The symbiotic transcription factor MtEFD and cytokinins are positively acting in the Medicago truncatula and Ralstonia solanacearum pathogenic interaction

• A plant-microbe dual biological system was set up involving the model legume Medicago truncatula and two bacteria, the soil-borne root pathogen Ralstonia solanacearum and the beneficial symbiont Sinorhizobium meliloti. • Comparison of transcriptomes under symbiotic and pathogenic conditions highlighted the transcription factor MtEFD (Ethylene response Factor required for nodule Differentiation) as being upregulated in both interactions, together with a set of cytokinin-related transcripts involved in metabolism, signaling and response. MtRR4 (Response Regulator), a cytokinin primary response gene negatively regulating cytokinin signaling and known as a target of MtEFD in nodulation processes, was retrieved in this set of transcripts. • Refined studies of MtEFD and MtRR4 expression during M. truncatula and R. solanacearum interaction indicated differential kinetics of induction and requirement of central regulators of bacterial pathogenicity, HrpG and HrpB. Similar to MtRR4, MtEFD upregulation during the pathogenic interaction was dependent on cytokinin perception mediated by the MtCRE1 (Cytokinin REsponse 1) receptor. • The use of M. truncatula efd-1 and cre1-1 mutants evidenced MtEFD and cytokinin perception as positive factors for bacterial wilt development. These factors therefore play an important role in both root nodulation and root disease development.

Lateral root formation and patterning in Medicago truncatula

The plant root system is crucial for anchorage and nutrition, and has a major role in plant adaptation, as well as in interactions with soil micro-organisms. Despite the agronomical and ecological importance of legume plants, whose roots can interact symbiotically with soil bacteria called rhizobia that fix atmospheric dinitrogen, and the evidence that lateral root (LR) development programmes are intercepted and influenced by symbiotic organisms, very little is known concerning the cellular and molecular events governing LR development in legumes. To better understand the interconnections between LR formation and symbiotic processes triggered by rhizobia or symbiotic molecules such as lipo-chitooligosaccharides (LCOs), we first need a detailed description of LR development mechanisms in legumes. Using thin sections, we have described the cellular events leading to the formation of a new LR primordium (LRP) in Medicago truncatula, and divided them into seven stages prior to LR emergence. To monitor auxin accumulation we generated transgenic DR5:GUS and DR5:VENUS-N7 reporter lines of M. truncatula, and used them to analyze early stages of LR development. Interesting differences were observed for LR ontogeny compared to Arabidopsis thaliana. Notably, we observed endodermal and cortical contributions to LRP formation, and the associated DR5:GUS expression profile indicated that endodermal and cortical cell divisions were correlated with auxin accumulation. As described for A. thaliana, we observed a preferential zone for LR initiation at 4.45 mm from the root tip. Finally, we studied LR emergence and showed that a significant proportion of new LRP do not emerge straight away and could thus be an additional source of root plasticity. Our results shed new light on the patterning and early development of LRs in M. truncatula.

STENOFOLIA recruits TOPLESS to repress ASYMMETRIC LEAVES2 at the leaf margin and promote leaf blade outgrowth in Medicago truncatula

The Medicago truncatula WUSCHEL-related homeobox (WOX) gene, STENOFOLIA (STF), plays a key role in leaf blade outgrowth by promoting cell proliferation at the adaxial-abaxial junction. STF functions primarily as a transcriptional repressor, but the underlying molecular mechanism is unknown. Here, we report the identification of a protein interaction partner and a direct target, shedding light on the mechanism of STF function. Two highly conserved motifs in the C-terminal domain of STF, the WUSCHEL (WUS) box and the STF box, cooperatively recruit TOPLESS (Mt-TPL) family corepressors, and this recruitment is required for STF function, as deletion of these two domains (STFdel) impaired blade outgrowth whereas fusing Mt-TPL to STFdel restored function. The homeodomain motif is required for direct repression of ASYMMETRIC LEAVES2 (Mt-AS2), silencing of which partially rescues the stf mutant phenotype. STF and LAMINALESS1 (LAM1) are functional orthologs. A single amino acid (Asn to Ile) substitution in the homeodomain abolished the repression of Mt-AS2 and STF's ability to complement the lam1 mutant of Nicotiana sylvestris. Our data together support a model in which STF recruits corepressors to transcriptionally repress its targets during leaf blade morphogenesis. We propose that recruitment of TPL/TPL-related proteins may be a common mechanism in the repressive function of modern/WUS clade WOX genes.

