Plant-Microbe Symbiosis
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Soil Carbon and Nitrogen and Evidence for Formation of Glomalin, a Recalcitrant Pool of Soil Organic Matter, in Developing Mount St. Helens Pyroclastic Substrates

Soil Carbon and Nitrogen and Evidence for Formation of Glomalin, a Recalcitrant Pool of Soil Organic Matter, in Developing Mount St. Helens Pyroclastic Substrates | Plant-Microbe Symbiosis | Scoop.it
Formation of stable soil organic matter is typically the result of a relatively slow series of decomposition processes that can be constrained in early successional sites. Alternatively, compounds such as glomalin, a glycoprotein produced by arbuscular mycorrhizal fungi, may form relatively early during soil development and improve aggregate stabilization, water infiltration, and carbon and nitrogen storage. After 31 years of development, significant amounts of soil C, N, and BRSP, an indicator of glomalin, had accrued in pyroclastic deposits, in patterns affected by both plant community type and soil depth. Mycorrhizal fungi are important, but incompletely understood,  drivers of pedogenic processes during primary succession and may exert disproportionate effects on soil processes and plant development prior to the accumulation of humified soil organic matter.

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Plant-Microbe Symbiosis
Beneficial associations between plants and microbes
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Sphagnum Species Modulate their Phenolic Profiles and Mycorrhizal Colonization of Surrounding Andromeda polifolia along Peatland Microhabitats

Sphagnum mosses mediate long-term carbon accumulation in peatlands. Given their functional role as keystone species, it is important to consider their responses to ecological gradients and environmental changes through the production of phenolics. We compared the extent to which Sphagnum phenolic production was dependent on species, microhabitats and season, and how surrounding dwarf shrubs responded to Sphagnum phenolics. We evaluated the phenolic profiles of aqueous extracts of Sphagnum fallax and Sphagnum magellanicum over a 6-month period in two microhabitats (wet lawns versus dry hummocks) in a French peatland. Phenolic profiles of water-soluble extracts were measured by UHPLC-QTOF-MS. Andromeda polifolia mycorrhizal colonization was quantified by assessing the intensity of global root cortex colonization. Phenolic profiles of both Sphagnum mosses were species-, season- and microhabitat- dependant. Sphagnum-derived acids were the phenolics mostly recovered; relative quantities were 2.5-fold higher in S. fallax than in S. magellanicum. Microtopography and vascular plant cover strongly influenced phenolic profiles, especially for minor metabolites present in low abundance. Higher mycorrhizal colonization of A. polifolia was found in lawns as compared to hummocks. Mycorrhizal abundance, in contrast to environmental parameters, was correlated with production of minor phenolics in S. fallax. Our results highlight the close interaction between mycorrhizae such as those colonizing A. polifolia and the release of Sphagnum phenolic metabolites and suggest that Sphagnum-derived acids and minor phenolics play different roles in this interaction. This work provides new insight into the ecological role of Sphagnum phenolics by proposing a strong association with mycorrhizal colonization of shrubs.

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The LaCEP1 peptide modulates cluster root morphology in Lupinus albus

White lupin cluster roots are specialized brush‐like root structures that are formed in some species under phosphorus (P)‐deficient conditions. They intensely secrete protons and organic acid anions for solubilization and acquisition of sparingly soluble phosphates. Phytohormones and sucrose modulate cluster root number, but the molecular mechanisms of cluster root formation have been elusive. Here, a novel peptide phytohormone was identified that affects cluster root development. It belongs to the C‐TERMINALLY-ENCODED PEPTIDE (CEP) family. Members of that family arrest root growth and modulate branching in model species. LaCEP1 was highly expressed in the pre‐emergence zone of clusters. Over‐expression of the gene encoding the LaCEP1 propeptide resulted in moderate inhibition of cluster root formation. The primary and lateral root lengths of lupin were little affected by the overexpression, but LaCEP1 reduced cluster rootlet and root hair elongation. Addition of a 15‐mer core peptide derived from LaCEP1 similarly altered root morphology and modified cluster activity, suggesting that a core sequence of the propeptide is functionally sufficient. Stable overexpression in Arabidopsis confirmed the LaCEP1 function in root growth inhibition across species. Taken together, the root inhibitory effects of the LaCEP1 phytohormone suggest a role as of a regulatory module involved in cluster root development in white lupin.

