Analysis of the structure and function of the LYK cluster of Medicago truncatula A17 and R108

The establishment of the Legume-Rhizobia symbiosis is generally dependent on the production of rhizobial lipochitooligosaccharidic Nod factors (NFs) and their perception by plant Lysin Motif Receptor-Like Kinases (LysM-RLKs). In this study, we characterized a cluster of LysM-RLK genes implicated in strain-specific recognition in two highly divergent and widely-studied Medicago truncatula genotypes, A17 and R108. We then used reverse genetic approaches and biochemical analyses to study the function of selected genes in the clusters and the ability of their encoded proteins to bind NFs. Our study has revealed that the LYK cluster exhibits a high degree of variability among M. truncatula genotypes, which in A17 and R108 includes recent recombination events within the cluster and a transposon insertion in A17. The essential role of LYK3 in nodulation in A17 is not conserved in R108 despite similar sequences and good nodulation expression profiles. Although, LYK2, LYK5 and LYK5bis are not essential for nodulation of the two genotypes, some evidence points to accessory roles in nodulation, but not through high-affinity NF binding. This work shows that recent evolution in the LYK cluster provides a source of variation for nodulation, and potential robustness of signaling through genetic redundancy.

Evolution of lipochitooligosaccharide binding to a LysM-RLK for nodulation in Medicago truncatula

Lysin motif receptor like kinases (LysM-RLKs) are involved in the perception of chitooligosaccharides (COs) and related lipochitooligosaccharides (LCOs) in plants. Expansion and divergence of the gene family during evolution have led to various roles in symbiosis and defence. By studying proteins of the LYR-IA subclass of LysM-RLKs of the Poaceae, we show here that they are high affinity LCO binding proteins with a lower affinity for COs, consistent with a role in LCO perception to establish arbuscular mycorrhiza (AM). In Papilionoid legumes whole genome duplication has resulted in two LYR-IA paralogs, MtLYR1 and MtNFP in Medicago truncatula, with MtNFP playing an essential role in the root nodule symbiosis with nitrogen-fixing rhizobia. We show that MtLYR1 has retained the ancestral LCO binding characteristic and is dispensable for AM. Domain swapping between the three Lysin motifs (LysMs) of MtNFP and MtLYR1 and mutagenesis in MtLYR1 suggest that the MtLYR1 LCO binding site is on the second LysM, and that divergence in MtNFP led to better nodulation, but surprisingly with decreased LCO binding. These results suggest that divergence of the LCO binding site has been important for the evolution of a role of MtNFP in nodulation with rhizobia.

LCO Receptors Involved in Arbuscular Mycorrhiza Are Functional for Rhizobia Perception in Legumes

Bacterial lipo-chitooligosaccharides (LCOs) are key mediators of the nitrogen-fixing root nodule symbiosis (RNS) in legumes. The isolation of LCOs from arbuscular mycorrhizal fungi suggested that LCOs are also signaling molecules in arbuscular mycorrhiza (AM). However, the corresponding plant receptors have remained uncharacterized. Here we show that petunia and tomato mutants in the LysM receptor-like kinases LYK10 are impaired in AM formation. Petunia and tomato LYK10 proteins have a high affinity for LCOs (Kd in the nM range) comparable to that previously reported for a legume LCO receptor essential for the RNS. Interestingly, the tomato and petunia LYK10 promoters, when introduced into a legume, were active in nodules similarly to the promoter of the legume orthologous gene. Moreover, tomato and petunia LYK10 coding sequences restored nodulation in legumes mutated in their orthologs. This combination of genetic and biochemical data clearly pinpoints Solanaceous LYK10 as part of an ancestral LCO perception system involved in AM establishment, which has been directly recruited during evolution of the RNS in legumes.

