance under low-nitrogen nutrient conditions. Here, we show that for one such maize landrace grown in nitrogen-depleted fields near Oaxaca, Mexico, 29%–82% of the plant nitrogen is derived from atmospheric nitrogen. High levels of nitrogen fixation are supported, at least in part, by the abundant production of a sugar-rich mucilage associated with aerial roots that provides a home to a complex nitrogen-fixing microbiome.

Via Jean-Michel Ané
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Jean-Michel Ané's curator insight, August 7, 9:22 PM

Our paper on the nitrogen-fixing maize is now publicly available!

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Transcriptome Profiles of Nod Factor-independent Symbiosis in the Tropical Legume Aeschynomene evenia

Transcriptome Profiles of Nod Factor-independent Symbiosis in the Tropical Legume Aeschynomene evenia | Regulation of the plant-microbe interactions | Scoop.it

Nod factors (NF) were assumed to be indispensable for the establishment of a rhizobium-legume symbiosis until the discovery that certain Bradyrhizobium strains interacting with certain Aeschynomene species lack the canonical nodABC genes required for their synthesis. So far, the molecular dialogue between Aeschynomene and its symbionts remains an open question. Here we report a time course transcriptional analysis of Aeschynomene evenia in response to inoculation with Bradyrhizobium ORS278. The NF-independent symbiotic process was monitored at five time points between bacterial infection and nodule maturity. The five time points correspond to three specific events, root infection by crack entry, nodule organogenesis, and the establishment of the nitrogen fixing process. During the third stage, about 80 NCR-like genes and eight symbiotic genes known to be involved in signaling, bacterial infection or nodulation regulation were highly expressed. Comparative gene expression analyses at the five time points also enabled the selection of genes with an expression profile that makes them promising markers to monitor early plant responses to bacteria. Such markers could be used in bioassays to identify the nature of the bacterial signal(s). Our data represent valuable resources for investigation of this Nod factor-independent symbiosis.

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ScienceDirect

ScienceDirect | Regulation of the plant-microbe interactions | Scoop.it
Modular Traits of the Rhizobiales Root Microbiota and Their Evolutionary Relationship with Symbiotic Rhizobia


Rhizobia are a paraphyletic group of soil-borne bacteria that induce nodule organogenesis in legume roots and fix atmospheric nitrogen for plant growth. In non-leguminous plants, species from the Rhizobiales order define a core lineage of the plant microbiota, suggesting additional functional interactions with plant hosts. In this work, genome analyses of 1,314 Rhizobiales isolates along with amplicon studies of the root microbiota reveal the evolutionary history of nitrogen-fixing symbiosis in this bacterial order. Key symbiosis genes were acquired multiple times, and the most recent common ancestor could colonize roots of a broad host range. In addition, root growth promotion is a characteristic trait of Rhizobiales in Arabidopsis thaliana, whereas interference with plant immunity constitutes a separate, strain-specific phenotype of root commensal Alphaproteobacteria. Additional studies with a tripartite gnotobiotic plant system reveal that these traits operate in a modular fashion and thus might be relevant to microbial homeostasis in healthy roots.
 
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Transcriptional control and exploitation of an immune‐responsive family of plant retrotransposons 

Mobilization of transposable elements (TEs) in plants has been recognized as a driving force of evolution and adaptation, in particular by providing genes with regulatory modules that impact their transcription. In this study, we employed an ATCOPIA93 long‐terminal repeat (LTR) promoter‐GUS fusion to show that this retrotransposon behaves like an immune‐responsive gene during pathogen defense in Arabidopsis. We also showed that the endogenous ATCOPIA93 copy “EVD”, which is activated in the presence of bacterial stress, is negatively regulated by both DNA methylation and polycomb‐mediated silencing, a mode of repression typically found at protein‐coding and microRNA genes. Interestingly, an ATCOPIA93‐derived soloLTR is located upstream of the disease resistance gene RPP4 and is devoid of DNA methylation and H3K27m3 marks. Through loss‐of‐function experiments, we demonstrate that this soloLTR is required for the proper expression of RPP4 during plant defense, thus linking the responsiveness of ATCOPIA93 to biotic stress and the co‐option of its LTR for plant immunity.
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Epidermal LysM receptor ensures robust symbiotic signalling in Lotus japonicus