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Creation and Characterization of a Genomically Hybrid Strain in the Nitrogen-Fixing Symbiotic Bacterium Sinorhizobium meliloti

Creation and Characterization of a Genomically Hybrid Strain in the Nitrogen-Fixing Symbiotic Bacterium Sinorhizobium meliloti | Plant-Microbe Symbiosis | Scoop.it
Many bacteria, often associated with eukaryotic hosts and of relevance for biotechnological applications, harbor a multipartite genome composed of more than one replicon. Biotechnologically relevant phenotypes are often encoded by genes residing on the secondary replicons. A synthetic biology approach to developing enhanced strains for biotechnological purposes could therefore involve merging pieces or entire replicons from multiple strains into a single genome. Here we report the creation of a genomic hybrid strain in a model multipartite genome species, the plant-symbiotic bacterium Sinorhizobium meliloti. We term this strain as cis-hybrid, since it is produced by genomic material coming from the same species’ pangenome. In particular, we moved the secondary replicon pSymA (accounting for nearly 20% of total genome content) from a donor S. meliloti strain to an acceptor strain. The cis-hybrid strain was screened for a panel of complex phenotypes (carbon/nitrogen utilization phenotypes, intra- and extracellular metabolomes, symbiosis, and various microbiological tests). Additionally, metabolic network reconstruction and constraint-based modeling were employed for in silico prediction of metabolic flux reorganization. Phenotypes of the cis-hybrid strain were in good agreement with those of both parental strains. Interestingly, the symbiotic phenotype showed a marked cultivar-specific improvement with the cis-hybrid strains compared to both parental strains. These results provide a proof-of-principle for the feasibility of genome-wide replicon-based remodelling of bacterial strains for improved biotechnological applications in precision agriculture.

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Diversity of Bradyrhizobia in Subsahara Africa: A Rich Resource

Diversity of Bradyrhizobia in Subsahara Africa: A Rich Resource | Plant-Microbe Symbiosis | Scoop.it
Making use of biological nitrogen fixation (BNF) with pulses and green manure legumes can help to alleviate nitrogen deficiencies and increase soil fertility, problems faced particularly in smallholder agriculture in Subsahara Africa (SSA). The isolation of indigenous rhizobia provides a basis for the formulation of rhizobial inoculants. Moreover, their identification and characterization contribute to the general understanding of species distribution and ecology. Here we discuss global species discovery of Bradyrhizobium spp. Although recently the number of validly published Bradyrhizobium species is rapidly increasing, their diversity in SSA is not well-represented. We summarize the recent knowledge on species diversity in the Bradyrhizobium yuanmingense lineage to which most SSA isolates belong, and their biogeographic distribution and adaptations. Most indigenous rhizobia appear to differ from species found on other continents. We stress that an as yet hidden diversity may be a rich resource for inoculant development in future. As some species are exceptionally temperature tolerant, they may be potential biofertilizer candidates for global warming scenarios.

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Transcriptomic analysis reveals the possible roles of sugar metabolism and export for positive mycorrhizal growth responses in soybean

To elucidate molecular mechanisms controlling differential growth responses to root colonization by arbuscular mycorrhizal (AM) fungi varying in colonization and cooperative behavior, a pot experiment was carried out using two soybean genotypes and three AM inocula. The results showed that inoculation by cooperative Rhizophagus irregularis (Ri) or less‐cooperative Glomus aggregatum with high AM colonization (Ga‐H) significantly promoted plant growth compared with inoculation by Glomus aggregatum with low AM colonization (Ga‐L). A comparative RNA sequencing analysis of the root transcriptomes showed that fatty acid synthesis pathway was significantly enriched in all three AM inoculation roots. However, sugar metabolism and transport were significantly enriched only in Ri and Ga‐H inoculation, which was consistent with positive growth responses in these two inoculation treatments. Accordingly, the expression levels of the key genes related to sugar metabolism and transport were also up‐regulated in Ri and Ga‐H roots compared with Ga‐L roots. Of them, two SWEET transporter genes, GmSWEET6 (Glyma.04G198600) and GmSWEET15 (Glyma.06G166800), and one invertase (Glyma.17G227900) gene were exclusively induced only in Ri and Ga‐H roots. Promoter analyses in transgenic soybean roots further demonstrated that GUS driven by the GmSWEET6 promoter was highly expressed in arbuscule‐containing cortical cells. Additionally, Ri and Ga‐H inoculation increased the contents of sucrose, glucose and fructose in both shoots and roots compared to those of Ga‐L and NM. These results imply that positive mycorrhizal growth responses in plants might mostly be due to the stimulation of photosynthate metabolism and transport by AM fungal inoculum with high colonization capabilities.

