The presence of nodules on legume root systems can alter phenotypic plasticity in response to internal nitrogen independent of nitrogen fixation.

All higher plants show developmental plasticity in response to the availability of nitrogen (N) in the soil. In legumes, N starvation causes the formation of root nodules, where symbiotic rhizobacteria fix atmospheric N2 for the host in exchange for fixed carbon (C) from the shoot. Here, we tested whether plastic responses to internal [N] of legumes are altered by their symbionts. Glasshouse experiments compared root phenotypes of three legumes, Medicago truncatula, Medicago sativa and Trifolium subterraneum, inoculated with their compatible symbiont partners and grown under four nitrate levels. In addition, six strains of rhizobia, differing in their ability to fix N2 in M. truncatula, were compared to test if plastic responses to internal [N] were dependent on the rhizobia or N2 -fixing capability of the nodules. We found that the presence of rhizobia affected phenotypic plasticity of the legumes to internal [N], particularly in root length and root mass ratio (RMR), in a plant species-dependent way. While root length responses of M. truncatula to internal [N] were dependent on the ability of rhizobial symbionts to fix N2 , RMR response to internal [N] was dependent only on initiation of nodules, irrespective of N2 -fixing ability of the rhizobia strains.

Functional specialization of duplicated AP3-like genes in Medicago truncatula.

The B-class of MADS box genes has been studied in a wide range of plant species, but has remained largely uncharacterized in legumes. Here we investigate the evolutionary fate of the duplicated AP3-like genes of a legume species. To obtain insight into the extent to which B-class MADS box gene functions are conserved or have diversified in legumes, we isolated and characterized the two members of the AP3 lineage in Medicago truncatula: MtNMH7 and MtTM6 (euAP3 and paleoAP3 genes, respectively). A non-overlapping and complementary expression pattern of both genes was observed in petals and stamens. MtTM6 was expressed predominantly in the outer cell layers of both floral organs, and MtNMH7 in the inner cell layers of petals and stamens. Functional analyses by reverse genetics approaches (RNAi and Tnt1 mutagenesis) showed that the contribution of MtNMH7 to petal identity is more important than that of MtTM6, whereas MtTM6 plays a more important role in stamen identity than its paralog MtNMH7. Our results suggest that the M. truncatula AP3-like genes have undergone a functional specialization process associated with complete partitioning of gene expression patterns of the ancestral gene lineage. We provide information regarding the similarities and differences in petal and stamen development among core eudicots.

Unexpectedly low nitrogen acquisition and absence of root architecture adaptation to nitrate supply in a Medicago truncatula highly branched root mutant.

To complement N2 fixation through symbiosis, legumes can efficiently acquire soil mineral N through adapted root architecture. However, root architecture adaptation to mineral N availability has been little studied in legumes. Therefore, this study investigated the effect of nitrate availability on root architecture in Medicago truncatula and assessed the N-uptake potential of a new highly branched root mutant, TR185. The effects of varying nitrate supply on both root architecture and N uptake were characterized in the mutant and in the wild type. Surprisingly, the root architecture of the mutant was not modified by variation in nitrate supply. Moreover, despite its highly branched root architecture, TR185 had a permanently N-starved phenotype. A transcriptome analysis was performed to identify genes differentially expressed between the two genotypes. This analysis revealed differential responses related to the nitrate acquisition pathway and confirmed that N starvation occurred in TR185. Changes in amino acid content and expression of genes involved in the phenylpropanoid pathway were associated with differences in root architecture between the mutant and the wild type.

Developmental analysis of a Medicago truncatula smooth leaf margin1 mutant reveals context-dependent effects on compound leaf development.

