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

Regulatory roles of the second messenger c-di-GMP in beneficial plant-bacteria interactions

The rhizosphere system of plants hosts a diverse consortium of bacteria that confer beneficial effects on plant, such as plant growth-promoting rhizobacteria (PGPR), biocontrol agents with disease-suppression activities, and symbiotic nitrogen fixing bacteria with the formation of root nodule. Efficient colonization in planta is of fundamental importance for promoting of these beneficial activities. However, the process of root colonization is complex, consisting of multiple stages, including chemotaxis, adhesion, aggregation, and biofilm formation. The secondary messenger, c-di-GMP (cyclic bis-(3'-5') dimeric guanosine monophosphate), plays a key regulatory role in a variety of physiological processes. This paper reviews recent progress on the actions of c-di-GMP in plant beneficial bacteria, with a specific focus on its role in chemotaxis, biofilm formation, and nodulation.

Bacterial synthase-dependent exopolysaccharide secretion: a focus on cellulose

Bacterial biofilms are a prevalent multicellular life form in which individual members can undergo significant functional differentiation and are typically embedded in a complex extracellular matrix of proteinaceous fimbriae, extracellular DNA, and exopolysaccharides (EPS). Bacteria have evolved at least four major mechanisms for EPS biosynthesis, of which the synthase-dependent systems for bacterial cellulose secretion (Bcs) represent not only key biofilm determinants in a wide array of environmental and host-associated microbes, but also an important model system for the studies of processive glycan polymerization, cyclic diguanylate (c-di-GMP)-dependent synthase regulation, and biotechnological polymer applications. The secreted cellulosic chains can be decorated with additional chemical groups or can pack with various degrees of crystallinity depending on dedicated enzymatic complexes and/or cytoskeletal scaffolds. Here, I review recent progress in our understanding of synthase-dependent EPS biogenesis with a focus on common and idiosyncratic molecular mechanisms across diverse cellulose secretion systems.

Glycosyl transferase GT2 genes mediate the biosynthesis of an unusual (1,3;1,4)-β-glucan exopolysaccharide in the bacterium Sarcina ventriculi

Linear, unbranched (1,3;1,4)-β-glucans (mixed-linkage glucans or MLGs) are commonly found in the cell walls of grasses, but have also been detected in basal land plants, algae, fungi and bacteria. Here we show that two family GT2 glycosyltransferases from the Gram-positive bacterium Sarcina ventriculi are capable of synthesizing MLGs. Immunotransmission electron microscopy demonstrates that MLG is secreted as an exopolysaccharide, where it may play a role in organizing individual cells into packets that are characteristic of Sarcina species. Heterologous expression of these two genes shows that they are capable of producing MLGs in planta, including an MLG that is chemically identical to the MLG secreted from S. ventriculi cells but which has regularly spaced (1,3)-β-linkages in a structure not reported previously for MLGs. The tandemly arranged, paralogous pair of genes are designated SvBmlgs1 and SvBmlgs2. The data indicate that MLG synthases have evolved different enzymic mechanisms for the incorporation of (1,3)-β- and (1,4)-β-glucosyl residues into a single polysaccharide chain. Amino acid variants associated with the evolutionary switch from (1,4)-β-glucan (cellulose) to MLG synthesis have been identified in the active site regions of the enzymes. The presence of MLG synthesis in bacteria could prove valuable for large-scale production of MLG for medical, food and beverage applications.

Covalent connectivity of glycogen in brewer's spent yeast cell walls revealed by enzymatic approaches and dynamic nuclear polarization NMR

Yeast cell walls undergo modifications during the brewing process, leading to a remodelling of their architecture. One significant change is the increased insolubility of the cell wall glycogen pool, likely due to the formation of covalent bonds between glycogen and cell wall polysaccharides. To verify this hypothesis, we extracted the brewer's spent yeast with 4 M KOH, obtaining an insoluble glucan fraction (AE.4 M) primarily composed of (α1 → 4)- and (1 → 3)-linked Glc residues. Dynamic nuclear polarization solid-state NMR of AE.4 M revealed distinct glucan resonances that helped to differentiate between α- and β glucosyl (1 → 4)-linked residues, and confirm covalent linkages between (β1 → 3)-glucans and glycogen through a (β1 → 4)-linkage. The hydrolysis with different endo-glucanases (zymolyase, cellulase, and lichenase) was used to obtain solubilized high molecular weight glycogen fractions. NMR analysis showed that covalent links between glycogen and (β1 → 6)-glucans through (α1 → 6) glycosidic linkage, with branching at the C6 position involving (β1 → 3), and (β1 → 6)-glucans. HPAEC-PAD analysis of the enzymatically released oligosaccharides confirmed covalent linkages of (β1 → 3), (β1 → 6)-, and (β1 → 4)-glucan motifs with (α1 → 4)-glucans. This combination of multiple enzymatic approaches and NMR methods shed light into the role of yeast cell wall glycogen as a structural core covalently linked to other cell wall components during the brewing process.

Quantification of Mixed-Linkage β-Glucan (MLG) in Bacteria

Prokaryotes are known to produce and secrete a broad range of biopolymers with a high functional and structural heterogeneity, often with critical duties in the bacterial physiology and ecology. Among these, exopolysaccharides (EPS) play relevant roles in the interaction of bacteria with eukaryotic hosts. EPS can help to colonize the host and assist in bacterial survival, making this interaction more robust by facilitating the formation of structured biofilms. In addition, they are often key molecules in the specific recognition mechanisms involved in both beneficial and pathogenic bacteria-host interactions. A novel EPS known as MLG (Mixed-Linkage β-Glucan) was recently discovered in rhizobia, where it participates in bacterial aggregation and biofilm formation and is required for efficient attachment to the roots of their legume host plants. MLG is the first and, so far, the only reported linear Mixed-Linkage β-glucan in bacteria, containing a perfect alternation of β (1 → 3) and β (1 → 4) bonds. A phylogenetic study of MLG biosynthetic genes suggests that far from being exclusive of rhizobia, different soil and plant-associated bacteria likely produce MLG, adding this novel polymer to the plethora of surface polysaccharides that help bacteria thrive in the changing environment and to establish successful interactions with their hosts.In this work, a quantification method for MLG is proposed. It relays on the hydrolysis of MLG by a specific enzyme (lichenase), and the subsequent quantification of the released disaccharide (laminaribiose) by the phenol-sulfuric acid method. The protocol has been set up and optimized for its use in 96-well plates, which makes it suitable for high-throughput screening (HTS) approaches. This method stands out by its fast processing, technical simplicity, and capability to handle multiple samples and biological replicates at a time.

Advanced strategies for metabolic engineering of Bacillus to produce extracellular polymeric substances

Extracellular polymeric substances are mainly synthesized via a variety of biosynthetic pathways in bacteria. Bacilli-sourced extracellular polymeric substances, such as exopolysaccharides (EPS) and poly-γ-glutamic acid (γ-PGA), can serve as active ingredients and hydrogels, and have other important industrial applications. However, the functional diversity and widespread applications of these extracellular polymeric substances, are hampered by their low yields and high costs. Biosynthesis of extracellular polymeric substances is very complex in Bacillus, and there is no detailed elucidation of the reactions and regulations among various metabolic pathways. Therefore, a better understanding of the metabolic mechanisms is required to broaden the functions and increase the yield of extracellular polymeric substances. This review systematically summarizes the biosynthesis and metabolic mechanisms of extracellular polymeric substances in Bacillus, providing an in-depth understanding of the relationships between EPS and γ-PGA synthesis. This review provides a better clarification of Bacillus metabolic mechanisms during extracellular polymeric substance secretion and thus benefits their application and commercialization.