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An efficient CRISPR vector toolbox for engineering large deletions in Arabidopsis thaliana

An efficient CRISPR vector toolbox for engineering large deletions in Arabidopsis thaliana | Plant-Microbe Symbiosis | Scoop.it
Background
Our knowledge of natural genetic variation is increasing at an extremely rapid pace, affording an opportunity to come to a much richer understanding of how effects of specific genes are dependent on the genetic background. To achieve a systematic understanding of such GxG interactions, it is desirable to develop genome editing tools that can be rapidly deployed across many different genetic varieties.

Results
We present an efficient CRISPR/Cas9 toolbox of super module (SM) vectors. These vectors are based on a previously described fluorescence protein marker expressed in seeds allowing identification of transgene-free mutants. We have used this vector series to delete genomic regions ranging from 1.7 to 13 kb in different natural accessions of the wild plant Arabidopsis thaliana. Based on results from 53 pairs of sgRNAs targeting individual nucleotide binding site leucine-rich repeat (NLR) genes, we provide a comprehensive overview of obtaining heritable deletions.

Conclusions
The SM series of CRISPR/Cas9 vectors enables the rapid generation of transgene-free, genome edited plants for a diversity of functional studies.
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Little evidence that farmers should consider abundance or diversity of arbuscular mycorrhizal fungi when managing crops

Arbuscular mycorrhizal fungi (AMF) are ubiquitous in agroecosystems and often stated to be critical for crop yield and agroecosystem sustainability. However, should farmers modify management to enhance the abundance and diversity of AMF? We address this question with a focus on field experiments that manipulated colonisation by indigenous AMF and report crop yield, or investigated community structure and diversity of AMF. We find that the literature presents an overly optimistic view of the importance of AMF in crop yield due, in part, to flawed methodology in field experiments. A small body of rigorous research only sometimes reports a positive impact of high colonisation on crop yield, even under phosphorus limitation. We suggest that studies vary due to the interaction of environment and genotype (crop and mycorrhizal fungal). We also find that the literature can be overly pessimistic about the impact of some common agricultural practices on mycorrhizal fungal communities and that interactions between AMF and soil microbes are complex and poorly understood. We provide a template for future field experiments and a list of research priorities, including phosphorus‐efficient agroecosystems. However, we conclude that management of AMF by farmers will not be warranted until benefits are demonstrated at the field scale under prescribed agronomic management.

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HcPT1.2 participates in Pi acquisition in Hebeloma cylindrosporum external hyphae of ectomycorrhizas under high and low phosphate conditions

Ectomycorrhizal fungi improve tree phosphorus nutrition through transporters specifically localized at soil-hyphae and symbiotic interfaces. In the model symbiosis between the fungus Hebeloma cylindrosporum and the maritime pine (Pinus pinaster), several transporters possibly involved in phosphate fluxes were identified, including three H+:Pi transporters. Among these three, we recently unraveled the function of one of them, named HcPT2, in both pure culture and symbiotic interaction with P. pinaster. Here we investigated the transporter named HcPT1.2, by analyzing inorganic phosphate transport ability in a yeast complementation assay, assessing its expression in the fungus associated or not with the plant, and immunolocalizing the proteins in ectomycorrhizas. We also evaluated the effect of external Pi concentration on expression and localization of HcPT1.2. Our results revealed that HcPT1.2 is involved in Pi acquisition by H. cylindrosporum mycelium, irrespective of the external Pi concentrations.

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An Array-based Comparative Genomic Hybridization Platform for Efficient Detection of Copy Number Variations in Fast Neutron-induced Medicago truncatula Mutants

This protocol provides experimental steps and information about reagents, equipment, and analysis tools for researchers who are interested in carrying out whole genome array-based comparative genomic hybridization (CGH) analysis of copy number variations in plants.

