A novel polar surface polysaccharide from Rhizobium leguminosarum binds host plant lectin

Enhancing T-DNA Transfer Efficiency in Barley (Hordeum vulgare L.) Cells Using Extracellular Cellulose and Lectin

A major limitation of transforming barley tissues by Agrobacterium tumefaciens is the low frequency of T-DNA transfer due to recalcitrance of barley as a host. The effect of extracellular cellulose and lectin on Agrobacterium transformation efficiency was investigated in this study. Barley callus cultures were transformed with the AGL1 strain containing the vector pBI121 in the presence of 10 mg mL(-1) cellulose or 0.001, 0.05 and 0.1 mg mL(-1) lectin. Addition of cellulose significantly (P ≤ 0.05) increased the number of GUS spots by 50 % compared to standard conditions in the presence of only 200 μM acetosyringone (AS). Frequency of G418-resistant aggregates on the surfaces of callus cultures was 29 and 71.5 %, following AS and AS + cellulose treatments, respectively, after 4 weeks of selection. Presence of 0.05 or 0.1 mg mL(-1) lectin also increased the number of GUS spots and frequency of G418-resistant cells in the selection period, but the increase in blue spots was not significant. We examined the effect of lectin and cellulose on bacterial attachment to callus tissues. Both cellulose and lectin were found to have a significant positive effect on the numbers of bacteria attached to barley callus. Epifluorescence microscopy revealed that Agrobacterium cells had accumulated in the scaffolds of irregular fibrous cellulose with a mean particle size of 200 μm. Expression of nptII in transformed callus lines confirmed the stable transformation of the gene. Our study showed for the first time the binding of Agrobacterium cells to fibrous cellulose and also demonstrated how polysaccharides and glycoproteins can be used to improve T-DNA transfer in monocotyledon transformation procedures.

Novel mixed-linkage β-glucan activated by c-di-GMP in Sinorhizobium meliloti

An artificial increase of cyclic diguanylate (c-di-GMP) levels in Sinorhizobium meliloti 8530, a bacterium that does not carry known cellulose synthesis genes, leads to overproduction of a substance that binds the dyes Congo red and calcofluor. Sugar composition and methylation analyses and NMR studies identified this compound as a linear mixed-linkage (1 → 3)(1 → 4)-β-D-glucan (ML β-glucan), not previously described in bacteria but resembling ML β-glucans found in plants and lichens. This unique polymer is hydrolyzed by the specific endoglucanase lichenase, but, unlike lichenan and barley glucan, it generates a disaccharidic → 4)-β-D-Glcp-(1 → 3)-β-D-Glcp-(1 → repeating unit. A two-gene operon bgsBA required for production of this ML β-glucan is conserved among several genera within the order Rhizobiales, where bgsA encodes a glycosyl transferase with domain resemblance and phylogenetic relationship to curdlan synthases and to bacterial cellulose synthases. ML β-glucan synthesis is subjected to both transcriptional and posttranslational regulation. bgsBA transcription is dependent on the exopolysaccharide/quorum sensing ExpR/SinI regulatory system, and posttranslational regulation seems to involve allosteric activation of the ML β-glucan synthase BgsA by c-di-GMP binding to its C-terminal domain. To our knowledge, this is the first report on a linear mixed-linkage (1 → 3)(1 → 4)-β-glucan produced by a bacterium. The S. meliloti ML β-glucan participates in bacterial aggregation and biofilm formation and is required for efficient attachment to the roots of a host plant, resembling the biological role of cellulose in other bacteria.

Structure and biological roles of Sinorhizobium fredii HH103 exopolysaccharide

Here we report that the structure of the Sinorhizobium fredii HH103 exopolysaccharide (EPS) is composed of glucose, galactose, glucuronic acid, pyruvic acid, in the ratios 5∶2∶2∶1 and is partially acetylated. A S. fredii HH103 exoA mutant (SVQ530), unable to produce EPS, not only forms nitrogen fixing nodules with soybean but also shows increased competitive capacity for nodule occupancy. Mutant SVQ530 is, however, less competitive to nodulate Vigna unguiculata. Biofilm formation was reduced in mutant SVQ530 but increased in an EPS overproducing mutant. Mutant SVQ530 was impaired in surface motility and showed higher osmosensitivity compared to its wild type strain in media containing 50 mM NaCl or 5% (w/v) sucrose. Neither S. fredii HH103 nor 41 other S. fredii strains were recognized by soybean lectin (SBL). S. fredii HH103 mutants affected in exopolysaccharides (EPS), lipopolysaccharides (LPS), cyclic glucans (CG) or capsular polysaccharides (KPS) were not significantly impaired in their soybean-root attachment capacity, suggesting that these surface polysaccharides might not be relevant in early attachment to soybean roots. These results also indicate that the molecular mechanisms involved in S. fredii attachment to soybean roots might be different to those operating in Bradyrhizobium japonicum.

The role of wheat germ agglutinin in the attachment of Pseudomonas sp. WS32 to wheat root

Wheat germ agglutinin (WGA), which is secreted on the surface of wheat root, has been defined as a protein that reversibly and non-enzymatically binds to specific carbohydrates. However, little attention has been paid to the function of WGA in the attachment of bacteria to their host plants. The aim of this study was to investigate the role of WGA in the attachment of Pseudomonas sp. WS32 to wheat roots. Wheat roots were initially treated with double-distilled water, WGA-H (WGA solution that was heated at 100°C for 15 min) and WGA, independently. Subsequently, the roots were co-incubated with cell solutions (10⁹ cells/ml). A dilution plate method using a solid nutrient medium was employed to determine the adsorption of WS32 to wheat roots. WGA was labeled with fluorescein isothiocyanate and detected using the fluorescent in situ hybridization (FISH) technique. The number of adsorptive WS32 cells on wheat roots was significantly increased when the wheat roots were pretreated with WGA, compared with the control treatment (p = 0.01). However, WGA-H failed to increase the amount of bacterial cells that attached to the wheat roots because of the loss of its physiological activity. The FISH assay also revealed that more cells adhered to WGA-treated wheat roots than to control or WGA-H-treated roots. The results indicated that WGA can mediate Pseudomonas strain WS32's adherence to wheat seedling roots. The findings of this study provide a better understanding of the processes involved in plant-microbe interactions.