Systemic regulation of sulfur homeostasis in Medicago truncatula

Sulfur (S) is an essential macronutrient for plants, and deficiency in soil S availability limits plant growth. Adaptive strategies have been evolved by plants to respond to S deficiency by coordinating systemic regulatory mechanism. A split-root experiment using legume model plant Medicago truncatula Gaertn. was conducted to investigate the systemic response to S deficiency. Plant growth, root morphology and S contents under varying conditions of S supply were determined, and the expression of genes encoding sulfate transporter (MtSULTRs) and MtAPR1 encoding an enzyme involved in S assimilation was monitored. Our results demonstrated that there was an apparent systemic response of M. truncatula to heterogeneous S supply in terms of root length, S contents, and S uptake and assimilation at the transcriptional level. When exposed to heterogeneous S supply, M. truncatula plants showed proliferation of lateral roots in S-rich medium and reduction in investment to S-depleted roots. Growth was stimulated with half-part of roots exposed to S-deficient medium. There were different expression patterns of MtSULTRs and MtAPR1 in response to heterogeneous S supply both in roots and shoots of M. truncatula. Expression of MtSULTR1.1 and MtSULTR1.3 was systemically responsive to S deficiency, leading to an enhancement of S uptake in roots exposed to S-sufficient medium. In addition, the response of S-deprived seedlings to re-supply of sulfate and Cys was also analyzed. It was shown that sulfate, but not Cys, may serve as a systemic signal to regulate the expression of genes associated with S absorption and assimilation in M. truncatula. These findings provide a comprehensive picture of systemic responses to S deficiency in leguminous species.

Elevated CO2 decreases the response of the ethylene signaling pathway in Medicago truncatula and increases the abundance of the pea aphid

The performance of herbivorous insects is greatly affected by plant nutritional quality and resistance, which are likely to be altered by rising concentrations of atmospheric CO2 . We previously reported that elevated CO2 enhanced biological nitrogen (N) fixation of Medicago truncatula, which could result in an increased supply of amino acids to the pea aphid (Acyrthosiphon pisum). The current study examined the N nutritional quality and aphid resistance of sickle, an ethylene-insensitive mutant of M. truncatula with supernodulation, and its wild-type control A17 under elevated CO2 in open-top field chambers. Regardless of CO2 concentration, growth and amino acid content were greater and aphid resistance was lower in sickle than in A17. Elevated CO2 up-regulated N assimilation and transamination-related enzymes activities and increased phloem amino acids in both genotypes. Furthermore, elevated CO2 down-regulated expression of 1-amino-cyclopropane-carboxylic acid (ACC), sickle gene (SKL) and ethylene response transcription factors (ERF) genes in the ethylene signaling pathway of A17 when infested by aphids and decreased resistance against aphids in terms of lower activities of superoxide dismutase (SOD), peroxidase (POD), and polyphenol oxidase (PPO). Our results suggest that elevated CO2 suppresses the ethylene signaling pathway in M. truncatula, which results in an increase in plant nutritional quality for aphids and a decrease in plant resistance against aphids.

Uncertain role of MtSEO-F3 in assembly of Medicago truncatula forisomes

Forisomes are specialized multimeric protein complexes found only in the papilionoid legumes. They undergo a reversible conformational change in response to phloem injury to enable the occlusion of sieve tubes, thus preventing the loss of photoassimilates. The individual subunits are designated by the letters SEO-F (sieve element occlusion by forisomes) and are part of the larger SEO protein family, which also includes the typical P-proteins found in most dicots and some monocots. When specific SEO-F subunits from different species are expressed in a heterologous background, they self-assemble into fully-functional artificial forisomes. However, with the exception of basal species such as Dipteryx panamensis, the geometry of these artificial forisomes differs from that of their native counterparts. Studies involving SEO-F proteins from the model legume Medicago truncatula have shown that a combination of 3 of the 4 subunits can fine-tune the geometry of artificial forisomes. However, MtSEO-F3 was excluded from these studies because it was not incorporated into either the native or artificial forisomes in our original experiments. In this addendum, we present further data concerning the interactive properties of the SEO-F proteins and confirm that all 4 MtSEO-F proteins interact in all possible pairwise combinations. These data indicate that the exclusion of MtSEO-F3 from the compact forisome may reflect the steric hindrance of binding sites rather than an inability to interact with other forisome subunits.