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A plant chitinase controls cortical infection thread progression and nitrogen-fixing symbiosis

Morphogens provide positional information and their concentration is key to the organized development of multicellular organisms. Nitrogen-fixing root nodules are unique organs induced by Nod factor-producing bacteria. Localized production of Nod factors establishes a developmental field within the root where plant cells are reprogrammed to form infection threads and primordia. We found that regulation of Nod factor levels by Lotus japonicus is required for the formation of nitrogen-fixing organs, determining the fate of this induced developmental program. Our analysis of plant and bacterial mutants shows that a host chitinase modulates Nod factor levels possibly in a structure-dependent manner. In Lotus, this is required for maintaining Nod factor signalling in parallel with the elongation of infection threads within the nodule cortex, while root hair infection and primordia formation are not influenced. Our study shows that infected nodules require balanced levels of Nod factors for completing their transition to functional, nitrogen-fixing organs.

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The dynamics of root cap sloughing in Arabidopsis is regulated by peptide signalling

The dynamics of root cap sloughing in Arabidopsis is regulated by peptide signalling | Plant-Microbe Symbiosis | Scoop.it
The root cap protects the stem cell niche of angiosperm roots from damage. In Arabidopsis, lateral root cap (LRC) cells covering the meristematic zone are regularly lost through programmed cell death, while the outermost layer of the root cap covering the tip is repeatedly sloughed. Efficient coordination with stem cells producing new layers is needed to maintain a constant size of the cap. We present a signalling pair, the peptide IDA-LIKE1 (IDL1) and its receptor HAESA-LIKE2 (HSL2), mediating such communication. Live imaging over several days characterized this process from initial fractures in LRC cell files to full separation of a layer. Enhanced expression of IDL1 in the separating root cap layers resulted in increased frequency of sloughing, balanced with generation of new layers in a HSL2-dependent manner. Transcriptome analyses linked IDL1-HSL2 signalling to the transcription factors BEARSKIN1/2 and genes associated with programmed cell death. Mutations in either IDL1 or HSL2 slowed down cell division, maturation and separation. Thus, IDL1-HSL2 signalling potentiates dynamic regulation of the homeostatic balance between stem cell division and sloughing activity.

Via Christophe Jacquet
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The Rhizobia-Lotus Symbioses: Deeply Specific and Widely Diverse

The Rhizobia-Lotus Symbioses: Deeply Specific and Widely Diverse | Plant-Microbe Symbiosis | Scoop.it
The symbiosis between Lotus and rhizobia has been long considered very specific and only two bacterial species were recognized as the microsymbionts of Lotus: Mesorhizobium loti was considered the typical rhizobia for the L. corniculatus complex, whereas Bradyrhizobium sp. (Lotus) was the symbiont for L. uliginosus and related species. As discussed in this review, this situation has dramatically changed during the last 15 years, with the characterization of nodule bacteria from worldwide geographical locations and from previously unexplored Lotus spp. Current data support that the Lotus rhizobia are dispersed amongst nearly 20 species in five genera (Mesorhizobium, Bradyrhizobium, Rhizobium, Ensifer, and Aminobacter). As a consequence, M. loti could be regarded an infrequent symbiont of Lotus, and several plant–bacteria compatibility groups can be envisaged. Despite the great progress achieved with the model L. japonicus in understanding the establishment and functionality of the symbiosis, the genetic and biochemical bases governing the stringent host-bacteria compatibility pairships within the genus Lotus await to be uncovered. Several Lotus spp. are grown for forage, and inoculation with rhizobia is a common practice in various countries. However, the great diversity of the Lotus rhizobia is likely squandered, as only few bacterial strains are used as inoculants for Lotus pastures in very different geographical locations, with a great variety of edaphic and climatic conditions. The agroecological potential of the genus Lotus can not be fully harnessed without acknowledging the great diversity of rhizobia-Lotus interactions, along with a better understanding of the specific plant and bacterial requirements for optimal symbiotic nitrogen fixation under increasingly constrained environmental conditions.