Evolutionary History of Plant LysM Receptor Proteins Related to Root Endosymbiosis

LysM receptor-like kinases (LysM-RLKs), which are specific to plants, can control establishment of both the arbuscular mycorrhizal (AM) and the rhizobium-legume (RL) symbioses in response to signal molecules produced, respectively, by the fungal and bacterial symbiotic partners. While most studies on these proteins have been performed in legume species, there are also important findings that demonstrate the roles of LysM-RLKs in controlling symbiosis in non-legume plants. Phylogenomic studies, which have revealed the presence or absence of certain LysM-RLKs among different plant species, have provided insight into the evolutionary mechanisms underlying both the acquisition and the loss of symbiotic properties. The role of a key nodulation LysM-RLK, NFP/NFR5, in legume plants has thus probably been co-opted from an ancestral role in the AM symbiosis, and has been lost in most plant species that have lost the ability to establish the AM or the RL symbiosis. Another LysM-RLK, LYK3/NFR1, that controls the RL symbiosis probably became neo-functionalised following two rounds of gene duplication. Evidence suggests that a third LysM-RLK, LYR3/LYS12, is also implicated in perceiving microbial symbiotic signals, and this protein could have roles in symbiosis and/or plant immunity in different plant species. By focusing on these three LysM-RLKs that are widespread in plants we review their evolutionary history and what this can tell us about the evolution of both the RL and the AM symbioses.

Synthesis of lipo-chitooligosaccharide analogues and their interaction with LYR3, a high affinity binding protein for Nod factors and Myc-LCOs

Lipo-chitotetrasaccharide analogues have been synthesized from a derivative obtained by controlled chitin depolymerization and a functionalized N-acetyl-glucosamine.

LYR3, a high-affinity LCO-binding protein of Medicago truncatula, interacts with LYK3, a key symbiotic receptor

LYR3, LYK3, and NFP are lysin motif-containing receptor-like kinases (LysM-RLKs) from Medicago truncatula, involved in perception of symbiotic lipo-chitooligosaccharide (LCO) signals. Here, we show that LYR3, a high-affinity LCO-binding protein, physically interacts with LYK3, a key player regulating symbiotic interactions. In vitro, LYR3 is phosphorylated by the active kinase domain of LYK3. Fluorescence lifetime imaging/Förster resonance energy transfer (FLIM/FRET) experiments in tobacco protoplasts show that the interaction between LYR3 and LYK3 at the plasma membrane is disrupted or inhibited by addition of LCOs. Moreover, LYR3 attenuates the cell death response, provoked by coexpression of NFP and LYK3 in tobacco leaves.

Lipo-chitooligosaccharidic nodulation factors and their perception by plant receptors

Lipo-chitooligosaccharides produced by nitrogen-fixing rhizobia are signaling molecules involved in the establishment of an important agronomical and ecological symbiosis with plants. These compounds, known as Nod factors, are biologically active on plant roots at very low concentrations indicating that they are perceived by specific receptors. This article summarizes the main strategies developed for the syntheses of bioactive Nod factors and their derivatives in order to better understand their mode of perception. Different Nod factor receptors and LCO-binding proteins identified by genetic or biochemical approaches are also presented, indicating perception mechanisms that seem to be more complicated than expected, probably involving multi-component receptor complexes.

Lipo-chitooligosaccharidic symbiotic signals are recognized by LysM receptor-like kinase LYR3 in the legume Medicago truncatula

While chitooligosaccharides (COs) derived from fungal chitin are potent elicitors of defense reactions, structurally related signals produced by certain bacteria and fungi, called lipo-chitooligosaccharides (LCOs), play important roles in the establishment of symbioses with plants. Understanding how plants distinguish between friend and foe through the perception of these signals is a major challenge. We report the synthesis of a range of COs and LCOs, including photoactivatable probes, to characterize a membrane protein from the legume Medicago truncatula. By coupling photoaffinity labeling experiments with proteomics and transcriptomics, we identified the likely LCO-binding protein as LYR3, a lysin motif receptor-like kinase (LysM-RLK). LYR3, expressed heterologously, exhibits high-affinity binding to LCOs but not COs. Homology modeling, based on the Arabidopsis CO-binding LysM-RLK AtCERK1, suggests that LYR3 could accommodate the LCO in a conserved binding site. The identification of LYR3 opens up ways for the molecular characterization of LCO/CO discrimination.

