Further studies of the role of cyclic beta-glucans in symbiosis. An NdvC mutant of Bradyrhizobium japonicum synthesizes cyclodecakis-(1-->3)-beta-glucosyl

Maintaining osmotic balance in legume nodules

This article comments on: Tian L, Liu L, Xu S, Deng R, Wu P, Jiang H, Wu G, Chen Y. 2022. A d-pinitol transporter, LjPLT11, regulates plant growth and nodule development in Lotus japonicus. Journal of Experimental Botany 73, 351–365.

Legume-rhizobium dance: an agricultural tool that could be improved?

The specific interaction between rhizobia and legume roots leads to the development of a highly regulated process called nodulation, by which the atmospheric nitrogen is converted into an assimilable plant nutrient. This capacity is the basis for the use of bacterial inoculants for field crop cultivation. Legume plants have acquired tools that allow the entry of compatible bacteria. Likewise, plants can impose sanctions against the maintenance of nodules occupied by rhizobia with low nitrogen-fixing capacity. At the same time, bacteria must overcome different obstacles posed first by the environment and then by the legume. The present review describes the mechanisms involved in the regulation of the entire legume-rhizobium symbiotic process and the strategies and tools of bacteria for reaching the nitrogen-fixing state inside the nodule. Also, we revised different approaches to improve the nodulation process for a better crop yield.

Unraveling the sugar code: the role of microbial extracellular glycans in plant-microbe interactions

To defend against microbial invaders but also to establish symbiotic programs, plants need to detect the presence of microbes through the perception of molecular signatures characteristic of a whole class of microbes. Among these molecular signatures, extracellular glycans represent a structurally complex and diverse group of biomolecules that has a pivotal role in the molecular dialog between plants and microbes. Secreted glycans and glycoconjugates such as symbiotic lipochitooligosaccharides or immunosuppressive cyclic β-glucans act as microbial messengers that prepare the ground for host colonization. On the other hand, microbial cell surface glycans are important indicators of microbial presence. They are conserved structures normally exposed and thus accessible for plant hydrolytic enzymes and cell surface receptor proteins. While the immunogenic potential of bacterial cell surface glycoconjugates such as lipopolysaccharides and peptidoglycan has been intensively studied in the past years, perception of cell surface glycans from filamentous microbes such as fungi or oomycetes is still largely unexplored. To date, only few studies have focused on the role of fungal-derived cell surface glycans other than chitin, highlighting a knowledge gap that needs to be addressed. The objective of this review is to give an overview on the biological functions and perception of microbial extracellular glycans, primarily focusing on their recognition and their contribution to plant-microbe interactions.

Gas-Phase Fragmentation of Cyclic Oligosaccharides in Tandem Mass Spectrometry

Modern mass spectrometry, including electrospray and MALDI, is applied for analysis and structure elucidation of carbohydrates. Cyclic oligosaccharides isolated from different sources (bacteria and plants) have been known for decades and some of them (cyclodextrins and their derivatives) are widely used in drug design, as food additives, in the construction of nanomaterials, etc. The peculiarities of the first- and second-order mass spectra of cyclic oligosaccharides (natural, synthetic and their derivatives and modifications: cyclodextrins, cycloglucans, cyclofructans, cyclooligoglucosamines, etc.) are discussed in this minireview.

Cyclic β-glucans at the bacteria-host cells interphase: One sugar ring to rule them all

Cyclic β-1,2-D-glucans (CβG) are natural bionanopolymers present in the periplasmic space of many Proteobacteria. These molecules are sugar rings made of 17 to 25 D-glucose units linked exclusively by β-1,2-glycosidic bonds. CβG are important for environmental sensing and osmoadaptation in bacteria, but most importantly, they play key roles in complex host-cell interactions such as symbiosis, pathogenesis, and immunomodulation. In the last years, the identification and characterisation of the enzymes involved in the synthesis of CβG allowed to know in detail the steps necessary for the formation of these sugar rings. Due to its peculiar structure, CβG can complex large hydrophobic molecules, a feature possibly related to its function in the interaction with the host. The capabilities of the CβG to function as molecular boxes and to solubilise hydrophobic compounds are attractive for application in the development of drugs, in food industry, nanotechnology, and chemistry. More importantly, its excellent immunomodulatory properties led to the proposal of CβG as a new class of adjuvants for vaccine development.

Applications of nuclear magnetic resonance spectroscopy for the understanding of enantiomer separation mechanisms in capillary electrophoresis

This review deals with the applications of nuclear magnetic resonance (NMR) spectroscopy to understand the mechanisms of chiral separation in capillary electrophoresis (CE). It is accepted that changes observed in the separation process, including the reversal of enantiomer migration order (EMO), can be caused by subtle modifications in the molecular recognition mechanisms between enantiomer and chiral selector. These modifications may imply minor structural differences in those selector-selectand complexes that arise from the above mentioned interactions. Therefore, it is mandatory to understand the fine intermolecular interactions between analytes and chiral selectors. In other words, it is necessary to know in detail the structures of the complexes formed by the enantiomer (selectand) and the selector. Any differences in the structures of these complexes arising from either enantiomer should be detected, so that enantiomeric bias in the separation process could be explained. As to the nature of these interactions, those have been extensively reviewed, and it is not intended to be discussed here. These interactions contemplate ionic, ion-dipole and dipole-dipole interactions, hydrogen bonding, van der Waals forces, π-π stacking, steric and hydrophobic interactions. The main subject of this review is to describe how NMR spectroscopy helps to gain insight into the non-covalent intermolecular interactions between selector and selectand that lead to enantiomer separation by CE. Examples in which diastereomeric species are created by covalent (irreversible) derivatization will not be considered here. This review is structured upon the different structural classes of chiral selectors employed in CE, in which NMR spectroscopy has made substantial contributions to rationalize the observed enantioseparations. Cases in which other techniques complement NMR spectroscopic data are also mentioned.