Functional diversity of c-di-GMP receptors in prokaryotic and eukaryotic systems

Cyclic bis-(3', 5')-dimeric guanosine monophosphate (c-di-GMP) is ubiquitous in many bacterial species, where it functions as a nucleotide-based secondary messenger and is a vital regulator of numerous biological processes. Due to its ubiquity, most bacterial species possess a wide range of downstream receptors that has a binding affinity to c-di-GMP and elicit output responses. In eukaryotes, several enzymes and riboswitches operate as receptors that interact with c-di-GMP and transduce cellular or environmental signals. This review examines the functional variety of receptors in prokaryotic and eukaryotic systems that exhibit distinct biological responses after interacting with c-di-GMP. Evolutionary relationships and similarities in distance among the c-di-GMP receptors in various bacterial species were evaluated to understand their specificities. Furthermore, residues of receptors involved in c-di-GMP binding are summarized. This review facilitates the understanding of how distinct receptors from different origins bind c-di-GMP equally well, yet fulfill diverse biological roles at the interspecies, intraspecies, and interkingdom levels. Furthermore, it also highlights c-di-GMP receptors as potential therapeutic targets, particularly those found in pathogenic microorganisms. Video Abstract.

The Gram-positive bacterium Romboutsia ilealis harbors a polysaccharide synthase that can produce (1,3;1,4)-β-D-glucans

(1,3;1,4)-β-D-Glucans are widely distributed in the cell walls of grasses (family Poaceae) and closely related families, as well as some other vascular plants. Additionally, they have been found in other organisms, including fungi, lichens, brown algae, charophycean green algae, and the bacterium Sinorhizobium meliloti. Only three members of the Cellulose Synthase-Like (CSL) genes in the families CSLF, CSLH, and CSLJ are implicated in (1,3;1,4)-β-D-glucan biosynthesis in grasses. Little is known about the enzymes responsible for synthesizing (1,3;1,4)-β-D-glucans outside the grasses. In the present study, we report the presence of (1,3;1,4)-β-D-glucans in the exopolysaccharides of the Gram-positive bacterium Romboutsia ilealis CRIB^T. We also report that RiGT2 is the candidate gene of R. ilealis that encodes (1,3;1,4)-β-D-glucan synthase. RiGT2 has conserved glycosyltransferase family 2 (GT2) motifs, including D, D, D, QXXRW, and a C-terminal PilZ domain that resembles the C-terminal domain of bacteria cellulose synthase, BcsA. Using a direct gain-of-function approach, we insert RiGT2 into Saccharomyces cerevisiae, and (1,3;1,4)-β-D-glucans are produced with structures similar to those of the (1,3;1,4)-β-D-glucans of the lichen Cetraria islandica. Phylogenetic analysis reveals that putative (1,3;1,4)-β-D-glucan synthase candidate genes in several other bacterial species support the finding of (1,3;1,4)-β-D-glucans in these species.

Gluco-oligosaccharides as potential prebiotics: Synthesis, purification, structural characterization, and evaluation of prebiotic effect

Prebiotics have long been used to modulate the gut microbiota and improve host health. Most established prebiotics are nondigestible carbohydrates, especially short-chain oligosaccharides. Recently, gluco-oligosaccharides (GlcOS) with 2-10 glucose residues and one or more O-glycosidic linkage(s) have been found to exert prebiotic potentials (not fully established prebiotics) because of their selective fermentation by beneficial gut bacteria. However, the prebiotic effects (non-digestibility, selective fermentability, and potential health effects) of GlcOS are highly variable due to their complex structure originating from different synthesis processes. The relationship between GlcOS structure and their potential prebiotic effects has not been fully understood. To date, a comprehensive summary of the knowledge of GlcOS is still missing. Therefore, this review provides an overview of GlcOS as potential prebiotics, covering their synthesis, purification, structural characterization, and prebiotic effect evaluation. First, GlcOS with different structures are introduced. Then, the enzymatic and chemical processes for GlcOS synthesis are critically reviewed, including reaction mechanisms, substrates, catalysts, the structures of resultant GlcOS, and the synthetic performance (yield and selectivity). Industrial separation techniques for GlcOS purification and structural characterization methods are discussed in detail. Finally, in vitro and in vivo studies to evaluate the non-digestibility, selective fermentability, and associated health effects of different GlcOS are extensively reviewed with a special focus on the GlcOS structure-function relationship.

Arabidopsis immune responses triggered by cellulose- and mixed-linked glucan-derived oligosaccharides require a group of leucine-rich repeat malectin receptor kinases

The plant immune system perceives a diversity of carbohydrate ligands from plant and microbial cell walls through the extracellular ectodomains (ECDs) of pattern recognition receptors (PRRs), which activate pattern-triggered immunity (PTI). Among these ligands are oligosaccharides derived from mixed-linked β-1,3/β-1,4-glucans (MLGs; e.g. β-1,4-D-(Glc)₂ -β-1,3-D-Glc, MLG43) and cellulose (e.g. β-1,4-D-(Glc)₃ , CEL3). The mechanisms behind carbohydrate perception in plants are poorly characterized except for fungal chitin oligosaccharides (e.g. β-1,4-d-(GlcNAc)₆ , CHI6), which involve several receptor kinase proteins (RKs) with LysM-ECDs. Here, we describe the isolation and characterization of Arabidopsis thaliana mutants impaired in glycan perception (igp) that are defective in PTI activation mediated by MLG43 and CEL3, but not by CHI6. igp1-igp4 are altered in three RKs - AT1G56145 (IGP1), AT1G56130 (IGP2/IGP3) and AT1G56140 (IGP4) - with leucine-rich-repeat (LRR) and malectin (MAL) domains in their ECDs. igp1 harbors point mutation E906K and igp2 and igp3 harbor point mutation G773E in their kinase domains, whereas igp4 is a T-DNA insertional loss-of-function mutant. Notably, isothermal titration calorimetry (ITC) assays with purified ECD-RKs of IGP1 and IGP3 showed that IGP1 binds with high affinity to CEL3 (with dissociation constant K_D  = 1.19 ± 0.03 μm) and cellopentaose (K_D  = 1.40 ± 0.01 μM), but not to MLG43, supporting its function as a plant PRR for cellulose-derived oligosaccharides. Our data suggest that these LRR-MAL RKs are components of a recognition mechanism for both cellulose- and MLG-derived oligosaccharide perception and downstream PTI activation in Arabidopsis.

The Role of Two Linear β-Glucans Activated by c-di-GMP in Rhizobium etli CFN42

Simple Summary

Bacterial exopolysaccharides (EPS) are secreted biopolymers with often critical roles in bacterial physiology and ecology. In addition to their biological role, there is increasing interest for EPS in various industrial sectors. β-glucans are among the most important ones including cellulose as the most abundant organic polymer on earth, but also newcomers, such as the bacterial Mixed Linkage β-Glucan (MLG), displaying a unique repeating unit suggestive of biotechnological potential. In this work we describe Rhizobium etli as the first bacterium reported to be able to produce these two linear β-glucans cellulose and MLG. Rhizobium etli is an agronomic relevant rhizobacteria able to perform Biological Nitrogen Fixation (BNF) in a symbiotic association with common bean plants. The production and regulation of cellulose and MLG by Rhizobium etli CFN42 is discussed and their impact on its free-living and symbiotic lifestyles evaluated.

Abstract

Bacterial exopolysaccharides (EPS) have been implicated in a variety of functions that assist in bacterial survival, colonization, and host–microbe interactions. Among them, bacterial linear β-glucans are polysaccharides formed by D-glucose units linked by β-glycosidic bonds, which include curdlan, cellulose, and the new described Mixed Linkage β-Glucan (MLG). Bis-(3′,5′)-cyclic dimeric guanosine monophosphate (c-di-GMP) is a universal bacterial second messenger that usually promote EPS production. Here, we report Rhizobium etli as the first bacterium capable of producing cellulose and MLG. Significant amounts of these two β-glucans are not produced under free-living laboratory conditions, but their production is triggered upon elevation of intracellular c-di-GMP levels, both contributing to Congo red (CR⁺) and Calcofluor (CF⁺) phenotypes. Cellulose turned out to be more relevant for free-living phenotypes promoting flocculation and biofilm formation under high c-di-GMP conditions. None of these two EPS are essential for attachment to roots of Phaseolus vulgaris, neither for nodulation nor for symbiotic nitrogen fixation. However, both β-glucans separately contribute to the fitness of interaction between R. etli and its host. Overproduction of these β-glucans, particularly cellulose, appears detrimental for symbiosis. This indicates that their activation by c-di-GMP must be strictly regulated in time and space and should be controlled by different, yet unknown, regulatory pathways.

