Legume Model Plant Medicago Truncatula Research Articles

Integrated bioinformatic and physiological analyses reveal the pivotal role of hydrogen sulfide in enhancing low-temperature tolerance in alfalfa.

Hydrogen sulfide (H2 S) is an important gaseous signal molecule that regulates plant growth and stress resistance. However, research on the H2 S synthase (HSase) genes is still limited in the model legume plant Medicago truncatula Gaertn. In the present study, a total of 40 HSase family members were first identified and analyzed in the M. truncatula genome, and these genes distributed across eight chromosomes and were clustered into five groups (I-V) based on their conserved gene structures and protein motifs. Expression analysis revealed that the MtHSase genes were expressed in all the tested abiotic stresses, albeit with expression level differences. This study also showed that H2 S improves low temperature tolerance of alfalfa seedlings by regulating the antioxidant defense system and enhancing photosynthetic capacity. Thus, the study provides new insights into how the H2 S signal regulates tolerance to low-temperature stress and provides the basis for further gene function and detection.

Reorganization of three-dimensional chromatin architecture in Medicago truncatula under phosphorus deficiency.

Emerging evidence reveals that the three-dimensional (3D) chromatin architecture plays a key regulatory role in various biological processes of plants. However, information on the 3D chromatin architecture of the legume model plant Medicago truncatula and its potential roles in the regulation of response to mineral nutrient deficiency are very limited. Using high-resolution chromosome conformation capture sequencing, we identified the 3D genome structure of M. truncatula in terms of A/B compartments, topologically associated domains (TADs) and chromatin loops. The gene density, expressional level, and active histone modification were higher in A compartments than in B compartments. Moreover, we analysed the 3D chromatin architecture reorganization in response to phosphorus (P) deficiency. The intra-chromosomal cis-interaction proportion was increased by P deficiency, and a total of 748 A/B compartment switch regions were detected. In these regions, density changes in H3K4me3 and H3K27ac modifications were associated with expression of P deficiency-responsive genes involved in root system architecture and hormonal responses. Furthermore, these genes enhanced P uptake and mobilization by increasing root surface area and strengthening signal transduction under P deficiency. These findings advance our understanding of the potential roles of 3D chromatin architecture in responses of plants in general, and in particular in M. truncatula, to P deficiency.

Core clock component MtLUX controls shoot architecture through repression of MtTB1/MtTCP1A in Medicago truncatula

Plants are capable of regulating their shoot architecture in response to diverse internal and external environments. The circadian clock is an adaptive mechanism that integrates information from internal and ambient conditions to help plants cope with recurring environmental fluctuations. Despite the current understanding of plant circadian clock and genetic framework underlying plant shoot architecture, the intricate connection between these two adaptive mechanisms remains largely unclear. In this study, we elucidated how the core clock gene LUX ARRHYTHMO (LUX) regulates shoot architecture in the model legume plant Medicago truncatula. We show that mtlux mutant displays increased main stem height, reduced lateral shoot length, and decreased the number of lateral branches and biomass yield. Gene expression analysis revealed that MtLUX regulated shoot architecture by repressing the expression of strigolactone receptor MtD14 and MtTB1/MtTCP1A, a TCP gene that functions centrally in modulating shoot architecture. In vivo and in vitro experiments showed that MtLUX directly binds to a cis-element in the promoter of MtTB1/MtTCP1A, suggesting that MtLUX regulates branching by rhythmically suppressing MtTB1/MtTCP1A. This work demonstrates the regulatory effect of the circadian clock on shoot architecture, offering a new understanding underlying the genetic basis towards the flexibility of plant shoot architecture.

Identification and characterization of long noncoding RNAs involved in the aluminum stress response in Medicago truncatula via genome-wide analysis.