Modification of linear (β1→3)-linked gluco-oligosaccharides with a novel recombinant β-glucosyltransferase (trans-β-glucosidase) enzyme from Bradyrhizobium diazoefficiens

Recently, we have shown that glycoside hydrolases enzymes of family GH17 from proteobacteria (genera Pseudomonas, Azotobacter) catalyze elongation transfer reactions with laminari-oligosaccharides generating (β1→3) linkages preferably and to a lesser extent (β1→6) or (β1→4) linkages. In the present study, the cloning and characterization of the gene encoding the structurally very similar GH17 domain of the NdvB enzyme from Bradyrhizobium diazoefficiens, designated Glt20, as well as its catalytic properties are described. The Glt20 enzyme was strikingly different from the previously investigated bacterial GH17 enzymes, both regarding substrate specificity and product formation. The Azotobacter and Pseudomonas enzymes cleaved the donor laminari-oligosaccharide substrates three or four moieties from the non-reducing end, generating linear oligosaccharides. In contrast, the Glt20 enzyme cleaved donor laminari-oligosaccharide substrates two glucose moieties from the reducing end, releasing laminaribiose and transferring the remainder to laminari-oligosaccharide acceptor substrates creating only (β1→3)(β1→6) branching points. This enables Glt20 to transfer larger oligosaccharide chains than the other type of bacterial enzymes previously described, and helps explain the biologically significant formation of cyclic β-glucans in B. diazoefficiens.

Bacterial Molecular Signals in the Sinorhizobium fredii-Soybean Symbiosis

Sinorhizobium (Ensifer) fredii (S. fredii) is a rhizobial species exhibiting a remarkably broad nodulation host-range. Thus, S. fredii is able to effectively nodulate dozens of different legumes, including plants forming determinate nodules, such as the important crops soybean and cowpea, and plants forming indeterminate nodules, such as Glycyrrhiza uralensis and pigeon-pea. This capacity of adaptation to different symbioses makes the study of the molecular signals produced by S. fredii strains of increasing interest since it allows the analysis of their symbiotic role in different types of nodule. In this review, we analyze in depth different S. fredii molecules that act as signals in symbiosis, including nodulation factors, different surface polysaccharides (exopolysaccharides, lipopolysaccharides, cyclic glucans, and K-antigen capsular polysaccharides), and effectors delivered to the interior of the host cells through a symbiotic type 3 secretion system.

Establishing a Role for Bacterial Cellulose in Environmental Interactions: Lessons Learned from Diverse Biofilm-Producing Proteobacteria

Bacterial cellulose (BC) serves as a molecular glue to facilitate intra- and inter-domain interactions in nature. Biosynthesis of BC-containing biofilms occurs in a variety of Proteobacteria that inhabit diverse ecological niches. The enzymatic and regulatory systems responsible for the polymerization, exportation, and regulation of BC are equally as diverse. Though the magnitude and environmental consequences of BC production are species-specific, the common role of BC-containing biofilms is to establish close contact with a preferred host to facilitate efficient host-bacteria interactions. Universally, BC aids in attachment, adherence, and subsequent colonization of a substrate. Bi-directional interactions influence host physiology, bacterial physiology, and regulation of BC biosynthesis, primarily through modulation of intracellular bis-(3'→5')-cyclic diguanylate (c-di-GMP) levels. Depending on the circumstance, BC producers exhibit a pathogenic or symbiotic relationship with plant, animal, or fungal hosts. Rhizobiaceae species colonize plant roots, Pseudomonadaceae inhabit the phyllosphere, Acetobacteriaceae associate with sugar-loving insects and inhabit the carposphere, Enterobacteriaceae use fresh produce as vehicles to infect animal hosts, and Vibrionaceae, particularly Aliivibrio fischeri, colonize the light organ of squid. This review will highlight the diversity of the biosynthesis and regulation of BC in nature by discussing various examples of Proteobacteria that use BC-containing biofilms to facilitate host-bacteria interactions. Through discussion of current data we will establish new directions for the elucidation of BC biosynthesis, its regulation and its ecophysiological roles.

Evaluation of the Role of the opgGH Operon in Yersinia pseudotuberculosis and Its Deletion during the Emergence of Yersinia pestis

The opgGH operon encodes glucosyltransferases that synthesize osmoregulated periplasmic glucans (OPGs) from UDP-glucose, using acyl carrier protein (ACP) as a cofactor. OPGs are required for motility, biofilm formation, and virulence in various bacteria. OpgH also sequesters FtsZ in order to regulate cell size according to nutrient availability. Yersinia pestis (the agent of flea-borne plague) lost the opgGH operon during its emergence from the enteropathogen Yersinia pseudotuberculosis. When expressed in OPG-negative strains of Escherichia coli and Dickeya dadantii, opgGH from Y. pseudotuberculosis restored OPGs synthesis, motility, and virulence. However, Y. pseudotuberculosis did not produce OPGs (i) under various growth conditions or (ii) when overexpressing its opgGH operon, its galUF operon (governing UDP-glucose), or the opgGH operon or Acp from E. coli. A ΔopgGH Y. pseudotuberculosis strain showed normal motility, biofilm formation, resistance to polymyxin and macrophages, and virulence but was smaller. Consistently, Y. pestis was smaller than Y. pseudotuberculosis when cultured at ≥ 37°C, except when the plague bacillus expressed opgGH. Y. pestis expressing opgGH grew normally in serum and within macrophages and was fully virulent in mice, suggesting that small cell size was not advantageous in the mammalian host. Lastly, Y. pestis expressing opgGH was able to infect Xenopsylla cheopis fleas normally. Our results suggest an evolutionary scenario whereby an ancestral Yersinia strain lost a factor required for OPG biosynthesis but kept opgGH (to regulate cell size). The opgGH operon was presumably then lost because OpgH-dependent cell size control became unnecessary.