NFP, a LysM protein controlling Nod factor perception, also intervenes in Medicago truncatula resistance to pathogens

Plant LysM proteins control the perception of microbial-derived N-acetylglucosamine compounds for the establishment of symbiosis or activation of plant immunity. This raises questions about how plants, and notably legumes, can differentiate friends and foes using similar molecular actors and whether any receptors can intervene in both symbiosis and resistance. To study this question, nfp and lyk3 LysM-receptor like kinase mutants of Medicago truncatula that are affected in the early steps of nodulation, were analysed following inoculation with Aphanomyces euteiches, a root oomycete. The role of NFP in this interaction was further analysed by overexpression of NFP and by transcriptome analyses. nfp, but not lyk3, mutants were significantly more susceptible than wildtype plants to A. euteiches, whereas NFP overexpression increased resistance. Transcriptome analyses on A. euteiches inoculation showed that mutation in the NFP gene led to significant changes in the expression of c. 500 genes, notably involved in cell dynamic processes previously associated with resistance to pathogen penetration. nfp mutants also showed an increased susceptibility to the fungus Colletotrichum trifolii. These results demonstrate that NFP intervenes in M. truncatula immunity, suggesting an unsuspected role for NFP in the perception of pathogenic signals.

Cell autonomous and non-cell autonomous control of rhizobial and mycorrhizal infection in Medicago truncatula

Legumes can form a nitrogen fixing symbiosis with soil bacteria called rhizobia (the RL symbiosis). They can also, like most plants, form symbiotic associations with arbuscular mycorrhizal (AM) fungi, which facilitate plants' phosphate nutrition. In both interactions, the symbionts are hosted inside the plant root. Nitrogen-fixing rhizobia are housed in intracellular symbiotic structures within nodules, while AM fungi form intracellular symbiotic structures, called arbuscules, within cortical root cells. These two endosymbioses present other similarities, including production by the microsymbionts of lipo-chitooligosaccharidic signals (Nod Factors and Myc-LCOs), and the involvement of common plant signaling elements. In Medicago truncatula, DMI3 encodes a calcium and calmodulin dependent protein kinase that is part of this common signaling pathway, while NFP encodes a LysM domain receptor-like kinase involved in Nod Factor perception. Using tissue specific promoters, we recently uncoupled the roles of NFP and DMI3 in the cortex and the epidermis of the root during the RL symbiosis. (1) Here, we provide additional data showing a cell autonomous tissular contribution of DMI3 in the AM symbiosis, and we comment on a non-cell autonomous cortical role of NFP during rhizobial infection.

Epidermal and cortical roles of NFP and DMI3 in coordinating early steps of nodulation in Medicago 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.

Biochemical and phylogenetic analysis of CEBiP-like LysM domain-containing extracellular proteins in higher plants

The chitin elicitor-binding protein (CEBiP) from rice was the first plant lysin motif (LysM) protein for which the biological and biochemical function had been established. It belongs to a plant-specific family of extracellular LysM proteins (LYMs) for which we analyzed the phylogeny. LYMs are present in vascular plants only, where an early gene duplication event might have resulted in two types which were retained in present day genomes. LYMs consist of a signal peptide, three consecutive LysMs, separated by cysteine pairs, and a C-terminal region without any known signature, whose length allows the distinction between the two types, and which may be followed by a glycosylphosphatidylinositol (GPI) anchor motif. We analyzed a representative of each type, MtLYM1 and MtLYM2, from Medicago truncatula at the biochemical level and with respect to their expression patterns and observed some similarities but also marked differences. MtLYM1 and MtLYM2 proved to be very different with regard to abundance and apparent molecular mass on SDS-PAGE. Both undergo several post-translational modifications, including N-glycosylation and the addition of a GPI anchor, which would position the proteins at the outer face of the plasma membrane. Only MtLYM2, but not MtLYM1, showed specific binding to biotinylated N-acetylchitooctaose in a manner similar to CEBiP, which belongs to the same type. We postulate that LYM2-type proteins likely function in the perception of chitin-related molecules, whereas possible functions of LYM1-type proteins remain to be elucidated.

