Stimulation of clover root hair infection by lectin-binding oligosaccharides from the capsular and extracellular polysaccharides of Rhizobium trifolii

Synthesis of Exopolysaccharide by Bradyrhizobium japonicum during Growth on Hydroaromatic Substrates

The hydroaromatic acids shikimate and quinate, which may be available as carbon sources in the soil, supported production of only low levels of acidic exopolysaccharide by Bradyrhizobium japonicum. Exopolysaccharide production (micrograms per 10 cells) was 4.9 on quinate and 4.5 on shikimate; in comparison, it was 128 on adipate, 18 on l-arabinose, and 39 on d-glucose.

Involvement of exo5 in production of surface polysaccharides in Rhizobium leguminosarum and its role in nodulation of Vicia sativa subsp. nigra

Analysis of two exopolysaccharide-deficient mutants of Rhizobium leguminosarum, RBL5808 and RBL5812, revealed independent Tn5 transposon integrations in a single gene, designated exo5. As judged from structural and functional homology, this gene encodes a UDP-glucose dehydrogenase responsible for the oxidation of UDP-glucose to UDP-glucuronic acid. A mutation in exo5 affects all glucuronic acid-containing polysaccharides and, consequently, all galacturonic acid-containing polysaccharides. Exo5-deficient rhizobia do not produce extracellular polysaccharide (EPS) or capsular polysaccharide (CPS), both of which contain glucuronic acid. Carbohydrate composition analysis and nuclear magnetic resonance studies demonstrated that EPS and CPS from the parent strain have very similar structures. Lipopolysaccharide (LPS) molecules produced by the mutant strains are deficient in galacturonic acid, which is normally present in the core and lipid A portions of the LPS. The sensitivity of exo5 mutant rhizobia to hydrophobic compounds shows the involvement of the galacturonic acid residues in the outer membrane structure. Nodulation studies with Vicia sativa subsp. nigra showed that exo5 mutant rhizobia are impaired in successful infection thread colonization. This is caused by strong agglutination of EPS-deficient bacteria in the root hair curl. Root infection could be restored by simultaneous inoculation with a Nod factor-defective strain which retained the ability to produce EPS and CPS. However, in this case colonization of the nodule tissue was impaired.

Immunochemical and biological characterization of three capsular polysaccharides from a single Bacteroides fragilis strain

Although Bacteroides fragilis accounts for only 0.5% of the normal human colonic flora, it is the anaerobic species most frequently isolated from intra-abdominal and other infections with an intestinal source. The capsular polysaccharides of B. fragilis are part of a complex of surface polysaccharides and are the organism's most important virulence factors in the formation of intra-abdominal abscesses. Two capsular polysaccharides from strain NCTC 9343, PS A1 and PS B1, have been characterized structurally. Their most striking feature is a zwitterionic charge motif consisting of both positively and negatively charged substituent groups on each repeating unit. This zwitterionic motif is essential for abscess formation. In this study, we sought to elucidate structural features of the capsular polysaccharide complex of a commonly studied B. fragilis strain, 638R, that is distinct from strain 9343. We sought a more general picture of the species to establish basic structure-activity and structure-biosynthesis relationships among abscess-inducing polysaccharides. Strain 638R was found to have a capsular polysaccharide complex from which three distinct carbohydrates could be isolated by a complex purification procedure. Compositional and immunochemical studies demonstrated a zwitterionic charge motif common to all of the capsular polysaccharides that correlated with their ability to induce experimental intra-abdominal abscesses. Of interest is the range of net charges of the isolated polysaccharides-from positive (PS C2) to balanced (PS A2) to negative (PS 3). Relationships among structural components of the zwitterionic polysaccharides and their molecular biosynthesis loci were identified.

Physiological aspects of fungi isolated from root nodules of faba bean (Vicia faba L.)