Extra-slow-growing Tardiphaga strains isolated from nodules of Vavilovia formosa (Stev.) Fed

Eleven extra-slow-growing strains were isolated from nodules of the relict legume Vavilovia formosa growing in North Ossetia (Caucasus) and Armenia. All isolates formed a single rrs cluster together with the type strain Tardiphaga robiniae LMG 26467(T), while the sequencing of the 16S-23S rDNA intergenic region (ITS) and housekeeping genes glnII, atpD, dnaK, gyrB, recA and rpoB divided them into three groups. North Ossetian isolates (in contrast to the Armenian ones) were clustered separately from the type strain LMG 26467(T). However, all isolates were classified as T. robiniae because the DNA-DNA relatedness between them and the type strain LMG 26467(T) was 69.6% minimum. Two symbiosis-related genes (nodM and nodT) were amplified in all isolated Tardiphaga strains. It was shown that the nodM gene phylogeny is similar to that of ITS and housekeeping genes. The presence of the other symbiosis-related genes in described Tardiphaga strains, which is recently described genus of rhizobia, as well as their ability to form nodules on any plants are under investigation.

Comparative genomics of Bradyrhizobium japonicum CPAC 15 and Bradyrhizobium diazoefficiens CPAC 7: elite model strains for understanding symbiotic performance with soybean

BACKGROUND

The soybean-Bradyrhizobium symbiosis can be highly efficient in fixing nitrogen, but few genomic sequences of elite inoculant strains are available. Here we contribute with information on the genomes of two commercial strains that are broadly applied to soybean crops in the tropics. B. japonicum CPAC 15 (=SEMIA 5079) is outstanding in its saprophytic capacity and competitiveness, whereas B. diazoefficiens CPAC 7 (=SEMIA 5080) is known for its high efficiency in fixing nitrogen. Both are well adapted to tropical soils. The genomes of CPAC 15 and CPAC 7 were compared to each other and also to those of B. japonicum USDA 6T and B. diazoefficiens USDA 110T.

RESULTS

Differences in genome size were found between species, with B. japonicum having larger genomes than B. diazoefficiens. Although most of the four genomes were syntenic, genome rearrangements within and between species were observed, including events in the symbiosis island. In addition to the symbiotic region, several genomic islands were identified. Altogether, these features must confer high genomic plasticity that might explain adaptation and differences in symbiotic performance. It was not possible to attribute known functions to half of the predicted genes. About 10% of the genomes was composed of exclusive genes of each strain, but up to 98% of them were of unknown function or coded for mobile genetic elements. In CPAC 15, more genes were associated with secondary metabolites, nutrient transport, iron-acquisition and IAA metabolism, potentially correlated with higher saprophytic capacity and competitiveness than seen with CPAC 7. In CPAC 7, more genes were related to the metabolism of amino acids and hydrogen uptake, potentially correlated with higher efficiency of nitrogen fixation than seen with CPAC 15.

CONCLUSIONS

Several differences and similarities detected between the two elite soybean-inoculant strains and between the two species of Bradyrhizobium provide new insights into adaptation to tropical soils, efficiency of N2 fixation, nodulation and competitiveness.

Alteration of the exopolysaccharide production and the transcriptional profile of free-living Frankia strain CcI3 under nitrogen-fixing conditions

We investigated the effect of different nitrogen (N) sources on exopolysaccharide (EPS) production and composition by Frankia strain CcI3, a N2-fixing actinomycete that forms root nodules with Casuarina species. Frankia cells grown in the absence of NH4Cl (i.e., under N2-fixing conditions) produced 1.7-fold more EPS, with lower galactose (45.1 vs. 54.7 mol%) and higher mannose (17.3 vs. 9.7 mol%) contents than those grown in the presence of NH4Cl as a combined N-source. In the absence of the combined N-source, terminally linked and branched residue contents were nearly twice as high with 32.8 vs. 15.1 mol% and 15.1 vs. 8.7 mol%, respectively, than in its presence, while the content of linearly linked residues was lower with 52.1 mol% compared to 76.2 mol%. To find out clues for the altered EPS production at the transcriptional level, we performed whole-gene expression profiling using quantitative reverse transcription PCR and microarray technology. The transcription profiles of Frankia strain CcI3 grown in the absence of NH4Cl revealed up to 2 orders of magnitude higher transcription of nitrogen fixation-related genes compared to those of CcI3 cells grown in the presence of NH4Cl. Unexpectedly, microarray data did not provide evidence for transcriptional regulation as a mechanism for differences in EPS production. These findings indicate effects of nitrogen fixation on the production and composition of EPS in Frankia strain CcI3 and suggest posttranscriptional regulation of enhanced EPS production in the absence of the combined N-source.

Mechanisms of microbial escape from phagocyte killing

Phagocytosis and phagosome maturation are crucial processes in biology. Phagocytosis and the subsequent digestion of phagocytosed particles occur across a huge diversity of eukaryotes and can be achieved by many different cells within one organism. In parallel, diverse groups of pathogens have evolved mechanisms to avoid killing by phagocytic cells. The present review discusses a key innate immune cell, the macrophage, and highlights the myriad mechanisms microbes have established to escape phagocytic killing.

HrpM is involved in glucan biosynthesis, biofilm formation and pathogenicity in Xanthomonas citri ssp. citri

Xanthomonas citri ssp. citri (Xcc) is the causal agent of citrus canker. This bacterium develops a characteristic biofilm on both biotic and abiotic surfaces. A biofilm-deficient mutant was identified in a screening of a transposon mutagenesis library of the Xcc 306 strain constructed using the commercial Tn5 transposon EZ-Tn5 <KAN-2> Tnp Transposome (Epicentre). Sequence analysis of a mutant obtained in the screening revealed that a single copy of the EZ-Tn5 was inserted at position 446 of hrpM, a gene encoding a putative enzyme involved in glucan synthesis. We demonstrate for the first time that the product encoded by the hrpM gene is involved in β-1,2-glucan synthesis in Xcc. A mutation in hrpM resulted in no disease symptoms after 4 weeks of inoculation in lemon and grapefruit plants. The mutant also showed reduced ability to swim in soft agar and decreased resistance to H(2)O(2) in comparison with the wild-type strain. All defective phenotypes were restored to wild-type levels by complementation with the plasmid pBBR1-MCS containing an intact copy of the hrpM gene and its promoter. These results indicate that the hrpM gene contributes to Xcc growth and adaptation in its host plant.

