Mechanism of cellulose synthesis in Agrobacterium tumefaciens

Upcycling Food By-products: Characteristics and Applications of Nanocellulose

Rising global food prices and the increasing prevalence of food insecurity highlight the imprudence of food waste and the inefficiencies of the current food system. Upcycling food by-products holds significant potential for mitigating food loss and waste within the food supply chain. Food by-products can be utilized to extract nanocellulose, a material that has obtained substantial attention recently due to its renewability, biocompatibility, bioavailability, and a multitude of remarkable properties. Cellulose nanomaterials have been the subject of extensive research and have shown promise across a wide array of applications, including the food industry. Notably, nanocellulose possesses unique attributes such as a surface area, aspect ratio, rheological behavior, water absorption capabilities, crystallinity, surface modification, as well as low possibilities of cytotoxicity and genotoxicity. These qualities make nanocellulose suitable for diverse applications spanning the realms of food production, biomedicine, packaging, and beyond. This review aims to provide an overview of the outcomes and potential applications of cellulose nanomaterials derived from food by-products. Nanocellulose can be produced through both top-down and bottom-up approaches, yielding various types of nanocellulose. Each of these variants possesses distinctive characteristics that have the potential to significantly enhance multiple sectors within the commercial market.

Strategy to combat biofilms: a focus on biofilm dispersal enzymes

Bacterial biofilms, which consist of three-dimensional extracellular polymeric substance (EPS), not only function as signaling networks, provide nutritional support, and facilitate surface adhesion, but also serve as a protective shield for the residing bacterial inhabitants against external stress, such as antibiotics, antimicrobials, and host immune responses. Biofilm-associated infections account for 65-80% of all human microbial infections that lead to serious mortality and morbidity. Tremendous effort has been spent to address the problem by developing biofilm-dispersing agents to discharge colonized microbial cells to a more vulnerable planktonic state. Here, we discuss the recent progress of enzymatic eradicating strategies against medical biofilms, with a focus on dispersal mechanisms. Particularly, we review three enzyme classes that have been extensively investigated, namely glycoside hydrolases, proteases, and deoxyribonucleases.

Media component bovine serum albumin facilitates the formation of mycobacterial biofilms in response to reductive stress

BACKGROUND

Mycobacterium tuberculosis (Mtb) forms physiologically relevant biofilms harboring drug-tolerant bacteria. This observation has brought the study of mycobacterial biofilms to the forefront of tuberculosis research. We established earlier that dithiothreitol (DTT) mediated reductive stress induces cellulose-rich biofilm formation in Mtb cultures. The molecular events associated with the DTT-induced biofilm formation are not known. Furthermore, there are only limited tools for monitoring the presence of cellulose in biofilms.

RESULTS

To decipher the molecular events associated with DTT-induced biofilm formation, we used Mtb and non-pathogenic, fast-growing Mycobacterium smegmatis (Msm). We observed that DTT induces biofilm formation in Msm cultures. We explored whether media components facilitate biofilm formation in mycobacteria upon exposure to DTT. We observed that media component bovine serum albumin promotes mycobacterial biofilm formation in response to DTT. Furthermore, we analyzed the composition of extracellular polymeric substances of Msm biofilms. We found that, like Mtb biofilms, Msm biofilms are also rich in polysaccharides and proteins. We also developed a novel protein-based molecular probe for imaging cellulose by utilizing the cellulose-binding domain of cellulase CenA from Cellulomonas fimi and fusing it to fluorescent reporter mCherry. Characterization of this new probe revealed that it has a high affinity for cellulose and could be used for visualizing cellulose biosynthesis during the development of Agrobacterium biofilms. Furthermore, we have demonstrated that biological macromolecule cellulose is present in the extracellular polymeric substances of Msm biofilms using this novel probe.

CONCLUSIONS

This study indicates that DTT-mediated reduction of media component BSA leads to the formation of nucleating foci. These nucleating foci are critical for subsequent attachment of bacterial cells and induction of EPS production. Furthermore, this new tool, IMT-CBD-mC, could be used for monitoring cellulose incorporation in plant cells, understanding cellulose biosynthesis dynamics during biofilm formation, etc.