Lipopolysaccharide O-chain core region required for cellular cohesion and compaction of in vitro and root biofilms developed by Rhizobium leguminosarum

The formation of biofilms is an important survival strategy allowing rhizobia to live on soil particles and plant roots. Within the microcolonies of the biofilm developed by Rhizobium leguminosarum, rhizobial cells interact tightly through lateral and polar connections, forming organized and compact cell aggregates. These microcolonies are embedded in a biofilm matrix, whose main component is the acidic exopolysaccharide (EPS). Our work shows that the O-chain core region of the R. leguminosarum lipopolysaccharide (LPS) (which stretches out of the cell surface) strongly influences bacterial adhesive properties and cell-cell cohesion. Mutants defective in the O chain or O-chain core moiety developed premature microcolonies in which lateral bacterial contacts were greatly reduced. Furthermore, cell-cell interactions within the microcolonies of the LPS mutants were mediated mostly through their poles, resulting in a biofilm with an altered three-dimensional structure and increased thickness. In addition, on the root epidermis and on root hairs, O-antigen core-defective strains showed altered biofilm patterns with the typical microcolony compaction impaired. Taken together, these results indicate that the surface-exposed moiety of the LPS is crucial for proper cell-to-cell interactions and for the formation of robust biofilms on different surfaces.

Mutation of praR in Rhizobium leguminosarum enhances root biofilms, improving nodulation competitiveness by increased expression of attachment proteins

In Rhizobium leguminosarum bv. viciae, quorum-sensing is regulated by CinR, which induces the cinIS operon. CinI synthesizes an AHL, whereas CinS inactivates PraR, a repressor. Mutation of praR enhanced biofilms in vitro. We developed a light (lux)-dependent assay of rhizobial attachment to roots and demonstrated that mutation of praR increased biofilms on pea roots. The praR mutant out-competed wild-type for infection of pea nodules in mixed inoculations. Analysis of gene expression by microarrays and promoter fusions revealed that PraR represses its own transcription and mutation of praR increased expression of several genes including those encoding secreted proteins (the adhesins RapA2, RapB and RapC, two cadherins and the glycanase PlyB), the polysaccharide regulator RosR, and another protein similar to PraR. PraR bound to the promoters of several of these genes indicating direct repression. Mutations in rapA2, rapB, rapC, plyB, the cadherins or rosR did not affect the enhanced root attachment or nodule competitiveness of the praR mutant. However combinations of mutations in rapA, rapB and rapC abolished the enhanced attachment and nodule competitiveness. We conclude that relief of PraR-mediated repression determines a lifestyle switch allowing the expression of genes that are important for biofilm formation on roots and the subsequent initiation of infection of legume roots.

Attachment of Agrobacterium to plant surfaces

Agrobacterium tumefaciens binds to the surfaces of inanimate objects, plants, and fungi. These bacteria are excellent colonizers of root surfaces. In addition, they also bind to soil particles and to the surface of artificial or man-made substances, such as polyesters and plastics. The mechanisms of attachment to these different surfaces have not been completely elucidated. At least two types of binding have been described unipolarpolysaccharide-dependent polar attachment and unipolar polysaccharide-independent attachment (both polar and lateral). The genes encoding the enzymes for the production of the former are located on the circular chromosome, while the genes involved in the latter have not been identified. The expression of both of these types of attachment is regulated in response to environmental signals. However, the signals to which they respond differ so that the two types of attachment are not necessarily expressed coordinately.

Mechanisms and regulation of surface interactions and biofilm formation in Agrobacterium

For many pathogenic bacteria surface attachment is a required first step during host interactions. Attachment can proceed to invasion of host tissue or cells or to establishment of a multicellular bacterial community known as a biofilm. The transition from a unicellular, often motile, state to a sessile, multicellular, biofilm-associated state is one of the most important developmental decisions for bacteria. Agrobacterium tumefaciens genetically transforms plant cells by transfer and integration of a segment of plasmid-encoded transferred DNA (T-DNA) into the host genome, and has also been a valuable tool for plant geneticists. A. tumefaciens attaches to and forms a complex biofilm on a variety of biotic and abiotic substrates in vitro. Although rarely studied in situ, it is hypothesized that the biofilm state plays an important functional role in the ecology of this organism. Surface attachment, motility, and cell division are coordinated through a complex regulatory network that imparts an unexpected asymmetry to the A. tumefaciens life cycle. In this review, we describe the mechanisms by which A. tumefaciens associates with surfaces, and regulation of this process. We focus on the transition between flagellar-based motility and surface attachment, and on the composition, production, and secretion of multiple extracellular components that contribute to the biofilm matrix. Biofilm formation by A. tumefaciens is linked with virulence both mechanistically and through shared regulatory molecules. We detail our current understanding of these and other regulatory schemes, as well as the internal and external (environmental) cues mediating development of the biofilm state, including the second messenger cyclic-di-GMP, nutrient levels, and the role of the plant host in influencing attachment and biofilm formation. A. tumefaciens is an important model system contributing to our understanding of developmental transitions, bacterial cell biology, and biofilm formation.

Seaweed-microbial interactions: key functions of seaweed-associated bacteria

Seaweed-associated bacteria play a crucial role in morphogenesis and growth of seaweeds (macroalgae) in direct and/or indirect ways. Bacterial communities belonging to the phyla Proteobacteria and Firmicutes are generally the most abundant on seaweed surfaces. Associated bacterial communities produce plant growth-promoting substances, quorum sensing signalling molecules, bioactive compounds and other effective molecules that are responsible for normal morphology, development and growth of seaweeds. Also, bioactive molecules of associated bacteria determine the presence of other bacterial strains on seaweeds and protect the host from harmful entities present in the pelagic realm. The ecological functions of cross-domain signalling between seaweeds and bacteria have been reported as liberation of carpospores in the red seaweeds and settlement of zoospores in the green seaweeds. In the present review, the role of extracellular polymeric substances in growth and settlement of seaweeds spores is also highlighted. To elucidate the functional roles of associated bacteria and the molecular mechanisms underlying reported ecological phenomena in seaweeds requires a combined ecological, microbiological and biochemical approach.