Epidermal LysM receptor ensures robust symbiotic signalling in Lotus japonicus | Regulation of the plant-microbe interactions | Scoop.it
Recognition of Nod factors by LysM receptors is crucial for nitrogen-fixing symbiosis in most legumes. The large families of LysM receptors in legumes suggest concerted functions, yet only NFR1 and NFR5 and their closest homologs are known to be required. Here we show that an epidermal LysM receptor (NFRe), ensures robust signalling in L. japonicus. Mutants of Nfre react to Nod factors with increased calcium spiking interval, reduced transcriptional response and fewer nodules in the presence of rhizobia. NFRe has an active kinase capable of phosphorylating NFR5, which in turn, controls NFRe downstream signalling. Our findings provide evidence for a more complex Nod factor signalling mechanism than previously anticipated. The spatio-temporal interplay between Nfre and Nfr1, and their divergent signalling through distinct kinases suggests the presence of an NFRe-mediated idling state keeping the epidermal cells of the expanding root system attuned to rhizobia.
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A Lotus japonicus E3 ligase interacts with the Nod factor receptor 5 and positively regulates nodulation

Post-translational modification of receptor proteins is involved in activation and de-activation of signaling systems in plants. Both ubiquitination and deubiquitination have been implicated in plant interactions with pathogens and symbionts. Here we present LjPUB13, a PUB ARMADILLO repeat E3 ligase that specifically ubiquitinates the kinase domain of the Nod Factor receptor NFR5 and has a direct role in nodule organogenesis events in Lotus japonicus. Phenotypic analyses of three LORE1 retroelement insertion plant lines revealed that pub13 plants display delayed and reduced nodulation capacity and retarded growth. LjPUB13 expression is spatially regulated during symbiosis with Mesorhizobium loti, with increased levels in young developing nodules. Thus, LjPUB13 is an E3 ligase with a positive regulatory role during the initial stages of nodulation in L. japonicus.

Via Jean-Michel Ané
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The Lotus japonicus acyl‐acyl carrier protein thioesterase FatM is required for mycorrhiza formation and lipid accumulation of Rhizophagus irregularis 

Arbuscular mycorrhiza (AM) fungi establish symbiotic interactions with plants, providing the host plant with minerals, i.e. phosphate, in exchange for organic carbon. Arbuscular mycorrhiza fungi of the order Glomerales produce vesicles which store lipids as an energy and carbon source. Acyl‐acyl carrier protein (ACP) thioesterases (Fat) are essential components of the plant plastid‐localized fatty acid synthase and determine the chain length of de novo synthesized fatty acids. In addition to the ubiquitous FatA and FatB thioesterases, AM‐competent plants contain an additional, AM‐specific, FatM gene. Here, we characterize FatM from Lotus japonicus by phenotypically analyzing fatm mutant lines and by studying the biochemical function of the recombinant FatM protein. Reduced shoot phosphate content in fatm indicates compromised symbiotic phosphate uptake due to reduced arbuscule branching, and the fungus shows reduced lipid accumulation accompanied by the occurrence of smaller and less frequent vesicles. Lipid profiling reveals a decrease in mycorrhiza‐specific phospholipid forms, AM fungal signature fatty acids (e.g. 16:1ω5, 18:1ω7 and 20:3) and storage lipids. Recombinant FatM shows preference for palmitoyl (16:0)‐ACP, indicating that large amounts of 16:0 fatty acid are exported from the plastids of arbuscule‐containing cells. Stable isotope labeling with [13C2]acetate showed reduced incorporation into mycorrhiza‐specific fatty acids in the fatm mutant. Therefore, colonized cells reprogram plastidial de novo fatty acid synthesis towards the production of extra amounts of 16:0, which is in agreement with previous results that fatty acid‐containing lipids are transported from the plant to the fungus.
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Systemic control of legume susceptibility to rhizobial infection by a mobile microRNA