Emerging Roles of β-Glucanases in Plant Development and Adaptative Responses

Plant β-glucanases are enzymes involved in the synthesis, remodelling and turnover of cell wall components during multiple physiological processes. Based on the type of the glycoside bond they cleave, plant β-glucanases have been grouped into three categories: (i) β-1,4-glucanases degrade cellulose and other polysaccharides containing 1,4-glycosidic bonds to remodel and disassemble the wall during cell growth. (ii) β-1,3-glucanases are responsible for the mobilization of callose, governing the symplastic trafficking through plasmodesmata. (iii) β-1,3-1,4-glucanases degrade mixed linkage glucan, a transient wall polysaccharide found in cereals, which is broken down to obtain energy during rapid seedling growth. In addition to their roles in the turnover of self-glucan structures, plant β-glucanases are crucial in regulating the outcome in symbiotic and hostile plant-microbe interactions by degrading non-self glucan structures. Plants use these enzymes to hydrolyse β-glucans found in the walls of microbes, not only by contributing to a local antimicrobial defence barrier, but also by generating signalling glucans triggering the activation of global responses. As a counterpart, microbes developed strategies to hijack plant β-glucanases to their advantage to successfully colonize plant tissues. This review outlines our current understanding on plant β-glucanases, with a particular focus on the latest advances on their roles in adaptative responses.

From Cancer Therapy to Winemaking: The Molecular Structure and Applications of β-Glucans and β-1, 3-Glucanases

β-glucans are a diverse group of polysaccharides composed of β-1,3 or β-(1,3-1,4) linked glucose monomers. They are mainly synthesized by fungi, plants, seaweed and bacteria, where they carry out structural, protective and energy storage roles. Because of their unique physicochemical properties, they have important applications in several industrial, biomedical and biotechnological processes. β-glucans are also major bioactive molecules with marked immunomodulatory and metabolic properties. As such, they have been the focus of many studies attesting to their ability to, among other roles, fight cancer, reduce the risk of cardiovascular diseases and control diabetes. The physicochemical and functional profiles of β-glucans are deeply influenced by their molecular structure. This structure governs β-glucan interaction with multiple β-glucan binding proteins, triggering myriad biological responses. It is then imperative to understand the structural properties of β-glucans to fully reveal their biological roles and potential applications. The deconstruction of β-glucans is a result of β-glucanase activity. In addition to being invaluable tools for the study of β-glucans, these enzymes have applications in numerous biotechnological and industrial processes, both alone and in conjunction with their natural substrates. Here, we review potential applications for β-glucans and β-glucanases, and explore how their functionalities are dictated by their structure.

Structural elucidation of the fucose containing polysaccharide of Paenibacillus polymyxa DSM 365

Paenibacillus polymyxa is an avid producer of exopolysaccharides of industrial interest. However, due to the complexity of the polymer composition, structural elucidation of the polysaccharide remained unfeasible for a long time. By using a CRISPR-Cas9 mediated knock-out strategy, all single glycosyltransferases as well as the Wzy polymerases were individually deleted in the corresponding gene cluster for the first time. Thereby, it was observed that the main polymer fraction was completely suppressed (or deleted) and a pure minor fucose containing polysaccharide could be isolated, which was named paenan II. Applying this combinatorial approach, the monosaccharide composition, sequence and linkage pattern of this novel polymer was determined via HPLC-MS, GC-MS and NMR. Furthermore, we demonstrated that the knock-out of the glycosyltransferases PepQ, PepT, PepU and PepV as well as of the Wzy polymerase PepG led to the absence of paenan II, attributing those enzymes to the assembly of the repeating unit.

Biosynthesis and applications of curdlan

Curdlan is widely applied in the food and pharmaceutical industries. This review focuses on the biosynthetic pathways, regulatory mechanisms and metabolic engineering strategies for curdlan production. Firstly, curdlan biosynthesis is discussed. Furthermore, various strategies to increase curdlan production are summarized from four aspects, including the overexpression of genes for curdlan biosynthesis, weakening/knockdown of genes from competing pathways, increasing the supply of curdlan precursors, and optimization of fermentation conditions. Moreover, the emerging and advanced applications of curdlan are introduced. Finally, the challenges that are frequently encountered during curdlan biosynthesis are noted with a discussion of directions for curdlan production.

Identification of a Novel Pyruvyltransferase Using 13C Solid-State Nuclear Magnetic Resonance To Analyze Rhizobial Exopolysaccharides

The alphaproteobacterium Sinorhizobium meliloti secretes two acidic exopolysaccharides (EPSs), succinoglycan (EPSI) and galactoglucan (EPSII), which differentially enable it to adapt to a changing environment. Succinoglycan is essential for invasion of plant hosts and, thus, for the formation of nitrogen-fixing root nodules. Galactoglucan is critical for population-based behaviors such as swarming and biofilm formation and can facilitate invasion in the absence of succinoglycan on some host plants. The biosynthesis of galactoglucan is not as completely understood as that of succinoglycan. We devised a pipeline to identify putative pyruvyltransferase and acetyltransferase genes, construct genomic deletions in strains engineered to produce either succinoglycan or galactoglucan, and analyze EPS from mutant bacterial strains. EPS samples were examined by ¹³C cross-polarization magic-angle spinning (CPMAS) solid-state nuclear magnetic resonance (NMR). CPMAS NMR is uniquely suited to defining chemical composition in complex samples and enables the detection and quantification of distinct EPS functional groups. Galactoglucan was isolated from mutant strains with deletions in five candidate acyl/acetyltransferase genes (exoZ, exoH, SMb20810, SMb21188, and SMa1016) and a putative pyruvyltransferase (wgaE or SMb21322). Most samples were similar in composition to wild-type EPSII by CPMAS NMR analysis. However, galactoglucan produced from a strain lacking wgaE exhibited a significant reduction in pyruvylation. Pyruvylation was restored through the ectopic expression of plasmid-borne wgaE. Our work has thus identified WgaE as a galactoglucan pyruvyltransferase. This exemplifies how the systematic combination of genetic analyses and solid-state NMR detection is a rapid means to identify genes responsible for modification of rhizobial exopolysaccharides. IMPORTANCE Nitrogen-fixing bacteria are crucial for geochemical cycles and global nitrogen nutrition. Symbioses between legumes and rhizobial bacteria establish root nodules, where bacteria convert dinitrogen to ammonia for plant utilization. Secreted exopolysaccharides (EPSs) produced by Sinorhizobium meliloti (succinoglycan and galactoglucan) play important roles in soil and plant environments. The biosynthesis of galactoglucan is not as well characterized as that of succinoglycan. We employed solid-state nuclear magnetic resonance (NMR) to examine intact EPS from wild-type and mutant S. meliloti strains. NMR analysis of EPS isolated from a wgaE gene mutant revealed a novel pyruvyltransferase that modifies galactoglucan. Few EPS pyruvyltransferases have been characterized. Our work provides insight into the biosynthesis of an important S. meliloti EPS and expands the knowledge of enzymes that modify polysaccharides.