The role of bacterial biofilms and surface components in plant-bacterial associations

The role of bacterial surface components in combination with bacterial functional signals in the process of biofilm formation has been increasingly studied in recent years. Plants support a diverse array of bacteria on or in their roots, transport vessels, stems, and leaves. These plant-associated bacteria have important effects on plant health and productivity. Biofilm formation on plants is associated with symbiotic and pathogenic responses, but how plants regulate such associations is unclear. Certain bacteria in biofilm matrices have been found to induce plant growth and to protect plants from phytopathogens (a process termed biocontrol), whereas others are involved in pathogenesis. In this review, we systematically describe the various components and mechanisms involved in bacterial biofilm formation and attachment to plant surfaces and the relationships of these mechanisms to bacterial activity and survival.

Holdfast spreading and thickening during Caulobacter crescentus attachment to surfaces

Background

Adhesion to surfaces facilitates many crucial functions of microbes in their natural habitats. Thus understanding the mechanism of microbial adhesion is of broad interest to the microbiology research community.

Results

We report a study by fluorescence imaging and atomic force microscopy on the growth in size and thickness of the holdfast of synchronized Caulobacter crescentus cells as they attach to a glass surface. We found that the holdfast undergoes a two-stage process of spreading and thickening during its morphogenesis. The holdfast first forms a thin plate on the surface. The diameter of the holdfast plate reaches its final average value of 360 nm by the cell age of ~ 30 min, while its thickness further increases until the age of ~ 60 min. Our AFM analysis indicates that the holdfast is typically thicker in the middle, with gradual falloff in thickness towards the outer edge.

Conclusions

We propose that the newly secreted holdfast substance is fluid-like. It has strong affinity to the surface and cures to form a plate-like holdfast capable of supporting strong and permanent adhesion.

Bypassing the need for subcellular localization of a polysaccharide export-anchor complex by overexpressing its protein subunits

Subcellular protein localization is thought to promote protein-protein interaction by increasing the effective concentration and enabling spatial co-ordination and proper segregation of proteins. We found that protein overexpression allowed the assembly of a productive polysaccharide biosynthesis-export-anchoring complex in the absence of polar localization in Caulobacter crescentus. Polar localization of the holdfast export protein, HfsD, depends on the presence of the other export proteins, HfsA and HfsB, and on the polar scaffold protein PodJ. The holdfast deficiency of hfsB and podJ mutants is suppressed by the overexpression of export proteins. Restored holdfasts are randomly positioned and colocalize with a holdfast anchor protein in these strains, indicating that functional complexes can form at non-polar sites. Therefore, overexpression of export proteins surpasses a concentration threshold necessary for holdfast synthesis. Restoration of holdfast synthesis at non-polar sites reduces surface adhesion, consistent with the need to spatially co-ordinate the holdfast synthesis machinery with the flagellum and pili. These strains lack the cell-specific segregation of the holdfast, resulting in the presence of holdfasts in motile daughter cells. Our results highlight the fact that multiple facets of subcellular localization can be coupled to improve the phenotypic outcome of a protein assembly.

Effect of leguminous lectins on the growth of Rhizobium tropici CIAT899

Rhizobium tropici is a Gram-negative bacterium that induces nodules and fixed atmospheric nitrogen in symbiotic association with Phaseolus vulgaris (common bean) and some other leguminous species. Lectins are proteins that specifically bind to carbohydrates and, consequently, modulate different biological functions. In this study, the d-glucose/d-mannose-binding lectins (from seeds of Dioclea megacarpa, D. rostrata and D. violacea) and d-galactose-binding lectins (from seeds of Bauhinia variegata, Erythina velutina and Vatairea macrocarpa) were purified using chromatographic techniques and evaluated for their effect on the growth of R. tropici CIAT899. All lectins were assayed with a satisfactory degree of purity according to SDS-PAGE analysis, and stimulated bacterial growth; in particular, the Dioclea rostrata lectin was the most active among all tested proteins. As confirmed in the present study, both d-galactose- and d-glucose/d-mannose-binding lectins purified from the seeds of leguminous plants may be powerful biotechnological tools to stimulate the growth of R. tropici CIAT99, thus improving symbiotic interaction between rhizobia and common bean and, hence, the production of this field crop.

Genome of the R-body producing marine alphaproteobacterium Labrenzia alexandrii type strain (DFL-11(T))

Labrenzia alexandrii Biebl et al. 2007 is a marine member of the family Rhodobacteraceae in the order Rhodobacterales, which has thus far only partially been characterized at the genome level. The bacterium is of interest because it lives in close association with the toxic dinoflagellate Alexandrium lusitanicum. Ultrastructural analysis reveals R-bodies within the bacterial cells, which are primarily known from obligate endosymbionts that trigger "killing traits" in ciliates (Paramecium spp.). Genomic traits of L. alexandrii DFL-11(T) are in accordance with these findings, as they include the reb genes putatively involved in R-body synthesis. Analysis of the two extrachromosomal elements suggests a role in heavy-metal resistance and exopolysaccharide formation, respectively. The 5,461,856 bp long genome with its 5,071 protein-coding and 73 RNA genes consists of one chromosome and two plasmids, and has been sequenced in the context of the Marine Microbial Initiative.