The present study was made to isolate and assess some physiological characteristics of root nodule-colonizing fungi. During this study, 17 fungal species were isolated from root nodule samples taken from faba bean plants (Vicia faba L.) collected from different sites at Assiut area (Egypt). The growth of faba bean plants in pots was significantly promoted by soil inoculation with most fungi. Growth was checked in pots with inocula of Cladosporium cladosporioides, Fusarium moniliforme, F: oxysporium, F solani, Macrophominia phaseolina and Rhizoctonia solani which were added separately. All growth-promoting fungi were capable of producing cellulase, pectin lyase, polygalacturonase, protease, urease, amidase, acid phosphatase, alkaline phosphatase and arylsulfatase in growth medium supplemented with the corresponding substrates. Four fungal species, Aspergillus awamori, A. flavus, Penicillium chrysogenum and Trichoderma koningii showed the highest rates of enzyme formation. The effect of the addition of six trace elements to the growth media at 30 micromol/ml on enzyme production revealed some dependency on species, enzyme and metal ion. Cd2+, Hg2+ and Zn2+ generally inhibited enzyme activity. Cu(1+), Fe3+ and Al3+ showed a stimulatory effect. Fungicides (afugan and tilt) and herbicides (brominal and fusilade) at 50 ppm generally promoted enzyme activity, but insecticides (kelthane and fenvalerate) caused some inhibition to enzyme activities. Salinization of the growth media with NaCl strongly inhibited the enzymatic activity of all fungi at concentrations between 0.5 and 1.5%.

Surface Properties and Motility of Rhizobium and Azospirillum in Relation to Plant Root Attachment

Plant growth promotion by rhizobacteria is a widely spread phenomenon. However only a few rhizobacteria have been studied thoroughly. Rhizobium is the best-studied rhizobacterium. It forms a symbiosis with a restricted host range. Azospirillum is another plant-growth-promoting rhizobacterium which forms rhizocoenoses with a wide range of plants. In both bacteria, the interaction with the plant involves the attraction toward the host plant and the attachment to the surface of the root. Both bacteria are attracted to plant roots, but differ in specificity. Attachment to plant roots occurs in two steps for both bacteria: a quick, reversible adsorption, and a slow, irreversible anchoring to the plant root surface. However, for the two systems under study, the bacterial surface molecules involved in plant root attachment are not necessarily the same.

Glycoconjugate and lipid components ofRhizobium"hedysari" IS123, a root-nodule symbiont of the stress-tolerant legumeHedysarum coronarium

We have examined the diversity of glycoconjugates and cellular lipids of Rhizobium "hedysari" IS123, a bacterial symbiont that specifically nodulates the drought-tolerant forage legume Hedysarum coronarium. IS123 develops a complete capsule consisting of a loose fibrillar network of ruthenium-red-staining acidic polymers and produces two different exopolysaccharides (EPS). EPS-A contains glucose, galactose, mannose, and a noncarbohydrate substitution tentatively identified as a lactyl ester. The composition of EPS-B, which includes glucose and galactose, as well as O-acetyl, pyruvyl, and succinyl substituents, is very similar to that of the EPS-II described in Rhizobium meliloti. IS123 also makes an O-acetylated heterooligosaccharide and unsubstituted β-1,2-glucans. The cellular fatty acid composition of IS123 is dominated by 18:1 and also includes 14:0, 16:0, 16:1, 3OH-16:0, 17:0Δ, 18:0, 3OH-18:0, and 19:0Δ. Phospholipids of IS123 include phosphatidylethanolamine, N-methyl phosphatidylethanolamine, N,N-dimethyl phosphatidylethanolamine, phosphatidylcholine, and phosphatidylglycerol.Key words: Rhizobium, capsule, exopolysaccharides, oligosaccharides, lipids.