Systemic control of legume susceptibility to rhizobial infection by a mobile microRNA | Regulation of the plant-microbe interactions | Scoop.it
Nitrogen-fixing root nodules on legumes result from two developmental processes, bacterial infection and nodule organogenesis. To balance symbiosis and plant growth, legume hosts restrict nodule numbers through an inducible autoregulatory process. Here, we present a mechanism where repression of a negative regulator ensures symbiotic susceptibility of uninfected roots of the host Lotus japonicus . We show that microRNA miR2111 undergoes shoot-to-root translocation to control rhizobial infection through posttranscriptional regulation of the symbiosis suppressor TOO MUCH LOVE in roots. miR2111 maintains a susceptible default status in uninfected hosts and functions as an activator of symbiosis downstream of LOTUS HISTIDINE KINASE1–mediated cytokinin perception in roots and HYPERNODULATION ABERRANT ROOT FORMATION1, a shoot factor in autoregulation. The miR2111- TML node ensures activation of feedback regulation to balance infection and nodulation events.
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A neglected alliance in battles against parasitic plants: arbuscular mycorrhizal and rhizobial symbioses alleviate damage to a legume host by root hemiparasitic Pedicularis species

A neglected alliance in battles against parasitic plants: arbuscular mycorrhizal and rhizobial symbioses alleviate damage to a legume host by root hemiparasitic Pedicularis species | Regulation of the plant-microbe interactions | Scoop.it
Despite their ubiquitous distribution and significant ecological roles, soil microorganisms have long been neglected in investigations addressing parasitic plant–host interactions. Because nutrient deprivation is a primary cause of host damage by parasitic plants, we hypothesized that beneficial soil microorganisms conferring nutrient benefits to parasitized hosts may play important roles in alleviating damage.
We conducted a pot cultivation experiment to test the inoculation effect of an arbuscular mycorrhizal fungus (Glomus mosseae), a rhizobium (Rhizobium leguminosarum) and their interactive effects, on alleviation of damage to a legume host (Trifolium repens) by two root hemiparasitic plants with different nutrient requirements (N‐demanding Pedicularis rex and P‐demanding P. tricolor).
Strong interactive effects between inoculation regimes and hemiparasite identity were observed. The relative benefits of microbial inoculation were related to hemiparasite nutrient requirements. Dual inoculation with the rhizobium strongly enhanced promotional arbuscular mycorrhizal effects on hosts parasitized by P. rex, but reduced the arbuscular mycorrhizal promotion on hosts parasitized by P. tricolor.
Our results demonstrate substantial contribution of arbuscular mycorrhizal and rhizobial symbioses to alleviating damage to the legume host by root hemiparasites, and suggest that soil microorganisms are critical factors regulating host–parasite interactions and should be taken into account in future studies.
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The Evolutionary Moulding in plant-microbial symbiosis: matching population diversity of rhizobial nodA and legume NFR5 genes