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Positive and negative regulation of transferred nif genes mediated by indigenous GlnR in Gram-positive Paenibacillus polymyxa

Positive and negative regulation of transferred nif genes mediated by indigenous GlnR in Gram-positive Paenibacillus polymyxa | Plant-Microbe Symbiosis | Scoop.it
Ammonia is a major signal that regulates nitrogen fixation in most diazotrophs. Regulation of nitrogen fixation by ammonia in the Gram-negative diazotrophs is well-characterized. In these bacteria, this regulation occurs mainly at the level of nif (nitrogen fixation) gene transcription, which requires a nif-specific activator, NifA. Although Gram-positive and diazotrophic Paenibacilli have been extensively used as a bacterial fertilizer in agriculture, how nitrogen fixation is regulated in response to nitrogen availability in these bacteria remains unclear. An indigenous GlnR and GlnR/TnrA-binding sites in the promoter region of the nif cluster are conserved in these strains, indicating the role of GlnR as a regulator of nitrogen fixation. In this study, we for the first time reveal that GlnR of Paenibacillus polymyxa WLY78 is essentially required for nif gene transcription under nitrogen limitation, whereas both GlnR and glutamine synthetase (GS) encoded by glnA within glnRA operon are required for repressing nif expression under excess nitrogen. Dimerization of GlnR is necessary for binding of GlnR to DNA. GlnR in P. polymyxa WLY78 exists in a mixture of dimers and monomers. The C-terminal region of GlnR monomer is an autoinhibitory domain that prevents GlnR from binding DNA. Two GlnR-biding sites flank the -35/-10 regions of the nif promoter of the nif operon (nifBHDKENXhesAnifV). The GlnR-binding site Ⅰ (located upstream of -35/-10 regions of the nif promoter) is specially required for activating nif transcription, while GlnR-binding siteⅡ (located downstream of -35/-10 regions of the nif promoter) is for repressing nif expression. Under nitrogen limitation, GlnR dimer binds to GlnR-binding siteⅠ in a weak and transient association way and then activates nif transcription. During excess nitrogen, glutamine binds to and feedback inhibits GS by forming the complex FBI-GS. The FBI-GS interacts with the C-terminal domain of GlnR and stabilizes the binding affinity of GlnR to GlnR-binding site Ⅱ and thus represses nif transcription.

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The Role of Plant Transporters in Mycorrhizal Symbioses

The Role of Plant Transporters in Mycorrhizal Symbioses | Plant-Microbe Symbiosis | Scoop.it
Membrane transport systems are crucial elements for plant nutrition and development as they play a key role in the absorption of mineral nutrients and water at the root level but also in the translocation within the plant. Moreover, membrane transport is involved in signalling and communication e.g. to adapt and interact with the environment. Most plants live in tight contact with beneficial soil microbes, such as bacteria and mycorrhizal fungi, which contribute to plant nutrition in part through modulation of the expression and functioning of plant transporter systems, as ion channels and transporters. In addition, mycorrhizal fungi largely increase the absorption surface of roots thereby promoting plant's access to soil resources as minerals and water. In turn, plants “reward” mycorrhizal fungi with sugars and/or lipids. This “fair trade” requires specific communication and a series of exchanges between the two symbiotic partners enabled by the adaptability and plasticity of their transporters. Here, we summarize recent advances allowing molecular insight in the impact of mycorrhizal symbiosis on the plant “transportome”. We highlight results obtained in ecto- and endomycorrhizal associations for plant transporters involved in the absorption of mineral nutrients and water released by the fungus at the symbiotic interface, and molecular players responsible for carbon and lipid nutrition of the fungal partner. We focus also on plant membrane transport systems implicated in early communication between plant and fungal partners.

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Great job Kevin and Sabine!