A novel nuclear protein interacts with the symbiotic DMI3 calcium- and calmodulin-dependent protein kinase of Medicago truncatula

Many higher plants establish symbiotic relationships with arbuscular mycorrhizal (AM) fungi that improve their ability to acquire nutrients from the soil. In addition to establishing AM symbiosis, legumes also enter into a nitrogen-fixing symbiosis with bacteria known as rhizobia that results in the formation of root nodules. Several genes involved in the perception and transduction of bacterial symbiotic signals named "Nod factors" have been cloned recently in model legumes through forward genetic approaches. Among them, DMI3 (Doesn't Make Infections 3) is a calcium- and calmodulin-dependent kinase required for the establishment of both nodulation and AM symbiosis. We have identified, by a yeast two-hybrid system, a novel protein interacting with DMI3 named IPD3 (Interacting Protein of DMI3). IPD3 is predicted to interact with DMI3 through a C-terminal coiled-coil domain. Chimeric IPD3::GFP is localized to the nucleus of transformed Medicago truncatula root cells, in which split yellow fluorescent protein assays suggest that IPD3 and DMI3 physically interact in Nicotiana benthamiana. Like DMI3, IPD3 is extremely well conserved among the angiosperms and is absent from Arabidopsis. Despite this high level of conservation, none of the homologous proteins have a demonstrated biological or biochemical function. This work provides the first evidence of the involvement of IPD3 in a nuclear interaction with DMI3.

Phosphoproteins analysis in plants: a proteomic approach

The study of phosphoproteome on a global scale represents one of the challenges in the post-genomic era. Here, we propose an integrated procedure starting from the crude protein extract, that consists of sequential purification steps, and ending up in the identification of phosphorylation sites. This involves (i) an enrichment in phosphoproteins with a commercially available chromatography matrix, (ii) a 2-D gel analysis of the enriched fraction followed by the selective staining with the phosphospecific fluorescent dye Pro-Q Diamond, (iii) a phosphopeptide capture, from the tryptic lysate of 2-D spots, using IMAC micro-columns. In the end, the identification of the phosphoproteins and their corresponding phosphorylation sites were achieved by MALDI-TOF-TOF spectrometry. The method was applied to contrasting samples prepared from cell suspension cultures of Arabidopsis thaliana and roots of Medicago truncatula. The results obtained, demonstrated the robustness of the combination of two enrichment stages, sequentially at the protein and at the peptide levels, to analyse phosphoproteins in plants.

The DMI1 and DMI2 early symbiotic genes of medicago truncatula are required for a high-affinity nodulation factor-binding site associated to a particulate fraction of roots

The establishment of the legume-rhizobia symbiosis between Medicago spp. and Sinorhizobium meliloti is dependent on the production of sulfated lipo-chitooligosaccharidic nodulation (Nod) factors by the bacterial partner. In this article, using a biochemical approach to characterize putative Nod factor receptors in the plant host, we describe a high-affinity binding site (Kd = 0.45 nm) for the major Nod factor produced by S. meliloti. This site is termed Nod factor-binding site 3 (NFBS3). NFBS3 is associated to a high-density fraction prepared from roots of Medicago truncatula and shows binding specificity for lipo-chitooligosaccharidic structures. As for the previously characterized binding sites (NFBS1 and NFBS2), NFBS3 does not recognize the sulfate group on the S. meliloti Nod factor. Studies of Nod factor binding in root extracts of early symbiotic mutants of M. truncatula reveals that the new site is present in Nod factor perception and does not make infections 3 (dmi3) mutants but is absent in dmi1 and dmi2 mutants. Roots and cell cultures of all these mutants still contain sites similar to NFBS1 and NFBS2, respectively. These results suggest that NFBS3 is different from NFBS2 and NFBS1 and is dependent on the common symbiotic genes DMI1 and DMI2 required for establishment of symbioses with both rhizobia and arbuscular mycorrhizal fungi. The potential role of this site in the establishment of root endosymbioses is discussed.