Early transcriptional responses to mercury: a role for ethylene in mercury-induced stress

Understanding the cellular mechanisms of plant tolerance to mercury (Hg) is important for developing phytoremediation strategies of Hg-contaminated soils. The early responses of alfalfa (Medicago sativa) seedlings to Hg were studied using transcriptomics analysis. A Medicago truncatula microarray was hybridized with high-quality root RNA from M. sativa treated with 3 μM Hg for 3, 6 and 24 h. The transcriptional pattern data were complementary to the measurements of root growth inhibition, lipid peroxidation, hydrogen peroxide (H2 O2 ) accumulation and NADPH-oxidase activity as stress indexes. Of 559 differentially expressed genes (DEGs), 91% were up-regulated. The majority of DEGs were shared between the 3 and 6 h (60%) time points, including the 'stress', 'secondary metabolism' and 'hormone metabolism' functional categories. Genes from ethylene metabolism and signalling were highly represented, suggesting that this phytohormone may be relevant for metal perception and homeostasis. Ethylene-insensitive alfalfa seedlings preincubated with the ethylene signalling inhibitor 1-methylcyclopronene and Arabidopsis thaliana ein2-5 mutants confirmed that ethylene participates in the early perception of Hg stress. It modulates root growth inhibition, NADPH-oxidase activity and Hg-induced apoplastic H2 O2 accumulation. Therefore, ethylene signalling attenuation could be useful in future phytotechnological applications to ameliorate stress symptoms in Hg-polluted plants.

DELLA proteins regulate arbuscule formation in arbuscular mycorrhizal symbiosis

Most flowering plants are able to form endosymbioses with arbuscular mycorrhizal fungi. In this mutualistic association, the fungus colonizes the root cortex and establishes elaborately branched hyphae, called arbuscules, within the cortical cells. Arbuscule development requires the cellular reorganization of both symbionts, and the resulting symbiotic interface functions in nutrient exchange. A plant symbiosis signaling pathway controls the development of the symbiosis. Several components of the pathway have been identified, but transcriptional regulators that control downstream pathways for arbuscule formation are still unknown. Here we show that DELLA proteins, which are repressors of gibberellic acid (GA) signaling and function at the nexus of several signaling pathways, are required for arbuscule formation. Arbuscule formation is severely impaired in a Medicago truncatula Mtdella1/Mtdella2 double mutant; GA treatment of wild-type roots phenocopies the della double mutant, and a dominant DELLA protein (della1-Δ18) enables arbuscule formation in the presence of GA. Ectopic expression of della1-Δ18 suggests that DELLA activity in the vascular tissue and endodermis is sufficient to enable arbuscule formation in the inner cortical cells. In addition, expression of della1-Δ18 restores arbuscule formation in the symbiosis signaling pathway mutant cyclops/ipd3, indicating an intersection between DELLA and symbiosis signaling for arbuscule formation. GA signaling also influences arbuscule formation in monocots, and a Green Revolution wheat variety carrying dominant DELLA alleles shows enhanced colonization but a limited growth response to arbuscular mycorrhizal symbiosis.

Fate of polyfluoroalkyl phosphate diesters and their metabolites in biosolids-applied soil: biodegradation and plant uptake in greenhouse and field experiments