Compound leaf development requires highly regulated cell proliferation, differentiation, and expansion patterns. We identified loss-of-function alleles at the SMOOTH LEAF MARGIN1 (SLM1) locus in Medicago truncatula, a model legume species with trifoliate adult leaves. SLM1 encodes an auxin efflux carrier protein and is the ortholog of Arabidopsis thaliana PIN-FORMED1 (PIN1). Auxin distribution is impaired in the slm1 mutant, resulting in pleiotropic phenotypes in different organs. The most striking change in slm1 is the increase in the number of terminal leaflets and a simultaneous reduction in the number of lateral leaflets, accompanied by reduced expression of SINGLE LEAFLET1 (SGL1), an ortholog of LEAFY. Characterization of the mutant indicates that distinct developmental domains exist in the formation of terminal and lateral leaflets. In contrast with the pinnate compound leaves in the wild type, the slm1 sgl1 double mutant shows nonpeltately palmate leaves, suggesting that the terminal leaflet primordium in M. truncatula has a unique developmental mechanism. Further investigations on the development of leaf serrations reveal different ontogenies between distal serration and marginal serration formation as well as between serration and leaflet formation. These data suggest that regulation of the elaboration of compound leaves and serrations is context dependent and tightly correlated with the auxin/SLM1 module in M. truncatula.

STENOFOLIA regulates blade outgrowth and leaf vascular patterning in Medicago truncatula and Nicotiana sylvestris.

Dicot leaf primordia initiate at the flanks of the shoot apical meristem and extend laterally by cell division and cell expansion to form the flat lamina, but the molecular mechanism of lamina outgrowth remains unclear. Here, we report the identification of STENOFOLIA (STF), a WUSCHEL-like homeobox transcriptional regulator, in Medicago truncatula, which is required for blade outgrowth and leaf vascular patterning. STF belongs to the MAEWEST clade and its inactivation by the transposable element of Nicotiana tabacum cell type1 (Tnt1) retrotransposon insertion leads to abortion of blade expansion in the mediolateral axis and disruption of vein patterning. We also show that the classical lam1 mutant of Nicotiana sylvestris, which is blocked in lamina formation and stem elongation, is caused by deletion of the STF ortholog. STF is expressed at the adaxial-abaxial boundary layer of leaf primordia and governs organization and outgrowth of lamina, conferring morphogenetic competence. STF does not affect formation of lateral leaflets but is critical to their ability to generate a leaf blade. Our data suggest that STF functions by modulating phytohormone homeostasis and crosstalk directly linked to sugar metabolism, highlighting the importance of coordinating metabolic and developmental signals for leaf elaboration.

A combined histology and transcriptome analysis unravels novel questions on Medicago truncatula seed coat.

The seed coat is involved in the determination of seed quality traits such as seed size, seed composition, seed permeability, and hormonal regulation. Understanding seed coat structure is therefore a prerequisite to deciphering the genetic mechanisms that govern seed coat functions. By combining histological and transcriptomic data analyses, cellular and molecular events occurring during Medicago truncatula seed coat development were dissected in order to relate structure to function and pinpoint target genes potentially involved in seed coat traits controlling final seed quality traits. The analyses revealed the complexity of the seed coat transcriptome, which contains >30 000 genes. In parallel, a set of genes showing a preferential expression in seed coat that may be involved in more specific functions was identified. The study describes how seed coat anatomy and morphological changes affect final seed quality such as seed size, seed composition, seed permeability, and hormonal regulation. Putative regulator genes of different processes have been identified as potential candidates for further functional genomic studies to improve agronomical seed traits. The study also raises new questions concerning the implication of seed coat endopolyploidy in cell expansion and the participation of the seed coat in de novo abscisic acid biosynthesis at early seed filling.

Control of dissected leaf morphology by a Cys(2)His(2) zinc finger transcription factor in the model legume Medicago truncatula.

Plant leaves are diverse in their morphology, reflecting to a large degree the plant diversity in the natural environment. How different leaf morphology is determined is not yet understood. The leguminous plant Medicago truncatula exhibits dissected leaves with three leaflets at the tip. We show that development of the trifoliate leaves is determined by the Cys(2)His(2) zinc finger transcription factor PALM1. Loss-of-function mutants of PALM1 develop dissected leaves with five leaflets clustered at the tip. We demonstrate that PALM1 binds a specific promoter sequence and down-regulates the expression of the M. truncatula LEAFY/UNIFOLIATA orthologue SINGLE LEAFLET1 (SGL1), encoding an indeterminacy factor necessary for leaflet initiation. Our data indicate that SGL1 is required for leaflet proliferation in the palm1 mutant. Interestingly, ectopic expression of PALM1 effectively suppresses the lobed leaf phenotype from overexpression of a class 1 KNOTTED1-like homeobox protein in Arabidopsis plants. Taken together, our results show that PALM1 acts as a determinacy factor, regulates the spatial-temporal expression of SGL1 during leaf morphogenesis and together with the LEAFY/UNIFOLIATA orthologue plays an important role in orchestrating the compound leaf morphology in M. truncatula.