Numerous studies have shown that plant long noncoding RNAs (lncRNAs) play an important regulatory role in the plant response to environmental stress. However, there are no reports on lncRNAs regulating and enhancing aluminum (Al) stress tolerance in legumes. This study analyzed the role of lncRNAs in response to Al stress in the legume model plant Medicago truncatula. A total of 219.49 Gb clean data were generated: 3,284 lncRNA genes were identified, of which 515 were differentially expressed, and 1,254 new genes were functionally annotated through database alignment. We further predicted and classified putative targets of these lncRNAs and found that they were enriched in biological processes and metabolic pathways such as plant hormone signal transduction, cell wall modification and the tricarboxylic acid (TCA) cycle. Finally, we characterized the functions of 2 Al-activated-malate-transporter-related lncRNAs in yeast. The recombinant plasmids of MSTRG.12506.5 and MSTRG.34338.20 were transformed into yeast, and these yeast exhibited better growth than those carrying empty vectors on medium supplemented with 10 μM AlCl3 and showed that they have biological functions affording Al stress tolerance. These findings suggest that lncRNAs are involved in regulating plant responses to Al stress. Our findings help to understand the role of lncRNAs in the response to Al stress in legumes and provide candidate lncRNAs for further studies.

Tolerant mechanism of model legume plant Medicago truncatula to drought, salt, and cold stresses.

Legume plants produce one-third of the total yield of primary crops and are important food sources for both humans and animals worldwide. Frequent exposure to abiotic stresses, such as drought, salt, and cold, greatly limits the production of legume crops. Several morphological, physiological, and molecular studies have been conducted to characterize the response and adaptation mechanism to abiotic stresses. The tolerant mechanisms of the model legume plant Medicago truncatula to abiotic stresses have been extensively studied. Although many potential genes and integrated networks underlying the M. truncatula in responding to abiotic stresses have been identified and described, a comprehensive summary of the tolerant mechanism is lacking. In this review, we provide a comprehensive summary of the adaptive mechanism by which M. truncatula responds to drought, salt, and cold stress. We also discuss future research that need to be explored to improve the abiotic tolerance of legume plants.

Overexpression of MtRAV3 enhances osmotic and salt tolerance and inhibits growth of Medicago truncatula

Related to ABI3/VP1 (RAV) transcription factors play important roles in regulating plant growth and stress tolerance, which have been studied in many plant species, but have remained largely unidentified in legumes. To functionally characterize RAV in legumes, MtRAV3 from legume model plant Medicago truncatula was isolated and its function was investigated by overexpressing MtRAV3 in M. truncatula. Expression analysis demonstrated that MtRAV3 was markedly induced by NaCl and polyethylene glycol (PEG). MtRAV3 overexpression enhanced tolerance of transgenic M. truncatula to mannitol, drought and salt stresses, and induced the expression of adversity-related genes, including MtWRKY76, MtMYB61, cold-acclimation specific protein 31 (MtCAS31), alternative oxidase 1 (MtAOX1) and ethylene response factor 1 (MtERF1). There were lower relative electrolyte leakage and higher chlorophyll content of leaves in the MtRAV3 overexpression plants than in wild type plants under both salt and drought stress. MtRAV3 overexpression M. truncatula were featured by some phenotypes of dwarfing, late flowering, more branches, smaller flower and leaf organs. Further investigations showed that the expression levels of DWARF14 (MtD14), CAROTENOID CLEAVAGE DIOXYGENASES 7 (MtCCD7) and GA3-oxidase1 (MtGA3ox1), which related to dwarf and branch phenotype, were obviously reduced, as well as MtGA3ox1’ (MTR_1g011580), GA20-oxidase1 (MtGA20ox1), MtGA20ox1’ (MTR_1g102070) and GA20-oxidase2 (MtGA20ox2) involved in gibberellins (GAs) pathway. Overall, our results revealed that MtRAV3 exerted an important role in adversity response and plant growth, was a multifunctional gene in M. truncatula, which provided reference for genetic improvement of alfalfa (Medicago sativa).