Significant contamination of perfluoroalkyl acids (PFAAs) in wastewater treatment plant (WWTP) sludge implicates the practice of applying treated sludge or biosolids as a potential source of these chemicals onto agricultural farmlands. Recent efforts to characterize the sources of PFAAs in the environment have unveiled a number of fluorotelomer-based materials that are capable of degrading to the perfluoroalkyl carboxylates (PFCAs), such as the polyfluoroalkyl phosphate diesters (diPAPs), which have been detected in WWTP and paper fiber biosolids. Here, a greenhouse microcosm was used to investigate the fate of endogenous diPAPs and PFCAs present in WWTP and paper fiber biosolids upon amendment of these materials with soil that had been sown with Medicago truncatula plants. Biodegradation pathways and plant uptake were further elucidated in a separate greenhouse microcosm supplemented with high concentrations of 6:2 diPAP. Biosolid-amended soil exhibited increased concentrations of diPAPs (4-83 ng/g dry weight (dw)) and PFCAs (0.1-19 ng/g dw), as compared to control soils (nd-1.4 ng/g dw). Both plant uptake and biotransformation contributed to the observed decline in diPAP soil concentrations over time. Biotransformation was further evidenced by the degradation of 6:2 diPAP to its corresponding fluorotelomer intermediates and C4-C7 PFCAs. Substantial plant accumulation of endogenous PFCAs present in the biosolids (0.1-138 ng/g wet weight (ww)) and those produced from 6:2 diPAP degradation (100-58 000 ng/g ww) were observed within 1.5 months of application, with the congener profile dominated by the short-chain PFCAs (C4-C6). This pattern was corroborated by the inverse relationship observed between the plant-soil accumulation factor (PSAF, Cplant/Csoil) and carbon chain length (p < 0.05, r = 0.90-0.97). These results were complemented by a field study in which the fate of diPAPs and PFCAs was investigated upon application of compost and paper fiber biosolids to two farm fields. Together, these studies provide the first evidence of soil biodegradation of diPAPs and the subsequent uptake of these chemicals and their metabolites into plants.

Elevated CO(2) modifies N acquisition of Medicago truncatula by enhancing N fixation and reducing nitrate uptake from soil

The effects of elevated CO2 (750 ppm vs. 390 ppm) were evaluated on nitrogen (N) acquisition and assimilation by three Medicago truncatula genotypes, including two N-fixing-deficient mutants (dnf1-1 and dnf1-2) and their wild-type (Jemalong). The proportion of N acquisition from atmosphere and soil were quantified by (15)N stable isotope, and N transportation and assimilation-related genes and enzymes were determined by qPCR and biochemical analysis. Elevated CO2 decreased nitrate uptake from soil in all three plant genotypes by down-regulating nitrate reductase (NR), nitrate transporter NRT1.1 and NR activity. Jemalong plant, however, produced more nodules, up-regulated N-fixation-related genes and enhanced percentage of N derived from fixation (%Ndf) to increase foliar N concentration and N content in whole plant (Ntotal Yield) to satisfy the requirement of larger biomass under elevated CO2. In contrast, both dnf1 mutants deficient in N fixation consequently decreased activity of glutamine synthetase/glutamate synthase (GS/GOGAT) and N concentration under elevated CO2. Our results suggest that elevated CO2 is likely to modify N acquisition of M. truncatula by simultaneously increasing N fixation and reducing nitrate uptake from soil. We propose that elevated CO2 causes legumes to rely more on N fixation than on N uptake from soil to satisfy N requirements.

The landscape of cytokinin binding by a plant nodulin

Nodulation is an extraordinary symbiotic interaction between leguminous plants and nitrogen-fixing bacteria (rhizobia) that assimilate atmospheric nitrogen (in root nodules) and convert it into compounds suitable for the plant host. A class of plant hormones called cytokinins are involved in the nodulation process. In the model legume Medicago truncatula, nodulin 13 (MtN13), which belongs to the pathogenesis-related proteins of class 10 (PR-10), is expressed in the outer cortex of the nodules. In general, PR-10 proteins are small and monomeric and have a characteristic fold with an internal hydrophobic cavity formed between a seven-stranded antiparallel β-sheet and a C-terminal α-helix. Previously, some PR-10 proteins not related to nodulation were found to bind cytokinins such as trans-zeatin. Here, four crystal structures of the MtN13 protein are reported in complexes with several cytokinins, namely trans-zeatin, N6-isopentenyladenine, kinetin and N6-benzyladenine. All four phytohormones are bound in the hydrophobic cavity in the same manner and have excellent definition in the electron-density maps. The binding of the cytokinins appears to be strong and specific and is reinforced by several hydrogen bonds. Although the binding stoichiometry is 1:1, the complex is actually dimeric, with a cytokinin molecule bound in each subunit. The ligand-binding site in each cavity is formed with the participation of a loop element from the other subunit, which plugs the only entrance to the cavity. Interestingly, a homodimer of MtN13 is also formed in solution, as confirmed by small-angle X-ray scattering (SAXS).

