Infection Threads Research Articles

Genotype Delimitation in the Nod-Independent Model Legume Aeschynomene evenia

Research on the nitrogen-fixing symbiosis has been so far focused on two model legumes, Medicago truncatula and Lotus japonicus, which use a sophisticated infection process involving infection thread formation. However, in 25% of the legumes, the bacterial entry occurs more simply in an intercellular fashion. Among them, some semi-aquatic Aeschynomene species present the distinctive feature to form nitrogen-fixing nodules on both roots and stems following elicitation by photosynthetic bradyrhizobia that do not produce Nod factors. This interaction is believed to represent a living testimony of the ancestral state of the rhizobium-legume symbiosis. To decipher the molecular mechanisms of this unique Nod-independent nitrogen-fixing symbiosis, we previously identified A. evenia C. Wright as an appropriate model legume, because it displays all the requisites for molecular and genetic approaches. To advance the use of this new model legume species, here we characterized the intraspecific diversity found in A. evenia. For this, the accessions available in germplasm banks were collected and subjected to morphological investigations, genotyping with RAPD and SSR markers, molecular phylogenies using ITS and single nuclear gene sequences, and cross-compatibility tests. These combined analyses revealed an important intraspecific differentiation that led us to propose a new taxonomic classification for A. evenia comprising two subspecies and four varieties. The A. evenia ssp. evenia contains var. evenia and var. pauciciliata whereas A. evenia ssp. serrulata comprises var. serrulata and var. major. This study provides information to exploit efficiently the diversity encountered in A. evenia and proposes subsp. evenia as the most appropriate subspecies for future projects aimed at identifying plant determinants of the Nod-independent symbiotic process.

Microtubules and biotic interactions

Plant microtubules undergo extensive reorganization in response to symbiotic and pathogenic organisms. During the development of successful symbioses with rhizobia and mycorrhizal fungi, novel microtubule arrays facilitate the progression of infection threads and hyphae, respectively, from the plant surface through epidermal and cortical cells. During viral and nematode infections, plant microtubules appear to be commandeered by the pathogen. Viruses use plant microtubules for intra and intercellular movement, as well as for interhost transmission. Nematodes manipulate spindle and phragmoplast microtubules to enhance mitosis and partial cytokinesis during the development of syncytia and giant cells. Pathogenic bacteria, fungi and oomycetes induce a range of alterations to microtubule arrays and dynamics. In many situations, the pathogen, or the elicitor or effector proteins derived from them, induce depolymerization of plant cortical microtubule arrays. In some cases, microtubule disruption is associated with the plant defence response and resistance. In other cases, microtubule depolymerization increases plant susceptibility to the invading pathogen. The reasons for this apparent inconsistency may depend on a number of factors, in particular on the identity of the organism orchestrating the microtubule changes. Overall, the weight of evidence indicates that microtubules play an important role in both the establishment of functional symbioses and in defence against invading pathogens. Research is beginning to unravel details about the nature of both the chemical and the mechanical signals to which the plant microtubule arrays respond during biotic interactions.

Role of IAA metabolizing enzymes on production of IAA in root, nodule of Cajanus cajan and its PGP Rhizobium sp

Abstract Indole acetic acid (IAA) is a phytohormone which plays a central role in plant growth and development as a regulator of numerous biological processes. The metabolism of IAA in nodules and its symbiont is not fully elucidated. The present work was undertaken to shed some light on IAA metabolism in Cajanus cajan roots, nodules, the Rhizobium symbiont and its possible role in Rhizobia–Legume interaction. The mature root nodules of C. cajan (L) were found to contain more IAA than non nodulated root. A tryptophan pull was present in the mature nodules and young roots might have served as a precursor for IAA production. The levels of tryptophan content were also higher in nodule than root. Comparative study of IAA metabolizing enzymes, such as IAA oxidase IAA peroxidase (EC 1.11.1.7) and polyphenol oxidase (E.C. 1.14.18.1) indicated that they might have some significant role in metabolism of IAA in roots and nodules. A symbiont designated as P2 was isolated from fresh nodules of C. cajan (L) and was identified as Rhizobium sp. by phenotypic and 16S rDNA sequence analysis. The strain produced significant amount of IAA (142.5 µg/ml) under in vitro condition when l -tryptophan (0.2%) was added to the medium as precursor and also possess ACC deaminase (E.C. 4.1.99.4) activity that might be helpful for successful entry of infection thread. The presence of significant amount of IAA catabolising enzyme in nodule, root and ACC deaminase activity of its symbiont might have physiological importance in plant-microbes interaction of symbiotic phenomenon and plant growth promotion.