Unique secreted-surface protein complex of Lactobacillus rhamnosus, identified by phage display

Proteins are the most diverse structures on bacterial surfaces; hence, they are candidates for species- and strain-specific interactions of bacteria with the host, environment, and other microorganisms. Genomics has decoded thousands of bacterial surface and secreted proteins, yet the function of most cannot be predicted because of the enormous variability and a lack of experimental data that would allow deduction of function through homology. Here, we used phage display to identify a pair of interacting extracellular proteins in the probiotic bacterium Lactobacillus rhamnosus HN001. A secreted protein, SpcA, containing two bacterial immunoglobulin-like domains type 3 (Big-3) and a domain distantly related to plant pathogen response domain 1 (PR-1-like) was identified by screening of an L. rhamnosus HN001 library using HN001 cells as bait. The SpcA-"docking" protein, SpcB, was in turn detected by another phage display library screening, using purified SpcA as bait. SpcB is a 3275-residue cell-surface protein that contains general features of large glycosylated Serine-rich adhesins/fibrils from gram-positive bacteria, including the hallmark signal sequence motif KxYKxGKxW. Both proteins are encoded by genes within a L. rhamnosus-unique gene cluster that distinguishes this species from other lactobacilli. To our knowledge, this is the first example of a secreted-docking protein pair identified in lactobacilli.

Arabinogalactan proteins in root and pollen-tube cells: distribution and functional aspects

BACKGROUND

Arabinogalactan proteins (AGPs) are complex proteoglycans of the cell wall found in the entire plant kingdom and in almost all plant organs. AGPs encompass a large group of heavily glycosylated cell-wall proteins which share common features, including the presence of glycan chains especially enriched in arabinose and galactose and a protein backbone particularly rich in hydroxyproline residues. However, AGPs also exhibit strong heterogeneities among their members in various plant species. AGP ubiquity in plants suggests these proteoglycans are fundamental players for plant survival and development.

SCOPE

In this review, we first present an overview of current knowledge and specific features of AGPs. A section devoted to major tools used to study AGPs is also presented. We then discuss the distribution of AGPs as well as various aspects of their functional properties in root tissues and pollen tubes. This review also suggests novel directions of research on the role of AGPs in the biology of roots and pollen tubes.

Think pink: photosynthesis, plasmids and the Roseobacter clade

Aerobic anoxygenic photosynthesis providing additional ATP for a photoheterotrophic lifestyle is characteristic for several representatives of the marine Roseobacter clade. The patchy distribution of photosynthesis gene clusters (PGCs) within this lineage probably results from horizontal transfers and this explanation is supported by two cases of plasmid-located PGCs. In this study sequencing of the three Sulfitobacter guttiformis plasmids (pSG4, pSG53, pSG118) was initiated with the objective to analyse the 118 kb-sized photosynthetic replicon, but our annotation revealed several additional important traits including key genes of the primary metabolism. The comparison of the two photosynthesis plasmids from S. guttiformis and Roseobacter litoralis showed that their replication modules are located at precisely the same position within the 45 kb-sized PGC. However, comprehensive phylogenetic analyses of the non-homologous replicases (RepB-III, DnaA-like I) and the two ParAB partitioning proteins unequivocally document an independent origin of their extrachromosomal replicons. The analogous positioning within the two photosynthesis super-operons can be explained by a two-step recombination scenario and seems to be the ultimate result of stabilizing selection. Our exemplary analyses of 'pink' plasmids document that chromosomal outsourcing is a common phenomenon in the Roseobacter clade and subsequent horizontal exchanges offer rapid access to the marine pan-genome.

Symbiosis specificity in the legume: rhizobial mutualism

Legume plants are able to engage in root nodule symbiosis with nitrogen-fixing soil bacteria, collectively called rhizobia. This mutualistic association is highly specific, such that each rhizobial species/strain interacts with only a specific group of legumes, and vice versa. Symbiosis specificity can occur at multiple phases of the interaction, ranging from initial bacterial attachment and infection to late nodule development associated with nitrogen fixation. Genetic control of symbiosis specificity is complex, involving fine-tuned signal communication between the symbiotic partners. Here we review our current understanding of the mechanisms used by the host and bacteria to choose their symbiotic partners, with a special focus on the role that the host immunity plays in controlling the specificity of the legume - rhizobial symbiosis.

A plant arabinogalactan-like glycoprotein promotes a novel type of polar surface attachment by Rhizobium leguminosarum

Rhizobium leguminosarum bv. viciae can attach to the roots of legume and non-legume plants. We wanted to determine whether root exudates could affect in vitro surface attachment in a confocal microscopy assay. Root exudate from pea, other legumes, wheat, and Arabidopsis induced R. leguminosarum bv. viciae to attach end-on (in a polar manner) to glass in hexagonal close-packed arrays, rather than attaching along their long axis. This did not involve a reorientation but was probably due to altered growth. The polar attachment involves a novel bacterial component because it occurred in mutants lacking a symbiosis plasmid (and hence nodulation genes) and polar glucomannan. The major surface (acidic) exopolysaccharide was required, and mutations affecting exported proteins and flagella delayed but did not block polar attachment. The polar attachment activity was purified as a high molecular weight fraction from pea root exudate and is an arabinogalactan protein (AGP) based on its carbohydrate content, reactivity with AGP-specific monoclonal antibodies and Yariv reagent, and sensitivity to enzymes that degrade proteins and carbohydrates. We propose that this novel mode of AGP-induced attachment may be important for growth of these bacteria on the roots of both legumes and non-legumes.

Surface contact stimulates the just-in-time deployment of bacterial adhesins

The attachment of bacteria to surfaces provides advantages such as increasing nutrient access and resistance to environmental stress. Attachment begins with a reversible phase, often mediated by surface structures such as flagella and pili, followed by a transition to irreversible attachment, typically mediated by polysaccharides. Here we show that the interplay between pili and flagellum rotation stimulates the rapid transition between reversible and polysaccharide-mediated irreversible attachment. We found that reversible attachment of Caulobacter crescentus cells is mediated by motile cells bearing pili and that their contact with a surface results in the rapid pili-dependent arrest of flagellum rotation and concurrent stimulation of polar holdfast adhesive polysaccharide. Similar stimulation of polar adhesin production by surface contact occurs in Asticcacaulis biprosthecum and Agrobacterium tumefaciens. Therefore, single bacterial cells respond to their initial contact with surfaces by triggering just-in-time adhesin production. This mechanism restricts stable attachment to intimate surface interactions, thereby maximizing surface attachment, discouraging non-productive self-adherence, and preventing curing of the adhesive.