Global Transcriptional Repression of Diguanylate Cyclases by MucR1 Is Essential for Sinorhizobium-Soybean Symbiosis

The ubiquitous bacterial second messenger c-di-GMP is intensively studied in pathogens but less so in mutualistic bacteria. Here, we report a genome-wide investigation of functional diguanylate cyclases (DGCs) synthesizing c-di-GMP from two molecules of GTP in Sinorhizobium fredii CCBAU45436, a facultative microsymbiont fixing nitrogen in nodules of diverse legumes, including soybean. Among 25 proteins harboring a putative GGDEF domain catalyzing the biosynthesis of c-di-GMP, eight functional DGCs were identified by heterogenous expression in Escherichia coli in a Congo red binding assay. This screening result was further verified by in vitro enzymatic assay with purified full proteins or the GGDEF domains from representative functional and nonfunctional DGCs. In the same in vitro assay, a functional EAL domain catalyzing the degradation of c-di-GMP into pGpG was identified in a protein that has an inactive GGDEF domain but with an active phosphodiesterase (PDE) function. The identified functional DGCs generally exhibited low transcription levels in soybean nodules compared to free-living cultures, as revealed in transcriptomes. An engineered upregulation of a functional DGC in nodules led to a significant increase of c-di-GMP level and symbiotic defects, which were not observed when a functional EAL domain was upregulated at the same level. Further transcriptional analysis and gel shift assay demonstrated that these functional DGCs were all transcriptionally repressed in nodules by a global pleiotropic regulator, MucR1, that is essential in Sinorhizobium-soybean symbiosis. These findings shed novel insights onto the systematic regulation of c-di-GMP biosynthesis in mutualistic symbiosis. IMPORTANCE The ubiquitous second messenger c-di-GMP is well-known for its role in biofilm formation and host adaptation of pathogens, whereas it is less investigated in mutualistic symbioses. Here, we reveal a cocktail of eight functional diguanylate cyclases (DGCs) catalyzing the biosynthesis of c-di-GMP in a broad-host-range Sinorhizobium that can establish nitrogen-fixing nodules on soybean and many other legumes. These functional DGCs are generally transcribed at low levels in soybean nodules compared to free-living conditions. The engineered nodule-specific upregulation of DGC can elevate the c-di-GMP level and cause symbiotic defects, while the upregulation of a phosphodiesterase that quenches c-di-GMP has no detectable symbiotic defects. Moreover, eight functional DGCs located on two different replicons are all directly repressed in nodules by a global silencer, MucR1, that is essential for Sinorhizobium-soybean symbiosis. These findings represent a novel mechanism of a strategic regulation of the c-di-GMP biosynthesis arsenal in prokaryote-eukaryote interactions.

Bacterial Extracellular Polymers: A Review

Prokaryotic microbial cells especially bacteria are highly emphases for their exopolysaccharides (EPS) production. EPS are the higher molecular weight natural extracellular compounds observe at the surface of the bacterial cells. Nowadays bacterial EPS represent rapidly emerging as new and industrially important biomaterials because it having tremendous physical and chemical properties with novel functionality. Due to its industrial demand as well as research studies the different extraction processes have been discovered to remove the EPS from the microbial biofilm. The novelties of EPS are also based on the microbial habitat conditions such as higher temperature, lower temperature, acidic, alkaliphilic, saline, etc. Based on its chemical structure they can be homopolysaccharide or heteropolysaccharide. EPSs have a wide range of applications in various industries such as food, textile, pharmaceutical, heavy metal recovery, agriculture, etc. So, this review focus on the understanding of the structure, different extraction processes, biosynthesis and genetic engineering of EPS as well as their desirable biotechnological applications.

Cyclic di-GMP signaling controlling the free-living lifestyle of alpha-proteobacterial rhizobia

Cyclic-di-GMP (c-di-GMP) is a ubiquitous bacterial second messenger which has been associated with a motile to sessile lifestyle switch in many bacteria. Here, we review recent insights into c-di-GMP regulated processes related to environmental adaptations in alphaproteobacterial rhizobia, which are diazotrophic bacteria capable of fixing nitrogen in symbiosis with their leguminous host plants. The review centers on Sinorhizobium meliloti, which in the recent years was intensively studied for its c-di-GMP regulatory network.

Cell wall-derived mixed-linked β-1,3/1,4-glucans trigger immune responses and disease resistance in plants

Pattern-triggered immunity (PTI) is activated in plants upon recognition by pattern recognition receptors (PRRs) of damage- and microbe-associated molecular patterns (DAMPs and MAMPs) derived from plants or microorganisms, respectively. To understand better the plant mechanisms involved in the perception of carbohydrate-based structures recognized as DAMPs/MAMPs, we have studied the ability of mixed-linked β-1,3/1,4-glucans (MLGs), present in some plant and microbial cell walls, to trigger immune responses and disease resistance in plants. A range of MLG structures were tested for their capacity to induce PTI hallmarks, such as cytoplasmic Ca²⁺ elevations, reactive oxygen species production, phosphorylation of mitogen-activated protein kinases and gene transcriptional reprogramming. These analyses revealed that MLG oligosaccharides are perceived by Arabidopsis thaliana and identified a trisaccharide, β-d-cellobiosyl-(1,3)-β-d-glucose (MLG43), as the smallest MLG structure triggering strong PTI responses. These MLG43-mediated PTI responses are partially dependent on LysM PRRs CERK1, LYK4 and LYK5, as they were weaker in cerk1 and lyk4 lyk5 mutants than in wild-type plants. Cross-elicitation experiments between MLG43 and the carbohydrate MAMP chitohexaose [β-1,4-d-(GlcNAc)₆ ], which is also perceived by these LysM PRRs, indicated that the mechanism of MLG43 recognition could differ from that of chitohexaose, which is fully impaired in cerk1 and lyk4 lyk5 plants. MLG43 treatment confers enhanced disease resistance in A. thaliana to the oomycete Hyaloperonospora arabidopsidis and in tomato and pepper to different bacterial and fungal pathogens. Our data support the classification of MLGs as a group of carbohydrate-based molecular patterns that are perceived by plants and trigger immune responses and disease resistance.

Structures, Biosynthesis, and Physiological Functions of (1,3;1,4)-β-D-Glucans

(1,3;1,4)-β-d-Glucans, also named as mixed-linkage glucans, are unbranched non-cellulosic polysaccharides containing both (1,3)- and (1,4)-β-linkages. The linkage ratio varies depending upon species origin and has a significant impact on the physicochemical properties of the (1,3;1,4)-β-d-glucans. (1,3;1,4)-β-d-Glucans were thought to be unique in the grasses family (Poaceae); however, evidence has shown that (1,3;1,4)-β-d-glucans are also synthesized in other taxa, including horsetail fern Equisetum, algae, lichens, and fungi, and more recently, bacteria. The enzyme involved in (1,3;1,4)-β-d-glucan biosynthesis has been well studied in grasses and cereal. However, how this enzyme is able to assemble the two different linkages remains a matter of debate. Additionally, the presence of (1,3;1,4)-β-d-glucan across the species evolutionarily distant from Poaceae but absence in some evolutionarily closely related species suggest that the synthesis is either highly conserved or has arisen twice as a result of convergent evolution. Here, we compare the structure of (1,3;1,4)-β-d-glucans present across various taxonomic groups and provide up-to-date information on how (1,3;1,4)-β-d-glucans are synthesized and their functions.

Exopolysaccharide Characterization of Rhizobium favelukesii LPU83 and Its Role in the Symbiosis With Alfalfa

One of the greatest inputs of available nitrogen into the biosphere occurs through the biological N2-fixation to ammonium as result of the symbiosis between rhizobia and leguminous plants. These interactions allow increased crop yields on nitrogen-poor soils. Exopolysaccharides (EPS) are key components for the establishment of an effective symbiosis between alfalfa and Ensifer meliloti, as bacteria that lack EPS are unable to infect the host plants. Rhizobium favelukesii LPU83 is an acid-tolerant rhizobia strain capable of nodulating alfalfa but inefficient to fix nitrogen. Aiming to identify the molecular determinants that allow R. favelukesii to infect plants, we studied its EPS biosynthesis. LPU83 produces an EPS I identical to the one present in E. meliloti, but the organization of the genes involved in its synthesis is different. The main gene cluster needed for the synthesis of EPS I in E. meliloti, is split into three different sections in R. favelukesii, which probably arose by a recent event of horizontal gene transfer. A R. favelukesii strain devoided of all the genes needed for the synthesis of EPS I is still able to infect and nodulate alfalfa, suggesting that attention should be directed to other molecules involved in the development of the symbiosis.

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.