The phosphomimetic mutation of an evolutionarily conserved serine residue affects the signaling properties of Rho of plants (ROPs).

Plant ROP (Rho of plants) proteins form a unique subgroup within the family of Rho-type small G-proteins of eukaryotes. In this paper we demonstrate that the phosphomimetic mutation of a serine residue conserved in all Rho proteins affects the signaling properties of plant ROPs. We found that the S74E mutation in Medicago ROP6 and Arabidopsis ROP4 prevented the binding of these proteins to their plant-specific upstream activator the plant-specific ROP nucleotide exchanger (PRONE)-domain-containing RopGEF (guanine nucleotide exchange factor) protein and abolished the PRONE-mediated nucleotide exchange reaction in vitro. Structural modeling supported the hypothesis that potential phosphorylation of the S74 residue interferes with the binding of the PRONE-domain to the adjacent plant-specific R76 residue which plays an important role in functional ROP-PRONE interaction. Moreover, we show that while the binding of constitutively active MsROP6 to the effector protein RIC (ROP-interactive CRIB-motif-containing protein) was not affected by the S74E mutation, the capability of this mutated protein to bind and activate the RRK1 kinase in vitro was reduced. These observations are in agreement with the morphology of tobacco pollen tubes expressing mutant forms of yellow fluorescent protein (YFP):MsROP6. The S74E mutation in MsROP6 had no influence on pollen tube morphology and attenuated the phenotype of a constitutively active form of MsROP6. The presented Medicago and Arabidopsis data support the notion that the phosphorylation of the serine residue in ROPs corresponding to S74 in Medicago ROP6 could be a general principle for regulating ROP activation and signaling in plants.

MRM Microcoil Performance Calibration and Usage Demonstrated on Medicago truncatula Roots at 22 T.

This protocol describes a signal-to-noise ratio (SNR) calibration and sample preparation method for solenoidal microcoils combined with biological samples, designed for high-resolution magnetic resonance imaging (MRI), also referred to as MR microscopy (MRM). It may be used at pre-clinical MRI spectrometers, demonstrated on Medicago truncatula root samples. Microcoils increase sensitivity by matching the size of the RF resonator to the size of the sample of interest, thereby enabling higher image resolutions in a given data acquisition time. Due to the relatively simple design, solenoidal microcoils are straightforward and cheap to construct and can be easily adapted to the sample requirements. Systematically, we explain how to calibrate new or home-built microcoils, using a reference solution. The calibration steps include: pulse power determination using a nutation curve; estimation of RF-field homogeneity; and calculating a volume-normalized signal-to-noise ratio (SNR) using standard pulse sequences. Important steps in sample preparation for small biological samples are discussed, as well as possible mitigating factors such as magnetic susceptibility differences. The applications of an optimized solenoid coil are demonstrated by high-resolution (13 x 13 x 13 µm3, 2.2 pL) 3D imaging of a root sample.

Soil origin and plant genotype structure distinct microbiome compartments in the model legume Medicago truncatula.

BACKGROUND: Understanding the genetic and environmental factors that structure plant microbiomes is necessary for leveraging these interactions to address critical needs in agriculture, conservation, and sustainability. Legumes, which form root nodule symbioses with nitrogen-fixing rhizobia, have served as model plants for understanding the genetics and evolution of beneficial plant-microbe interactions for decades, and thus have added value as models of plant-microbiome interactions. Here we use a common garden experiment with 16S rRNA gene amplicon and shotgun metagenomic sequencing to study the drivers of microbiome diversity and composition in three genotypes of the model legume Medicago truncatula grown in two native soil communities. RESULTS: Bacterial diversity decreased between external (rhizosphere) and internal plant compartments (root endosphere, nodule endosphere, and leaf endosphere). Community composition was shaped by strong compartment × soil origin and compartment × plant genotype interactions, driven by significant soil origin effects in the rhizosphere and significant plant genotype effects in the root endosphere. Nevertheless, all compartments were dominated by Ensifer, the genus of rhizobia that forms root nodule symbiosis with M. truncatula, and additional shotgun metagenomic sequencing suggests that the nodulating Ensifer were not genetically distinguishable from those elsewhere in the plant. We also identify a handful of OTUs that are common in nodule tissues, which are likely colonized from the root endosphere. CONCLUSIONS: Our results demonstrate strong host filtering effects, with rhizospheres driven by soil origin and internal plant compartments driven by host genetics, and identify several key nodule-inhabiting taxa that coexist with rhizobia in the native range. Our results set the stage for future functional genetic experiments aimed at expanding our pairwise understanding of legume-rhizobium symbiosis toward a more mechanistic understanding of plant microbiomes. Video Abstract.