Sinorhizobium medicae 419 vs S. meliloti 1021: differences in root nodules induced by these two strains on the Medicago truncatula host

Symbiosis between plants of family Fabaceae and bacteria genus Rhizobium is the most widespread interaction, in which atmospheric nitrogen is reduced into ammonia. This highly oxygen-sensitive process is performed by endosymbiotc forms of rhizobia called bacteroids, which occupy newly formed plant organs called root nodules. The goal in this paper was to explore the differences in the (ultra)structure and Reactive Oxygen Species (ROS) localization in two symbiotic interactions between legume model plant Medicago truncatula Gaertn. and bacteria from genus Sinorhizobium. Two bacterial strains—Sinorhizobium meliloti 1021 and Sinorhizobium medicae 419 have the ability of inducing root nodules on the Medicago truncatula, however, such nodules differ in effectiveness of biological nitrogen fixation. We demonstrated that root nodules made of S. medicae 419 [fully effective (F++)] induce a lower number of nodule per plant in comparison to S. meliloti 1021—(partially effective F±). Although the differences in the structure are scant, there are some discrepancies in the ultrastructure. In the meristem of F± nodules, there are Cajal bodies and a lower number of vacuole profiles by larger size in comparison to the F++. Nodules F++ have a thicker interzone and store less starch than F+/, while they have higher esterase activity. Localization of different ROS seems to be comparable in the two type of nodules but interestingly, there is not any Nitric Oxide (NO) in the infection thread of F± root nodules.

Specific tissue proteins 1 and 6 are involved in root biology during normal development and under symbiotic and pathogenic interactions in Medicago truncatula.

ST1 and ST6 are possibly involved in primary and lateral root and symbiotic nodule development, but only ST6 participates in the interaction with hemibiotrophic fungi. Specific tissue (ST) proteins have been shown to be involved in several processes related to plant nutritional status, development, and responses to biotic agents. In particular, ST1 and ST6 are mainly expressed in roots throughout plant development. Here, we analyze where and how the expression of the genes encoding both proteins are modulated in the legume model plant Medicago truncatula in response to the plant developmental program, nodulation induced by a beneficial nitrogen-fixing bacterium (Sinorhizobium meliloti) and the defense response triggered by a pathogenic hemibiotrophic fungus (Fusarium oxysporum). Gene expression results show that ST1 and ST6 participate in the vasculature development of both primary and lateral roots, although only ST6 is related to meristem activity. ST1 and ST6 clearly display different roles in the biotic interactions analyzed, where ST1 is activated in response to a N2-fixing bacterium and ST6 is up-regulated after inoculation with F. oxysporum. The role of ST1 and ST6 in the nodulation process may be related to nodule organogenesis rather than to the establishment of the interaction itself, and an increase in ST6 correlates with the activation of the salicylic acid signaling pathway during the infection and colonization processes. These results further support the role of ST6 in response to hemibiotrophic fungi. This research contributes to the understanding of the complex network that controls root biology and strengthens the idea that ST proteins are involved in several processes such as primary and lateral root development, nodule organogenesis, and the plant-microbe interaction.

Comprehensive identification and characterization of abiotic stress and hormone responsive glycosyl hydrolase family 1 genes in Medicago truncatula

β-glucosidases (BGLUs) hydrolyze the β-D-glycosidic bond with retention of anomeric configuration. BGLUs were associated with many aspects of plant physiological processes, in particular biotic and abiotic stresses through the activation of phytohormones and defense compounds. However, studies on systematic analysis of the stress- or hormone-responsive BGLU genes in plant are still rare. In this study, total 51 BGLU genes of the glycoside hydrolase family 1 were identified in the genome of the model legume plant Medicago truncatula, and they were classified into five distinct clusters. Sequence alignments revealed several conserved and characteristic motifs among these MtBGLU proteins. Analyses of their putative signal peptides and N-glycosylation site suggested that the majority of MtBGLU members have dual targeting to the vacuole and/or chloroplast. Many regulatory elements possibly related with abiotic stresses and phytohormones were identified in MtBGLU genes. Moreover, Microarray and qPCR analyses showed that these MtBGLU genes exhibited distinct expression patterns in various tissues, and in response to different abiotic stress and hormonal treatments. Notably, MtBGLU21, MtBGLU22, MtBGLU28, and MtBGLU30 in cluster I were dramatically activated by NaCl, PEG, IAA, ABA, SA and GA3 treatments. Collectively, our genome-wide characterization, evolutionary analysis, and expression pattern analysis of MtBGLU genes suggested that BGLU genes play crucial roles in response to various abiotic stresses and hormonal cues in M. truncatula.

Cloning and Functional Analysis of Dwarf Gene Mini Plant 1 (MNP1) in Medicago truncatula.