A novel GRAS protein gene MtSymSCL1 plays a role in regulating nodule number in Medicago truncatula

Legume-rhizobium symbiosis requires the expression of a series of plant genes playing roles in nodulation signaling that leads to the formation of functional nodules. A few plant-specific GRAS proteins have been shown to be involved in early nodulation signaling. Here, we isolated a novel SCARECROW-like GRAS protein gene, MtSymSCL1, from the model legume Medicago truncatula. MtSymSCL1 has unique gene and protein domain structures, and its homologs are identified from various plant species. Phylogenetic analysis showed that MtSymSCL1 could be classified into a novel subclass in the GRAS protein family. MtSymSCL1 transcript accumulated highly in nodulated roots at 14 days after inoculation of Sinorhizobium meliloti. Translationally fused MtSymSCL1-GFP expressed from a viral or the native promoter was co-localized with nuclear localization signal-fused RFP in leaf mesophyll protoplasts, Agrobacterium rhizogenes-transformed hairy root cells, and epidermal cells of nodules that developed on the transformed roots. Unlike SCARECROW of Arabidopsis, however, RNA silencing of MtsymSCL1 did not affect root growth, nor root hair curling and infection thread growth. Nevertheless, MtSymSCL1-silenced roots developed significantly fewer nodules than GUS-silenced roots, as shown by replicated nodulation assay with pot- or agar plate-grown plants. The structure of MtSymSCL1-silenced root nodules appeared similar to that of the wild type, as shown by sectioned nodules stained with green fluorescent dye. Together, these results suggest that MtSymSCL1 plays a role in regulating nodule number during legume-rhizobium symbiosis.

Rhizobial Infection Is Associated with the Development of Peripheral Vasculature in Nodules ofMedicago truncatula

Nodulation in legumes involves the coordination of epidermal infection by rhizobia with cell divisions in the underlying cortex. During nodulation, rhizobia are entrapped within curled root hairs to form an infection pocket. Transcellular tubes called infection threads then develop from the pocket and become colonized by rhizobia. The infection thread grows toward the developing nodule primordia and rhizobia are taken up into the nodule cells, where they eventually fix nitrogen. The epidermal and cortical developmental programs are synchronized by a yet-to-be-identified signal that is transmitted from the outer to the inner cell layers of the root. Using a new allele of the Medicago truncatula mutant Lumpy Infections, lin-4, which forms normal infection pockets but cannot initiate infection threads, we show that infection thread initiation is required for normal nodule development. lin-4 forms nodules with centrally located vascular bundles similar to that found in lateral roots rather than the peripheral vasculature characteristic of legume nodules. The same phenomenon was observed in M. truncatula plants inoculated with the Sinorhizobium meliloti exoY mutant, and the M. truncatula vapyrin-2 mutant, all cases where infections arrest. Nodules on lin-4 have reduced expression of the nodule meristem marker MtCRE1 and do not express root-tip markers. In addition, these mutant nodules have altered patterns of gene expression for the cytokinin and auxin markers CRE1 and DR5. Our work highlights the coordinating role that bacterial infection exerts on the developing nodule and allows us to draw comparisons with primitive actinorhizal nodules and rhizobia-induced nodules on the nonlegume Parasponia andersonii.