A molecular framework underlying the compound leaf pattern of Medicago truncatula.

Compound leaves show more complex patterns than simple leaves, and this is mainly because of a specific morphogenetic process (leaflet initiation and arrangement) that occurs during their development. How the relevant morphogenetic activity is established and modulated to form a proper pattern of leaflets is a central question. Here we show that the trifoliate leaf pattern of the model leguminous plant Medicago truncatula is controlled by the BEL1-like homeodomain protein PINNATE-LIKE PENTAFOLIATA1 (PINNA1). We identify PINNA1 as a determinacy factor during leaf morphogenesis that directly represses transcription of the LEAFY (LFY) orthologue SINGLE LEAFLET1 (SGL1), which encodes an indeterminacy factor key to the morphogenetic activity maintenance. PINNA1 functions alone in the terminal leaflet region and synergizes with another determinacy factor, the C2H2 zinc finger protein PALMATE-LIKE PENTAFOLIATA1 (PALM1), in the lateral leaflet regions to define the spatiotemporal expression of SGL1, leading to an elaborate control of morphogenetic activity. This study reveals a framework for trifoliate leaf-pattern formation and sheds light on mechanisms generating diverse leaf forms.

The model legume Medicago truncatula A17 is poorly matched for N2 fixation with the sequenced microsymbiont Sinorhizobium meliloti 1021.

Medicago truncatula (barrel medic) A17 is currently being sequenced as a model legume, complementing the sequenced root nodule bacterial strain Sinorhizobium meliloti 1021 (Sm1021). In this study, the effectiveness of the Sm1021-M. truncatula symbiosis at fixing N(2) was evaluated. N(2) fixation effectiveness was examined with eight Medicago species and three accessions of M. truncatula with Sm1021 and two other Sinorhizobium strains. Plant shoot dry weights, plant nitrogen content and nodule distribution, morphology and number were analysed. Compared with nitrogen-fed controls, Sm1021 was ineffective or partially effective on all hosts tested (excluding M. sativa), as measured by reduced dry weights and shoot N content. Against an effective strain, Sm1021 on M. truncatula accessions produced more nodules, which were small, pale, more widely distributed on the root system and with fewer infected cells. The Sm1021-M. truncatula symbiosis is poorly matched for N(2) fixation and the strain could possess broader N(2) fixation deficiencies. A possible origin for this reduction in effectiveness is discussed. An alternative sequenced strain, effective at N(2) fixation on M. truncatula A17, is Sinorhizobium medicae WSM419.

Functional characterisation of brassinosteroid receptor MtBRI1 in Medicago truncatula.

Brassinosteroids are phytohormones involved in plant development and physiological processes. Brassinosteroids Insensitive 1 (BRI1) is required for BR perception and initiation of subsequent signal transduction in Arabidopsis. In this study, the orthologue of BRI1 in the model legume species Medicago truncatula, MtBRI1, was identified and characterised. Three allelic Tnt1 insertion mutants, mtbri1-1, mtbri1-2, and mtbri1-3, were obtained from the M. truncatula Tnt1 insertion population. mtbri1 mutants displayed characteristic bri1 mutant phenotypes: extreme dwarfness, dark green curled leaves, short primary roots, less lateral roots, and insensitive to exogenous brassinolide (BL). Moreover, mtbri1 mutants show decreased total nodule number and defects in nitrogen fixation. MtBRI1 is able to complement an Arabidopsis BRI1 mutant, bri1-5. Similar to the interaction of BRI1 and BAK1 in Arabidopsis, MtBRI1 interacts with MtSERK1 in vivo. Global gene expression profiling revealed that the expression of BR biosynthesis genes and SAUR genes are significantly altered in mtbri1 mutants. MapMan analysis indicated that genes involved in signaling, hormone, cell wall, and biotic stress responses are over-represented in differentially expressed genes. Taken together, the results indicate that MtBRI1 is the BR receptor in M. truncatula and that BR signaling may play a conserved role in balancing plant growth and defenses.