The relative orientation of the lipid and carbohydrate moieties of lipochitooligosaccharides related to nodulation factors depends on lipid chain saturation

Lipochitooligosaccharides (LCOs) signal the symbiosis of rhizobia with legumes and the formation of nitrogen-fixing root nodules. LCOs 1 and 2 share identical tetrasaccharide scaffolds but different lipid moieties (1, LCO-IV(C16:1[9Z], SNa) and , LCO-IV(C16:2[2E,9Z], SNa)). The conformational behaviors of both LCOs were studied by molecular modeling and NMR. Modeling predicts that a small lipid modification would result in a different relative orientation of the lipid and tetrasaccharide moieties. Diffusion ordered spectroscopy reports that both LCOs form small aggregates above 1 mM. Nuclear Overhauser spectroscopy (NOESY) data, collected under monomeric conditions, reveals lipid-carbohydrate contacts only for 1, in agreement with the modeling data. The distinct molecular structures of 1 and 2 have the potential to contribute to their selective binding by legume proteins.

Radiolabeling of lipo-chitooligosaccharides using the NodH sulfotransferase: a two-step enzymatic procedure

Background

The NodH sulfotransferase from Sinorhizobium meliloti has been used to radiolabel lipochitooligosaccharidic (LCO) Nod factor signals with ³⁵S from inorganic sulfate in a two-step enzymatic procedure. The first step involved the production of 3'-phosphoadenosine 5'-phosphosulfate (PAPS), a sulphate donor, using enzymes contained in a yeast extract, and the second step used the NodH enzyme. However with this established procedure, only a low incorporation of the initial inorganic sulfate into the Nod factors was obtained (about 7% after purification of the labeled compounds). The aim of this work was to optimize the radiolabelling of Nod factors with ³⁵S.

Results

The limiting step has been shown to be the sulfation of ATP and its subsequent conversion into PAPS (first step), the sulfate donor for the NodH sulfotransferase activity (second step). By the addition of GTP to the reaction mixture and by manipulating the [ATP]/[Mg²⁺] ratio the yield of PAPS has been increased from 13% to 80%. Using the radiolabeled PAPS we have shown that the efficiency of sulfate transfer to LCOs, by the recombinant S. meliloti NodH sulfotransferase is strongly influenced by the length of the oligosaccharide chain. Variations in the substitutions on the non-reducing sugar, including the structure of the fatty acyl chain, had little effect and Nod factors from the heterologous bacterium Rhizobium tropici could be sulfated by NodH from S. meliloti.

Conclusions

By characterizing the two steps we have optimized the procedure to radiolabel biologically-important, lipo-chitooligosaccharide (LCO) Nod factors to a specific radioactivity of about 800 Ci.mmol^-1 with an incorporation of 60% of the initial inorganic sulfate. The two-step sulfation procedure may be used to radiolabel a variety of related LCO molecules.

New access to lipo-chitooligosaccharide nodulation factors

Sulfonated and non-sulfonated lipo-chitooligosaccharides involved in Sinorhizobium meliloti-legume symbiosis are efficiently obtained on a multi mg scale by a 2-step procedure combining biotechnological and chemical approaches.

Characterization of four lectin-like receptor kinases expressed in roots of Medicago truncatula. Structure, location, regulation of expression, and potential role in the symbiosis with Sinorhizobium meliloti