Environmental signals and regulatory pathways that influence exopolysaccharide production in rhizobia

Rhizobia are Gram-negative bacteria that can exist either as free-living bacteria or as nitrogen-fixing symbionts inside root nodules of leguminous plants. The composition of the rhizobial outer surface, containing a variety of polysaccharides, plays a significant role in the adaptation of these bacteria in both habitats. Among rhizobial polymers, exopolysaccharide (EPS) is indispensable for the invasion of a great majority of host plants which form indeterminate-type nodules. Various functions are ascribed to this heteropolymer, including protection against environmental stress and host defense, attachment to abiotic and biotic surfaces, and in signaling. The synthesis of EPS in rhizobia is a multi-step process regulated by several proteins at both transcriptional and post-transcriptional levels. Also, some environmental factors (carbon source, nitrogen and phosphate starvation, flavonoids) and stress conditions (osmolarity, ionic strength) affect EPS production. This paper discusses the recent data concerning the function of the genes required for EPS synthesis and the regulation of this process by several environmental signals. Up till now, the synthesis of rhizobial EPS has been best studied in two species, Sinorhizobium meliloti and Rhizobium leguminosarum. The latest data indicate that EPS synthesis in rhizobia undergoes very complex hierarchical regulation, in which proteins engaged in quorum sensing and the regulation of motility genes also participate. This finding enables a better understanding of the complex processes occurring in the rhizosphere which are crucial for successful colonization and infection of host plant roots.

Dual activity of pyocyanin from Pseudomonas aeruginosa--antibiotic against phytopathogen and signal molecule for biofilm development by rhizobia

The purified pyocyanin from Pseudomonas aeruginosa TO3 was investigated for its antagonistic activity against Macrophomina phaseolina and as a signaling molecule for development of biofilm by rhizobial strain Ca2. The antagonistic activity of purified pyocyanin, as determined by a dry mass method, showed inhibition of M. phaseolina. Biofilm formation by strain Ca2 was performed by crystal violet assay. There was an increase in biofilm development by Ca2 with an increase in pyocyanin concentration up to 0.12 nmol·L(-1), followed by a reduction. Using a well-diffusion method, we determined the effect of pyocyanin on disease suppression and biofilm formation by strain Ca2 on radicles of groundnut ( Arachis hypogaea L. ) placed in three concentric whorls on water agar plates. Pyocyanin suppressed disease better at high concentration; however, at lower concentrations increased colony-forming units of Ca2 on radicles of seedlings was observed. A field study in soil infested with M. phaseolina showed that a coinoculant of P. aeruginosa TO3 and rhizobial strain Ca2 enhanced nodule mass and nitrogenase activity by 264.38% and 269.06%, respectively, over that of the control. This study reports that application of pyocyanin-producing pseudomonads together with rhizobia contributes to the enhancement of nodulation ability and better sustains the growth and productivity of groundnut even in the presence of M. phaseolina.

Inducing salt tolerance in mung bean through coinoculation with rhizobia and plant-growth-promoting rhizobacteria containing 1-aminocyclopropane-1-carboxylate deaminase

Twenty-five strains of plant-growth-promoting rhizobacteria (PGPR) containing 1-aminocyclopropane-1-carboxylate (ACC) deaminase and 10 strains of rhizobia were isolated from rhizosphere soil samples and nodules of mung bean. They were screened in separate trials under salt-stressed axenic conditions. The three most effective strains of PGPR (Mk1, Pseudomonas syringae ; Mk20, Pseudomonas fluorescens ; and Mk25, Pseudomonas fluorescens biotype G) and Rhizobium phaseoli strains M1, M6, and M9 were evaluated in coinoculation for their growth-promoting activity at three salinity levels (original, 4 dS·m(-1), and 6 dS·m(-1)) under axenic conditions. The results showed that salinity stress significantly reduced plant growth but inoculation with PGPR containing ACC deaminase and rhizobia enhanced plant growth, thus reducing the inhibitory effect of salinity. However, their combined application was more effective under saline conditions, and the combination Mk20 × M6 was the most efficient for improving seedling growth and nodulation. The effect of high ethylene concentrations on plant growth and the performance of these strains for reducing the negative impact of saline stress was also evaluated by conducting a classical triple-response bioassay. The intensity of the classical triple response decreased owing to inoculation with these strains, with the root and shoot lengths of inoculated mung bean seedlings increasing and stem diameter decreasing, which is a typical response to the dilution in a classical triple response bioassay. Thus, coinoculation with PGPR containing ACC deaminase and Rhizobium spp. could be a useful approach for inducing salt tolerance and thus improving growth and nodulation in mung bean under salt-affected conditions.

Poles apart: prokaryotic polar organelles and their spatial regulation

While polar organelles hold the key to understanding the fundamentals of cell polarity and cell biological principles in general, they have served in the past merely for taxonomical purposes. Here, we highlight recent efforts in unraveling the molecular basis of polar organelle positioning in bacterial cells. Specifically, we detail the role of members of the Ras-like GTPase superfamily and coiled-coil-rich scaffolding proteins in modulating bacterial cell polarity and in recruiting effector proteins to polar sites. Such roles are well established for eukaryotic cells, but not for bacterial cells that are generally considered diffusion-limited. Studies on spatial regulation of protein positioning in bacterial cells, though still in their infancy, will undoubtedly experience a surge of interest, as comprehensive localization screens have yielded an extensive list of (polarly) localized proteins, potentially reflecting subcellular sites of functional specialization predicted for organelles.