Sugar-binding activity of pea (Pisum sativum) lectin is essential for heterologous infection of transgenic white clover hairy roots by Rhizobium leguminosarum biovar viciae

Legume lectin stimulates infection of roots in the symbiosis between leguminous plants and bacteria of the genus Rhizobium. Introduction of the Pisum sativum lectin gene (psl) into white clover hairy roots enables heterologous infection and nodulation by the pea symbiont R. leguminosarum biovar viciae (R.l. viciae). Legume lectins contain a specific sugar-binding site. Here, we show that inoculation of white clover hairy roots co-transformed with a psl mutant encoding a non-sugar-binding lectin (PSL N125D) with R.l. viciae yielded only background pseudo-nodule formation, in contrast to the situation after transformation with wild type psl or with a psl mutant encoding sugar-binding PSL (PSL A126V). For every construct tested, nodulation by the homologous symbiont R.l. trifolii was normal. These results strongly suggest that (1) sugar-binding activity of PSL is necessary for infection of white clover hairy roots by R.l. viciae, and (2) the rhizobial ligand of host lectin is a sugar residue rather than a lipid.

Polysaccharide lyases

Polysaccharide lyases are the products of various microorganisms, bacteriophage and some eukaryotes. All such enzymes cleave a hexose-1,4-alpha- or beta-uronic acid sequence by beta-elimination. They are in some examples, the only known type of enzymes degrading their polyanionic substrates. Although only a small number of these enzymes have been exhaustively studied, the pectin lyases of bacterial origin have proved to be of interesting crystal structure containing a parallel beta-helix domain. Alginate and heparin lyases may yield products with biotechnological potential.

Characterization and symbiotic importance of acidic extracellular polysaccharides of Rhizobium sp. strain GRH2 isolated from acacia nodules

Rhizobium sp. wild-type strain GRH2 was originally isolated from root nodules of the leguminous tree Acacia cyanophylla and has a broad host range which includes herbaceous legumes, e.g., Trifolium spp. We examined the extracellular exopolysaccharides (EPSs) produced by strain GRH2 and found three independent glycosidic structures: a high-molecular-weight acidic heteropolysaccharide which is very similar to the acidic EPS produced by Rhizobium leguminosarum biovar trifolii ANU843, a low-molecular-weight native heterooligosaccharide resembling a dimer of the repeat unit of the high-molecular-weight EPS, and low-molecular-weight neutral beta (1,2)-glucans. A Tn5 insertion mutant derivative of GRH2 (exo-57) that fails to form acidic heteropolysaccharides was obtained. This Exo- mutant formed nitrogen-fixing nodules on Acacia plants but infected a smaller proportion of cells in the central zone of the nodules than did wild-type GRH2. In addition, the exo-57 mutant failed to nodulate several herbaceous legume hosts that are nodulated by wild-type strain GRH2.

Immunochemical characterization of two surface polysaccharides of Bacteroides fragilis

Immunochemical analysis of the capsular polysaccharide from Bacteroides fragilis NCTC 9343 revealed a novel structure composed of two distinct polysaccharides. Immunoelectrophoresis of an extract of purified surface polysaccharide from fermenter-grown organisms showed a complex precipitin profile with varying anodal mobility. DEAE-Sephacel anion-exchange chromatography of the polysaccharide extract failed to separate the majority of this aggregate. Disaggregation of this complex was accomplished by very mild acid treatment; purification was achieved by DEAE-Sephacel anion-exchange chromatography. Polysaccharide A had a neutral charge at pH 7.3, a net negative charge at pH 8.6, and an average Mr = 110,000; chemical analysis showed it to contain galactose, galactosamine, and an unidentified amino sugar. Polysaccharide B eluted from the anion-exchange column with increased salt concentration; it had a net negative charge and an average Mr = 200,000, and contained fucose, galactose, quinovosamine, galacturonic acid, and glucosamine. Neither of these polysaccharides contained detectable 3-deoxy-D-manno-octolusonic acid, and both were recognized as distinct antigens on the basis of their reactivity with monoclonal antibodies CE3 and F10, which reacted with the complex before acid treatment. These data indicate that the capsule of B. fragilis NCTC 9343 comprises two discrete, surface-exposed polysaccharides with differing physiochemical properties that are distinct from the lipopolysaccharide of this organism. The finding of two surface polysaccharides has not been described for other bacteria pathogenic to humans.