To study the role of LecRK (lectin-like receptor kinase) genes in the legumerhizobia symbiosis, we have characterized the four Medicago truncatula Gaernt. LecRK genes that are most highly expressed in roots. Three of these genes, MtLecRK7;1, MtLecRK7;2, and MtLecRK7;3, encode proteins most closely related to the Class A LecRKs of Arabidopsis, whereas the protein encoded by the fourth gene, MtLecRK1;1, is most similar to a Class B Arabidopsis LecRK. All four genes show a strongly enhanced root expression, and detailed studies on MtLecRK1;1 and MtLecRK7;2 revealed that the levels of their mRNAs are increased by nitrogen starvation and transiently repressed after either rhizobial inoculation or addition of lipochitooligosaccharidic Nod factors. Studies of the MtLecRK1;1 and MtLecRK7;2 proteins, using green fluorescent protein fusions in transgenic M. truncatula roots, revealed that they are located in the plasma membrane and that their central transmembrane-spanning helix is required for correct sorting. Moreover, their lectin-like domains appear to be highly glycosylated. Of the four proteins, only MtLecRK1;1 shows a high conservation of key residues implicated in monosaccharide binding, and molecular modeling revealed that this protein may be capable of interacting with Nod factors. However, no increase in Nod factor binding was found in roots overexpressing a fusion in which the kinase domain of this protein had been replaced with green fluorescent protein. Roots expressing this fusion protein however showed an increase in nodule number, suggesting that expression of MtLecRK1;1 influences nodulation. The potential role of LecRKs in the legume-rhizobia symbiosis is discussed.

Identical accumulation and immobilization of sulfated and nonsulfated Nod factors in host and nonhost root hair cell walls

Nod factors are signaling molecules secreted by Rhizobium bacteria. These lipo-chitooligosaccharides (LCOs) are required for symbiosis with legumes and can elicit specific responses at subnanomolar concentrations on a compatible host. How plants perceive LCOs is unclear. In this study, using fluorescent Nod factor analogs, we investigated whether sulfated and nonsulfated Nod factors were bound and perceived differently by Medicago truncatula and Vicia sativa root hairs. The bioactivity of three novel sulfated fluorescent LCOs was tested in a root hair deformation assay on M. truncatula, showing bioactivity down to 0.1 to 1 nM. Fluorescence microscopy of plasmolyzed M. truncatula root hairs shows that sulfated fluorescent Nod factors accumulate in the cell wall of root hairs, whereas they are absent from the plasma membrane when applied at 10 nM. When the fluorescent Nod factor distribution in medium surrounding a root was studied, a sharp decrease in fluorescence close to the root hairs was observed, visualizing the remarkable capacity of root hairs to absorb Nod factors from the medium. Fluorescence correlation microscopy was used to study in detail the mobilities of sulfated and nonsulfated fluorescent Nod factors which are biologically active on M. truncatula and V. sativa, respectively. Remarkably, no difference between sulfated and nonsulfated Nod factors was observed: both hardly diffuse and strongly accumulate in root hair cell walls of both M. truncatula and V. sativa. The implications for the mode of Nod factor perception are discussed.

Rapid colorimetric quantification of lipo-chitooligosaccharides from Mesorhizobium loti and Sinorhizobium meliloti

Nod factors are lipids with a chitinlike headgroup produced by gram-negative Rhizobium bacteria. These lipo-chitooligosaccharides (LCOs) are essential signaling molecules for accomplishing symbiosis between the bacteria and roots of legume plants. Despite their important role in the Rhizobium-legume interaction, no fast and sensitive Nod factor quantification methods exist. Here, we report two different quantification methods. The first is based on the enzymatic hydrolysis of Nod factors to release N-acetylglucosamine (GlcNAc), which can subsequently be quantified. It is shown that the degrading enzyme, glusulase, releases exactly two GlcNAc units per pentameric nodulation factor from Mesorhizobium loti factor, allowing quantification of LCOs from Mesorhizobium loti. The second method is based on a specific type of Nod factors that are sulfated on the reducing GlcNAc, allowing quantification analogous to the quantification of sulfolipids. Here, a two-phase extraction method is used in the presence of methylene blue, which specifically forms an ion pair with sulfated lipids. The blue ion pair partitions into the organic phase, after which the methylene blue signal can be quantified. To enable Nod factor quantification with this method, the organic phase was modified and the partitioning was evaluated using fluorescent and radiolabeled sulfated Nod factors. It is shown that sulfated LCOs can be quantified with this method, using sodium dodecyl sulfate for calibration. Both methods allow Nod factor quantification in parallel enabling a fast and easy detection of nanomole quantities of Nod factors. Accurate Nod factor quantification will be crucial for characterization and cross-comparison of the affinity for Nod factors of newly identified Nod factor binding proteins or putative Nod factor receptors.