BcsQ is an essential component of the Escherichia coli cellulose biosynthesis apparatus that localizes at the bacterial cell pole

Biofilms are microbial communities characterized by three-dimensional growth resulting from the ability of individual cells to adhere to each other as well as to produce an extracellular matrix that ensures biofilm physical cohesion. Numerous bacteria produce cellulose as a biofilm matrix polymer, a property relying on the expression of bacterial cellulose synthesis (Bcs) proteins and their post-translational activation upon binding of cyclic di-guanosine mono-phosphate second messenger (c-di-GMP) produced by diguanylate cyclases. In Escherichia coli and other Enterobacteriaceae, two genes of unknown function, yhjR and yhjQ, are located upstream of the bcs genes. Here, we show that yhjQ, but not yhjR, is essential for cellulose biosynthesis; it has therefore been renamed bcsQ. Using a green fluorescent protein (GFP) fusion approach, we demonstrate that BcsQ, a MinD homologue, displays a polar localization and that cell-to-cell adhesion is initiated through production of cellulose at the BcsQ-labelled pole. Although we did not detect a similar localization for other Bcs proteins, immunogold labelling of cellulose itself at the pole of individual bacteria indicates the localized activity of the cellulose biosynthesis apparatus. These results therefore suggest that BcsQ could participate in spatial restriction of cellulose biosynthesis activity in Enterobacteriaceae.

Structures of the lipopolysaccharides from Rhizobium leguminosarum RBL5523 and its UDP-glucose dehydrogenase mutant (exo5)

Rhizobial lipopolysaccharide (LPS) is required to establish an effective symbiosis with its host plant. An exo5 mutant of Rhizobium leguminosarum RBL5523, strain RBL5808, is defective in UDP-glucose (Glc) dehydrogenase that converts UDP-Glc to UDP-glucuronic acid (GlcA). This mutant is unable to synthesize either UDP-GlcA or UDP-galacturonic acid (GalA) and is unable to synthesize extracellular and capsular polysaccharides, lacks GalA in its LPS and is defective in symbiosis (Laus MC, Logman TJ, van Brussel AAN, Carlson RW, Azadi P, Gao MY, Kijne JW. 2004. Involvement of exo5 in production of surface polysaccharides in Rhizobium leguminosarum and its role in nodulation of Vicia sativa subsp. nigra. J Bacteriol. 186:6617-6625). Here, we determined and compared the structures of the RBL5523 parent and RBL5808 mutant LPSs. The parent LPS core oligosaccharide (OS), as with other R. leguminosarum and Rhizobium etli strains, is a Gal(1)Man(1)GalA(3)Kdo(3) octasaccharide in, which each of the GalA residues is terminally linked. The core OS from the mutant lacks all three GalA residues. Also, the parent lipid A consists of a fatty acylated GlcNGlcNonate or GlcNGlcN disaccharide that has a GalA residue at the 4'-position, typical of other R. leguminosarum and R. etli lipids A. The mutant lipid A lacks the 4'-GalA residue, and the proximal glycosyl residue was only present as GlcNonate. In spite of these alterations to the lipid A and core OSs, the mutant was still able to synthesize an LPS containing a normal O-chain polysaccharide (OPS), but at reduced levels. The structure of the OPS of the mutant LPS was identical to that of the parent and consists of an O-acetylated →4)-α-d-Glcp-(1→3)-α-d-QuipNAc-(1→ repeating unit.

Agrobacterium tumefaciens ExoR represses succinoglycan biosynthesis and is required for biofilm formation and motility

The ubiquitous plant pathogen Agrobacterium tumefaciens attaches efficiently to plant tissues and abiotic surfaces and can form complex biofilms. A genetic screen for mutants unable to form biofilms on PVC identified disruptions in a homologue of the exoR gene. ExoR is a predicted periplasmic protein, originally identified in Sinorhizobium meliloti, but widely conserved among alphaproteobacteria. Disruptions in the A. tumefaciens exoR gene result in severely compromised attachment to abiotic surfaces under static and flow conditions, and to plant tissues. These mutants are hypermucoid due to elevated production of the exopolysaccharide succinoglycan, via derepression of the exo genes that direct succinoglycan synthesis. In addition, exoR mutants have lost flagellar motility, do not synthesize detectable flagellin and are diminished in flagellar gene expression. The attachment deficiency is, however, complex and not solely attributable to succinoglycan overproduction or motility disruption. A. tumefaciens ExoR can function independently of the ChvG–ChvI two component system, implicated in ExoR-dependent regulation in S. meliloti. Mutations that suppress the exoR motility defect suggest a branched regulatory pathway controlling succinoglycan synthesis, motility and biofilm formation.

The roles of extracellular proteins, polysaccharides and signals in the interactions of rhizobia with legume roots

Rhizobia adopt many different lifestyles including survival in soil, growth in the rhizosphere, attachment to root hairs and infection and growth within legume roots, both in infection threads and in nodules where they fix nitrogen. They are actively involved in extracellular signalling to their host legumes to initiate infection and nodule morphogenesis. Rhizobia also use quorum-sensing gene regulation via N-acyl-homoserine lactone signals and this can enhance their interaction with legumes as well as their survival under stress and their ability to induce conjugation of plasmids and symbiotic islands, thereby spreading their symbiotic capacity. They produce several surface polysaccharides that are critical for attachment and biofilm formation; some of these polysaccharides are specific for their growth on root hairs and can considerably enhance their ability to infect their host legumes. Different rhizobia use several different types of protein secretion mechanisms (Types I, III, IV, V and VI), and many of the secreted proteins play an important role in their interaction with plants. This review summarizes many of the aspects of the extracellular biology of rhizobia, in particular in relation to their symbiotic interaction with legumes.

An integrated view of biofilm formation in rhizobia

Biofilms are bacterial communities enclosed within an extracellular matrix of polysaccharides produced by the bacteria, which adhere to a living or an inert macrosurface. In nature, biofilms constitute a protected growth modality allowing bacteria to survive in hostile environments. Studies of environmental isolates have revealed a highly ordered, three-dimensional organization of the extracellular matrix, which has important implications for biofilm physiology. The zone of soil immediately surrounding a plant root where complex biological and ecological processes occur, termed rhizosphere, forms an environment that fulfills the requirements for biofilm formation, including sufficient moisture and supply of nutrients, which are provided by the plant. Biofilm formation on plants appears to be associated with symbiotic and pathogenic responses, but it is unclear how plants regulate the association. Biofilms function as structures resistant against stress factors such as desiccation, UV radiation, predation, and antibiosis, which help create protective niches for rhizobia. However, the role of biofilms in rhizobial-legume symbiosis remains to be clarified. Here, the mechanisms involved in bacterial biofilm formation and attachment on plant roots, and the relation of these mechanisms to rhizobial function and survival are reviewed.