Plant height is a vital agronomic trait that greatly determines crop yields because of the close relationship between plant height and lodging resistance. Legumes play a unique role in the worldwide agriculture; however, little attention has been given to the molecular basis of their height. Here, we characterized the first dwarf mutant mini plant 1 (mnp1) of the model legume plant Medicago truncatula. Our study found that both cell length and the cell number of internodes were reduced in a mnp1 mutant. Using the forward genetic screening and subsequent whole-genome resequencing approach, we cloned the MNP1 gene and found that it encodes a putative copalyl diphosphate synthase (CPS) implicated in the first step of gibberellin (GA) biosynthesis. MNP1 was highly homologous to Pisum sativum LS. The subcellular localization showed that MNP1 was located in the chloroplast. Further analysis indicated that GA3 could significantly restore the plant height of mnp1-1, and expression of MNP1 in a cps1 mutant of Arabidopsis partially rescued its mini-plant phenotype, indicating the conservation function of MNP1 in GA biosynthesis. Our results provide valuable information for understanding the genetic regulation of plant height in M. truncatula.

Transforming compound leaf patterning by manipulating REVOLUTA in Medicago truncatula.

Leaves are derived from the shoot apical meristem with three distinct axes: dorsoventral, proximodistal and mediolateral. Different regulators are involved in the establishment of leaf polarity. Members of the class III homeodomain-leucine zipper (HD-ZIPIII) gene family are critical players in the determination of leaf adaxial identity mediated by microRNA165/166. However, their roles in compound leaf development are still unclear. By screening of a retrotransposon-tagged mutant population of the model legume plant Medicago truncatula, a mutant line with altered leaflet numbers was isolated and characterized. Mutant leaves partially lost their adaxial identity. Leaflet numbers in the mutant were increased along the proximodistal axis, showing pinnate pentafoliate leaves in most cases, in contrast to the trifoliate leaves of the wild type. Detailed characterization revealed that a lesion in a HD-ZIPIII gene, REVOLUTA (MtREV1), resulted in the defects of the mutant. Overexpression of MtMIR166-insensitive MtREV1 led to adaxialized leaves and ectopic leaflets along the dorsoventral axis. Accompanying the abnormal leaf patterning, the free auxin content was affected. Our results demonstrate that MtREV1 plays a key role in determination of leaf adaxial-abaxial polarity and compound leaf patterning, which is associated with proper auxin homeostasis.

Cloning and functional characterization of epidermis-specific promoter MtML1 from Medicago truncatula

Epidermis-specific promoters are necessary for ectopic expression of specific functional genes such as the cuticle-related genes. Previous studies indicated that both ECERIFERUM 6 (AtCER6) and MERISTEM L1 LAYER (ATML1) promoters from Arabidopsis thaliana can drive gene expression specifically in the epidermis of shoot apical meristems (SAMs) and leaves. However, the epidermis-specific promoters from legume plants have not been reported. Here, we cloned a 5′ flanking sequence from the upstream -2150 bp to the translational start ATG codon of MtML1 gene of legume model plant Medicago truncatula. PlantCARE analysis indicated that this sequence matches the characteristics of a promoter, having TATA box and CAAT box, as well as contains some conserved elements of epidermis-specific promoters like AtCER6 and ATML1 promoters. The β-glucuronidase (GUS) histochemical analysis showed that MtML1 promoter can drive GUS gene expression in transiently transformed Nicotiana tabacum leaves under non-inducing condition. Furthermore, it can also control GUS expression in leaves and siliques rather than roots of the stably transformed Arabidopsis. More importantly, the leaf cross-section observations indicated that MtML1 exclusively expressed in the epidermis of leaves. These results suggested that MtML1 promoter performed the epidermis-specific in plant shoot. Our study establishes the foundation for driving the cuticle-related gene to express in epidermis, which may be very useful in genetic engineering of legume plants.