Genetic potential for exopolysaccharide synthesis in activated sludge bacteria uncovered by genome-resolved metagenomics

A good floc formation of activated sludge (AS) is crucial for solid-liquid separation and production of clean effluent during wastewater treatment. Floc formation is partly controlled by self-produced extracellular polymeric substances (EPS) such as exopolysaccharides, proteins, and nucleic acids. Little is known about the composition, structure, and function of EPS in AS and which bacteria produce them. To address this knowledge gap for the exopolysaccharides, we took advantage of 1083 high-quality metagenome-assembled genomes (MAGs) obtained from 23 Danish wastewater treatment plants. We investigated the genomic potential for exopolysaccharide biosynthesis in bacterial species typical in AS systems based on genome mining and gene synteny analyses. Putative gene clusters associated with the biosynthesis of alginate, cellulose, curdlan, diutan, hyaluronic acids, Pel, poly-β-1,6-N-acetyl-d-glucosamine (PNAG), Psl, S88 capsular polysaccharide, salecan, succinoglycan, and xanthan were identified and linked to individual MAGs, providing a comprehensive overview of the genome-resolved potential for these exopolysaccharides in AS bacteria. The approach and results provide a starting point for a more comprehensive understanding of EPS composition in wastewater treatment systems, which may facilitate a more refined regulation of the activated sludge process for improved stability.

The activity of BcsZ of Salmonella Typhimurium and its role in Salmonella-plants interactions

Salmonella enterica is one of the most common human pathogens associated with fresh produce outbreaks. The present study suggests that expression of BcsZ, one of the proteins in the bcs complex, enhances the survival of Salmonella Typhimurium on parsley. BcsZ demonstrated glucanase activity with the substrates carboxymethylcellulose and crystalline cellulose, and was responsible for a major part of the S. Typhimurium CMCase activity. Moreover, there was constitutive expression of BcsZ, which was also manifested after exposure to plant polysaccharides and parsley-leaf extract. In an in-planta model, overexpression of BcsZ significantly improved the epiphytic and endophytic survival of S. Typhimurium on/in parsley leaves compared with the wild-type strain and bcsZ null mutant. Interestingly, necrotic lesions appeared on the parsley leaf after infiltration of Salmonella overexpressing BcsZ, while infiltration of the wild-type S. Typhimurium did not cause any visible symptoms. Infiltration of purified BcsZ enzyme, or its degradation products also caused symptoms on parsley leaves. We suggest that the BcsZ degradation products trigger the plant’s defense response, causing local necrotic symptoms. These results indicate that BcsZ plays an important role in the Salmonella-plant interactions, and imply that injured bacteria may take part in these interactions.

Bacterial Cellulose: Production, Characterization, and Application as Antimicrobial Agent

Bacterial cellulose (BC) is recognized as a multifaceted, versatile biomaterial with abundant applications. Groups of microorganisms such as bacteria are accountable for BC synthesis through static or agitated fermentation processes in the presence of competent media. In comparison to static cultivation, agitated cultivation provides the maximum yield of the BC. A pure cellulose BC can positively interact with hydrophilic or hydrophobic biopolymers while being used in the biomedical domain. From the last two decades, the reinforcement of biopolymer-based biocomposites and its applicability with BC have increased in the research field. The harmony of hydrophobic biopolymers can be reduced due to the high moisture content of BC in comparison to hydrophilic biopolymers. Mechanical properties are the important parameters not only in producing green composite but also in dealing with tissue engineering, medical implants, and biofilm. The wide requisition of BC in medical as well as industrial fields has warranted the scaling up of the production of BC with added economy. This review provides a detailed overview of the production and properties of BC and several parameters affecting the production of BC and its biocomposites, elucidating their antimicrobial and antibiofilm efficacy with an insight to highlight their therapeutic potential.

Weaving of bacterial cellulose by the Bcs secretion systems

Cellulose is the most abundant biological compound on Earth and while it is the predominant building constituent of plants, it is also a key extracellular matrix component in many diverse bacterial species. While bacterial cellulose was first described in the 19th century, it was not until this last decade that a string of structural works provided insights into how the cellulose synthase BcsA, assisted by its inner-membrane partner BcsB, senses c-di-GMP to simultaneously polymerize its substrate and extrude the nascent polysaccharide across the inner bacterial membrane. It is now established that bacterial cellulose can be produced by several distinct types of cellulose secretion systems and that in addition to BcsAB, they can feature multiple accessory subunits, often indispensable for polysaccharide production. Importantly, the last years mark significant progress in our understanding not only of cellulose polymerization per se but also of the bigger picture of bacterial signaling, secretion system assembly, biofilm formation and host tissue colonization, as well as of structural and functional parallels of this dominant biosynthetic process between the bacterial and eukaryotic domains of life. Here, we review current mechanistic knowledge on bacterial cellulose secretion with focus on the structure, assembly and cooperativity of Bcs secretion system components.