Mechanisms and regulation of polar surface attachment in Agrobacterium tumefaciens

Agrobacterium tumefaciens is a plant pathogen that transfers a segment of its own DNA into host plants to cause Crown Gall disease. The infection process requires intimate contact between the infecting bacteria and the host tissue. A. tumefaciens attaches efficiently to plant tissues and to abiotic surfaces, and can establish complex biofilms at colonization sites. The dominant mode of attachment is via a single pole in contact with the surface. Several different appendages, adhesins and adhesives play roles during attachment, and foster the transition from free-swimming to sessile growth. This polar surface interaction reflects a more fundamental cellular asymmetry in A. tumefaciens that influences and is congruent with its attached lifestyle.

Rhizobium leguminosarum biovar viciae 3841, deficient in 27-hydroxyoctacosanoate-modified lipopolysaccharide, is impaired in desiccation tolerance, biofilm formation and motility

The lipopolysaccharide (LPS) of the Gram-negative legume symbiont Rhizobium leguminosarum biovar viciae 3,841 contains several unique modifications, including the addition of a 27-hydroxyoctacosanoic acid (27OHC28 : 0), also termed the very long chain fatty acid (VLCFA), attached at the 2' position of lipid A. A transposon mutant that lacks expression of two putative 3-oxo-acyl [acyl-carrier protein] synthase II genes, fabF1 and fabF2, from the VLCFA biosynthetic cluster, was isolated and characterized. MS indicated that the lipid A of the mutant lacked the VLCFA modification, and sodium deoxycholate (DOC)-PAGE of the LPS indicated further structural alterations. The mutant was characteristically sensitive to several stresses that would be experienced in the soil environment, such as desiccation and osmotic stresses. An increase in the excretion of neutral surface polysaccharides was observed in the mutant. This mutant was also altered in its attachment to solid surfaces, and was non-motile, with most of the mutant cells lacking flagella. Despite the pleiotropic effects of the mutation, these mutants were still able to nodulate legumes and fix atmospheric nitrogen. This report emphasizes that a structurally intact VLCFA-containing lipid A is critical to cellular traits that are important for survival in the rhizosphere.

Rhizobium leguminosarum bv. trifolii rosR gene expression is regulated by catabolic repression

Rhizobium leguminosarum bv. trifolii rosR gene encodes a transcriptional regulator involved in the positive regulation of exopolysaccharide synthesis. Transcription of rosR is directed by two promoters, distal P1 and proximal P2, of different strengths. We demonstrated that rosR P1 functions as the main promoter and, besides the -35 and -10 sequences, it contains two other important regulatory elements, an extended -10 motif and an upstream promoter element, that play a significant role in the initiation of transcription. Two cAMP-CRP binding sites (I and II) have been identified upstream of P1, both necessary for optimal rosR expression. cAMP-CRP binding site III is located within the P2 promoter and also influences rosR transcription. rosR transcription levels are dependent both on the presence of the cAMP-CRP complex and on the carbon source, indicating regulation of transcription and exopolysaccharide production by catabolite repression.

New taxonomic markers for identification of Rhizobium leguminosarum and discrimination between closely related species

Rhizobia, producing species-specific exopolysaccharides (EPSs), comprise a very diverse group of soil bacteria that are able to establish nitrogen-fixing symbioses with legumes. Based on the sequences of R. leguminosarum EPS synthesis genes, a sensitive and reliable PCR-based method for identification and subsequent discrimination between Rhizobium species has been developed and tested. For identification of R. leguminosarum, primer sets I-III complementary to sequences of rosR, pssA and pssY genes were proposed. Further sets of primers (IV-VII) were designed for discrimination between R. leguminosarum biovars. The usefulness of the method was examined using a wide range of R. leguminosarum strains isolated from different host plants nodules originating from different regions of Poland. We demonstrate a high discriminating power of primer sets I-III that allow distinguishing R. leguminosarum and two closely related species, R. etli and R. gallicum. This new approach is applicable to identification of R. leguminosarum strains, originating from nodules or soil, where many other closely related bacteria are expected to be present. Based on the nucleotide sequence of rosR and pssA genes, phylogenetic relationships of selected R. leguminosarum isolates were determined. Our results indicate that both rosR and pssA might be useful markers to differentiate and define relationships within a group of R. leguminosarum strains.

Tsr-GFP accumulates linearly with time at cell poles, and can be used to differentiate 'old' versus 'new' poles, in Escherichia coli

SUMMARY

In Escherichia coli, the chemotaxis receptor protein Tsr localizes abundantly to cell poles. The current study, utilizing a Tsr-GFP fusion protein and time-lapse fluorescence microscopy of individual cell lineages, demonstrates that Tsr accumulates approximately linearly with time at the cell poles and that, in consequence, more Tsr is present at the old pole of each cell than at its newborn pole. The rate of pole-localized Tsr accumulation is large enough that old and new poles can always be reliably distinguished, even for cells whose old poles have had only one generation to accumulate signal. Correspondingly, Tsr-GFP can be reliably used to assign new and old poles to any cell without use of information regarding pole heritage, thus providing a useful tool to analyse cells whose prior history is not available. The absolute level of Tsr-GFP at the old pole of a cell also provides a rough estimate of pole (and thus cell) age.