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Plant Promoter Analysis: Identification and Characterization of Root Nodule Specific Promoter in the Common Bean

Plant Promoter Analysis: Identification and Characterization of Root Nodule Specific Promoter in the Common Bean | Plant-Microbe Symbiosis | Scoop.it
The upstream sequences of gene coding sequences are termed as promoter sequences. Studying the expression patterns of promoters are very significant in understanding the gene regulation and spatiotemporal expression patterns of target genes. On the other hand, it is also critical to establish promoter evaluation tools and genetic transformation techniques that are fast, efficient, and reproducible. In this study, we investigated the spatiotemporal expression pattern of the rhizobial symbiosis-specific nodule inception (NIN) promoter of Phaseolus vulgaris in the transgenic hairy roots. Using plant genome databases and analysis tools we identified, isolated, and cloned the P. vulgaris NIN promoter in a transcriptional fusion to the chimeric reporter β-glucuronidase (GUS) GUS-enhanced::GFP. Further, this protocol describes a rapid and versatile system of genetic transformation in the P. vulgaris using Agrobacterium rhizogenes induced hairy roots. This system generates ≥2 cm hairy roots at 10 to 12 days after transformation. Next, we assessed the spatiotemporal expression of NIN promoter in Rhizobium inoculated hairy roots at periodic intervals of post-inoculation. Our results depicted by GUS activity show that the NIN promoter was active during the process of nodulation. Together, the present protocol demonstrates how to identify, isolate, clone, and characterize a plant promoter in the common bean hairy roots. Moreover, this protocol is easy to use in non-specialized laboratories.

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Contrasted evolutionary constraints on carbohydrate active enzymes (CAZymes) in selected Frankia strains

Carbohydrate active enzymes (CAZymes) are capable of breaking complex polysaccharides into simpler form. In plant-host-associated microorganisms CAZymes are known to be involved in plant cell wall degradation. However, the biology and evolution of Frankia CAZymes are largely unknown. In the present study, we took a genomic approach to evaluate the presence and putative roles of CAZymes in Frankia. The CAZymes were found to be potentially highly expressed (PHX) proteins and contained more aromatic amino acids, which increased their biosynthetic energy cost. These energy rich amino acids were present in the active sites of CAZymes aiding in their carbohydrate binding capacity. Phylogenetic and evolutionary analyses showed that, in Frankia strains with the capacity to nodulate host plants, CAZymes were evolving slower than the other PHX genes, whereas similar genes from non-nodulating (or ineffectively nodulating) Frankia strains showed little variation in their evolutionary constraints compared to other PHX genes. Thus, the present study revealed the persistence of a strong purifying selection on CAZymes of Frankia indicating their crucial role.

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Does mycorrhizal colonisation vary between maize and sunflower under limitations to radiation source or carbohydrate sink?

The aim of this work was to analyse and compare indigenous arbuscular mycorrhizal colonisation (AMC) in relation to growth and total soluble carbohydrates (TSC) in two major, physiologically contrasting crop species: maize (Zea mays L.) and sunflower (Helianthus annuus L.). In order to promote contrasting TSC concentrations, we modified the radiation source by shading and the carbohydrate sink by manipulating reproductive sinks at different phenological stages during the grain-filling period in two field experiments. We assessed plant dry matter, TSC in stems, and root AMC from flowering until final harvest. AMC during the grain-filling period decreased in maize and increased in sunflower. A sink limitation increased AMC in maize, and reduced it in sunflower. A source limitation decreased AMC in both species, especially in sunflower. AMC was positively related to TSC in maize, but negatively in sunflower. The relationship was affected by shading in sunflower, but not in maize. In both species, a different linear model described the relationship between AMC and TSC in plants submitted to the removal of the reproductive organs. The results highlight the role of carbohydrates in mediating mycorrhizal formation, and show for the first time the opposite AMC–TSC relationships in maize and sunflower.

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A Lotus japonicus E3 ligase interacts with the Nod Factor Receptor 5 and positively regulates nodulation

A Lotus japonicus E3 ligase interacts with the Nod Factor Receptor 5 and positively regulates nodulation | Plant-Microbe Symbiosis | Scoop.it
Background
Post-translational modification of receptor proteins is involved in activation and de-activation of signalling systems in plants. Both ubiquitination and deubiquitination have been implicated in plant interactions with pathogens and symbionts.

Results
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.