Adaptive and progressive evolution of plant-microbe symbiosis

In N2-fixing symbionts of leguminous plants (rhizobia) evolution of the host-beneficial (“altruistic”) traits occurs in populations colonizing the subcellular compartments in nodules (infection threads, symbiosomes). These compartments are developed as a result of partners’ coevolution related to complications of trophic and regulatory interactions elevating the ecological efficiency of symbiosis. Their analysis enables us to study correlations between genetic mechanisms of adaptive and progressive symbiosis evolution which remain obscure in free-living organisms

Conditional Requirement for Exopolysaccharide in the Mesorhizobium–Lotus Symbiosis

Rhizobial surface polysaccharides are required for nodule formation on the roots of at least some legumes but the mechanisms by which they act are yet to be determined. As a first step to investigate the function of exopolysaccharide (EPS) in the formation of determinate nodules, we isolated Mesorhizobium loti mutants affected in various steps of EPS biosynthesis and characterized their symbiotic phenotypes on two Lotus spp. The wild-type M. loti R7A produced both high molecular weight EPS and lower molecular weight (LMW) polysaccharide fractions whereas most mutant strains produced only LMW fractions. Mutants affected in predicted early biosynthetic steps (e.g., exoB) formed nitrogen-fixing nodules on Lotus corniculatus and L. japonicus 'Gifu', whereas mutants affected in mid or late biosynthetic steps (e.g., exoU) induced uninfected nodule primordia and, occasionally, a few infected nodules following a lengthy delay. These mutants were disrupted at the stage of infection thread (IT) development. Symbiotically defective EPS and Nod factor mutants functionally complemented each other in co-inoculation experiments. The majority of full-length IT observed harbored only the EPS mutant strain and did not show bacterial release, whereas the nitrogen-fixing nodules contained both mutants. Examination of the symbiotic proficiency of the exoU mutant on various L. japonicus ecotypes revealed that both host and environmental factors were linked to the requirement for EPS. These results reveal a complex function for M. loti EPS in determinate nodule formation and suggest that EPS plays a signaling role at the stages of both IT initiation and bacterial release.

ANALYSIS OF INTERACTION OF PEA (Pisum sativum L.) SYMBIOTIC GENES Sym33 AND Sym42 WHOSE MUTATIONS RESULT IN ABNORMALITES DURING INFECTION THREAD DEVELOPMENT

Using pea single mutant lines SGEFix–-2 (sym33) and RisFixV (sym42), which are characterized by different abnormalities during symbiotic nodule development, including thickening of infection threads’ walls, a double mutant RBT4 line, carrying a pair of symbiotic genes sym33 and sym42 was constructed. The epistasis of the mutant allele sym33 over the mutant allele sym42 with respect to the histological and ultrastructural organisation of nodules was shown. Thus, it was demonstrated that Sym33 gene functions earlier in symbiotic nodule development than Sym42 gene.

Structural changes in Medicago truncatula root nodules caused by short-term aluminum stress

Aluminum in the form of Al3+ is one of the most toxic heavy metal pollutants in nature and its effects are primarily root-related. Roots of Medicago truncatula exposed to 50 μM of AlCl3 for 2 h and 24 h were examined by light and electron microscopy. Changes in the appearance of the host cells, infection threads and bacteroidal tissue occurred during the first 2 h of Al stress. Microscopic observations showed that aluminum: (1) induced thickening of plant cell and infection threads (ITs) walls, (2) stimulated IT enlargement, (3) caused disturbances in bacterial release from the ITs, (4) modified cell vacuolation and induced synthesis of granular material and its deposition in the cytoplasm, (5) and caused structural alterations of organella and bacteroids.