We propose the Evolutionary Moulding hypothesis that population diversities of partners in nitrogen-fixing rhizobium-legume symbiosis are matched, and tested it in nucleotide polymorphism of symbiotic genes encoding two components of the plant-bacteria signalling system. The first component is the rhizobial nodA acyltransferase involved in the fatty acid tail decoration of Nod factor (rhizobia signalling molecule). The second component is the plant NFR5 receptor, putatively required for Nod-factor binding. We collected three wild growing legume species together with soil samples adjacent to the roots (soil pool) from one large 25-year fallow: Vicia sativa, Lathyrus pratensis and Trifolium hybridum nodulated by one of the two Rhizobium leguminosarum biovars (viciae and trifolii). For each plant species we prepared three pools for DNA extraction: the plant pool (30 plant indiv.), the nodule pool (90 nodules) and the soil pool (30 samples). NFR5 gene libraries from the plant pool and nodA gene libraries from nodule and soil pools were sequenced by Sanger technology and High-throughput pyrosequencing, respectively. Analysis of the data demonstrated concordance in population diversities of one symbiotic partner (rhizobia) the second partner (legume host), in line with the Evolutionary Moulding hypothesis. This effect was evinced by the following observations for each plant species: (1) significantly increased diversity in the nodule nodA popset (set of gene sequences derived from the nodule population) compared to the soil popset; (2) a monotonic relationship between the diversity in the plant NFR5 gene popset and the nodule rhizobial nodA gene popset; and (3) higher topological similarity of the NFR5 gene tree with the nodA gene tree of the nodule popset, than with the nodA gene tree of the soil popset. Both nonsynonymous diversity and Tajima's D were increased in the nodule popsets compared to the soil popsets, consistent with relaxation of negative selection and/or admixture of balancing selection underlying the Evolutionary Moulding effect. We propose that the observed genetic concordance arises from the selection of particular characteristics of the nodule nodA genes by the host plant.
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EmptTranscriptomic analysis with the progress of symbiosis in ‘crack-entry’ legume Arachis hypogaea highlights its contrast with ‘infection thread’ adapted legumesy title 

In root-nodule symbiosis, rhizobial invasion and nodule organogenesis is host controlled. In most legumes, rhizobia enter through infection-threads and nodule primordium in the cortex is induced from a distance. But in dalbergoid legumes like Arachis hypogaea, rhizobia directly invade cortical cells through epidermal cracks to generate the primordia. Herein we report the transcriptional dynamics with the progress of symbiosis in A. hypogaea at 1dpi: invasion; 4dpi: nodule primordia; 8dpi: spread of infection in nodule-like structure; 12dpi: immature nodules containing rod-shaped rhizobia; and 21dpi: mature nodules with spherical symbiosomes. Expression of putative orthologue of symbiotic genes in ‘crack-entry’ legume A. hypogaea was compared with infection thread adapted model legumes. The contrasting features were (i) higher expression of receptors like LYR3, EPR3 as compared to canonical NFRs (ii) late induction of transcription factors like NIN, NSP2 and constitutive high expression of ERF1, EIN2, bHLH476 and (iii) induction of divergent pathogenesis responsive PR-1 genes. Additionally, symbiotic orthologues of SymCRK, ROP6, RR9, SEN1 and DNF2 were not detectable and microsynteny analysis indicated the absence of RPG homologue in diploid parental genomes of A. hypogaea. The implications are discussed and a molecular framework that guide ‘crack-entry’ symbiosis in A. hypogaea is proposed.
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Variation in bradyrhizobial NopP effector determines symbiotic incompatibility with Rj2- soybeans via effector-triggered immunity

Variation in bradyrhizobial NopP effector determines symbiotic incompatibility with Rj2- soybeans via effector-triggered immunity | Regulation of the plant-microbe interactions | Scoop.it
Genotype-specific incompatibility in legume–rhizobium symbiosis has been suggested to be controlled by effector-triggered immunity underlying pathogenic host-bacteria interactions. However, the rhizobial determinant interacting with the host resistance protein (e.g., Rj2) and the molecular mechanism of symbiotic incompatibility remain unclear. Using natural mutants of Bradyrhizobium diazoefficiens USDA 122, we identified a type III-secretory protein NopP as the determinant of symbiotic incompatibility with Rj2-soybean. The analysis of nopP mutations and variants in a culture collection reveal that three amino acid residues (R60, R67, and H173) in NopP are required for Rj2-mediated incompatibility. Complementation of rj2-soybean by the Rj2 allele confers the incompatibility induced by USDA 122-type NopP. In response to incompatible strains, Rj2-soybean plants activate defense marker gene PR-2 and suppress infection thread number at 2 days after inoculation. These results suggest that Rj2-soybeans monitor the specific variants of NopP and reject bradyrhizobial infection via effector-triggered immunity mediated by Rj2 protein.
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Modulating plant growth–metabolism coordination for sustainable agriculture