Complete Genome Sequence of the Soybean Symbiont Bradyrhizobium japonicum Strain USDA6T

The complete nucleotide sequence of the genome of the soybean symbiont Bradyrhizobium japonicum strain USDA6^T was determined. The genome of USDA6^T is a single circular chromosome of 9,207,384 bp. The genome size is similar to that of the genome of another soybean symbiont, B. japonicum USDA110 (9,105,828 bp). Comparison of the whole-genome sequences of USDA6^T and USDA110 showed colinearity of major regions in the two genomes, although a large inversion exists between them. A significantly high level of sequence conservation was detected in three regions on each genome. The gene constitution and nucleotide sequence features in these three regions indicate that they may have been derived from a symbiosis island. An ancestral, large symbiosis island, approximately 860 kb in total size, appears to have been split into these three regions by unknown large-scale genome rearrangements. The two integration events responsible for this appear to have taken place independently, but through comparable mechanisms, in both genomes.

The determinants of the actinorhizal symbiosis

The actinorhizal symbiosis is a major contributor to the global nitrogen budget, playing a dominant role in ecological successions following disturbances. The mechanisms involved are still poorly known but there emerges the vision that on the plant side, the kinases that transmit the symbiotic signal are conserved with those involved in the transmission of the Rhizobium Nod signal in legumes. However, on the microbial side, complementation with Frankia DNA of Rhizobium nod mutants failed to permit identification of symbiotic genes. Furthermore, analysis of three Frankia genomes failed to permit identification of canonical nod genes and revealed symbiosis-associated genes such as nif, hup, suf and shc to be spread around the genomes. The present review explores some recently published approaches aimed at identifying bacterial symbiotic determinants.

Periplasmic glucans isolated from Proteobacteria

Periplasmic glucans (PGs) are general constituents in the periplasmic space of Proteobacteria. PGs from bacterial strains are found in larger amounts during growth on medium with low osmolarity and thus are often been specified as osmoregulated periplasmic glucans (OPGs). Furthermore, they appear to play crucial roles in pathogenesis and symbiosis. PGs have been classified into four families based on the structural features of their backbones, and they can be modified by a variety of non-sugar substituents. It has also recently been confirmed that novel PGs with various degrees of polymerization (DPs) and/or different substituents are produced under different growth conditions among Proteobacteria. In addition to their biological functions as regulators of low osmolarity, PGs have a variety of physico-chemical properties due to their inherent three-dimensional structures, hydrogen-bonding and complex-forming abilities. Thus, much attention has recently been focused on their physico-chemical applications. In this review, we provide an updated classification of PGs, as well as a description of the occurrences of novel PGs with substituents under various bacterial growth environments, the genes involved in PG biosynthesis and the various physico-chemical properties of PGs.

High-level antibiotic resistance in Pseudomonas aeruginosa biofilm: the ndvB gene is involved in the production of highly glycerol-phosphorylated beta-(1->3)-glucans, which bind aminoglycosides

Pseudomonas aeruginosa is an opportunistic pathogen that affects immunocompromised individuals and causes life-threatening infections in cystic fibrosis (CF) patients. Colonization of CF lung by P. aeruginosa involves a biofilm mode of growth, which is promoted by the production of exopolysaccharides. These polymers are essential components of the extracellular biofilm matrix. P. aeruginosa possesses several clusters contributing to the formation of the matrix, including the pel or psl genes. In the present study, we identified anionic cyclic glucans produced by P. aeruginosa, which are associated with the matrix of strains PAKDeltaretS and PA14. Their structure has been elucidated using chemical analysis, 1- and 2D nuclear magnetic resonance techniques and mass spectrometry. They belong to a family of cyclic beta-(1-->3)-linked glucans of 12-16 glucose residues with 30-50% of glucose units substituted by 1-phosphoglycerol at O-6. These glucans were also recovered in pel mutant strains, which indicated that their biosynthesis was pel independent. In an effort to understand the biogenesis of these glucans, we analyzed the matrix components of a previously characterized P. aeruginosa PA14 mutant, the PA14::ndvB mutant strain. The ndvB gene was predicted to be involved in the synthesis of perisplasmic glucans, capable of physically interacting with aminoglycoside antibiotics. We revealed that the highly glycerol-phosphorylated beta-(1-->3)-glucans are lacking in the ndvB mutant, and we showed that these glucans are capable of direct binding with the aminoglycoside antibiotic kanamycin. This observation fills a gap in our understanding of the relationship between biofilm, cyclic glucans and high-level antibiotic resistance.

Study of free oligosaccharides derived from the bacterial N-glycosylation pathway

The food-borne pathogen Campylobacter jejuni is one of the leading causes of bacterial gastroenteritis worldwide and the most frequent antecedent in neuropathies such as the Guillain-Barré and Miller Fisher syndromes. C. jejuni was demonstrated to possess an N-linked protein glycosylation pathway that adds a conserved heptasaccharide to >40 periplasmic and membrane proteins. Recently, we showed that C. jejuni also produces free heptasaccharides derived from the N-glycan pathway reminiscent of the free oligosaccharides (fOS) produced by eukaryotes. Herein, we demonstrate that C. jejuni fOS are produced in response to changes in the osmolarity of the environment and bacterial growth phase. We provide evidence showing the conserved WWDYG motif of the oligosaccharyltransferase, PglB, is necessary for fOS release into the periplasm. This report demonstrates that fOS from an N-glycosylation pathway in bacteria are potentially equivalent to osmoregulated periplasmic glucans in other Gram-negative organisms.

Osmoregulated periplasmic glucans of Salmonella enterica serovar Typhimurium are required for optimal virulence in mice

We purified osmoregulated periplasmic glucans (OPGs) fromSalmonella entericaserovar Typhimurium and found them to be composed of 100 % glucose with 2-linked glucose as the most abundant residue, with terminal glucose, 2,3-linked and 2,6-linked glucose also present in high quantities. The two structural genes for OPG biosynthesis,opgGandopgH, form a bicistronic operon, and insertion of a kanamycin resistance gene cassette into this operon resulted in a strain devoid of OPGs. TheopgGHmutant strain was impaired in motility and growth under low osmolarity conditions. TheopgGHmutation also resulted in a 2 log increase in the LD50in mice compared to the wild-type strain SL1344. Inability to synthesize OPGs had no significant impact on the organism's lipopolysaccharide pattern or its ability to survive antimicrobial peptides-, detergent-, pH- and nutrient-stress conditions. We observed that theopgGH-defective strain respired at a reduced rate under acidic growth conditions (pH 5.0) and had lower ATP levels compared to the wild-type strain. These data indicate that OPGs ofS.Typhimurium contribute towards mouse virulence as well as growth and motility under low osmolarity growth conditions.