Evolution of symbiotic bacteria within the extra- and intra-cellular plant compartments: experimental evidence and mathematical simulation (Mini-review)

Using the example of nodular legume-rhizobia symbiosis (LRS), we discuss the evolution in plant micro-symbionts of mutualistic traits that are apparently host-beneficial and therefore the products of inter-species evolution. These traits include: in planta activation of N2 fixation machinery; exporting the products of nitrogenase reaction into the plant cells/tissues; and the terminal differentiation of bacteria into non-reproductive N2-fixing bacteroids. It seems probable that such adaptive traits evolved by natural selection within the populations of endosymbiotic bacteria that colonize the extra- and intra-cellular compartments provided by the hosts (i.e., infection threads and symbiosomes). This evolution would occur under the impacts of group (inter-deme, kin) selection pressures induced by the partners’ metabolic and regulatory feedbacks that ensure the high activity of symbiotic N2 fixation. These important feedbacks include: progressive allocation of C compounds into N2-fixing nodules; maintenance of micro-aerobic intracellular environments that are indispensable for intensive N2 fixation; and stringent control by the host over bacterial reproduction in planta. A computational simulation of the associated co-evolutionary processes reveals the trade-off between inter-species and individual species components of progressive and adaptive LRS evolution. This is expressed as a correlated increase of ecological efficiency, functional integrity and genotypic specificity of mutualistic symbiosis. Thus, the evolution of rhizobia in symbiosis may be represented by a progressive multi-level scenario based on increasing the dependency of bacteria on the host-provided nutrients accompanied by increasing complexity of the bacterial genomes and of the symbiosis-encoding gene networks.

Medicago truncatula ERN Transcription Factors: Regulatory Interplay with NSP1/NSP2 GRAS Factors and Expression Dynamics throughout Rhizobial Infection

Rhizobial nodulation factors (NFs) activate a specific signaling pathway in Medicago truncatula root hairs that involves the complex interplay of Nodulation Signaling Pathway1 (NSP1)/NSP2 GRAS and Ethylene Response Factor Required for Nodulation1 (ERN1) transcription factors (TFs) to achieve full ENOD11 transcription. ERN1 acts as a direct transcriptional regulator of ENOD11 through the activation of the NF-responsive "NF box." Here, we show that NSP1, when combined with NSP2, can act as a strong positive regulator of ERN1 and ENOD11 transcription. Although ERN1 and NSP1/NSP2 both activate ENOD11, two separate promoter regions are involved that regulate expression during consecutive symbiotic stages. Our findings indicate that ERN1 is required to activate NF-elicited ENOD11 expression exclusively during early preinfection, while NSP1/NSP2 mediates ENOD11 expression during subsequent rhizobial infection. The relative contributions of ERN1 and the closely related ERN2 to the rhizobial symbiosis were then evaluated by comparing their regulation and in vivo dynamics. ERN1 and ERN2 exhibit expression profiles compatible with roles during NF signaling and subsequent infection. However, differences in expression levels and spatiotemporal profiles suggest specialized functions for these two TFs, ERN1 being involved in stages preceding and accompanying infection thread progression while ERN2 is only involved in certain stages of infection. By cross complementation, we show that ERN2, when expressed under the control of the ERN1 promoter, can restore both NF-elicited ENOD11 expression and nodule formation in an ern1 mutant background. This indicates that ERN1 and ERN2 possess similar biological activities and that functional diversification of these closely related TFs relies primarily on changes in tissue-specific expression patterns.

Identifying abnormalities in symbiotic development between Trifolium spp. and Rhizobium leguminosarum bv. trifolii leading to sub-optimal and ineffective nodule phenotypes.