Modulating plant growth–metabolism coordination for sustainable agriculture | Regulation of the plant-microbe interactions | Scoop.it

Enhancing global food security by increasing the productivity of green revolution varieties of cereals risks increasing the collateral environmental damage produced by inorganic nitrogen fertilizers. Improvements in the efficiency of nitrogen use of crops are therefore essential; however, they require an in-depth understanding of the co-regulatory mechanisms that integrate growth, nitrogen assimilation and carbon fixation. Here we show that the balanced opposing activities and physical interactions of the rice GROWTH-REGULATING FACTOR 4 (GRF4) transcription factor and the growth inhibitor DELLA confer homeostatic co-regulation of growth and the metabolism of carbon and nitrogen. GRF4 promotes and integrates nitrogen assimilation, carbon fixation and growth, whereas DELLA inhibits these processes. As a consequence, the accumulation of DELLA that is characteristic of green revolution varieties confers not only yield-enhancing dwarfism, but also reduces the efficiency of nitrogen use. However, the nitrogen-use efficiency of green revolution varieties and grain yield are increased by tipping the GRF4–DELLA balance towards increased GRF4 abundance. Modulation of plant growth and metabolic co-regulation thus enables novel breeding strategies for future sustainable food security and a new green revolution.

 
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MtNODULE ROOT1 and MtNODULE ROOT2 are essential for indeterminate nodule identity

MtNODULE ROOT1 and MtNODULE ROOT2 are essential for indeterminate nodule identity | Regulation of the plant-microbe interactions | Scoop.it
Symbiotic interactions between legume plants and rhizobia result in the formation of nitrogen-fixing nodules, but the molecular actors and the mechanisms allowing for the maintenance of nodule identity are poorly understood. Medicago truncatula NODULE ROOT1 (MtNOOT1), Pisum sativum COCHLEATA1 (PsCOCH1) and Lotus japonicus NOOT-BOP-COCH-LIKE1 (LjNBCL1) are orthologs of Arabidopsis (Arabidopsis thaliana) AtBLADE-ON-PETIOLE1/2 and are members of the NOOT-BOP-COCH-LIKE (NBCL) gene family, which has conserved roles in plant development and is essential for indeterminate and determinate nodule identity in legumes. The loss-of-function of MtNOOT1, PsCOCH1 and LjNBCL1 triggers a partial loss of nodule identity characterized by the development of ectopic roots arising from nodule vascular meristems. Here, we report the identification and characterization of a second gene involved in regulating indeterminate nodule identity in M. truncatula, MtNODULE ROOT2 (MtNOOT2). MtNOOT2 is the paralog of MtNOOT1 and belongs to a second legume-specific NBCL sub-clade, the NOOT-BOP-COCH-LIKE2 (NBCL2) clade. MtNOOT2 expression was induced during early nodule formation, and it was primarily expressed in the nodule central meristem. Mtnoot2 mutants did not present any particular symbiotic phenotype; however, the loss-of-function of both MtNOOT1 and MtNOOT2 resulted in the complete loss of nodule identity and was accompanied by drastic changes in the expression of symbiotic, defense, and root apical meristem marker genes. Mtnoot1 noot2 double mutants developed only non-fixing root-like structures that were no longer able to host symbiotic rhizobia. This study provides original insights into the molecular basis underlying nodule identity in legumes forming indeterminate nodules.
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Role of a receptor-like kinase K1 in pea Rhizobium symbiosis development