Quorum Sensing Signaling Molecules Positively Regulate c-di-GMP Effector PelD Encoding Gene and PEL Exopolysaccharide Biosynthesis in Extremophile Bacterium Acidithiobacillus thiooxidans

Acidithiobacillus species are fundamental players in biofilm formation by acidophile bioleaching communities. It has been previously reported that Acidithiobacillus ferrooxidans possesses a functional quorum sensing mediated by acyl-homoserine lactones (AHL), involved in biofilm formation, and AHLs naturally produced by Acidithiobacillus species also induce biofilm formation in Acidithiobacillus thiooxidans. A c-di-GMP pathway has been characterized in Acidithiobacillus species but it has been pointed out that the c-di-GMP effector PelD and pel-like operon are only present in the sulfur oxidizers such as A. thiooxidans. PEL exopolysaccharide has been recently involved in biofilm formation in this Acidithiobacillus species. Here, by comparing wild type and ΔpelD strains through mechanical analysis of biofilm-cells detachment, fluorescence microscopy and qPCR experiments, the structural role of PEL exopolysaccharide and the molecular network involved for its biosynthesis by A. thiooxidans were tackled. Besides, the effect of AHLs on PEL exopolysaccharide production was assessed. Mechanical resistance experiments indicated that the loss of PEL exopolysaccharide produces fragile A. thiooxidans biofilms. qRT-PCR analysis established that AHLs induce the transcription of pelA and pelD genes while epifluorescence microscopy studies revealed that PEL exopolysaccharide was required for the development of AHL-induced biofilms. Altogether these results reveal for the first time that AHLs positively regulate pel genes and participate in the molecular network for PEL exopolysaccharide biosynthesis by A. thiooxidans.

Mixed Linkage β-1,3/1,4-Glucan Oligosaccharides Induce Defense Responses in Hordeum vulgare and Arabidopsis thaliana

Plants detect conserved microbe-associated molecular patterns (MAMPs) and modified "self" molecules produced during pathogen infection [danger associated molecular patterns (DAMPs)] with plasma membrane-resident pattern recognition receptors (PRRs). PRR-mediated MAMP and/or DAMP perception activates signal transduction cascades, transcriptional reprogramming and plant immune responses collectively referred to as pattern-triggered immunity (PTI). Potential sources for MAMPs and DAMPs are microbial and plant cell walls, which are complex extracellular matrices composed of different carbohydrates and glycoproteins. Mixed linkage β-1,3/1,4-glucan (β-1,3/1,4-MLG) oligosaccharides are abundant components of monocot plant cell walls and are present in symbiotic, pathogenic and apathogenic fungi, oomycetes and bacteria, but have not been detected in the cell walls of dicot plant species so far. Here, we provide evidence that the monocot crop plant H. vulgare and the dicot A. thaliana can perceive β-1,3/1,4-MLG oligosaccharides and react with prototypical PTI responses. A collection of Arabidopsis innate immunity signaling mutants and >100 Arabidopsis ecotypes showed unaltered responses upon treatment with β-1,3/1,4-MLG oligosaccharides suggesting the employment of a so far unknown and highly conserved perception machinery. In conclusion, we postulate that β-1,3/1,4-MLG oligosaccharides have the dual capacity to act as immune-active DAMPs and/or MAMPs in monocot and dicot plant species.

Polyamines produced by Sinorhizobium meliloti Rm8530 contribute to symbiotically relevant phenotypes ex planta and to nodulation efficiency on alfalfa

In nitrogen-fixing rhizobia, emerging evidence shows significant roles for polyamines in growth and abiotic stress resistance. In this work we show that a polyamine-deficient ornithine decarboxylase null mutant (odc2) derived from Sinorhizobium meliloti Rm8530 had significant phenotypic differences from the wild-type, including greatly reduced production of exopolysaccharides (EPS; ostensibly both succinoglycan and galactoglucan), increased sensitivity to oxidative stress and decreased swimming motility. The introduction of the odc2 gene borne on a plasmid into the odc2 mutant restored wild-type phenotypes for EPS production, growth under oxidative stress and swimming. The production of calcofluor-binding EPS (succinoglycan) by the odc2 mutant was also completely or mostly restored in the presence of exogenous spermidine (Spd), norspermidine (NSpd) or spermine (Spm). The odc2 mutant formed about 25 % more biofilm than the wild-type, and its ability to form biofilm was significantly inhibited by exogenous Spd, NSpd or Spm. The odc2 mutant formed a less efficient symbiosis with alfalfa, resulting in plants with significantly less biomass and height, more nodules but less nodule biomass, and 25 % less nitrogen-fixing activity. Exogenously supplied Put was not able to revert these phenotypes and caused a similar increase in plant height and dry weight in uninoculated plants and in those inoculated with the wild-type or odc2 mutant. We discuss ways in which polyamines might affect the phenotypes of the odc2 mutant.

Differences in protein structural regions that impact functional specificity in GT2 family β-glucan synthases

Most cell wall and secreted β-glucans are synthesised by the CAZy Glycosyltransferase 2 family (www.cazy.org), with different members catalysing the formation of (1,4)-β-, (1,3)-β-, or both (1,4)- and (1,3)-β-glucosidic linkages. Given the distinct physicochemical properties of each of the resultant β-glucans (cellulose, curdlan, and mixed linkage glucan, respectively) are crucial to their biological and biotechnological functions, there is a desire to understand the molecular evolution of synthesis and how linkage specificity is determined. With structural studies hamstrung by the instability of these proteins to solubilisation, we have utilised in silico techniques and the crystal structure for a bacterial cellulose synthase to further understand how these enzymes have evolved distinct functions. Sequence and phylogenetic analyses were performed to determine amino acid conservation, both family-wide and within each sub-family. Further structural analysis centred on comparison of a bacterial curdlan synthase homology model with the bacterial cellulose synthase crystal structure, with molecular dynamics simulations performed with their respective β-glucan products bound in the trans-membrane channel. Key residues that differentially interact with the different β-glucan chains and have sub-family-specific conservation were found to reside at the entrance of the trans-membrane channel. The linkage-specific catalytic activity of these enzymes and hence the type of β-glucan chain built is thus likely determined by the different interactions between the proteins and the first few glucose residues in the channel, which in turn dictates the position of the acceptor glucose. The sequence-function relationships for the bacterial β-glucan synthases pave the way for extending this understanding to other kingdoms, such as plants.

A partner-switching system controls activation of mixed-linkage β-glucan synthesis by c-di-GMP in Sinorhizobium meliloti

Sinorhizobium meliloti synthesizes a linear mixed-linkage (1 → 3)(1 → 4)-β-d-glucan (ML β-glucan, MLG) in response to high levels of cyclic diguanylate (c-di-GMP). Two proteins BgsA and BgsB are required for MLG synthesis, BgsA being the glucan synthase which is activated upon c-di-GMP binding to its C-terminal domain. Here we report that the product of bgrR (SMb20447) is a diguanylate cyclase (DGC) that provides c-di-GMP for the synthesis of MLG by BgsA. bgrR is the first gene of a hexacistronic bgrRSTUWV operon, likely encoding a partner-switching regulatory network where BgrR is the final target. Using different approaches, we have determined that the products of genes bgrU (containing a putative PP2C serine phosphatase domain) and bgrW (with predicted kinase effector domain), modulate the phosphorylation status and the activity of the STAS domain protein BgrV. We propose that unphosphorylated BgrV inhibits BgrR DGC activity, perhaps through direct protein-protein interactions as established for other partner switchers. A bgrRSTUWV operon coexists with MLG structural bgsBA genes in many rhizobial genomes but is also present in some MLG non-producers, suggesting a role of this partner-switching system in other processes besides MLG biosynthesis.