High-affinity nod factor binding site from Phaseolus vulgaris cell suspension cultures

The lipo-chitooligosaccharidic Nod factors produced by rhizobia are key molecules in the establishment of symbiosis with legumes and probably are recognized by the host plant via specific receptors. Here, we report on the presence of a binding site in cell cultures of Phaseolus vulgaris displaying a high affinity for Nod factors from Rhizobium tropici (NodRt-V) (Me, S, C18:1), a symbiont of this legume. The binding site shares common properties with NFBS2, a Nod-factor binding site previously characterised in Medicago varia, in terms of affinity, preferential plasma-membrane location, and sensitivity to proteases and lysine reactive reagents. However, the bean site poorly recognizes the Nod factors produced by Sinorhizobium meliloti, the symbiont of Medicago. The study of selectivity toward the Nod factors reveals that the length and degree of unsaturation of the acyl chain and the length of the oligosaccharidic moiety are important determinants of high affinity binding to the bean site; whereas, the N-methyl and O-sulfuryl groups play a minor role. Thus, the common characteristics of P. vulgaris and M. varia Nod-factor binding sites suggest that they probably correspond to structurally related proteins, but their different selectivity suggests that they may be involved in a differential perception system for Nod factors in legumes.

Perception of lipo-chitooligosaccharidic Nod factors in legumes

Lipo-chitooligosaccharides produced by rhizobia are a class of signalling molecules that mediate recognition and nodule organogenesis in the legume-rhizobia symbiosis. Their synthesis is specified by the nodulation genes of rhizobia and hence they are commonly known as Nod factors. They are amphiphilic molecules and induce a variety of responses in the roots of the legume hosts. Studies using plant and rhizobial mutants and purified molecules suggest that Nod factors are recognized by more than one receptor. In this article, we review evidence about the affinity, specificity and location of these putative receptors and describe recent studies with regard to their identification.

Recent advances in the study of nod factor perception and signal transduction

Rhizobial lipochitooligosaccharidic Nod factors mediate the specific recognition between leguminous plants and their prokaryotic symbionts. This review summarizes recent findings on the way plants could perceive and transduce these bacterial signals. It starts by summarizing knowledge about Nod factor binding sites, before moving to the potential implications in Nod factor signal transduction of G proteins, root-hair plasma membrane depolarisation, cytoplasmic and extracellular alkalinisation and finally variations in cytoplasmic calcium concentration.

Ligand specificity of a high-affinity binding site for lipo-chitooligosaccharidic Nod factors in Medicago cell suspension cultures

Rhizobial lipo-chitooligosaccharides (LCOs) are signaling molecules involved in host-range recognition for the establishment of the symbiosis with leguminous plants. The major LCO of Rhizobium meliloti, the symbiont of Medicago plants contains four or five N-acetylglucosamines, O-acetylated and N-acylated with a C16:2 fatty acid on the terminal nonreducing sugar and O-sulfated on the reducing sugar. In this paper, the ligand specificity of a high-affinity binding site (Nod factor binding site 2 or NFBS2), enriched in a plasma membrane-enriched fraction of Medicago cell suspension cultures, is reported. By using chemically synthesized LCOs, the role of structural elements, important for symbiotic activities, as recognition motifs for NFBS2 was determined. The results show that the substitutions on the nonreducing sugar of the LCOs (the O-acetate group, the fatty acid, and the hydroxyl group on the C4 of the sugar) are determinants for high-affinity binding to NFBS2. In contrast, the sulfate group, which is necessary for all biological activities on Medicago, is not discriminated by NFBS2. However, the reducing sugar of the LCO seems to interact with NFBS2, because ligand binding is affected by the reduction of the free anomeric carbon and depends on the number of N-acetyl glucosamine residues. These results suggest that the recognition of the LCOs by NFBS2 is mediated by structural elements in both the lipid and oligosaccharidic moities, but not by the sulfate group.