Role of a receptor-like kinase K1 in pea Rhizobium symbiosis development | Regulation of the plant-microbe interactions | Scoop.it

The ability of the crop legume Pisum sativum L. to perceive the Nod factor rhizobial signals may depend on several receptors that differ in ligand structure specificity. Identification of pea mutants defective in two types of LysM receptor-like kinases (LysM-RLKs), SYM10 and SYM37, featuring different phenotypic manifestations and impaired at various stages of symbiosis development, corresponds well to this assumption. There is evidence that one of the receptor proteins involved in symbiosis initiation, SYM10, has an inactive kinase domain. This implies the presence of an additional component in the receptor complex, together with SYM10, that remains unknown. Here, we describe a new LysM-RLK, K1, which may serve as an additional component of the receptor complex in pea. To verify the function of K1 in symbiosis, several P. sativum non-nodulating mutants in the k1 gene were identified using the TILLING approach. Phenotyping revealed the blocking of symbiosis development at an appropriately early stage, strongly suggesting the importance of LysM-RLK K1 for symbiosis initiation. Moreover, the analysis of pea mutants with weaker phenotypes provides evidence for the additional role of K1 in infection thread distribution in the cortex and rhizobia penetration. The interaction between K1 and SYM10 was detected using transient leaf expression in Nicotiana benthamiana and in the yeast two-hybrid system. Since the possibility of SYM10/SYM37 complex formation was also shown, we tested whether the SYM37 and K1 receptors are functionally interchangeable using a complementation test. The interaction between K1 and other receptors is discussed.

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Nutrient- and Dose-Dependent Microbiome-Mediated Protection against a Plant Pathogen

Nutrient- and Dose-Dependent Microbiome-Mediated Protection against a Plant Pathogen | Regulation of the plant-microbe interactions | Scoop.it

Plant-associated microbial communities can promote plant nutrient uptake, growth, and resistance to pathogens [1, 2, 3]. Host resistance to infection can increase directly through commensal-pathogen interactions or indirectly through modulation of host defenses [4, 5, 6], the mechanisms of which are best described for rhizosphere-associated bacteria. For example, Arabidopsis plants infected with the foliar pathogen, Pseudomonas syringae pathovar tomato (Pst), increase their root secretion of malate, which attracts Bacillus subtillis to the roots and leads to a stronger host response against Pst [7]. Although there are numerous examples of individual defensive symbionts (e.g., [8]), it is less clear whether this type of protection is an emergent property of whole microbial communities. In particular, relatively little is known about whether and how the presence of phyllosphere (above-ground) microbial communities can increase host resistance against pathogens. In this study, we examined the ability of augmented tomato phyllosphere microbiomes to confer resistance against the causal agent of bacterial speck, Pst. Across five independent experiments, the augmented phyllosphere microbiome was found to decrease pathogen colonization. Furthermore, the dose of commensal bacteria applied affected the degree of protection conferred, and although the effect is dependent on microbial composition, it is not clearly related to overall bacterial diversity. Finally, our results suggest that resources available to the phyllosphere microbial community may play an important role in protection, as the addition of fertilizer abolished the observed microbiome-mediated protection. Together, these results have clear relevance to microbiome-mediated protection within agricultural settings and the use of plant probiotics to increase disease resistance.