Bacterial cellulose: a versatile biopolymer for wound dressing applications

Although several therapeutic approaches are available for wound and burn treatment and much progress has been made in this area, room for improvement still exists, driven by the urgent need of better strategies to accelerate wound healing and recovery, mostly for cases of severe burned patients. Bacterial cellulose (BC) is a biopolymer produced by bacteria with several advantages over vegetal cellulose, such as purity, high porosity, permeability to liquid and gases, elevated water uptake capacity and mechanical robustness. Besides its biocompatibility, BC can be modified in order to acquire antibacterial response and possible local drug delivery features. Due to its intrinsic versatility, BC is the perfect example of a biotechnological response to a clinical problem. In this review, we assess the BC main features and emphasis is given to a specific biomedical application: wound dressings. The production process and the physical-chemical properties that entitle this material to be used as wound dressing namely for burn healing are highlighted. An overview of the most common BC composites and their enhanced properties, in particular physical and biological, is provided, including the different production processes. A particular focus is given to the biochemistry and genetic manipulation of BC. A summary of the current marketed BC-based wound dressing products is presented, and finally, future perspectives for the usage of BC as wound dressing are foreseen.

Transcriptomic Studies Reveal that the Rhizobium leguminosarum Serine/Threonine Protein Phosphatase PssZ has a Role in the Synthesis of Cell-Surface Components, Nutrient Utilization, and Other Cellular Processes

Rhizobium leguminosarum bv. trifolii is a soil bacterium capable of establishing symbiotic associations with clover plants (Trifolium spp.). Surface polysaccharides, transport systems, and extracellular components synthesized by this bacterium are required for both the adaptation to changing environmental conditions and successful infection of host plant roots. The pssZ gene located in the Pss-I region, which is involved in the synthesis of extracellular polysaccharide, encodes a protein belonging to the group of serine/threonine protein phosphatases. In this study, a comparative transcriptomic analysis of R. leguminosarum bv. trifolii wild-type strain Rt24.2 and its derivative Rt297 carrying a pssZ mutation was performed. RNA-Seq data identified a large number of genes differentially expressed in these two backgrounds. Transcriptome profiling of the pssZ mutant revealed a role of the PssZ protein in several cellular processes, including cell signalling, transcription regulation, synthesis of cell-surface polysaccharides and components, and bacterial metabolism. In addition, we show that inactivation of pssZ affects the rhizobial ability to grow in the presence of different sugars and at various temperatures, as well as the production of different surface polysaccharides. In conclusion, our results identified a set of genes whose expression was affected by PssZ and confirmed the important role of this protein in the rhizobial regulatory network.

Alkaline Extraction, Structural Characterization, and Bioactivities of (1→6)-β-d-Glucan from Lentinus edodes

The purpose of this study is to develop a robust approach to obtain β glucans from Lentinus edodes and to characterize their structural and biological properties for sustainable utilization. The alkali extraction was optimized with an orthogonal experimental design, and a concise process for obtaining specific targeting polysaccharides from Lentinus edodes was developed in this study. After purification with a Q-Sepharose Fast Flow strong anion-exchange column, the monosaccharide composition, a methylation analysis, and NMR spectroscopy were employed for their structural characterizations. LeP-N2 was found to be composed of (1→6)-β-d-glucans with minor β-(1→3) glucosidic side chains. Atomic force microscopy (AFM) and high-performance gel permeation chromatography–refractive index–multi-angle laser light scattering (HPGPC-RI-MALLS) also revealed LeP-N2 exhibiting a compact unit in aqueous solution. This (1→6)-β-d-glucan was tested for antioxidant activities with IC₅₀ at 157 μg/mL. Moreover, RAW 264.7 macrophage activation indicated that the release of nitric oxide (NO) and reactive oxygen species (ROS) were markedly increased with no cytotoxicity at a dose of 100 μg/mL. These findings suggest that the (1→6)-β-d-glucans obtained from Lentinus edodes could serve as potential agents in the fields of functional foods or medicine.

A Bifunctional UDP-Sugar 4-Epimerase Supports Biosynthesis of Multiple Cell Surface Polysaccharides in Sinorhizobium meliloti

Sinorhizobium meliloti produces multiple extracellular glycans, including among others, lipopolysaccharides (LPS), and the exopolysaccharides (EPS) succinoglycan (SG) and galactoglucan (GG). These polysaccharides serve cell protective roles. Furthermore, SG and GG promote the interaction of S. meliloti with its host Medicago sativa in root nodule symbiosis. ExoB has been suggested to be the sole enzyme catalyzing synthesis of UDP-galactose in S. meliloti (A. M. Buendia, B. Enenkel, R. Köplin, K. Niehaus, et al. Mol Microbiol 5:1519-1530, 1991, https://doi.org/10.1111/j.1365-2958.1991.tb00799.x). Accordingly, exoB mutants were previously found to be affected in the synthesis of the galactose-containing glycans LPS, SG, and GG and consequently, in symbiosis. Here, we report that the S. meliloti Rm2011 uxs1-uxe-apsS-apsH1-apsE-apsH2 (SMb20458-63) gene cluster directs biosynthesis of an arabinose-containing polysaccharide (APS), which contributes to biofilm formation, and is solely or mainly composed of arabinose. Uxe has previously been identified as UDP-xylose 4-epimerase. Collectively, our data from mutational and overexpression analyses of the APS biosynthesis genes and in vitro enzymatic assays indicate that Uxe functions as UDP-xylose 4- and UDP-glucose 4-epimerase catalyzing UDP-xylose/UDP-arabinose and UDP-glucose/UDP-galactose interconversions, respectively. Overexpression of uxe suppressed the phenotypes of an exoB mutant, evidencing that Uxe can functionally replace ExoB. We suggest that under conditions stimulating expression of the APS biosynthesis operon, Uxe contributes to the synthesis of multiple glycans and thereby to cell protection, biofilm formation, and symbiosis. Furthermore, we show that the C₂H₂ zinc finger transcriptional regulator MucR counteracts the previously reported CuxR-c-di-GMP-mediated activation of the APS biosynthesis operon. This integrates the c-di-GMP-dependent control of APS production into the opposing regulation of EPS biosynthesis and swimming motility in S. meliloti IMPORTANCE Bacterial extracellular polysaccharides serve important cell protective, structural, and signaling roles. They have particularly attracted attention as adhesives and matrix components promoting biofilm formation, which significantly contributes to resistance against antibiotics. In the root nodule symbiosis between rhizobia and leguminous plants, extracellular polysaccharides have a signaling function. UDP-sugar 4-epimerases are important enzymes in the synthesis of the activated sugar substrates, which are frequently shared between multiple polysaccharide biosynthesis pathways. Thus, these enzymes are potential targets to interfere with these pathways. Our finding of a bifunctional UDP-sugar 4-epimerase in Sinorhizobium meliloti generally advances the knowledge of substrate promiscuity of such enzymes and specifically of the biosynthesis of extracellular polysaccharides involved in biofilm formation and symbiosis in this alphaproteobacterium.

AmrZ and FleQ Co-regulate Cellulose Production in Pseudomonas syringae pv. Tomato DC3000

Pseudomonas syringae pv. tomato DC3000 carries the wssABCDEFGHI operon for the synthesis of acetylated cellulose, whose production is stimulated by increasing the intracellular levels of the second messenger c-di-GMP. This enhances air-liquid biofilm formation and generates a wrinkly colony morphotype in solid media. In the present study we show that cellulose production is a complex process regulated at multiple levels and involving different players in this bacterium. Using different in vitro approaches, including Electrophoretic Mobility Shift Assay (EMSA) and footprint analysis, we demonstrated the interrelated role of two transcriptional regulators, AmrZ and FleQ, over cellulose production in Pto DC3000 and the influence of c-di-GMP in this process. Under physiological c-di-GMP levels, both regulators bind directly to adjacent regions at the wss promoter inhibiting its expression. However, just FleQ responds to c-di-GMP releasing from its wss operator site and converting from a repressor to an activator of cellulose production. The additive effect of the double amrZ/fleQ mutation on the expression of wss, together with the fact that they are not cross-regulated at the transcriptional level, suggest that FleQ and AmrZ behave as independent regulators, unlike what has been described in other Pseudomonas species. Furthermore, this dual co-regulation exerted by AmrZ and FleQ is not limited to cellulose production, but also affects other important phenotypes in Pto DC3000, such as motility and virulence.