A Mesorhizobium loti mutant with reduced glucan content shows defective invasion of its host plant Lotus japonicus

Random transposon mutagenesis led to the isolation of a novel Mesorhizobium loti mutant that is defective in nitrogen fixation during symbiosis with Lotus japonicus. The mutated locus, designated cep, encodes a putative cell-envelope protein displaying no significant sequence similarity to proteins with known functions. This mutant elicits the formation of nodule-like bumps and root-hair curling, but not the elongation of infection threads, on L. japonicus roots. This is reminiscent of the phenotypes of rhizobial mutants impaired in cyclic beta-glucan biosynthesis. The cep mutant exhibits partially reduced content of cell-associated glucans and intermediate deficiency of motility under hypo-osmotic conditions as compared to a glucan-deficient mutant. Second-site pseudorevertants of the cep mutant were isolated by selecting for restoration of symbiotic nitrogen fixation. A subset of pseudorevertants restored both symbiotic capability and glucan content to levels comparable to that of the wild-type. These results suggest that the Cep product acts on a successful symbiosis by affecting cell-associated glucan content.

Defects in rhizobial cyclic glucan and lipopolysaccharide synthesis alter legume gene expression during nodule development

cDNA array technology was used to compare transcriptome profiles of Lotus japonicus roots inoculated with a Mesorhizobium loti wild-type and two mutant strains affected in cyclic beta(1-2) glucan synthesis (cgs) and in lipopolysaccharide synthesis (lpsbeta2). Expression of genes associated with the development of a fully functional nodule was significantly affected in plants inoculated with the cgs mutant. Array results also revealed that induction of marker genes for nodule development was delayed when plants were inoculated with the lpsbeta2 mutant. Quantitative real-time reverse-transcriptase polymerase chain reaction was used to quantify gene expression of a subset of genes involved in plant defense response, redox metabolism, or genes that encode for nodulins. The majority of the genes analyzed in this study were more highly expressed in roots inoculated with the wild type compared with those inoculated with the cgs mutant strain. Some of the genes exhibited a transient increase in transcript levels during intermediate steps of normal nodule development while others displayed induced expression during the final steps of nodule development. Ineffective nodules induced by the glucan mutant showed higher expression of phenylalanine ammonia lyase than wild-type nodules. Differences in expression pattern of genes involved in early recognition and signaling were observed in plants inoculated with the M. loti mutant strain affected in the synthesis of cyclic glucan.

Molecular determinants of a symbiotic chronic infection

Rhizobial bacteria colonize legume roots for the purpose of biological nitrogen fixation. A complex series of events, coordinated by host and bacterial signal molecules, underlie the development of this symbiotic interaction. Rhizobia elicit de novo formation of a novel root organ within which they establish a chronic intracellular infection. Legumes permit rhizobia to invade these root tissues while exerting control over the infection process. Once rhizobia gain intracellular access to their host, legumes also strongly influence the process of bacterial differentiation that is required for nitrogen fixation. Even so, symbiotic rhizobia play an active role in promoting their goal of host invasion and chronic persistence by producing a variety of signal molecules that elicit changes in host gene expression. In particular, rhizobia appear to advocate for their access to the host by producing a variety of signal molecules capable of suppressing a general pathogen defense response.

How rhizobial symbionts invade plants: the Sinorhizobium-Medicago model

Nitrogen-fixing rhizobial bacteria and leguminous plants have evolved complex signal exchange mechanisms that allow a specific bacterial species to induce its host plant to form invasion structures through which the bacteria can enter the plant root. Once the bacteria have been endocytosed within a host-membrane-bound compartment by root cells, the bacteria differentiate into a new form that can convert atmospheric nitrogen into ammonia. Bacterial differentiation and nitrogen fixation are dependent on the microaerobic environment and other support factors provided by the plant. In return, the plant receives nitrogen from the bacteria, which allows it to grow in the absence of an external nitrogen source. Here, we review recent discoveries about the mutual recognition process that allows the model rhizobial symbiont Sinorhizobium meliloti to invade and differentiate inside its host plant alfalfa (Medicago sativa) and the model host plant barrel medic (Medicago truncatula).

Bacterial cyclic beta-(1,2)-glucan acts in systemic suppression of plant immune responses

Although cyclic glucans have been shown to be important for a number of symbiotic and pathogenic bacterium-plant interactions, their precise roles are unclear. Here, we examined the role of cyclic beta-(1,2)-glucan in the virulence of the black rot pathogen Xanthomonas campestris pv campestris (Xcc). Disruption of the Xcc nodule development B (ndvB) gene, which encodes a glycosyltransferase required for cyclic glucan synthesis, generated a mutant that failed to synthesize extracellular cyclic beta-(1,2)-glucan and was compromised in virulence in the model plants Arabidopsis thaliana and Nicotiana benthamiana. Infection of the mutant bacterium in N. benthamiana was associated with enhanced callose deposition and earlier expression of the PATHOGENESIS-RELATED1 (PR-1) gene. Application of purified cyclic beta-(1,2)-glucan prior to inoculation of the ndvB mutant suppressed the accumulation of callose deposition and the expression of PR-1 in N. benthamiana and restored virulence in both N. benthamiana and Arabidopsis plants. These effects were seen when cyclic glucan and bacteria were applied either to the same or to different leaves. Cyclic beta-(1,2)-glucan-induced systemic suppression was associated with the transport of the molecule throughout the plant. Systemic suppression is a novel counterdefensive strategy that may facilitate pathogen spread in plants and may have important implications for the understanding of plant-pathogen coevolution and for the development of phytoprotection measures.