Conclusion
LjPUB13 is an E3 ligase with a positive regulatory role during the initial stages of nodulation in L. japonicus.
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NIN interacts with NLPs to mediate nitrate inhibition of nodulation in Medicago truncatula

NIN interacts with NLPs to mediate nitrate inhibition of nodulation in Medicago truncatula | Plant-Microbe Symbiosis | Scoop.it
Legume plants can assimilate inorganic nitrogen and have access to fixed nitrogen through symbiotic interaction with diazotrophic bacteria called rhizobia. Symbiotic nitrogen fixation is an energy-consuming process and is strongly inhibited when sufficient levels of fixed nitrogen are available, but the molecular mechanisms governing this regulation are largely unknown. The transcription factor nodule inception (NIN) is strictly required for nodulation and belongs to a family of NIN-like proteins (NLPs), which have been implicated in the regulation of nitrogen homeostasis in Arabidopsis. Here, we show that mutation or downregulation of NLP genes prevents nitrate inhibition of infection, nodule formation and nitrogen fixation. We find that NIN and NLPs physically interact through their carboxy-terminal PB1 domains. Furthermore, we find that NLP1 is required for the expression of nitrate-responsive genes and that nitrate triggers NLP1 re-localization from the cytosol to the nucleus. Finally, we show that NLP1 can suppress NIN activation of CRE1 expression in Nicotiana benthamiana and Medicago truncatula. Our findings highlight a central role for NLPs in the suppression of nodulation by nitrate.

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Great paper... I love it.

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Mixotrophic orchids do not use photosynthates for perennial underground organs

Mixotrophic orchids do not use photosynthates for perennial underground organs | Plant-Microbe Symbiosis | Scoop.it
Most plants are autotrophic and interact with soil fungi, forming mycorrhizal symbioses (van der Heijden et al., 2015) where plants gain mineral nutrients and provide photosynthates to fungi. Yet, plants repeatedly evolve heterotrophy (Tĕšitel et al., 2018), and several lineages, especially in orchids, import carbon from their mycorrhizal fungi, a strategy called mycoheterotrophy (Merckx, 2013). Green plants that are photosynthetic but also import carbon from their mycorrhizal fungi have raised considerable interest over the past two decades (Julou et al., 2005; Selosse & Roy, 2009). These plants with two carbon sources are termed mixotrophic, and pave the evolutionary way to full mycoheterotrophy (Selosse & Roy, 2009). Mixotrophy enables them to adapt to shaded conditions (Julou et al., 2005; Preiss et al., 2010; with some exceptions: Girlanda et al., 2011; Schiebold et al., 2017) and sometimes drives a reduction of their photosynthetic abilities (Girlanda et al., 2006). Their study is therefore of crucial interest to an understanding of the evolution to full mycoheterotrophy.

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Digging deeper: why we need more proterozoic algal fossils and how to get them

Known Proterozoic algal fossils raise compelling questions about the origin and diversification of cyanobacteria and eukaryotic algae, and their ecological influence in deep time. This perspectives article describes particular examples of persistent evolutionary and biogeochemical issues whose resolution would be aided by additional algal fossil evidence from Proterozoic deposits, which have been the subjects of recent intensive study. New Proterozoic geosciences literature relevant to the early diversification of algae is surveyed. Previously underappreciated algal traits that might improve taxonomic attributions of fossil remains are highlighted. Processes that phycologists could use to improve detection of algal fossils are recommended. Potential geological sources of new Proterozoic fossils are suggested.

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Nitrate inhibition of nodule formation in Medicago truncatula is mediated by ACC SYNTHASE 10

Legumes form a mutualistic endosymbiosis with nitrogen-fixing rhizobia. These rhizobia are housed intracellularly in specialised lateral root organs, called nodules. Initiation of these nodules is triggered by bacterial derived signalling molecules, lipochitooligosaccharides (LCO). The process of nitrogen fixation is highly energy-demanding and therefore nodule initiation is tightly regulated. Nitrate is a potent inhibitor of nodulation. However, the precise mechanisms by which nitrate inhibits nodulation is poorly understood. Here, we demonstrate that in Medicago truncatula nitrate interferes with the transcriptional regulation of the ethylene biosynthesis gene ACC SYNTHASE 10. ACSs commit the rate limiting step in ethylene biosynthesis and in M. truncatula ACS10 is highly expressed in the zone of the root where nodulation occurs. Our results show that a reduction in ACS10 expression in response to LCO exposure correlates with the ability to form nodules. In addition, RNAi-mediated knockdown of ACS10 confers nodulation ability under otherwise inhibitory nitrate conditions. This discovery sheds new light on how ethylene is involved in the inhibition of nodulation by nitrate, bringing us one step closer to understanding how plants regulate their susceptibility towards rhizobia.
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Nice work @KohlenW