Enzymatic radiolabelling to a high specific activity of legume lipo-oligosaccharidic nodulation factors from Rhizobium meliloti

In this paper we describe the two-step coupled 35S-radiolabelling of the lipo-oligosaccharidic nodulation (Nod) factors of the bacterium Rhizobium meliloti to a specific radioactivity of 800 Ci/mmol. These radiolabelled Nod factors bind to a particulate fraction from roots of the bacterium's symbiotic host, Medicago truncatula, with an equilibrium dissociation constant (KD) of 117 nM, similar to that observed with a synthetic tritiated ligand. The first step of the 35S-labelling involves the synthesis of 3'-phosphoadenosine 5'-phospho[35S]sulphate ([35S]PAPS) from ATP and [35S]sulphate using yeast enzymes. The second step exploits the sulphotransferase activity of the R. meliloti NodH protein, which has been expressed in Escherichia coli, to transfer the labelled sulphate group from PAPS to non-sulphated Nod factors. This enzyme was found to be active in E. coli cultured at 18 degrees C but not 37 degrees C. NodH could also transfer the sulphate group from PAPS to a model substrate, tetra-N-acetyl chitotetraose, with apparent Km values of 56 and 70 microM respectively, and exhibited an apparent Km value for non-sulphated Nod factors of 28 microM. Coupling the two steps of the radiolabelling resulted in an efficiency of 35S incorporation from inorganic sulphate to the Nod factors of approximately 10%. These labelled factors will be a valuable tool in the search for high-affinity receptors for the lipo-oligosaccharidic nodulation factors.

Characterization of a binding site for chemically synthesized lipo-oligosaccharidic NodRm factors in particulate fractions prepared from roots

This paper describes the characteristics of a binding site for the major, lipo-oligosaccharide Nod factor of Rhizobium meliloti in roots of the symbiotic host plant, Medicago truncatula. Chemically synthesized NodRm-IV(Ac, S, C16:2) was labelled by tritiation to a specific activity of 56 Ci mmol-1 and this ligand was shown to be biologically active in the root hair deformation assay at 10(-11) M. Binding of the ligand to a particulate fraction from roots of M. truncatula was found to be saturable and reversible with an affinity (Kd) of 86 nM and the binding characteristics were consistent with a single class of binding sites. Competition with modified Nod factors showed that the binding was independent of both the O-acetyl and the sulphyl group and did not depend on the unsaturation of the fatty acid. However, both moieties of the lipo-oligosaccharide are required for high-affinity binding since tetra-N-acetyl-chitotetraose and palmitate were found to be poor competitors of ligand binding. A binding site with analogous characteristics was also found in a similarly prepared particulate fraction of tomato roots. This binding site for Nod factors, termed NFBS1, which is present in both a leguminous and a non-leguminous plant, may have a more general role than symbiosis.

Effect of Mn(II) on reactions catalyzed by lignin peroxidase from Phanerochaete chrysosporium

The effect of manganese on lignin peroxidase activity was studied. The enzyme was produced with a new process using an air-lift-type reactor. The experiments were performed with veratryl alcohol and a dimeric lignin model compound. It was shown that when Mn(II).lactate complex was present the amount of veratraldehyde formed and the uptake of oxygen were significantly enhanced during the aerobic oxidation of veratryl alcohol. A similar effect can be obtained with superoxide dismutase. These results strongly suggest that the superoxide anion can occur during the reaction; its scavenging by Mn(II) or superoxide dismutase generates H2O2. In contrast, no evidence for the formation of superoxide anion was found during oxidation of the lignin model, compound veratrylglycerol-beta-guaiacyl ether.

Teaching the informed consent process to residents

We believe the focus and emphasis of this committee on informed consent is unique, and that the development of an informed consent teaching process for residents will focus on the moral and ethical issues regarding informed consent. Even more important, we believe we will emphasize the patient care aspects of informed consent and also teach residents their responsibilities for long-term patient care with regard to patients' satisfaction and understanding of their disease process.