Mycorrhiza symbiosis increases the surface for sunlight capture in Medicago truncatula for better photosynthetic production.

Arbuscular mycorrhizal (AM) fungi play a prominent role in plant nutrition by supplying mineral nutrients, particularly inorganic phosphate (Pi), and also constitute an important carbon sink. AM stimulates plant growth and development, but the underlying mechanisms are not well understood. In this study, Medicago truncatula plants were grown with Rhizophagus irregularis BEG141 inoculum (AM), mock inoculum (control) or with P(i) fertilization. We hypothesized that AM stimulates plant growth through either modifications of leaf anatomy or photosynthetic activity per leaf area. We investigated whether these effects are shared with P(i) fertilization, and also assessed the relationship between levels of AM colonization and these effects. We found that increased P(i) supply by either mycorrhization or fertilization led to improved shoot growth associated with increased nitrogen uptake and carbon assimilation. Both mycorrhized and P(i)-fertilized plants had more and longer branches with larger and thicker leaves than the control plants, resulting in an increased photosynthetically active area. AM-specific effects were earlier appearance of the first growth axes and increased number of chloroplasts per cell section, since they were not induced by P(i) fertilization. Photosynthetic activity per leaf area remained the same regardless of type of treatment. In conclusion, the increase in growth of mycorrhized and P(i)-fertilized Medicago truncatula plants is linked to an increase in the surface for sunlight capture, hence increasing their photosynthetic production, rather than to an increase in the photosynthetic activity per leaf area.

Medicago truncatula ecotypes A17 and R108 differed in their response to iron deficiency.

Medicago truncatula Gaertn is a model legume species with a wide genetic diversity. To evaluate the responses of the two M. truncatula ecotypes, the effect of Fe deficiency on ecotype A17 and ecotype R108, which have been widely used in physiological and molecular studies, was investigated. A greater reduction in shoot Fe concentration of R108 plants than that of A17 plants was observed under Fe-deficient conditions. Exposure to Fe-deficient medium led to a greater increase in ferric chelate reductase (FCR) activity in roots of A17 than those of R108 plants, while expression of genes encoding FCR in roots of A17 and R108 plants was similarly up-regulated by Fe deficiency. Exposure of A17 plants to Fe-deficient medium evoked an ethylene evolution from roots, while the same treatment had no effect on ethylene evolution from R108 roots. There was a significant increase in expression of MtIRT encoding a Fe transporter in A17, but not in R108 plants, upon exposure to Fe-deficient medium. Transcripts of MtFRD3 that is responsible for loading of iron chelator citrate into xylem were up-regulated by Fe deficiency in A17, but not in R108 plants. These results suggest that M. truncatula ecotypes A17 and R108 differed in their response and adaptation to Fe deficiency, and that ethylene may play an important role in regulation of greater tolerance of A17 plant to Fe deficiency. These findings provide important clues for further elucidation of molecular mechanism by which legume plants respond and adapt to low soil Fe availability.

Seed shape in model legumes: approximation by a cardioid reveals differences in ethylene insensitive mutants of Lotus japonicus and Medicago truncatula.

Seed shape in the model legumes Lotus japonicus and Medicago truncatula is described. Based in previous work with Arabidopsis, the outline of the longitudinal sections of seeds is compared with a cardioid curve. L. japonicus seeds adjust well to an unmodified cardioid, whereas accurate adjustment in M. truncatula is obtained by the simple transformation of scaling the vertical axis by a factor equal to the Golden Ratio. Adjustments of seed shape measurements with simple geometrical forms are essential tools for the statistical analysis of variations in seed shape under different conditions or in mutants. The efficiency of the adjustment to a cardioid in the model plants suggests that seed morphology may be related to genome complexity. Seeds of ethylene insensitive mutants present differences in size and shape as well as altered responses to imbibition. The biological implication and meaning of these relationships are discussed.