Calmodulin-like gene MtCML40 is involved in salt tolerance by regulating MtHKTs transporters in Medicago truncatula

Calcium (Ca2+) is a universal messenger mediating numerous physiological processes in responses to developmental and environmental cues in plant cells. Calmodulin (CaM) and calmodulin-like proteins (CMLs) are important plant Ca2+ sensors involved in decoding Ca2+ signatures to execute downstream physiological responses. Despite the involvements of CML proteins in the regulation of developmental processes, little is known about the function of CMLs in response to abiotic stresses in plants. To characterize CML proteins, we isolated and functionally characterized a gene encoding a CML protein from legume model plant Medicago truncatula, referred to as MtCML40. The MtCML40 belonged to subgroup VI of CML family. Expression of MtCML40 was up-regulated by salt, cold and osmotic stress as well as ABA treatment, suggesting a role of MtCML40 in abiotic stress. To test this hypothesis, we generated MtCML40 overexpressing transgenic lines in M. truncatula. Overexpression of MtCML40 rendered seed germination more sensitive to salt stress as evidenced by greater inhibition of seed germination of transgenic lines than wild-type seeds when exposed to NaCl, while seed germination of WT and transgenic lines was comparable under control conditions. In addition to seed germination, exposure to salt stress led to greater inhibition of shoot and root growth, reduction in chlorophyll and carotenoid concentrations and photosynthetic rates in the transgenic lines than WT plants, suggesting a negative regulation of salt tolerance by MtCML40. The greater accumulation of Na+ in shoots of transgenic lines may account for the greater sensitivity to salt stress. We further found that overexpression of MtCML40 resulted in down-regulation of MtHKT1;1 and MtHKT1;2 that encoded proteins associated with removal of Na+ from shoots. Taken together, our results demonstrate that MtCML40 is involved in the regulation of salt tolerance by targeting MtHKT-dependent Na+ accumulation in M. truncatula.

The Impact of Simulated Microgravity on the Growth of Different Genotypes of the Model Legume Plant Medicago truncatula

Simulated microgravity has been a useful tool to help understand plant development in altered gravity conditions. Thirty-one genotypes of the legume plant Medicago truncatula were grown in either simulated microgravity on a rotating clinostat, or in a static, vertical environment. Twenty morphological features were measured and compared between these two gravity treatments. Within-species genotypic variation was a significant predictor of the phenotypic response to gravity treatment in 100% of the measured morphological and growth features. In addition, there was a genotype–environment interaction (G × E) for 45% of the response variables, including shoot relative growth rate (p < 0.0005), median number of roots (p ∼ 0.02), and root dry mass (p < 0.005). Our studies demonstrate that genotype does play a significant role in M. truncatula morphology and affects the response of plants to the gravity treatment, influencing both the magnitude and direction of the gravity response. These findings are discussed in the context of improving future studies in plant space biology by controlling for genotypic differences. Thus, manipulation of genotype effects, in combination with M. truncatula’s symbiotic relationships with bacteria and fungi, will be important for optimizing legumes for cultivation on long-term space missions.

Crystal structure of thermospermine synthase from Medicago truncatula and substrate discriminatory features of plant aminopropyltransferases.

Polyamines are linear polycationic compounds that play a crucial role in the growth and development of higher plants. One triamine (spermidine, SPD) and two tetraamine isomers (spermine, SPM, and thermospermine, TSPM) are obtained by the transfer of the aminopropyl group from decarboxylated S-adenosylmethionine to putrescine and SPD. These reactions are catalyzed by the specialized aminopropyltransferases. In that respect, plants are unique eukaryotes that have independently evolved two enzymes, thermospermine synthase (TSPS), encoded by the gene ACAULIS5, and spermine synthase, which produce TSPM and SPM, respectively. In this work, we structurally characterize the ACAULIS5 gene product, TSPS, from the model legume plant Medicago truncatula (Mt). Six crystal structures of MtTSPS - one without ligands and five in complexes with either reaction substrate (SPD), reaction product (TSPM), or one of three cofactor analogs (5'-methylthioadenosine, S-adenosylthiopropylamine, and adenosine) - give detailed insights into the biosynthesis of TSPM. Combined with small-angle X-ray scattering data, the crystal structures show that MtTSPS is a symmetric homotetramer with an interdomain eight-stranded β-barrel. Such an assembly and the presence of a hinge-like feature between N-terminal and C-terminal domains give the protein additional flexibility which potentially improves loading substrates and discarding products after the catalytic event. We also discuss the sequence and structural features around the active site of the plant aminopropyltransferases that distinguish them from each other and determine their characteristic substrate discrimination.