Which steps are essential for the formation of functional legume nodules?

Nodulation is reviewed in terms of the phenotypes proposed by Vincent (1980). Individual legumes may be infectible by one or more of the three bacterial genera (collectively known as rhizobia) Rhizobium, Bradyrhizobium, or Azorhizobium. The type of infection process by which rhizobia gain entry is largely governed by the host genotype. In addition to the widely studied root-hair pathway, infections may be associated with lateral root emergence or occur between root epidermal cells. The exact chemical and physical nature of the root hair/epidermal cell wall is likely to be a critical factor in determining whether infections can proceed. In addition to differing with species, wall composition may be influenced by soil chemical (e.g. Ca²⁺ ) and biotic factors (e.g. bacteria). Rhizobial features essential for infection include particular surface polysaccharides and the induction of nodulation genes by plant root exudates. Neither of these is likely to be a major barrier to the extension of nodulation to new hosts. Dissemination of rhizobia within developing nodules may be intercellular, via infection threads or by division of a small number of infected cells. All functional symbioses eventually have 'intracellular' bacteria, in the sense that rhizobia are geographically located within the boundary of the host cell walls. However, they remain extracellular in the sense that they are always confined by a membrane which is largely of host cell origin. In some genera they are also surrounded by infection thread walls, probably modified forms of 'invasive' infection thread walls, which allow differentiation of rhizobia into the nitrogen-fixing form. Thus, natural, functional, symbioses may (a) never involve a stage in which bacteria are confined within tubular infection threads or (b) never release bacteria from infection threads. These features are determined by host genotype. The one feature of legume nodules so far found never to vary is the stem-like character of a peripheral vascular system. This contrasts with the central vascular system of actinorhizas and the rhizobial-induced nodules on the Ulmaceous genus Parasponia. Although of great intrinsic interest, this character is unlikely to present an insurmountable barrier to the extension of nodulation to new species. Other features, such as the ability to produce haemoglobin are now known to the in the genetic makeup of many higher plants. The discovery of the wide range of nodule structures occurring in nature, together with work on mutant rhizobia which may bypass critical stages in the nodulation process, suggest various ways in which the extension of nodulation to non-nodulated legumes and to other (initially at least, dicotyledonous) plants may be engineered. CONTENTS Summary 129 I. Introduction 130 II. The symbionts 130 III. Stages in nodulation 132 IV. Stems and nodules 143 V. Prospects for finding/making new symbioses 144 VI. Conclusions 145 Acknowledgements 147 References 147.

Lectin-enhanced accumulation of manganese-limited Rhizobium leguminosarum cells on pea root hair tips

The ability of Rhizobium leguminosarum 248 to attach to developing Pisum sativum root hairs was investigated during various phases of bacterial growth in yeast extract-mannitol medium. Direct cell counting revealed that growth of the rhizobia transiently stopped three successive times during batch culture in yeast extract-mannitol medium. These interruptions of growth, as well as the simultaneous autoagglutination of the bacteria, appeared to be caused by manganese limitation. Rhizobia harvested during the transient phases of growth inhibition appeared to have a better attachment ability than did exponentially growing rhizobia. The attachment characteristics of these manganese-limited rhizobia were compared with those of carbon-limited rhizobia (G. Smit, J. W. Kijne, and B. J. J. Lugtenberg, J. Bacteriol. 168:821-827, 1986, and J. Bacteriol. 169:4294-4301, 1987). In contrast to the attachment of carbon-limited cells, accumulation of manganese-limited rhizobia (cap formation) was already in full progress after 10 min of incubation; significantly delayed by 3-O-methyl-D-glucose, a pea lectin haptenic monosaccharide; partially resistant to sodium chloride; and partially resistant to pretreatment of the bacteria with cellulase. Binding of single bacteria to the root hair tips was not inhibited by 3-O-methyl-D-glucose. Whereas attachment of single R. leguminosarum cells to the surface of pea root hair tips seemed to be similar for both carbon- and manganese-limited cells, the subsequent accumulation of manganese-limited rhizobia at the root hair tips is apparently accelerated by pea lectin molecules. Moreover, spot inoculation tests with rhizobia grown under various culture conditions indicated that differences in attachment between manganese- and carbon-limited R. leguminosarum cells are correlated with a significant difference in infectivity in that manganese-limited rhizobia, in contrast to carbon-limited rhizobia, are infective. This growth-medium-dependent behavior offers and explanation for the seemingly conflicting data on the involvement of host plant lectins in attachment of rhizobia to root hairs of leguminous plants. Sym plasmid-borne genes do not play a role in manganese-limitation-induced attachment of R. leguminosarum.