Inhibition of nitrogen fixation in symbiotic Medicago truncatula upon Cd exposure is a local process involving leghaemoglobin

Leguminous biological nitrogen fixation (BNF) is very sensitive to environmental fluctuations. It is still contentious how BNF is regulated under stress conditions. The local or systemic control of BNF and the role played by reactive oxygen species (ROS) in such regulation have still not been elucidated completely. Cadmium, which belongs to the so-called heavy metals, is one of the most toxic substances released into the environment. The mechanisms involved in Cd toxicity are still not completely understood but the overproduction of ROS is one of its characteristic symptoms. In this work, we used a split-root system approach to study nodule BNF and the antioxidant machinery's response to the application of a mild Cd treatment on one side of a nodulated Medicago truncatula root system. Cd induced the majority of nodule antioxidants without generating any oxidative damage. Cd treatment also provoked BNF inhibition exclusively in nodules directly exposed to Cd, without provoking any effect on plant shoot biomass or chlorophyll content. The overall data suggest that the decline in BNF was not due to a generalized breakdown of the plant but to control exerted through leghaemoglobin/oxygen availability, affecting nitrogenase function.

Calcium/Calmodulin-dependent protein kinase is negatively and positively regulated by calcium, providing a mechanism for decoding calcium responses during symbiosis signaling

The establishment of symbiotic associations in plants requires calcium oscillations that must be decoded to invoke downstream developmental programs. In animal systems, comparable calcium oscillations are decoded by calmodulin (CaM)-dependent protein kinases, but symbiotic signaling involves a calcium/CaM-dependent protein kinase (CCaMK) that is unique to plants. CCaMK differs from the animal CaM kinases by its dual ability to bind free calcium, via calcium binding EF-hand domains on the protein, or to bind calcium complexed with CaM, via a CaM binding domain. In this study, we dissect this dual regulation of CCaMK by calcium. We find that calcium binding to the EF-hand domains promotes autophosphorylation, which negatively regulates CCaMK by stabilizing the inactive state of the protein. By contrast, calcium-dependent CaM binding overrides the effects of autophosphorylation and activates the protein. The differential calcium binding affinities of the EF-hand domains compared with those of CaM suggest that CCaMK is maintained in the inactive state at basal calcium concentrations and is activated via CaM binding during calcium oscillations. This work provides a model for decoding calcium oscillations that uses differential calcium binding affinities to create a robust molecular switch that is responsive to calcium concentrations associated with both the basal state and with oscillations.

Structural and functional characterization of annexin 1 from Medicago truncatula

Annexins are calcium- and membrane-binding proteins that have been shown to have diverse properties such as actin, integrin and GTP binding, both in animals and plants. Recently, Medicago truncatula annexin 1 (AnnMt1) has been suggested to participate in nodulation (Nod factor signaling) and mycorrhization in legume plants. In this report we demonstrate for the first time that recombinant AnnMt1 (rec-AnnMt1) mediates membrane permeabilization to cations with conductance ranging from 16 pS to 329 pS. In agreement with other structurally determined annexins, homology modeling of AnnMt1 suggests that most of the functional determinants are found on the convex surface of the modeled structure. In conclusion, we propose a potential constitutive role of AnnMt1 in Nod factor signaling as a non-specific ion channel.

Contrasting calcium localization and speciation in leaves of the Medicago truncatula mutant cod5 analyzed via synchrotron X-ray techniques

Oxalate-producing plants accumulate calcium oxalate crystals (CaOx(c)) in the range of 3-80% w/w of their dry weight, reducing calcium (Ca) bioavailability. The calcium oxalate deficient 5 (cod5) mutant of Medicago truncatula has been previously shown to contain similar Ca concentrations to wild-type (WT) plants, but lower oxalate and CaOx(c) concentrations. We imaged the Ca distribution in WT and cod5 leaflets via synchrotron X-ray fluorescence mapping (SXRF). We observed a difference in the Ca distribution between cod5 and WT leaflets, manifested as an abundance of Ca in the interveinal areas and a lack of Ca along the secondary veins in cod5, i.e. the opposite of what is observed in WT. X-ray microdiffraction (μXRD) of M. truncatula leaves confirmed that crystalline CaOx(c) (whewellite; CaC2 O4 · H2 O) was present in the WT only, in cells sheathing the secondary veins. Together with μXRD, microbeam Ca K-edge X-ray absorption near-edge structure spectroscopy (μXANES) indicated that, among the forms of CaOx, i.e. crystalline or amorphous, only amorphous CaOx was present in cod5. These results demonstrate that deletion of COD5 changes both Ca localization and the form of CaOx within leaflets.