Novel phosphate deficiency-responsive long non-coding RNAs in the legume model plant Medicago truncatula.

Emerging evidence indicates that long non-coding RNAs (lncRNAs) play important roles in the regulation of many biological processes. Inhibition of plant growth due to deficiency in soil inorganic phosphate (Pi) occurs widely across natural and agricultural ecosystems; however, we know little about the function of plant lncRNAs in response to Pi deficiency. To address this issue, we first identified 10 785 lncRNAs in the legume model species Medicago truncatula by sequencing eight strand-specific libraries. Out of these lncRNAs, 358 and 224 were responsive to Pi deficiency in the leaves and roots, respectively. We further predicted and classified the putative targets of those lncRNAs and the results revealed that they may be involved in the processes of signal transduction, energy synthesis, detoxification, and Pi transport. Finally, we functionally characterized three Phosphate Deficiency-Induced LncRNAs (PDILs) using their corresponding Tnt1 mutants. The results showed that PDIL1 suppressed degradation of MtPHO2, which encodes a ubiquitin-conjugating E2 enzyme regulated by miR399, while PDIL2 and PDIL3 directly regulated Pi transport at the transcriptional level. These findings demonstrate that PDILs can regulate Pi-deficiency signaling and Pi transport, highlighting the involvement of lncRNAs in the regulation of responses of plants to Pi deficiency.

Epigenomic diversification within the genus Lupinus.

Deciphering the various chemical modifications of both DNA and the histone compound of chromatin not only leads to a better understanding of the genome-wide organisation of epigenetic landmarks and their impact on gene expression but may also provide some insights into the evolutionary processes. Although both histone modifications and DNA methylation have been widely investigated in various plant genomes, here we present the first study for the genus Lupinus. Lupins, which are members of grain legumes (pulses), are beneficial for food security, nutrition, health and the environment. In order to gain a better understanding of the epigenetic organisation of genomes in lupins we applied the immunostaining of methylated histone H3 and DNA methylation as well as whole-genome bisulfite sequencing. We revealed variations in the patterns of chromatin modifications at the chromosomal level among three crop lupins, i.e. L. angustifolius (2n = 40), L. albus (2n = 50) and L. luteus (2n = 52), and the legume model plant Medicago truncatula (2n = 16). Different chromosomal patterns were found depending on the specific modification, e.g. H3K4me2 was localised in the terminal parts of L. angustifolius and M. truncatula chromosomes, which is in agreement with the results that have been obtained for other species. Interestingly, in L. albus and L. luteus this modification was limited to one arm in the case of all of the chromosomes in the complement. Additionally, H3K9me2 was detected in all of the analysed species except L. luteus. DNA methylation sequencing (CG, CHG and CHH contexts) of aforementioned crop but also wild lupins such as L. cosentinii (2n = 32), L. digitatus (2n = 36), L. micranthus (2n = 52) and L. pilosus (2n = 42) supported the range of interspecific diversity. The examples of epigenetic modifications illustrate the diversity of lupin genomes and could be helpful for elucidating further epigenetic changes in the evolution of the lupin genome.

Assessment of the function and expression pattern of auxin response factor B3 in the model legume plant Medicago truncatula

The phytohormone auxin is a critical signal molecule, regulating fundamental processes in plant growth and development, such as shaping the root and shoot architecture, organ patterning, and nodulation. Auxin regulates plant gene expression mainly through auxin response factors (ARFs), which bind to auxin response elements in the promoter, upstream of auxin-activated genes. Here we examine and assess the function and expression pattern of a gene described as an auxin response factor, containing a DNA-binding pseudobarrel and B3 DNA-binding domains, from Medicago truncatula (MtARF-B3). For the model legume species M. truncatula, stable transgenic plants with MtARF-B3 overexpression, downregulation, and transcriptional reporters were constructed. Phenotypic and morphological evaluation of the obtained transgenic plants confirmed the important role of MtARF-B3 in general plant growth and development, modeling of root architecture, and development of seeds. Detailed histochemical and transcriptional analysis revealed expression of the gene in various stages of somatic embryogenesis, during formation of plant organs and tissues, and symbiotic nodulation. The fact that MtARF-B3 was strongly expressed in stamens and pollen grains in M. truncatula suggests that this gene could play a role in the fertility of this model legume.