Structural insights into homotrimeric assembly of cellulose synthase CesA7 from Gossypium hirsutum

Cellulose is one of the most abundant organic polymers in nature. It contains multiple β-1,4-glucan chains synthesized by cellulose synthases (CesAs) on the plasma membrane of higher plants. CesA subunits assemble into a pseudo-sixfold symmetric cellulose synthase complex (CSC), known as a 'rosette complex'. The structure of CesA remains enigmatic. Here, we report the cryo-EM structure of the homotrimeric CesA7 from Gossypium hirsutum at 3.5-angstrom resolution. The GhCesA7 homotrimer shows a C3 symmetrical assembly. Each protomer contains seven transmembrane helices (TMs) which form a channel potentially facilitating the release of newly synthesized glucans. The cytoplasmic glycosyltransferase domain (GT domain) of GhCesA7 protrudes from the membrane, and its catalytic pocket is directed towards the TM pore. The homotrimer GhCesA7 is stabilized by the transmembrane helix 7 (TM7) and the plant-conserved region (PCR) domains. It represents the building block of CSCs and facilitates microfibril formation. This structure provides insight into how eukaryotic cellulose synthase assembles and provides a mechanistic basis for the improvement of cotton fibre quality in the future.

Competitive Exclusion and Metabolic Dependency among Microorganisms Structure the Cellulose Economy of an Agricultural Soil

Microorganisms that degrade cellulose utilize extracellular reactions that yield free by-products which can promote interactions with noncellulolytic organisms. We hypothesized that these interactions determine the ecological and physiological traits governing the fate of cellulosic carbon (C) in soil. We performed comparative genomics with genome bins from a shotgun metagenomic-stable isotope probing experiment to characterize the attributes of cellulolytic and noncellulolytic taxa accessing ¹³C from cellulose. We hypothesized that cellulolytic taxa would exhibit competitive traits that limit access, while noncellulolytic taxa would display greater metabolic dependency, such as signatures of adaptive gene loss. We tested our hypotheses by evaluating genomic traits indicative of competitive exclusion or metabolic dependency, such as antibiotic production, growth rate, surface attachment, biomass degrading potential, and auxotrophy. The most ¹³C-enriched taxa were cellulolytic Cellvibrio (Gammaproteobacteria) and Chaetomium (Ascomycota), which exhibited a strategy of self-sufficiency (prototrophy), rapid growth, and competitive exclusion via antibiotic production. Auxotrophy was more prevalent in cellulolytic Actinobacteria than in cellulolytic Proteobacteria, demonstrating differences in dependency among cellulose degraders. Noncellulolytic taxa that accessed ¹³C from cellulose (Planctomycetales, Verrucomicrobia, and Vampirovibrionales) were also more dependent, as indicated by patterns of auxotrophy and ¹³C labeling (i.e., partial labeling or labeling at later stages). Major ¹³C-labeled cellulolytic microbes (e.g., Sorangium, Actinomycetales, Rhizobiales, and Caulobacteraceae) possessed adaptations for surface colonization (e.g., gliding motility, hyphae, attachment structures) signifying the importance of surface ecology in decomposing particulate organic matter. Our results demonstrated that access to cellulosic C was accompanied by ecological trade-offs characterized by differing degrees of metabolic dependency and competitive exclusion.

An Integrated Analysis of the Rice Transcriptome and Metabolome Reveals Root Growth Regulation Mechanisms in Response to Nitrogen Availability

Nitrogen is an essential nutrient for plant growth and basic metabolic processes. Root systems play an important role in the ability of plants to obtain nutrients from the soil, and are closely related to the growth and development of above-ground plants. Root morphology analysis showed that root growth was induced under low-nitrogen conditions and inhibited under high-nitrogen conditions. To better understand the molecular mechanisms and metabolic basis underlying the rice root response to nitrogen availability, an integrated analysis of the rice root transcriptome and metabolome under three environmental conditions (low-, control, and high-nitrogen conditions) was conducted. A total of 262 and 262 differentially level metabolites were identified under low- and high-nitrogen conditions, respectively. A total of 696 and 808 differentially expressed genes were identified under low- and high-nitrogen conditions, respectively. For both the differentially expressed genes and metabolites, KEGG pathway analysis indicated that amino acid metabolism, carbon and nitrogen metabolism, phenylpropanoid metabolism, and phytohormones’ signal transduction were significantly affected by nitrogen availability. Additionally, variable levels of 65 transcription factors (TFs) were identified in rice leaves exposed to high and low nitrogen, covering 22 TF families. These results also indicate that there is a significant difference in the transcriptional regulation mechanisms of rice roots between low and high nitrogen. In summary, our study provides new information for a further understanding of the response of rice roots to low-nitrogen and high-nitrogen conditions.