Host-specific Nod-factors associated with Medicago truncatula nodule infection differentially induce calcium influx and calcium spiking in root hairs

Rhizobial nodulation (Nod) factors activate both nodule morphogenesis and infection thread development during legume nodulation. Nod factors induce two different calcium responses: intra-nuclear calcium oscillations and a calcium influx at the root hair tip. Calcium oscillations activate nodule development; we wanted to test if the calcium influx is associated with infection. Sinorhizobium meliloti nodL and nodF mutations additively reduce infection of Medicago truncatula. Nod-factors made by the nodL mutant lack an acetyl group; mutation of nodF causes the nitrogen (N)-linked C16:2 acyl chain to be replaced by C18:1. We tested whether these Nod-factors differentially induced calcium influx and calcium spiking. The absence of the NodL-determined acetyl group greatly reduced the induction of calcium influx without affecting calcium spiking. The calcium influx was even further reduced if the N-linked C16:2 acyl group was replaced by C18:1. These additive effects on calcium influx correlate with the additive effects of mutations in nodF and nodL on legume infection. Infection thread development is inhibited by ethylene, which also inhibited Nod-factor-induced calcium influx. We conclude that Nod-factor perception differentially activates the two developmental pathways required for nodulation and that activation of the pathway involving the calcium influx is important for efficient infection.

The identification of novel loci required for appropriate nodule development in Medicago truncatula

BACKGROUND

The formation of functional symbiotic nodules is the result of a coordinated developmental program between legumes and rhizobial bacteria. Genetic analyses in legumes have been used to dissect the signaling processes required for establishing the legume-rhizobial endosymbiotic association. Compared to the early events of the symbiotic interaction, less attention has been paid to plant loci required for rhizobial colonization and the functioning of the nodule. Here we describe the identification and characterization of a number of new genetic loci in Medicago truncatula that are required for the development of effective nitrogen fixing nodules.

RESULTS

Approximately 38,000 EMS and fast neutron mutagenized Medicago truncatula seedlings were screened for defects in symbiotic nitrogen fixation. Mutant plants impaired in nodule development and efficient nitrogen fixation were selected for further genetic and phenotypic analysis. Nine mutants completely lacking in nodule formation (Nod-) represented six complementation groups of which two novel loci have been identified. Eight mutants with ineffective nodules (Fix-) represented seven complementation groups, out of which five were new monogenic loci. The Fix- M. truncatula mutants showed symptoms of nitrogen deficiency and developed small white nodules. Microscopic analysis of Fix- nodules revealed that the mutants have defects in the release of rhizobia from infection threads, differentiation of rhizobia and maintenance of persistence of bacteria in nodule cells. Additionally, we monitored the transcriptional activity of symbiosis specific genes to define what transcriptional stage of the symbiotic process is blocked in each of the Fix- mutants. Based on the phenotypic and gene expression analysis a functional hierarchy of the FIX genes is proposed.

CONCLUSIONS

The new symbiotic loci of M. truncatula isolated in this study provide the foundation for further characterization of the mechanisms underpinning nodulation, in particular the later stages associated with bacterial release and nodule function.

Sl-IAA27 gene expression is induced during arbuscular mycorrhizal symbiosis in tomato and in Medicago truncatula

Aux/IAA genes play a pivotal role in auxin transcriptional regulation. Their functions were mainly studied in Arabidopsis through analysis of gain-of-function mutants. In the tomato, the Solanaceae reference species, different studies on Sl-IAA down-regulated lines showed specific role for Sl-IAA genes. Our recent work revealed that the Sl-IAA 27 gene displays a distinct behavior compared with most Aux/IAA genes, being down-regulated by auxin. Interestingly, the silencing of Sl-IAA27 leads to altered chlorophyll accumulation in leaves, reduced fertilization, altered fruit development and altered root formation. Here we report that IAA27 could be a key auxin signaling gene involved in AM in tomato and also in Medicago model plant. Indeed both Sl-IAA27 and its closest homolog in Medicago truncatula, Mt-IAA27, are overexpressed in mycorrhized roots. These data are in line with the putative role of auxin in arbuscular mycorrhization.