Enantioseparation of some chiral flavanones using microbial cyclic beta-(1-->3),(1-->6)-glucans as novel chiral additives in capillary electrophoresis

Cyclic beta-(1-->3),(1-->6)-glucans, microbial cyclooligosaccharides produced by Bradyrhizobium japonicum USDA 110, were used as novel chiral additives for the enantiomeric separation of some flavanones such as eriodictyol, homoeriodictyol, hesperetin, naringenin, and isosakuranetin in capillary electrophoresis (CE). Among the flavanones, eriodictyol was separated with the highest resolution (R(s) 5.66) and selectivity factor (alpha 1.18) when 20mM cyclic beta-(1-->3),(1-->6)-glucans were added to the background electrolyte (BGE) at pH 8.3.

Novel beta-1,3-, 1,6-oligoglucan elicitor from Alternaria alternata 102 for defense responses in tobacco

A novel elicitor that induces chitinases in tobacco BY-2 cells was isolated from Alternaria alternata 102. Six other fungi, including A. alternata IFO 6587, could not induce, or weakly induce chitinase activity. The purified elicitor was soluble in 75% methanol and showed the chitinase-inducing activity when applied at concentrations of as low as 25 ng x mL(-1). Structural determination by methylation analysis, reducing-end analysis, MALDI-TOF/MS, and NMR spectroscopy indicated that the elicitor was a mixture of beta-1,3-, 1,6-oligoglucans mostly with a degree of polymerization of between 8 and 17. Periodate oxidation of the elicitor suggested that the 1,6-linked and nonreducing terminal residues are essential for the elicitor activity. Further analysis of the elicitor responses in BY-2 cells indicated that the activity of this beta-1,3-, 1,6-glucan elicitor was about 1000 times more potent than that of laminarin, which is a known elicitor of defense responses in tobacco. Analyzing the expression of defense-related genes indicated that a phenylalanine ammonia-lyase gene and a coumaroyl-CoA O-methyltransferase gene were transiently expressed by this beta-1,3-, 1,6-glucan elicitor. The elicitor induced a weak oxidative burst but did not induce cell death in the BY-2 cells. In the tissue of tobacco plants, this beta-1,3-, 1,6-glucan elicitor induced the expression of basic PR-3 genes, the phenylpropanoid pathway genes, and the sesquiterpenoid pathway genes. In comparison with laminarin and laminarin sulfate, which are reported to be potent elicitors of defense responses in tobacco, the expression pattern of genes induced by the purified beta-1,3-, 1,6-glucan elicitor was more similar to that induced by laminarin than to that induced by laminarin sulfate.

Surviving inside a macrophage: The many ways of Brucella

Bacteria of the genus Brucella are intracellular pathogens capable of survival and replication within macrophages of mammalian hosts. Recent advances have shed light on virulence factors and host functions involved at various stages of the Brucella intracellular life cycle. This review focuses on how this pathogen uses multiple strategies to circumvent macrophage defense mechanisms and generate an organelle permissive for replication.

Cyclic beta-1,2-glucan is a Brucella virulence factor required for intracellular survival

Pathogenic brucella bacteria have developed strategies to persist for prolonged periods of time in host cells, avoiding innate immune responses. Here we show that the cyclic beta-1,2-glucans (CbetaG) synthesized by brucella is important for circumventing host cell defenses. CbetaG acted in lipid rafts found on host cell membranes. CbetaG-deficient mutants failed to prevent phagosome-lysosome fusion and could not replicate. However, when treated with purified CbetaG or synthetic methyl-beta-cyclodextrin, the mutants were able to control vacuole maturation by avoiding lysosome fusion, and this allowed intracellular brucella to survive and reach the endoplasmic reticulum. Fusion between the endoplasmic reticulum and the brucella-containing vacuole depended on the brucella virulence type IV secretion system but not on CbetaG. Brucella CbetaG is thus a virulence factor that interacts with lipid rafts and contributes to pathogen survival.

Nodule development induced by Mesorhizobium loti mutant strains affected in polysaccharide synthesis

The role of Mesorhizobium loti surface polysaccharides on the nodulation process is not yet fully understood. In this article, we describe the nodulation phenotype of mutants affected in the synthesis of lipopolysaccharide (LPS) and beta(1,2) cyclic glucan. M. loti lpsbeta2 mutant produces LPS with reduced amount of O-antigen, whereas M. loti lpsbeta1 mutant produces LPS totally devoid of O-antigen. Both genes are clustered in the chromosome. Based on amino acid sequence homology, LPS sugar composition, and enzymatic activity, we concluded that lpsbeta2 codes for an enzyme involved in the transformation of dTDP-glucose into dTDP-rhamnose, the sugar donor of rhamnose for the synthesis of O-antigen. On the other hand, lpsbeta1 codes for a glucosyltransferase involved in the biosynthesis of the O-antigen. Although LPS mutants elicited normal nodules, both show reduced competitiveness compared with the wild type. M. loti beta(1-2) cyclic glucan synthase (cgs) mutant induces white, empty, ineffective pseudonodules in Lotus tenuis. Cgs mutant induces normal root hair curling but is unable to induce the formation of infection threads. M. loti cgs mutant was more sensitive to deoxycholate and displayed motility impairment compared with the wild-type strain. This pleiotropic effect depends on calcium concentration and temperature.

Curdlan and other bacterial (1→3)-β-d-glucans

Three structural classes of (1-->3)-beta-D-glucans are encountered in some important soil-dwelling, plant-associated or human pathogenic bacteria. Linear (1-->3)-beta-glucans and side-chain-branched (1-->3,1-->2)-beta-glucans are major constituents of capsular materials, with roles in bacterial aggregation, virulence and carbohydrate storage. Cyclic (1-->3,1-->6)-beta-glucans are predominantly periplasmic, serving in osmotic adaptation. Curdlan, the linear (1-->3)-beta-glucan from Agrobacterium, has unique rheological and thermal gelling properties, with applications in the food industry and other sectors. This review includes information on the structure, properties and molecular genetics of the bacterial (1-->3)-beta-glucans, together with an overview of the physiology and biotechnology of curdlan production and applications of this biopolymer and its derivatives.