Screening of c-di-GMP-Regulated Exopolysaccharides in Host Interacting Bacteria

Bacterial exopolysaccharides (EPS) often confer a survival advantage by protecting the cell against abiotic and biotic stresses, including host defensive factors. They are also main components of the extracellular matrix involved in cell-cell recognition, surface adhesion and biofilm formation. Biosynthesis of a growing number of EPS has been reported to be regulated by the ubiquitous second messenger c-di-GMP, which promotes the transition to a biofilm mode of growth in an intimate association with the eukaryotic host. Here we describe a strategy based on the combination of an approach to artificially increase the intracellular level of c-di-GMP in virtually any gram-negative bacteria with a high throughput screening (HTS) for the identification of monosaccharide composition and carbohydrate fingerprinting of novel EPS, or modified variants, that can be involved in host-bacteria interactions.

Water-Soluble Humic Materials Regulate Quorum Sensing in Sinorhizobium meliloti Through a Novel Repressor of expR

Quorum sensing (QS) plays an important role in the growth, nodulation, and nitrogen fixation of rhizobia. In this study, we show that water-soluble humic materials (WSHM) repress the expression of the QS related genes sinI, sinR, and expR in Sinorhizobium meliloti. This decreased the production of N-acetyl homoserine lactones (AHL) and exopolysaccharides (EPS), and ultimately increased S. meliloti cell density. We also identified a novel regulator, SMc03890 (renamed QsrR), which binds directly to the expR promoter. Deletion of qsrR increased expR expression. WSHM repressed the expression of expR by augmenting the interaction between QsrR and the expR promoter; this was determined by a bacterial-one-hybrid assay. These effects of WSHM on the QS system in S. meliloti may be the underlying mechanism by which WSHM increase the symbiotic nitrogen fixation of Medicago sativa inoculated with S. meliloti. This study provides the first evidence that humic acids regulate the QS of rhizobia and suggests that WSHM could be used as fertilizers to improve the efficiency of symbiotic nitrogen fixation.

Functional Characterization of a Glycosyltransferase from the Moss Physcomitrella patens Involved in the Biosynthesis of a Novel Cell Wall Arabinoglucan

Mixed-linkage (1,3;1,4)-β-glucan (MLG), an abundant cell wall polysaccharide in the Poaceae, has been detected in ascomycetes, algae, and seedless vascular plants, but not in eudicots. Although MLG has not been reported in bryophytes, a predicted glycosyltransferase from the moss Physcomitrella patens (Pp3c12_24670) is similar to a bona fide ascomycete MLG synthase. We tested whether Pp3c12_24670 encodes an MLG synthase by expressing it in wild tobacco (Nicotiana benthamiana) and testing for release of diagnostic oligosaccharides from the cell walls by either lichenase or (1,4)-β-glucan endohydrolase. Lichenase, an MLG-specific endohydrolase, showed no activity against cell walls from transformed N. benthamiana, but (1,4)-β-glucan endohydrolase released oligosaccharides that were distinct from oligosaccharides released from MLG by this enzyme. Further analysis revealed that these oligosaccharides were derived from a novel unbranched, unsubstituted arabinoglucan (AGlc) polysaccharide. We identified sequences similar to the P. patens AGlc synthase from algae, bryophytes, lycophytes, and monilophytes, raising the possibility that other early divergent plants synthesize AGlc. Similarity of P. patens AGlc synthase to MLG synthases from ascomycetes, but not those from Poaceae, suggests that AGlc and MLG have a common evolutionary history that includes loss in seed plants, followed by a more recent independent origin of MLG within the monocots.

Regulation Mediated by N-Acyl Homoserine Lactone Quorum Sensing Signals in the Rhizobium-Legume Symbiosis

Soil-dwelling bacteria collectively referred to as rhizobia synthesize and perceive N-acyl-homoserine lactone (AHL) signals to regulate gene expression in a population density-dependent manner. AHL-mediated signaling in these bacteria regulates several functions which are important for the establishment of nitrogen-fixing symbiosis with legume plants. Moreover, rhizobial AHL act as interkingdom signals triggering plant responses that impact the plant-bacteria interaction. Both the regulatory mechanisms that control AHL synthesis in rhizobia and the set of bacterial genes and associated traits under quorum sensing (QS) control vary greatly among the rhizobial species. In this article, we focus on the well-known QS system of the alfalfa symbiont Sinorhizobium(Ensifer)meliloti. Bacterial genes, environmental factors and transcriptional and posttranscriptional regulatory mechanisms that control AHL production in this Rhizobium, as well as the effects of the signaling molecule on bacterial phenotypes and plant responses will be reviewed. Current knowledge of S. meliloti QS will be compared with that of other rhizobia. Finally, participation of the legume host in QS by interfering with rhizobial AHL perception through the production of molecular mimics will also be addressed.

Genetic identification of factors for extracellular cellulose accumulation in the thermophilic cyanobacterium Thermosynechococcus vulcanus: proposal of a novel tripartite secretion system

Cells of the thermophilic cyanobacterium Thermosynechococcus vulcanus strain RKN (NIES-2134) aggregate and produce extracellular cellulose under induced conditions of blue light and low temperature, and both aggregation and cellulose production require the cellulose synthase Tll0007 (XcsA) and photosensory diguanylate cyclases. However, overexpression of both the cellulose synthase and a constitutively active diguanylate cyclase was not sufficient to induce cellulose-mediated cell aggregation under normal growth conditions. Synteny analysis and gene knockout revealed that two putative genes, hlyD-like tlr0903 (xcsB) and endoglucanase-like tlr1902 (xcsC), are linked to tll0007, although they are located apart from tll0007 in the T. vulcanus genome. Gene knockdown revealed that tlr1605 (tolC) was essential for the cellulose-mediated cell aggregation. Low temperature induced marked upregulation of tlr0903, and overexpression of both tlr0903 (but not tlr1902) and diguanylate cyclase resulted in the strong cell aggregation and cellulose accumulation under normal conditions. Based on these and phylogenetic analysis, we propose that the cyanobacterial extracellular cellulose production is due to a novel variant of the bacterial tripartite secretion system.

Recent insights in microbial exopolysaccharide biosynthesis and engineering strategies

The distinct biosynthesis pathways for microbial exopolysaccharide production provide different engineering strategies to tailor the chemical structures of the final polymers. This review focuses on the latest insights in the various pathways and identifies bottlenecks as well as promising targets for tailoring microbial polysaccharide production. The main engineering strategies includes the combinatorial assembly of glycosyltransferases and engineering of the Wzx and Wzy proteins for flipping of repeating units as well as polymerization. In the case of synthase based polysaccharides, the use of epimerases or engineering approaches of the synthase itself as well as overexpression of c-di-GMP levels is identified as one of the most promising strategies. For sucrase-based biosynthesis, the in vitro production by engineered sucrase enzymes or adjusted production conditions is shown as a very promising method.

A novel c-di-GMP binding domain in glycosyltransferase BgsA is responsible for the synthesis of a mixed-linkage β-glucan

BgsA is the glycosyltransferase (GT) involved in the synthesis of a linear mixed-linkage β-glucan (MLG), a recently described exopolysaccharide activated by c-di-GMP in Sinorhizobium meliloti and other Rhizobiales. Although BgsA displays sequence and structural homology with bacterial cellulose synthases (CS), it does not contain any predictable c-di-GMP binding domain. In this work we demonstrate that the cytoplasmic C-terminal domain of BgsA (C-BgsA) binds c-di-GMP with both high affinity (K_D = 0.23 μM) and specificity. C-BgsA is structurally different to the otherwise equivalent cytoplasmic C-terminal domain of CS, and does not contain PilZ motifs for c-di-GMP recognition. A combination of random and site-directed mutagenesis with surface plasmon resonance (SPR) allowed identification of the C-BgsA residues which are important not only for c-di-GMP binding, but also for BgsA GT activity. The results suggest that the C-BgsA domain is important for both, c-di-GMP binding and GT activity of BgsA. In contrast to bacterial CS where c-di-GMP has been proposed as a derepressor of GT activity, we hypothesize that the C-terminal domain of BgsA plays an active role in BgsA GT activity upon binding c-di-GMP.