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Transient genetic transformation of Mougeotia scalaris (Zygnematophyceae) mediated by the endogenous α‐tubulin1 promoter

Mougeotia scalaris is a filamentous streptophyte alga renowned for light‐inducible plastid rotation and microtubule‐dependent polarity establishment. As a first step toward transgenic approaches we determined the 5,825 base pair genomic sequence encoding the α‐tubulin1 gene (MsTUA1) of M. scalaris (strain SAG 164.80). The subcloned MsTUA1 promoter facilitated strong transgene expression in M. scalaris and tobacco leaf cells, as shown by particle bombardment and the subsequent visualization of expressed fluorescent protein markers. Our results provide a route for the genetic transformation of the filamentous streptophyte alga M. scalaris based on the endogenous TUA1 promoter.

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Actinorhizal signaling molecules: Frankia root hair deforming factor shares properties with NIN inducing factor

Actinorhizal plants are able to establish a symbiotic relationship with Frankia bacteria leading to the formation of root nodules. The symbiotic interaction starts with the exchange of symbiotic signals in the soil between the plant and the bacteria. This molecular dialogue involves signaling molecules that are responsible for the specific recognition of the plant host and its endosymbiont. Here we studied two factors potentially involved in signaling between Frankia casuarinae and its actinorhizal host Casuarina glauca: (1) the Root Hair Deforming Factor (CgRHDF) detected using a test based on the characteristic deformation of C. glauca root hairs inoculated with F. casuarinae and (2) a NIN activating factor (CgNINA) which is able to activate the expression of CgNIN, a symbiotic gene expressed during preinfection stages of root hair development.
We showed that CgRHDF and CgNINA corresponded to small thermoresistant molecules. Both factors were also hydrophilic and resistant to a chitinase digestion indicating structural differences from rhizobial Nod factors or mycorrhizal Myc-LCOs. We also investigated the presence of CgNINA and CgRHDF in 16 Frankia strains representative of Frankia diversity. High levels of root hair deformation (RHD) and activation of ProCgNIN were detected for Casuarina-infective strains from clade Ic and closely-related strains from clade Ia unable to nodulate C. glauca. Lower levels were present for distantly related strains belonging to clade III. No CgRHDF or CgNINA could be detected for Frankia coriariae (Clade II) or for uninfective strains from clade IV.
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Genome Wide Transcriptome Analysis Reveals Complex Regulatory Mechanisms Underlying Phosphate Homeostasis in Soybean Nodules

Genome Wide Transcriptome Analysis Reveals Complex Regulatory Mechanisms Underlying Phosphate Homeostasis in Soybean Nodules | Plant-Microbe Symbiosis | Scoop.it
Phosphorus (P) deficiency is a major limitation for legume crop production. Although overall adaptations of plant roots to P deficiency have been extensively studied, only fragmentary information is available in regard to root nodule responses to P deficiency. In this study, genome wide transcriptome analysis was conducted using RNA-seq analysis in soybean nodules grown under P-sufficient (500 μM KH2PO4) and P-deficient (25 μM KH2PO4) conditions to investigate molecular mechanisms underlying soybean (Glycine max) nodule adaptation to phosphate (Pi) starvation. Phosphorus deficiency significantly decreased soybean nodule growth and nitrogenase activity. Nodule Pi concentrations declined by 49% in response to P deficiency, but this was well below the 87% and 88% decreases observed in shoots and roots, respectively. Nodule transcript profiling revealed that a total of 2055 genes exhibited differential expression patterns between Pi sufficient and deficient conditions. A set of (differentially expressed genes) DEGs appeared to be involved in maintaining Pi homeostasis in soybean nodules, including eight Pi transporters (PTs), eight genes coding proteins containing the SYG1/PHO81/XPR1 domain (SPXs), and 16 purple acid phosphatases (PAPs). The results suggest that a complex transcriptional regulatory network participates in soybean nodule adaption to Pi starvation, most notable a Pi signaling pathway, are involved in maintaining Pi homeostasis in nodules. View Full-Text
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