Expression profiling of virulence and pathogenicity genes of Xanthomonas axonopodis pv. citri

DNA macroarrays of 279 genes of Xanthomonas axonopodis pv. citri potentially associated with pathogenicity and virulence were used to compare the transcriptional alterations of this bacterium in response to two synthetic media. Data analysis indicated that 31 genes were up-regulated by synthetic medium XVM2, while only 7 genes were repressed. The results suggest that XVM2 could be used as an in vitro system to identify candidate genes involved in pathogenesis of X. axonopodis pv. citri.

Importance of opgHXcv of Xanthomonas campestris pv. vesicatoria in host-parasite interactions

Tn5 insertion mutants of Xanthomonas campestris pv. vesicatoria were inoculated into tomato and screened for reduced virulence. One mutant exhibited reduced aggressiveness and attenuated growth in planta. Southern blot analyses indicated that the mutant carried a single Tn5 insertion not associated with previously cloned pathogenicity-related genes of X. campestris pv. vesicatoria. The wild-type phenotype of this mutant was restored by one recombinant plasmid (pOPG361) selected from a genomic library of X. campestris pv. vesicatoria 91-118. Tn3-gus insertion mutagenesis and sequence analyses of a subclone of pOPG361 identified a 1,929-bp open reading frame (ORF) essential for complementation of the mutants. The predicted protein encoded by this ORF was highly homologous to the previously reported pathogenicity-related HrpM protein of Pseudomonas syringae pv. syringae and OpgH of Erwinia chrysanthemi. Based on homology, the new locus was designated opgHXcv. Manipulation of the osmotic potential in the intercellular spaces of tomato leaves by addition of mannitol at low concentrations (25 to 50 mM) compensates for the opgHXcv mutation.

An ancient enzyme domain hidden in the putative beta-glucan elicitor receptor of soybean may play an active part in the perception of pathogen-associated molecular patterns during broad host resistance

A successful defense against potential pathogens requires that a host organism is able to discriminate between self and nonself structures. Soybean (Glycine max L.) exploits a specific molecular pattern, a 1,6-beta-linked and 1,3-beta-branched heptaglucoside (HG), present in cell walls of the oomycetal pathogen Phytophthora sojae, as a signal compound eliciting the onset of defense reactions. The specific and high affinity HG-binding site is contained in the beta-glucan-binding protein (GBP), which in turn is part of a proposed receptor complex. The ability to perceive and respond to Phytophthora cell wall-derived beta-glucan elicitors is exclusive to plants that belong to the Fabaceae. However, we propose that the presence of the GBP is essential, but not sufficient for beta-glucan elicitor-dependent disease resistance because genes encoding GBP-related proteins can be retrieved from many plant species. Furthermore, we show that the GBP is composed of two different carbohydrateactive protein domains, one containing the beta-glucan-binding site, and the other related to glucan endoglucosidases of fungal origin. The glucan hydrolase displays most likely an endo-specific mode of action, cleaving only 1,3-beta-d-glucosidic linkages of oligoglucosides consisting of at least four moieties. Thus, the intrinsic endo-1,3-beta-glucanase activity of the GBP is perfectly suited during initial contact with Phytophthora to release oligoglucoside fragments enriched in motifs that constitute ligands for the high affinity binding site present in the same protein. The concept of innate immunity in plants receives substantial support by this highly sophisticated system using ancient enzyme modules as an active part of the recognition mechanism.

Organogenesis of legume root nodules

The N(2)-fixing nodules elicited by rhizobia on legume roots represent a useful model for studying plant development. Nodule formation implies a complex progression of temporally and spatially regulated events of cell differentiation/dedifferentiation involving several root tissues. In this review we describe the morphogenetic events leading to the development of these histologically well-structured organs. These events include (1) root hair deformation, (2) development and growth of infection threads, (3) induction of the nodule primordium, and (4) induction, activity, and persistence of the nodular meristem and/or of foci of meristematic activities. Particular attention is given to specific aspects of the symbiosis, such as the early stages of intracellular invasion and to differentiation of the intracellular form of rhizobia, called symbiosomes. These developmental aspects were correlated with (1) the regulatory signals exchanged, (2) the plant genes expressed in specific cell types, and (3) the staining procedures that allow the recognition of some cell types. When strictly linked with morphogenesis, the nodulation phenotypes of plant and bacterial mutants such as the developmental consequence of the treatment with metabolic inhibitors, metabolic intermediates, or the variation of physical parameters are described. Finally, some aspects of nodule senescence and of regulation of nodulation are discussed.

Isolation and characterization of periplasmic cyclic β-glucans ofAzorhizobium caulinodans

Oligoglucose molecules isolated from Azorhizobium caulinodans were characterized by compositional analysis, Smith degradation, matrix-assisted laser desorption/ionization time of flight mass spectrometry, and (1)H and (13)C nuclear magnetic resonance analysis. A. caulinodans produced nonbranched and unsubstituted cyclic glucans composed solely of glucose, with the degree of polymerization ranging from 10 to 13. A major fraction of the periplasmic glucans contains 11 glucose residues within rings. The glucose residues are linked by beta-(1,3) and beta-(1,6) glycosidic bonds. These molecules seem to be quite similar to the periplasmic beta-(1,3);(1,6)-glucans synthesized by the Bradyrhizobium strain and are substantially different from the cyclic beta-(1,2)-glucans produced by Agrobacterium and Sinorhizobium species. Azorhizobial cyclic glucan synthesis is not osmoregulated. The response to the osmotic stress in Azorhizobium can be regulated similarly to Brucella spp. It is probable that the biosynthesis of beta-glucans is subject to the feedback control mechanism.

Surface polysaccharide involvement in establishing the rhizobium-legume symbiosis

When the rhizosphere is nitrogen-starved, legumes and rhizobia (soil bacteria) enter into a symbiosis that enables the fixation of atmospheric dinitrogen. This implies a complex chemical dialogue between partners and drastic changes on both plant roots and bacteria. Several recent works pointed out the importance of rhizobial surface polysaccharides in the establishing of the highly specific symbiosis between symbionts. Exopolysaccharides appear to be essential for the early infection process. Lipopolysaccharides exhibit specific roles in the later stages of the nodulation processes such as the penetration of the infection thread into the cortical cells or the setting up of the nitrogen-fixing phenotype. More generally, even if active at different steps of the establishing of the symbiosis, all the polysaccharide classes seem to be involved in complex processes of plant defense inhibition that allow plant root invasion. Their chemistry is important for structural recognition as well as for physico-chemical properties.