Sodium extrusion associated with enhanced expression of SOS1 underlies different salt tolerance between Medicago falcata and Medicago truncatula seedlings

Medicago falcata L. is a perennial legume species native to semiarid grasslands in northern China, and distinguished by its superior tolerance to abiotic stresses such as drought and cold. But there have been few studies to investigate the physiological mechanism underlying salt tolerance in M. falcata. In the present study, we compared the response of non-nodulated M. falcata with legume model plant Medicago truncatula Garetn at physiological and molecular level by submitting them to a salt stock. M. falcata exhibited higher survival rate and less chlorosis than M. truncatula seedlings upon exposure to salt stress, suggesting that M. falcata is more tolerant to salt stress than M. truncatula. The ability to maintain a high K+ and low Na+ contents by plants is an important trait for tolerance to salt stress. M. falcata accumulated less amount of Na+ in their shoots than M. truncatula when exposed to NaCl, leading to a higher K+/(K++Na+) ratio in M. falcata than in M. truncatula under saline conditions. There was a similar difference in Na+ concentration and K+/(K++Na+) ratio in roots and xylem sap between the two species under saline conditions. Results from grafting experiments revealed that seedlings grafted with M. falcata either as scions or rootstocks can significantly reduce Na+ accumulation and increase K+/(K++Na+) ratio in both shoots and roots under saline conditions. Expression levels of SOS1 that encodes a Na+–H+ antiporter responsible for Na+ extrusion from cells in M. falcata were much higher than in M. truncatula under both control and saline conditions. Abundance of SOS1 transcripts in seedlings grafted with M. falcata as either rootstocks or scions was significantly higher than in seedlings self-grafted with M. truncatula. These results strongly suggested that SOS1-dependent Na+ extrusion is an important mechanism to maintain a relatively low Na+ concentration and high K+/(K++Na+) ratio in non-nodulated M. falcata under saline conditions, thus conferring tolerance to salt stress.

Genome variations account for different response to three mineral elements between Medicago truncatula ecotypes Jemalong A17 and R108.

BackgroundResequencing can be used to identify genome variations underpinning many morphological and physiological phenotypes. Legume model plant Medicago truncatula ecotypes Jemalong A17 (J. A17) and R108 differ in their responses to mineral toxicity of aluminum and sodium, and mineral deficiency of iron in growth medium. The difference may result from their genome variations, but no experimental evidence supports this hypothesis.ResultsA total of 12,750 structure variations, 135,045 short insertions/deletions and 764,154 single nucleotide polymorphisms were identified by resequencing the genome of R108. The suppressed expression of MtAACT that encodes a putative aluminum-induced citrate efflux transporter by deletion of partial sequence of the second intron may account for the less aluminum-induced citrate exudation and greater accumulation of aluminum in roots of R108 than in roots of J. A17, thus rendering R108 more sensitive to aluminum toxicity. The higher expression-level of MtZpt2-1 encoding a TFIIIA-related transcription factor in J. A17 than R108 under conditions of salt stress can be explained by the greater number of stress-responsive elements in its promoter sequence, thus conferring J. A17 more tolerant to salt stress than R108 plants by activating the expression of downstream stress-responsive genes. YSLs (Yellow Stripe-Likes) are involved in long-distance transport of iron in plants. We found that an YSL gene was deleted in the genome of R108 plants, thus rendering R108 less tolerance to iron deficiency than J. A17 plants.ConclusionsThe deletion or change in several genes may account for the different responses of M. truncatula ecotypes J. A17 and R108 to mineral toxicity of aluminum and sodium as well as iron deficiency. Uncovering genome variations by resequencing is an effective method to identify different traits between species/ecotypes that are genetically related. These findings demonstrate that analyses of genome variations by resequencing can shed important light on differences in responses of M. truncatula ecotypes to abiotic stress in general and mineral stress in particular.