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Rescooped by Oswaldo Valdes-Lopez from Plant-Microbe Symbiosis
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Legume nodulation: The host controls the party

Legume nodulation: The host controls the party | Regulation of the plant-microbe interactions | Scoop.it
Global demand to increase food production and simultaneously reduce synthetic nitrogen fertilizer inputs in agriculture are underpinning the need to intensify the use of legume crops. The symbiotic relationship that legume plants establish with nitrogen‐fixing rhizobia bacteria is central to their advantage. This plant–microbe interaction results in newly developed root organs, called nodules, where the rhizobia convert atmospheric nitrogen gas into forms of nitrogen the plant can use. However, the process of developing and maintaining nodules is resource intensive; hence, the plant tightly controls the number of nodules forming. A variety of molecular mechanisms are used to regulate nodule numbers under both favourable and stressful growing conditions, enabling the plant to conserve resources and optimize development in response to a range of circumstances. Using genetic and genomic approaches, many components acting in the regulation of nodulation have now been identified. Discovering and functionally characterizing these components can provide genetic targets and polymorphic markers that aid in the selection of superior legume cultivars and rhizobia strains that benefit agricultural sustainability and food security. This review addresses recent findings in nodulation control, presents detailed models of the molecular mechanisms driving these processes, and identifies gaps in these processes that are not yet fully explained.


Via Jean-Michel Ané
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Rescooped by Oswaldo Valdes-Lopez from Plant immunity and legume symbiosis
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Epidermal auxin biosynthesis facilitates rhizobial infection in Lotus japonicus - Nadzieja - 2018 - The Plant Journal -

Epidermal auxin biosynthesis facilitates rhizobial infection in Lotus japonicus - Nadzieja - 2018 - The Plant Journal - | Regulation of the plant-microbe interactions | Scoop.it
Symbiotic nitrogen fixation in legumes requires nodule organogenesis to be coordinated with infection by rhizobia. The plant hormone auxin influences symbiotic infection, but the precise timing of auxin accumulation and the genetic network governing it remain unclear. We used a Lotus japonicus optimised variant of the DII‐based auxin accumulation sensor and identified a rapid accumulation of auxin in the epidermis, specifically in the root hair cells. This auxin accumulation occurs in the infected root hairs during rhizobia invasion, while Nod factor application induces this response across a broader range of root hairs. Using the DR5 auxin responsive promoter, we demonstrate that activation of auxin signalling also occurs specifically in infected root hairs. Analysis of root hair transcriptome data identified induction of an auxin biosynthesis gene of the Tryptophan Amino‐transferase Related (LjTar1) family following both bacteria inoculation and Nod factor treatment. Genetic analysis showed that both expression of the LjTar1 biosynthesis gene and the auxin response requires Nod factor perception, while common symbiotic pathway transcription factors are only partially required or act redundantly to initiate auxin accumulation. Using a chemical genetics approach, we confirmed that auxin biosynthesis has a functional role in promoting symbiotic infection events in the epidermis.

Via Christophe Jacquet
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Partner communication and role of nutrients in the arbuscular mycorrhizal symbiosis 

Partner communication and role of nutrients in the arbuscular mycorrhizal symbiosis  | Regulation of the plant-microbe interactions | Scoop.it
The evolutionary and ecological success of the arbuscular mycorrhizal (AM) symbiosis relies on an efficient and multifactorial communication system for partner recognition, and on a fine‐tuned and reciprocal metabolic regulation of each symbiont to reach an optimal functional integration. Besides strigolactones, N‐acetylglucosamine‐derivatives released by the plant were recently suggested to trigger fungal reprogramming at the pre‐contact stage. Remarkably, N‐acetylglucosamine‐based diffusible molecules also are symbiotic signals produced by AM fungi (AMF) and clues on the mechanisms of their perception by the plant are emerging. AMF genomes and transcriptomes contain a battery of putative effector genes that may have conserved and AMF‐ or host plant‐specific functions. Nutrient exchange is the key feature of AM symbiosis. A mechanism of phosphate transport inside fungal hyphae has been suggested, and first insights into the regulatory mechanisms of root colonization in accordance with nutrient transfer and status were obtained. The recent discovery of the dependency of AMF on fatty acid transfer from the host has offered a convincing explanation for their obligate biotrophism. Novel studies highlighted the importance of plant and fungal genotypes for the outcome of the symbiosis. These findings open new perspectives for fundamental research and application of AMF in agriculture.
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