Morphological and genetic changes induced by excess Zn in roots of Medicago truncatula A17 and a Zn accumulating mutant.

BACKGROUND: Nutrient fluxes associated with legume-rhizobia symbioses are poorly understood and little is known regarding the influence of abiotic stresses on development and maintenance of N-fixing nodules and root system architecture (RSA). We examined effects of Zn on nodule development and structure, root architecture, and expression of nodulation-related miRNAs in Medicago truncatula and the mutant, raz (requires additional Zn). FINDINGS: Excess Zn increased root and shoot associated Zn in both genotypes, however, raz plants had lower root associated Zn than WT plants. Roots of raz plants exposed to excess Zn had less volume, surface area, and total length compared to WT plants. Raz plants had lower lateral root number than WT plants. Excess Zn was found to increase root diameter in both genotypes. The Mn Translocation Factor (TfMn) increased in response to Zn in both genotypes; this was more pronounced in raz plants. TfZn was higher in raz plants and reduced in both genotypes in response to Zn. Nodulation was not influenced by Zn treatment or plant genotype. MicroRNA166 was upregulated under excess Zn in WT plants. CONCLUSIONS: Neither the raz mutation nor Zn treatment affected nodulation, however, raz plants had altered RSA compared with WT and responded differently to Zn, implying the mutation potentially modulates RSA responses to Zn but doesn't play a direct role in nodulation. MicroRNA166 was significantly induced in WT plants by excess Zn, warranting further investigation into the potential role it plays in controlling RSA.

Identification of quantitative trait loci influencing aerial morphogenesis in the model legume Medicago truncatula.

In many legume crops, especially in forage legumes, aerial morphogenesis defined as growth and development of plant organs, is an essential trait as it determines plant and seed biomass as well as forage quality (protein concentration, dry matter digestibility). Medicago truncatula is a model species for legume crops. A set of 29 accessions of M. truncatula was evaluated for aerial morphogenetic traits. A recombinant inbred lines (RILs) mapping population was used for analysing quantitative variation in aerial morphogenetic traits and QTL detection. Genes described to be involved in aerial morphogenetic traits in other species were mapped to analyse co-location between QTLs and genes. A large variation was found for flowering date, morphology and dynamics of branch elongation among the 29 accessions and within the RILs population. Flowering date was negatively correlated to main stem and branch length. QTLs were detected for all traits, and each QTL explained from 5.2 to 59.2% of the phenotypic variation. A QTL explaining a large part of genetic variation for flowering date and branch growth was found on chromosome 7. The other chromosomes were also involved in the variation detected in several traits. Mapping of candidate genes indicates a co-location between a homologue of Constans gene or a flowering locus T (FT) gene and the QTL of flowering date on chromosome 7. Other candidate genes for several QTLs are described.

Dynamic of the genetic structure of bacterial and fungal communities at different developmental stages of Medicago truncatula Gaertn. cv. Jemalong line J5.

The genetic structure of bacterial and fungal communities was characterized in the rhizosphere of Medicago truncatula Gaertn. cv. Jemalong line J5 at five developmental stages (three vegetative and two reproductive stages), and in three compartments (bulk soil, rhizosphere soil and root tissues). The genetic structure of microbial communities was determined by cultivation-independent methods using directly extracted DNA that was characterized by automated ribosomal intergenic spacer analysis (ARISA). Principal component analyses (PCA) indicate that, for all developmental stages, the genetic structure of microbial communities differed significantly by compartment, with a major shift in the community in root tissues corresponding to the most intimate compartment with the plant. Differences were also recorded during plant development, the most significant being observed during the transition between vegetative and reproductive stages. Throughout this period, plants were shown to establish the highest level of symbiotic association (mycorrhization, nodulation) with arbuscular mycorrhizal fungi and Rhizobia. During the reproductive stages, the dynamics of the genetic structure differed between bacterial and fungal communities. At the last reproductive stage, the genetic structure of bacterial communities became close to that recorded during the first vegetative stages, suggesting a resilience phenomenon, whereas the genetic structure of fungal communities remained different from the vegetative stages and also from the early reproductive stages, suggesting a persistence of the rhizosphere effect.