Legumes overcome nitrogen limitations by entering into a mutualistic symbiosis with N(2)-fixing bacteria (rhizobia). Fully compatible associations (effective) between Trifolium spp. and Rhizobium leguminosarum bv. trifolii result from successful recognition of symbiotic partners in the rhizosphere, root hair infection and the formation of nodules where N(2)-fixing bacteroids reside. Poorly compatible associations can result in root nodule formation with minimal (sub-optimal) or no (ineffective) N(2)-fixation. Despite the abundance and persistence of strains in agricultural soils which are poorly compatible with the commercially grown clover species, little is known of how and why they fail symbiotically. The aims of this research were to determine the morphological aberrations occurring in sub-optimal and ineffective clover nodules and to determine whether reduced bacteroid numbers or reduced N(2)-fixing activity is the main cause for the Sub-optimal phenotype. Symbiotic effectiveness of four Trifolium hosts with each of four R. leguminosarum bv. trifolii strains was assessed by analysis of plant yields and nitrogen content; nodule yields, abundance, morphology and internal structure; and bacteroid cytology, quantity and activity. Effective nodules (Nodule Function 83-100 %) contained four developmental zones and N(2)-fixing bacteroids. In contrast, Sub-optimal nodules of the same age (Nodule Function 24-57 %) carried prematurely senescing bacteroids and a small bacteroid pool resulting in reduced shoot N. Ineffective-differentiated nodules carried bacteroids aborted at stage 2 or 3 in differentiation. In contrast, bacteroids were not observed in Ineffective-vegetative nodules despite the presence of bacteria within infection threads. Three major responses to N(2)-fixation incompatibility between Trifolium spp. and R. l. trifolii strains were found: failed bacterial endocytosis from infection threads into plant cortical cells, bacteroid differentiation aborted prematurely, and a reduced pool of functional bacteroids which underwent premature senescence. We discuss possible underlying genetic causes of these developmental abnormalities and consider impacts on N(2)-fixation of clovers.

Polymorphic infection and organogenesis patterns induced by a Rhizobium leguminosarum isolate from Lotus root nodules are determined by the host genotype

To sample the natural variation in genes controlling compatibility in the legume-rhizobium symbiosis, we isolated rhizobia from nodules of endemic Lotus species from 21 sites across Europe. The majority of isolates were identified as Mesorhizobium- or Bradyrhizobium-related and formed nitrogen-fixing root nodules on Lotus corniculatus and L.pendunculatus, respectively, thus confirming previously defined cross-inoculation groups. Rhizobium leguminosarum (Rl) strain Norway, isolated from L.corniculatus nodules, displayed an exceptional phenotypic variation on different Lotus genotypes. On L.burttii, Rl Norway formed infected nodules, whereas tumors and elongated infected swellings were induced on L.glaber and L.japonicus ecotype Nepal, respectively. A symbiosis- and Nod-factor-responsive promoter:uidA fusion was strongly and rapidly induced in L.japonicus Gifu, but infection threads or signs of nodule organogenesis were absent. This complex phenotypic pattern was not mimicked by either of three engineered R.leguminosarum bv viciae strains producing different Nod-factor variants. Intriguingly, Rl Norway formed infection threads on Pisum sativum cv Sparkle, but failed to induce organogenesis. Rl Norway thus uncovered variation in symbiotic capabilities among diploid Lotus species and ecotypes that are obscured by optimally adapted M.loti strains. These contrasting infection and organogenesis phenotypes reveal recent diversification of recognition determinants in Lotus.

A Phaseolus vulgaris NADPH Oxidase Gene is Required for Root Infection by Rhizobia

Plant NADPH oxidases [respiratory burst oxidase homologs (RBOHs)] have emerged as key players in the regulation of plant-pathogen interactions. Nonetheless, their role in mutualistic associations, such as the rhizobia-legume symbiosis, is poorly understood. In this work, nine members of the Phaseolus vulgaris Rboh gene family were identified. The transcript of one of these, PvRbohB, accumulated abundantly in shoots, roots and nodules. PvRbohB promoter activity was detected in meristematic regions of P. vulgaris roots, as well as during infection thread (IT) progression and nodule development. RNA interference (RNAi)-mediated PvRbohB down-regulation in transgenic roots reduced reactive oxygen species (ROS) production and lateral root density, and greatly impaired nodulation. Microscopy analysis revealed that progression of the ITs was impeded at the base of root hairs in PvRbohB-RNAi roots. Furthermore, the few nodules that formed in PvRbohB-down-regulated roots displayed abnormally wide ITs and reduced nitrogen fixation. These findings indicate that this common bean NADPH oxidase is crucial for successful rhizobial colonization and probably maintains proper IT growth and shape.