Nod factor perception protein carries weight in biotic interactions

Plant plasma membrane-bound receptors with extracellular lysin motif (LysM) domains participate in interactions with microorganisms. In Medicago truncatula, the LysM receptor-like kinase gene nodulation (Nod) factor perception (NFP) is a key gene that controls the perception of rhizobial lipochitooligosaccharide (LCO) Nod factors for the establishment of the Rhizobium-legume symbiosis. In this article, we review recent data that have refined our understanding of this function and that have revealed a role for NFP in the perception of arbuscular mycorrhizal (AM) symbiotic signals and plant pathogenic microorganisms. The dual role of NFP in symbiosis and immunity suggests that this receptor protein controls the perception of different signals and the activation of different downstream signalling pathways. These advances provide new insights into the evolution and functioning of this versatile plant protein.

Control of NO level in rhizobium-legume root nodules: not only a plant globin story

Nitric oxide (NO ) is a gaseous signaling molecule which plays both regulatory and defense roles in animals and plants. In the symbiosis between legumes and rhizobia, NO has been shown to be involved in bacterial infection and nodule development steps as well as in mature nodule functioning. We recently showed that an increase in NO level inside Medicago truncatula root nodules also could trigger premature nodule senescence. Here we discuss the importance of the bacterial Sinorhizobium meliloti flavohemoglobin to finely tune the NO level inside nodules and further, we demonstrate that S. meliloti possesses at least two non redundant ways to control NO and that both systems are necessary to maintain efficient nitrogen fixing activity.

miR393 and miR164 influence indeterminate but not determinate nodule development

The roles of auxin in the regulation of symbiotic legume nodule formation are unclear. We recently showed that enhanced sensitivity to auxin resulting from overexpression of miR160 inhibits determinate nodule formation in soybean. We examined the roles of miR393 and miR164 in soybean (that forms determinate nodules) and Medicago truncatula (that forms indeterminate nodules). Our results together with previous studies suggest that indeterminate nodule formation requires a higher, but narrow window of auxin sensitivity and that miR164 regulation is not crucial for determinate nodule formation.

Insight into the role of anthocyanin biosynthesis-related genes in Medicago truncatula mutants impaired in pigmentation in leaves

Flavonoids are the most common antioxidant compounds produced in plants. In this study, two wild types and two independent mutants of Medicago truncatula with altered anthocyanin content in leaves were characterized at the phenotype, metabolite profile, gene structure and transcript levels. Flavonoid profiles showed conserved levels of dihydroflavonols, leucoanthocyanidins and flavonols, while anthocyanidin, anthocyanin and isoflavone levels were lower in the mutants (up to 90% less) compared with the wild types. Genes encoding key enzymes of the anthocyanin pathway and transcriptional factors were analyzed by RT-PCR. Genes involved in the later steps of the anthocyanin pathway (dihydrokaempferol reductase 2, UDP-glucose:anthocyanin 3-O-glucosyltransferase and glutathione S-transferase) were found under-expressed in both mutants. Dihydrokaempferol reductase 1 was downregulated two-fold in the anthocyanin-less mutant while the UDP-glucose:anthocyanin 5-O-glucosyltransferase was strongly repressed only in the mutant with low pigmentation, suggesting a different regulation in the two genotypes. The common feature was that the first enzymes of the flavonoid biosynthesis pathway were not altered in rate of expression. A very high reduction in transcript accumulation was also found for two homologous R2R3 MYB genes, namely MtMYBA and AN2, suggesting that these genes have a role in anthocyanin accumulation in leaves. More evidence was found on analyzing their nucleotide sequence: several SNPs, insertions and deletions in the coding and non-coding regions of both MYB genes were found between mutants and wild types that could influence anthocyanin biosynthesis. Moreover, a subfamily of eight MYB genes with a high homology to MtMYBA was discovered in tandem on chromosome 5 of M. truncatula.