The complete structure of the trifoliin A lectin-binding capsular polysaccharide of Rhizobium trifolii 843

The complete structure of the acidic, extracellular, capsular polysaccharide of Rhizobium trifolii 843 has been elucidated by a combination of chemical, enzymic, and spectroscopic methods, confirming an earlier proposed sugar sequence and assigning the locations of the acyl substituents. The polysaccharide was depolymerized by a lyase into octasaccharide units which were uniform in carbohydrate composition and linkage. These units also contained a uniform distribution of acetyl and pyruvic acetal [O-(1-carboxyethylidene)] groups, and half of them were further acylated with D-3-hydroxybutanoyl groups. A much smaller proportion (less than 5%) of the oligomers was further acylated by a second D-3-hydroxybutanoyl group. The locations of the subtituents were determined chemically and by J-correlated, 1H-n.m.r. spectroscopy, proton nuclear Overhauser effect (n.O.e.) measurements, double-rd structure of the carbohydrate chain were determined by methylation analysis using g.l.c.-m.s. fast-atom-bombardment mass spectrometry, and n.m.r. studies on the reduced, deacylated oligomer. Structural studies were supplemented by n.m.r. analyses on the original polymer. The oligosaccharides were found to be branched octasaccharides with four sugar residues in each branch, and the carbohydrate sequence agreed well with that expected from earlier work. In the abbreviated sequence and structure (1a), the sugar residues are labelled "a" through "h". The main chain (a-d) is composed of a 4-deoxy-alpha-L-threo-hex-4-enopyranosyluronic acid group (a) that is linked to O-4 of a 3-O-acetyl-D-glucosyluronic acid residue (b) which is beta-linked to O-4 of a D-glucosyl residue (c). Residue c is beta-linked to O-4 of the branching D-glucose residue (d). The side chain consists of a substituted D-galactosyl group (h) which is beta-linked to O-3 of residue 9 of a beta-(1----4)-linked D-glucose trisaccharide (fragment e-f-g). The reducing end of the resulting tetrasaccharide (e-f-g-h) is beta-linked to O-6 of the branching D-glucose residue (d). In the native polymer, this branching residue is alpha-linked to O-4 of the modified D-glucuronic acid residue (a) which is the unsaturated sugar in the oligomer. A small proportion of the O-2 atoms of the acetylated D-glucosyluronic acid residues is acetylated because of ester migration. The two terminal sugars (g and h) of the branch chain bear 4,6-O-(1-carboxyethylidene) groups. The D-galactosyl groups of half of the oligomers are acylated by D-3-hydroxybutanoyl groups at O-3.(ABSTRACT TRUNCATED AT 400 WORDS)

Reexamination of the presence and linkage of 3-hydroxybutyryl substituents in the acidic capsular polysaccharide of Rhizobium trifolii 0403

We resolved previous conflicting results concerning the presence of 3-hydroxybutyryl substituents on the extracellular acidic polysaccharide from Rhizobium trifolii 0403. These substituents were indeed present in the polysaccharide and in the oligosaccharide fragments obtained by hydrogen fluoride solvolysis of the extracellular and capsular polysaccharides of the bacteria grown on plates. The 3-hydroxybutyrate substituent could be removed from the polysaccharide by 10 mM sodium deuteroxide without evidence of elimination, indicating that this substituent was ester linked.