Insights into the transcriptomic response of the plant engineering bacterium Ensifer adhaerens OV14 during transformation

The ability to engineer plant genomes has been primarily driven by the soil bacterium Agrobacterium tumefaciens but recently the potential of alternative rhizobia such as Rhizobium etli and Ensifer adhaerens OV14, the latter of which supports Ensifer Mediated Transformation (EMT) has been reported. Surprisingly, a knowledge deficit exists in regards to understanding the whole genome processes underway in plant transforming bacteria, irrespective of the species. To begin to address the issue, we undertook a temporal RNAseq-based profiling study of E. adhaerens OV14 in the presence/absence of Arabidopsis thaliana tissues. Following co-cultivation with root tissues, 2333 differentially expressed genes (DEGs) were noted. Meta-analysis of the RNAseq data sets identified a clear shift from plasmid-derived gene expression to chromosomal-based transcription within the early stages of bacterium-plant co-cultivation. During this time, the number of differentially expressed prokaryotic genes increased steadily out to 7 days co-cultivation, a time at which optimum rates of transformation were observed. Gene ontology evaluations indicated a role for both chromosomal and plasmid-based gene families linked specifically with quorum sensing, flagellin production and biofilm formation in the process of EMT. Transcriptional evaluation of vir genes, housed on the pCAMBIA 5105 plasmid in E. adhaerens OV14 confirmed the ability of E. adhaerens OV14 to perceive and activate its transcriptome in response to the presence of 200 µM of acetosyringone. Significantly, this is the first study to characterise the whole transcriptomic response of a plant engineering bacterium in the presence of plant tissues and provides a novel insight into prokaryotic genetic processes that support T-DNA transfer.

Function and Regulation of Agrobacterium tumefaciens Cell Surface Structures that Promote Attachment

Agrobacterium tumefaciens attaches stably to plant host tissues and abiotic surfaces. During pathogenesis, physical attachment to the site of infection is a prerequisite to infection and horizontal gene transfer to the plant. Virulent and avirulent strains may also attach to plant tissue in more benign plant associations, and as with other soil microbes, to soil surfaces in the terrestrial environment. Although most A. tumefaciens virulence functions are encoded on the tumor-inducing plasmid, genes that direct general surface attachment are chromosomally encoded, and thus this process is not obligatorily tied to virulence, but is a more fundamental capacity. Several different cellular structures are known or suspected to contribute to the attachment process. The flagella influence surface attachment primarily via their propulsive activity, but control of their rotation during the transition to the attached state may be quite complex. A. tumefaciens produces several pili, including the Tad-type Ctp pili, and several plasmid-borne conjugal pili encoded by the Ti and At plasmids, as well as the so-called T-pilus, involved in interkingdom horizontal gene transfer. The Ctp pili promote reversible interactions with surfaces, whereas the conjugal and T-pili drive horizontal gene transfer (HGT) interactions with other cells and tissues. The T-pilus is likely to contribute to physical association with plant tissues during DNA transfer to plants. A. tumefaciens can synthesize a variety of polysaccharides including cellulose, curdlan (β-1,3 glucan), β-1,2 glucan (cyclic and linear), succinoglycan, and a localized polysaccharide(s) that is confined to a single cellular pole and is called the unipolar polysaccharide (UPP). Lipopolysaccharides are also in the outer leaflet of the outer membrane. Cellulose and curdlan production can influence attachment under certain conditions. The UPP is required for stable attachment under a range of conditions and on abiotic and biotic surfaces. Other factors that have been reported to play a role in attachment include the elusive protein called rhicadhesin. The process of surface attachment is under extensive regulatory control and can be modulated by environmental conditions, as well as by direct responses to surface contact. Complex transcriptional and post-transcriptional control circuitry underlies much of the production and deployment of these attachment functions.