Insoluble (1 → 3), (1 → 4)-β-D-glucan is a component of cell walls in brown algae (Phaeophyceae) and is masked by alginates in tissues

Brown algae are photosynthetic multicellular marine organisms. They belong to the phylum of Stramenopiles, which are not closely related to land plants and green algae. Brown algae share common evolutionary features with other photosynthetic and multicellular organisms, including a carbohydrate-rich cell-wall. Brown algal cell walls are composed predominantly of the polyanionic polysaccharides alginates and fucose-containing sulfated polysaccharides. These polymers are prevalent over neutral and crystalline components, which are believed to be mostly, if not exclusively, cellulose. In an attempt to better understand brown algal cell walls, we performed an extensive glycan array analysis of a wide range of brown algal species. Here we provide the first demonstration that mixed-linkage (1 → 3), (1 → 4)-β-D-glucan (MLG) is common in brown algal cell walls. Ultra-Performance Liquid Chromatography analyses indicate that MLG in brown algae solely consists of trisaccharide units of contiguous (1 → 4)-β-linked glucose residues joined by (1 → 3)-β-linkages. This regular conformation may allow long stretches of the molecule to align and to form well-structured microfibrils. At the tissue level, immunofluorescence studies indicate that MLG epitopes in brown algae are unmasked by a pre-treatment with alginate lyases to remove alginates. These findings are further discussed in terms of the origin and evolution of MLG in the Stramenopile lineage.

Cohesive Properties of the Caulobacter crescentus Holdfast Adhesin Are Regulated by a Novel c-di-GMP Effector Protein

When encountering surfaces, many bacteria produce adhesins to facilitate their initial attachment and to irreversibly glue themselves to the solid substrate. A central molecule regulating the processes of this motile-sessile transition is the second messenger c-di-GMP, which stimulates the production of a variety of exopolysaccharide adhesins in different bacterial model organisms. In Caulobacter crescentus, c-di-GMP regulates the synthesis of the polar holdfast adhesin during the cell cycle, yet the molecular and cellular details of this control are currently unknown. Here we identify HfsK, a member of a versatile N-acetyltransferase family, as a novel c-di-GMP effector involved in holdfast biogenesis. Cells lacking HfsK form highly malleable holdfast structures with reduced adhesive strength that cannot support surface colonization. We present indirect evidence that HfsK modifies the polysaccharide component of holdfast to buttress its cohesive properties. HfsK is a soluble protein but associates with the cell membrane during most of the cell cycle. Coincident with peak c-di-GMP levels during the C. crescentus cell cycle, HfsK relocalizes to the cytosol in a c-di-GMP-dependent manner. Our results indicate that this c-di-GMP-mediated dynamic positioning controls HfsK activity, leading to its inactivation at high c-di-GMP levels. A short C-terminal extension is essential for the membrane association, c-di-GMP binding, and activity of HfsK. We propose a model in which c-di-GMP binding leads to the dispersal and inactivation of HfsK as part of holdfast biogenesis progression.

Exopolysaccharide (EPS) adhesins are important determinants of bacterial surface colonization and biofilm formation. Biofilms are a major cause of chronic infections and are responsible for biofouling on water-exposed surfaces. To tackle these problems, it is essential to dissect the processes leading to surface colonization at the molecular and cellular levels. Here we describe a novel c-di-GMP effector, HfsK, that contributes to the cohesive properties and stability of the holdfast adhesin in C. crescentus. We demonstrate for the first time that c-di-GMP, in addition to its role in the regulation of the rate of EPS production, also modulates the physicochemical properties of bacterial adhesins. By demonstrating how c-di-GMP coordinates the activity and subcellular localization of HfsK, we provide a novel understanding of the cellular processes involved in adhesin biogenesis control. Homologs of HfsK are found in representatives of different bacterial phyla, suggesting that they play important roles in various EPS synthesis systems.

Exploiting the commons: cyclic diguanylate regulation of bacterial exopolysaccharide production

Nowadays, there is increasing interest for bacterial polysaccharides in a wide variety of industrial sectors. This is due to their chemical and reological properties, and also the possibility to be obtained by fermentation processes. Biosynthesis of a growing number of exopolysaccharides (EPS) has been reported to be regulated by the ubiquitous second messenger c-di-GMP in a limited number of bacterial species. Since most bacteria are yet unexplored, it is likely that an unsuspected number and variety of EPS structures activated by c-di-GMP await to be uncovered. In the search of new EPS, manipulation of bacterial c-di-GMP metabolism can be combined with high throughput approaches for screening of large collections of bacteria. In addition, c-di-GMP activation of EPS production and promotion of cell aggregation may have direct applications in environmental industries related with biofuel production or wastewater treatments.

Transcriptome profiling of a Rhizobium leguminosarum bv. trifolii rosR mutant reveals the role of the transcriptional regulator RosR in motility, synthesis of cell-surface components, and other cellular processes

Background

Rhizobium leguminosarum bv. trifolii is a soil bacterium capable of establishing a symbiotic relationship with red clover (Trifolium pratense). The presence of surface polysaccharides and other extracellular components as well as motility and competitiveness are essential traits for both adaptation of this bacterium to changing environmental conditions and successful infection of host plant roots. The R. leguminosarum bv. trifolii rosR gene encodes a protein belonging to the family of Ros/MucR transcriptional regulators, which contain a Cys₂His₂-type zinc-finger motif and are involved in the regulation of exopolysaccharide synthesis in several rhizobial species. Previously, it was established that a mutation in the rosR gene significantly decreased exopolysaccharide synthesis, increased bacterial sensitivity to some stress factors, and negatively affected infection of clover roots.

Results

RNA-Seq analysis performed for the R. leguminosarum bv. trifolii wild-type strain Rt24.2 and its derivative Rt2472 carrying a rosR mutation identified a large number of genes which were differentially expressed in these two backgrounds. A considerable majority of these genes were up-regulated in the mutant (63.22 %), indicating that RosR functions mainly as a repressor. Transcriptome profiling of the rosR mutant revealed a role of this regulator in several cellular processes, including the synthesis of cell-surface components and polysaccharides, motility, and bacterial metabolism. Moreover, it was established that the Rt2472 strain was characterized by a longer generation time and showed an increased aggregation ability, but was impaired in motility as a result of considerably reduced flagellation of its cells.

Conclusions

The comparative transcriptome analysis of R. leguminosarum bv. trifolii wild-type Rt24.2 and the Rt2472 mutant identified a set of genes belonging to the RosR regulon and confirmed the important role of RosR in the regulatory network. The data obtained in this study indicate that this protein affects several cellular processes and plays an important role in bacterial adaptation to environmental conditions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2332-4) contains supplementary material, which is available to authorized users.

AmrZ regulates cellulose production in Pseudomonas syringae pv. tomato DC3000

In Pseudomonas syringae pv. tomato DC3000, the second messenger c-di-GMP has been previously shown to stimulate pellicle formation and cellulose biosynthesis. A screen for genes involved in cellulose production under high c-di-GMP intracellular levels led to the identification of insertions in two genes, wssB and wssE, belonging to the Pto DC3000 cellulose biosynthesis operon wssABCDEFGHI. Interestingly, beside cellulose-deficient mutants, colonies with a rougher appearance than the wild type also arouse among the transposants. Those mutants carry insertions in amrZ, a gene encoding a transcriptional regulator in different Pseudomonas. Here, we provide evidence that AmrZ is involved in the regulation of bacterial cellulose production at transcriptional level by binding to the promoter region of the wssABCDEFGHI operon and repressing cellulose biosynthesis genes. Mutation of amrZ promotes wrinkly colony morphology, increased cellulose production and loss of motility in Pto DC3000. AmrZ regulon includes putative c-di-GMP metabolising proteins, like AdcA and MorA, which may also impact those phenotypes. Furthermore, an amrZ but not a cellulose-deficient mutant turned out to be impaired in pathogenesis, indicating that AmrZ is a key regulator of Pto DC3000 virulence probably by controlling bacterial processes other than cellulose production.