Alteration of surface properties in a Tn5 mutant strain of Rhizobium trifolii 0403

A symbiotically defective mutant strain of Rhizobium trifolii, UR251, was obtained by transposon Tn5 mutagenesis of R. trifolii 0403 rif and recognized by its partially ineffective (Fix +/-) phenotype on white clover plants. UR251 had a single Tn5 insertion in plasmid DNA, a wild-type plasmid pattern, and no detectable Mu DNA sequences originally present in the vector used for Tn5 mutagenesis. Agglutination by the clover lectin trifoliin A and attachment to clover root hairs was higher with UR251 than with the wild-type strain. The capsular polysaccharide (CPS) of UR251 was altered, as shown by a slower rate of CPS depolymerization with a CPS beta-lyase, PD-I; more pyruvate and less acetate and 3-hydroxybutanoate noncarbohydrate substitutions as quantitated by 1H nuclear magnetic resonance; and a higher pyruvyl transferase activity (enzymatic pyruvylation of lipid-bound saccharides). The site of increased pyruvylation in the CPS of UR251 was on the terminal galactose of the branch of the repeating oligosaccharide unit. These results show that the level of noncarbohydrate substitutions of the CPS as well as pyruvyl transferase activity are altered in R. trifolii UR251 and that trifoliin A-binding ability and clover root hair attachment are improved in this mutant strain of R. trifolii 0403 rif.

A Rhizobium meliloti mutant that forms ineffective pseudonodules in alfalfa produces exopolysaccharide but fails to form beta-(1----2) glucan

A mutant of Rhizobium meliloti that elicited the formation of inactive nodules in alfalfa was found not to form beta-(1----2) glucan in vivo or in vitro. It was nonmotile because it lacks flagella. The 235-kilodalton protein which acts as an intermediate in beta-(1----2) glucan synthesis was undetectable in the mutant. These properties of the mutant are common to those of chvB mutants of Agrobacterium tumefaciens. Exopolysaccharide formation by the R. meliloti mutant was about double that by the wild type.

Biosynthesis of Rhizobium trifolii capsular polysaccharide: enzymatic transfer of pyruvate substitutions into lipid-bound saccharide intermediates

The activity of capsular polysaccharide pyruvyltransferase catalyzing the pyruvylation of acidic heteropolysaccharide was measured in Rhizobium trifolii 843 and 0403 rif. This enzyme activity was determined with EDTA-treated cells, uridine diphosphate-sugar precursors, and phosphoenol [1-14C]pyruvate. Activity was measured by the incorporation of radioactivity into organic solvent-soluble glycoconjugates. Enzymatic pyruvylation of capsular polysaccharide occurred from phosphoenolpyruvate at the lipid-bound saccharide stage.

Exopolysaccharide-deficient mutants of Rhizobium meliloti that form ineffective nodules

By screening with the fluorescent stain Calcofluor, we have isolated 26 independent transposon Tn5 insertion mutants of Rhizobium meliloti that are deficient in the production of a known extracellular polysaccharide (Exo-). The mutants belonged to six distinct genetic groups based on the ability of their Exo- phenotype to be complemented by different recombinant plasmids from a R. meliloti clone bank. With few exceptions, all of the mutants formed ineffective (non-nitrogen-fixing) nodules on alfalfa. For all but one group, the complementing plasmids restored effective nodulation. These results establish a firm and extensive correlation between the ability of Rhizobium to produce a particular polysaccharide and symbiotic proficiency. The ineffective nodules appeared to contain no bacteroids and to form without shepherds' crooks or infection threads; this symbiotic phenotype matches that described for a set of independently isolated mutants that belong phenotypically and genetically to the group B exopolysaccharide mutants described previously [Finan et al. (1985) Cell 40, 869-877]. Apparently the exopolysaccharide, although not required for nodule formation, is involved in wild-type nodule invasion.