Epidermal and cortical roles ofNFPandDMI3in coordinating early steps of nodulation inMedicago truncatula

Legumes have evolved the capacity to form a root nodule symbiosis with soil bacteria called rhizobia. The establishment of this symbiosis involves specific developmental events occurring both in the root epidermis (notably bacterial entry) and at a distance in the underlying root cortical cells (notably cell divisions leading to nodule organogenesis). The processes of bacterial entry and nodule organogenesis are tightly linked and both depend on rhizobial production of lipo-chitooligosaccharide molecules called Nod factors. However, how these events are coordinated remains poorly understood. Here, we have addressed the roles of two key symbiotic genes of Medicago truncatula, the lysin motif (LysM) domain-receptor like kinase gene NFP and the calcium- and calmodulin-dependent protein kinase gene DMI3, in the control of both nodule organogenesis and bacterial entry. By complementing mutant plants with corresponding genes expressed either in the epidermis or in the cortex, we have shown that epidermal DMI3, but not NFP, is sufficient for infection thread formation in root hairs. Epidermal NFP is sufficient to induce cortical cell divisions leading to nodule primordia formation, whereas DMI3 is required in both cell layers for these processes. Our results therefore suggest that a signal, produced in the epidermis under the control of NFP and DMI3, is responsible for activating DMI3 in the cortex to trigger nodule organogenesis. We integrate these data to propose a new model for epidermal/cortical crosstalk during early steps of nodulation.

Lotus japonicus ARPC1 Is Required for Rhizobial Infection

Remodeling of the plant cell cytoskeleton precedes symbiotic entry of nitrogen-fixing bacteria within the host plant roots. Here we identify a Lotus japonicus gene encoding a predicted ACTIN-RELATED PROTEIN COMPONENT1 (ARPC1) as essential for rhizobial infection but not for arbuscular mycorrhiza symbiosis. In other organisms ARPC1 constitutes a subunit of the ARP2/3 complex, the major nucleator of Y-branched actin filaments. The L. japonicus arpc1 mutant showed a distorted trichome phenotype and was defective in epidermal infection thread formation, producing mostly empty nodules. A few partially colonized nodules that did form in arpc1 contained abnormal infections. Together with previously described L. japonicus Nck-associated protein1 and 121F-specific p53 inducible RNA mutants, which are also impaired in the accommodation of rhizobia, our data indicate that ARPC1 and, by inference a suppressor of cAMP receptor/WASP-family verpolin homologous protein-ARP2/3 pathway, must have been coopted during evolution of nitrogen-fixing symbiosis to specifically mediate bacterial entry.

A Ubiquitin Ligase of Symbiosis Receptor Kinase Involved in Nodule Organogenesis

The symbiosis receptor kinase (SymRK) is required for morphological changes of legume root hairs triggered by rhizobial infection. How protein turnover of SymRK is regulated and how the nodulation factor signals are transduced downstream of SymRK are not known. In this report, a SymRK-interacting E3 ubiquitin ligase (SIE3) was shown to bind and ubiquitinate SymRK. The SIE3-SymRK interaction and the ubiquitination of SymRK were shown to occur in vitro and in planta. SIE3 represents a new class of plant-specific E3 ligases that contain a unique pattern of the conserved CTLH (for C-terminal to LisH), CRA (for CT11-RanBPM), and RING (for Really Interesting New Gene) domains. Expression of SIE3 was detected in all tested tissues of Lotus japonicus plants, and its transcript level in roots was enhanced by rhizobial infection. The SIE3 protein was localized to multiple subcellular locations including the nuclei and plasma membrane, where the SIE3-SymRK interaction took place. Overexpression of SIE3 promoted nodulation in transgenic hairy roots, whereas downregulation of SIE3 transcripts by RNA interference inhibited infection thread development and nodule organogenesis. These results suggest that SIE3 represents a new class of E3 ubiquitin ligase, acts as a regulator of SymRK, and is involved in rhizobial infection and nodulation in L. japonicus.