Biosynthesis of phosphatidylcholine in bacteria

Phosphatidylcholine (PC) is the major membrane-forming phospholipid in eukaryotes and can be synthesized by either of two pathways, the methylation pathway or the CDP-choline pathway. Many prokaryotes lack PC, but it can be found in significant amounts in membranes of rather diverse bacteria and based on genomic data, we estimate that more than 10% of all bacteria possess PC. Enzymatic methylation of phosphatidylethanolamine via the methylation pathway was thought to be the only biosynthetic pathway to yield PC in bacteria. However, a choline-dependent pathway for PC biosynthesis has been discovered in Sinorhizobium meliloti. In this pathway, PC synthase, condenses choline directly with CDP-diacylglyceride to form PC in one step. A number of symbiotic (Rhizobium leguminosarum, Mesorhizobium loti) and pathogenic (Agrobacterium tumefaciens, Brucella melitensis, Pseudomonas aeruginosa, Borrelia burgdorferi and Legionella pneumophila) bacteria seem to possess the PC synthase pathway and we suggest that the respective eukaryotic host functions as the provider of choline for this pathway. Pathogens entering their hosts through epithelia (Streptococcus pneumoniae, Haemophilus influenzae) require phosphocholine substitutions on their cell surface components that are biosynthetically also derived from choline supplied by the host. However, the incorporation of choline in these latter cases proceeds via choline phosphate and CDP-choline as intermediates. The occurrence of two intermediates in prokaryotes usually found as intermediates in the eukaryotic CDP-choline pathway for PC biosynthesis raises the question whether some bacteria might form PC via a CDP-choline pathway.

Establishment of biotrophy by parasitic fungi and reprogramming of host cells for disease resistance

Parasitic biotrophs such as mildews and rusts evolved specific mechanisms that keep host cells alive during infection. These fungi appear to absorb nutrients mainly by proton-symport-driven transporter proteins that reside in specialized feeding structures. Accumulating evidence suggests that biotrophic fungi both suppress induction of plant defense responses in physical proximity to infection sites and induce specific host genes for the establishment of biotrophy. The peculiarities of biotrophic pathogenesis likely reflect diverse types of plant disease-resistance responses. The cloning of race-specific resistance genes to powdery mildew infection and of genes required for their function provides first insights into molecular mechanisms enabling the host to recognize mildew effector components and suggests candidate mechanisms of resistance signaling. Resistance to powdery mildew fungi that result from mutations in host genes promises to shed light on mechanisms that are required for the establishment of disease susceptibility.

Characterization of ndvD, the third gene involved in the synthesis of cyclic beta-(1 --> 3),(1 --> 6)-D-glucans in Bradyrhizobium japonicum

Previously, we identified two genes in Bradyrhizobium japonicum (ndvB, ndvC) that are required for cyclic beta-(1 --> 3),(1 --> 6)-D-glucan synthesis and successful symbiotic interaction with soybean (Glycine max). In this study, we report a new open reading frame (ORF1) located in the intergenic region between ndvB and ndvC, which is essential for beta-glucan synthesis and effective nodulation of G. max. This new gene is designated ndvD (nodule development). The ndvD translation product has a predicted molecular mass of 26.4 kDa with one transmembrane domain. Genetic experiments involving gene deletion, Tn5 insertion, and gene complementation revealed that the mutation of ndvD generated pleiotropic phenotypes, including hypoosmotic sensitivity, reduced motility, and defects in conjugative gene transfer, in addition to symbiotic ineffectiveness. Although deficient in in vivo beta-glucan synthesis, membrane preparations from the ndvD mutant synthesized neutral beta-glucans in vitro. Therefore, ndvD does not appear to be a structural gene for beta-glucan synthesis. Our hypothesis for the mechanism of beta-(1 --> 3),(1 --> 6)-D-glucan synthesis is presented.

Suppression of plant defence in rhizobia-legume symbiosis

The symbiosis between rhizobia and legumes is characterized by the formation of dinitrogen-fixing root nodules. Although rhizobia colonize roots in a way that is reminiscent of pathogenic microorganisms, no host plant defence reactions are triggered during successful symbioses. Nevertheless, the plants obviously control the invading bacteria; failure in effective nodule formation or infections with rhizobia defective in surface polysaccharides often result in pathogenic responses. This article focuses on whether and how defence responses in effective symbiosis might be suppressed. Recent results suggest a central role for rhizobial polysaccharides acting as antagonists in the negative regulation of defence induction.

The opgGIH and opgC genes of Rhodobacter sphaeroides form an operon that controls backbone synthesis and succinylation of osmoregulated periplasmic glucans

Osmoregulated periplasmic glucans (OPGs) of Rhodobacter sphaeroides are anionic cyclic molecules that accumulate in large amounts in the periplasmic space in response to low osmolarity of the medium. Their anionic character is provided by the substitution of the glucosidic backbone by succinyl residues. A wild-type strain was subject to transposon mutagenesis, and putative mutant clones were screened for changes in OPGs by thin layer chromatography. One mutant deficient in succinyl substitution of the OPGs was obtained and the gene inactivated in this mutant was characterized and named opgC. opgC is located downstream of three ORFs, opgGIH, two of which are similar to the Escherichia coli operon, mdoGH, governing OPG backbone synthesis. Inactivation of opgG, opgI or opgH abolished OPG production and complementation analysis indicated that the three genes are necessary for backbone synthesis. In contrast, inactivation of a gene similar to ndvB, encoding the OPG-glucosyl transferase in Sinorhizobium meliloti, had no consequence on OPG synthesis in Rhodobacter sphaeroides. Cassette insertions in opgH had a polar effect on glucan substitution, indicating that opgC is in the same transcription unit. Expression of opgIHC in E. coli mdoB/mdoC and mdoH mutants allowed the production of slightly anionic and abnormally long linear glucans.