Medicago truncatula esn1 defines a genetic locus involved in nodule senescence and symbiotic nitrogen fixation

Symbiotic interaction between Medicago truncatula and Sinorhizobium meliloti results in the formation on the host roots of new organs, nodules, in which biological nitrogen fixation takes place. In infected cells, rhizobia enclosed in a plant-derived membrane, the symbiosome membrane, differentiate to nitrogen-fixing bacteroids. The symbiosome membrane serves as an interface for metabolite and signal exchanges between the host cells and endosymbionts. At some point during symbiosis, symbiosomes and symbiotic cells are disintegrated, resulting in nodule senescence. The regulatory mechanisms that underlie nodule senescence are not fully understood. Using a forward genetics approach, we have uncovered the early senescent nodule 1 (esn1) mutant from an M. truncatula fast neutron-induced mutant collection. Nodules on esn1 roots are spherically shaped, ineffective in nitrogen fixation, and senesce early. Atypical among fixation defective mutants isolated thus far, bacteroid differentiation and expression of nifH, Leghemoglobin, and DNF1 genes are not affected in esn1 nodules, supporting the idea that a process downstream of bacteroid differentiation and nitrogenase gene expression is affected in the esn1 mutant. Expression analysis shows that marker genes involved in senescence, macronutrient degradation, and remobilization are greatly upregulated during nodule development in the esn1 mutant, consistent with a role of ESN1 in nodule senescence and symbiotic nitrogen fixation.

MALDI mass spectrometry-assisted molecular imaging of metabolites during nitrogen fixation in the Medicago truncatula-Sinorhizobium meliloti symbiosis

Symbiotic associations between leguminous plants and nitrogen-fixing rhizobia culminate in the formation of specialized organs called root nodules, in which the rhizobia fix atmospheric nitrogen and transfer it to the plant. Efficient biological nitrogen fixation depends on metabolites produced by and exchanged between both partners. The Medicago truncatula-Sinorhizobium meliloti association is an excellent model for dissecting this nitrogen-fixing symbiosis because of the availability of genetic information for both symbiotic partners. Here, we employed a powerful imaging technique - matrix-assisted laser desorption/ionization (MALDI)/mass spectrometric imaging (MSI) - to study metabolite distribution in roots and root nodules of M. truncatula during nitrogen fixation. The combination of an efficient, novel MALDI matrix [1,8-bis(dimethyl-amino) naphthalene, DMAN] with a conventional matrix 2,5-dihydroxybenzoic acid (DHB) allowed detection of a large array of organic acids, amino acids, sugars, lipids, flavonoids and their conjugates with improved coverage. Ion density maps of representative metabolites are presented and correlated with the nitrogen fixation process. We demonstrate differences in metabolite distribution between roots and nodules, and also between fixing and non-fixing nodules produced by plant and bacterial mutants. Our study highlights the benefits of using MSI for detecting differences in metabolite distributions in plant biology.

Medicago glucosyltransferase UGT72L1: potential roles in proanthocyanidin biosynthesis

In the first reaction specific for proanthocyanidin (PA) biosynthesis in Arabidopsis thaliana and Medicago truncatula, anthocyanidin reductase (ANR) converts cyanidin to (-)-epicatechin. The glucosyltransferase UGT72L1 catalyzes formation of epicatechin 3'-O-glucoside (E3'OG), the preferred substrate for MATE transporters implicated in PA biosynthesis in both species. The mechanism of PA polymerization is still unclear, but may involve the laccase-like polyphenol oxidase TRANSPARENT TESTA 10 (TT10). We have employed a combination of cell biological, biochemical and genetic approaches to evaluate this PA pathway model. The promoter regions of UGT72L1 and MtANR share common cis-acting elements and direct overlapping, but partially distinct, expression patterns. UGT72L1 and MtANR are localized in the cytosol, whereas TT10 is localized to the vacuole. Over-expression of UGT72L1 in M. truncatula hairy roots results in increased accumulation of PA-like compounds, and loss of function of UGT72L1 partially reduces epicatechin, E3'OG and extractable PA levels in M. truncatula seeds. Expression of UGT72L1 in A. thaliana leads to a massive increase in E3'OG in immature seed, but reduced levels of extractable PAs. However, when UGT72L1 was expressed in the Arabidopsis tt10 mutant, extractable PA levels increased and seed coat browning was delayed. Our results suggest that glycosylation of epicatechin is important for both PA precursor transport and assembly, but that additional redundant pathways may exist.