Glucomannan-mediated attachment of Rhizobium leguminosarum to pea root hairs is required for competitive nodule infection

The Rhizobium leguminosarum biovar viciae genome contains several genes predicted to determine surface polysaccharides. Mutants predicted to affect the initial steps of polysaccharide synthesis were identified and characterized. In addition to the known cellulose (cel) and acidic exopolysaccharide (EPS) (pss) genes, we mutated three other loci; one of these loci (gmsA) determines glucomannan synthesis and one (gelA) determines a gel-forming polysaccharide, but the role of the other locus (an exoY-like gene) was not identified. Mutants were tested for attachment and biofilm formation in vitro and on root hairs; the mutant lacking the EPS was defective for both of these characteristics, but mutation of gelA or the exoY-like gene had no effect on either type of attachment. The cellulose (celA) mutant attached and formed normal biofilms in vitro, but it did not form a biofilm on root hairs, although attachment did occur. The cellulose-dependent biofilm on root hairs appears not to be critical for nodulation, because the celA mutant competed with the wild-type for nodule infection. The glucomannan (gmsA) mutant attached and formed normal biofilms in vitro, but it was defective for attachment and biofilm formation on root hairs. Although this mutant formed nodules on peas, it was very strongly outcompeted by the wild type in mixed inoculations, showing that glucomannan is critical for competitive nodulation. The polysaccharide synthesis genes around gmsA are highly conserved among other rhizobia and agrobacteria but are absent from closely related bacteria (such as Brucella spp.) that are not normally plant associated, suggesting that these genes may play a wide role in bacterium-plant interactions.

Influence of phosphate and ammonia on the growth, exopolysaccharide production and symbiosis of Rhizobium leguminosarum bv. trifolii TA1 with clover (Trifolium pratense)

The Rhizobium-legume interaction is sensitive to a number of environmental factors, among which phosphate (Pi) and ammonium availability are the most important. We investigated the effect of Pi and ammonia concentration on exopolysaccharide production and symbiosis Trifolium pratense with Rhizobium leguminosarum bv. trifolii TA1 (RtTA1). The optimal Pi concentration in the bacterial growth medium for RtTA1 growth and exopolysaccharide production was in the range from 0.9 mM to 8.1 mM. Independently of Pi concentration, ammonium (NH4Cl) concentration above 8.1 mM in the culture medium significantly decreased EPS production, indicating a regulatory role of this nutrient on the EPS production in the RtTA1 strain. Pi availability has a beneficial effect on both partners of symbiosis. Pi concentration in the plant medium in the range from 1.7 mM to 5 mM was optimal for nodule formation, nodule occupancy and nitrogen fixation ability. Despite of T. pratense cv. Bryza tolerance on high Pi concentration, 20 mM Pi occurs to be nearly phytotoxic, which negatively affects almost all symbiotic parameters. Large amounts of starch were accumulated in the nodules formed by clover grown on medium containing high Pi concentration.

Coordinating nodule morphogenesis with rhizobial infection in legumes

The formation of nitrogen-fixing nodules on legumes requires an integration of infection by rhizobia at the root epidermis and the initiation of cell division in the cortex, several cell layers away from the sites of infection. Several recent developments have added to our understanding of the signaling events in the epidermis associated with the perception of rhizobial nodulation factors and the role of plant hormones in the activation of cell division leading to nodule morphogenesis. This review focuses on the tissue-specific nature of the developmental processes associated with nodulation and the mechanisms by which these processes are coordinated during the formation of a nodule.

Biofilm formation by plant-associated bacteria

Plants support a diverse array of bacteria, including parasites, mutualists, and commensals on and around their roots, in the vasculature, and on aerial tissues. These microbes have a profound influence on plant health and productivity. Bacteria physically interact with surfaces to form complex multicellular and often multispecies assemblies, including biofilms and smaller aggregates. There is growing appreciation that the intensity, duration, and outcome of plant-microbe interactions are significantly influenced by the conformation of adherent microbial populations. Biofilms on different tissues have unique properties, reflecting the prevailing conditions at those sites. Attachment is required for biofilm formation, and bacteria interact with plant tissues through adhesins including polysaccharides and surface proteins, with initial contact often mediated by active motility. Recognition between lectins and their cognate carbohydrates is a common means of specificity. Biofilm development and the resulting intimate interactions with plants often require cell-cell communication between colonizing bacteria.

Attachment of bacteria to the roots of higher plants

Attachment of soil bacteria to plant cells is supposedly the very early step required in plant-microbe interactions. Attachment also is an initial step for the formation of microbial biofilms on plant roots. For the rhizobia-legume symbiosis, various mechanisms and diverse surface molecules of both partners have been proposed to mediate in this process. The first phase of attachment is a weak, reversible, and unspecific binding in which plant lectins, a Ca(+2)-binding bacterial protein (rhicadhesin), and bacterial surface polysaccharide appear to be involved. The second attachment step requires the synthesis of bacterial cellulose fibrils that cause a tight and irreversible binding of the bacteria to the roots. Cyclic glucans, capsular polysaccharide, and cellulose fibrils also appear to be involved in the attachment of Agrobacterium to plant cells. Attachment of Azospirillum brasilense to cereals roots also can be divided in two different steps. Bacterial surface proteins, capsular polysaccharide and flagella appear to govern the first binding step while extracellular polysaccharide is involved in the second step. Outer cell surface proteins and pili are implicated in the adherence of Pseudomonas species to plant roots.

Syntenic arrangements of the surface polysaccharide biosynthesis genes in Rhizobium leguminosarum

We applied a genomic approach in the identification of genes required for the biosynthesis of different polysaccharides in Rhizobium leguminosarum bv. trifolii TA1 (RtTA1). Pulsed-field gel electrophoresis analyses of undigested genomic DNA revealed that the RtTA1 genome is partitioned into a chromosome and four large plasmids. The combination of sequencing of RtTA1 library BAC clones and PCR amplification of polysaccharide genes from the RtTA1 genome led to the identification of five large regions and clusters, as well as many separate potential polysaccharide biosynthesis genes dispersed in the genome. We observed an apparent abundance of genes possibly linked to lipopolysaccharide biosynthesis. All RtTA1 polysaccharide biosynthesis regions showed a high degree of conserved synteny between R. leguminosarum bv. viciae and/or Rhizobium etli. A majority of the genes displaying a conserved order also showed high sequence identity levels.