Aeschynomene evenia, a Model Plant for Studying the Molecular Genetics of the Nod-Independent Rhizobium-Legume Symbiosis

Research on the nitrogen-fixing symbiosis has been focused, thus far, on two model legumes, Medicago truncatula and Lotus japonicus, which use a sophisticated infection process involving infection thread formation. However, in 25% of the legumes, the bacterial entry occurs more simply in an intercellular fashion. Among them, some Aeschynomene spp. are nodulated by photosynthetic Bradyrhizobium spp. that do not produce Nod factors. This interaction is believed to represent a living testimony of the ancestral state of the rhizobium-legume symbiosis. To decipher the mechanisms of this Nod-independent process, we propose Aeschynomene evenia as a model legume because it presents all the characteristics required for genetic and molecular analysis. It is a short-perennial and autogamous species, with a diploid and relatively small genome (2n=20; 460 Mb/1C). A. evenia 'IRFL6945' is nodulated by the well-characterized photosynthetic Bradyrhizobium sp. strain ORS278 and is efficiently transformed by Agrobacterium rhizogenes. Aeschynomene evenia is genetically homozygous but polymorphic accessions were found. A manual hybridization procedure has been set up, allowing directed crosses. Therefore, it should be relatively straightforward to unravel the molecular determinants of the Nod-independent process in A. evenia. This should shed new light on the evolution of rhizobium-legume symbiosis and could have important agronomic implications.

Increased Production of the Exopolysaccharide Succinoglycan Enhances Sinorhizobium meliloti 1021 Symbiosis with the Host Plant Medicago truncatula

The nitrogen-fixing rhizobial symbiont Sinorhizobium meliloti 1021 produces acidic symbiotic exopolysaccharides that enable it to initiate and maintain infection thread formation on host legume plants. The exopolysaccharide that is most efficient in mediating this process is succinoglycan (exopolysaccharide I [EPSI]), a polysaccharide composed of octasaccharide repeating units of 1 galactose and 7 glucose residues, modified with succinyl, acetyl, and pyruvyl substituents. Previous studies had shown that S. meliloti 1021 mutants that produce increased levels of succinoglycan, such as exoR mutants, are defective in symbiosis with host plants, leading to the hypothesis that high levels of succinoglycan production might be detrimental to symbiotic development. This study demonstrates that increased succinoglycan production itself is not detrimental to symbiotic development and, in fact, enhances the symbiotic productivity of S. meliloti 1021 with the host plant Medicago truncatula cv. Jemalong A17. Increased succinoglycan production was engineered by overexpression of the exoY gene, which encodes the enzyme responsible for the first step in succinoglycan biosynthesis. These results suggest that the level of symbiotic exopolysaccharide produced by a rhizobial species is one of the factors involved in optimizing the interaction with plant hosts.

SymRK and the nodule vascular system

Symbiotic legume-rhizobia relationship leads to the formation of nitrogen-fixing nodules. Successful nodulation depends on the expression and cross-talk of a batttery of genes, among them SymRK (symbiosis receptor-like kinase), a leucine-rich repeat receptor-like kinase. SymRK is required for the rhizobia invasion of root hairs, as well as for the infection thread and symbiosome formation. Using immunolocalization and downregulation strategies we have recently provided evidence of a new function of PvSymRK in nodulation. We have found that a tight regulation of PvSymRK expression is required for the accurate development of the vascular bundle system in Phaseolus vulgaris nodules.