A Study of the Physical and Mechanical Properties of Aerogels Obtained from Bacterial Cellulose

Aerogels with a density of 4.2-22.8 kg/m³ were obtained from bacterial cellulose synthesized under static and dynamic cultivation conditions on a molasses medium. The strength properties and porous structure of the aerogels strongly depended on their density. With an aerogel density of 22.8 kg/m³, the modulus of elasticity at 80% compression of the sample was 0.1 MPa. The decrease in the density of aerogels led to an increase in the pore sizes ranging from 20 to 1000 μm and a decrease in the modulus of elasticity. These characteristics were more pronounced in aerogels obtained from bacterial cellulose under static cultivation conditions. The aerogels had a low coefficient of thermal conductivity (0.0257 W m^-1 °C^-1), which is comparable to the thermal conductivity of air, and moderate thermal stability because the degradation processes of the aerogels began at 237 °C. The aerogels obtained from bacterial cellulose had high sound absorption coefficients in the frequency range of 200-5000 Hz, which makes it possible to use the aerogels as heat- and sound-insulating materials.

Genome sequence and characterization of the bcs clusters for the production of nanocellulose from the low pH resistant strain Komagataeibacter medellinensis ID13488

Komagataeibacter medellinensis ID13488 (formerly Gluconacetobacter medellinensis ID13488) is able to produce crystalline bacterial cellulose (BC) under high acidic growth conditions. These abilities make this strain desirable for industrial BC production from acidic residues (e.g. wastes generated from cider production). To explore the molecular bases of the BC biosynthesis in this bacterium, the genome has been sequenced revealing a sequence of 3.4 Mb containing three putative plasmids of 38.1 kb (pKM01), 4.3 kb (pKM02) and 3.3 Kb (pKM03). Genome comparison analyses of K. medellinensis ID13488 with other cellulose‐producing related strains resulted in the identification of the bcs genes involved in the cellulose biosynthesis. Genes arrangement and composition of four bcs clusters (bcs1, bcs2, bcs3 and bcs4) was studied by RT‐PCR, and their organization in four operons transcribed as four independent polycistronic mRNAs was determined. qRT‐PCR experiments demonstrated that mostly bcs1 and bcs4 are expressed under BC production conditions, suggesting that these operons direct the synthesis of BC. Genomic differences with the close related strain K. medellinensis NBRC 3288 unable to produce BC were also described and discussed.

Structure and function of a novel GH8 endoglucanase from the bacterial cellulose synthase complex of Raoultella ornithinolytica

Cellulose synthesis in bacteria is a complex process involving the concerted action of several enzymes whose genes are often organized in operons. This process influences many fundamental physiological aspects such as bacteria and host interaction, biofilm formation, among others. Although it might sound contradictory, the participation of cellulose-degrading enzymes is critical to this process. The presence of endoglucanases from family 8 of glycosyl hydrolases (GH8) in bacterial cellulose synthase (Bcs) complex has been described in different bacteria, including the model organism Komagataeibacter xylinus; however, their role in this process is not completely understood. In this study, we describe the biochemical characterization and three-dimensional structure of a novel GH8 member from Raoultella ornithinolytica, named AfmE1, which was previously identified by our group from the metagenomic analysis of the giant snail Achatina fulica. Our results demonstrated that AfmE1 is an endo-β-1,4-glucanase, with maximum activity in acidic to neutral pH over a wide temperature range. This enzyme cleaves cello-oligosaccharides with a degree of polymerization ≥ 5 and presents six glucosyl-binding subsites. The structural comparison of AfmE1 with other GH8 endoglucanases showed significant structural dissimilarities in the catalytic cleft, particularly in the subsite +3, which correlate with different functional mechanisms, such as the recognition of substrate molecules having different arrangements and crystallinities. Together, these findings provide new insights into molecular and structural features of evolutionarily conserved endoglucanases from the bacterial cellulose biosynthetic machinery.

The crystal structure of the endoglucanase Cel10, a family 8 glycosyl hydrolase from Klebsiella pneumoniae

Cellulases are produced by microorganisms that grow on cellulose biomass. Here, a cellulase, Cel10, was identified in a strain of Klebsiella pneumoniae isolated from Chinese bamboo rat gut. Analysis of substrate specificity showed that Cel10 is able to hydrolyze amorphous carboxymethyl cellulose (CMC) and crystalline forms of cellulose (Avicel and xylan) but is unable to hydrolyze p-nitrophenol β-D-glucopyranoside (p-NPG), proving that Cel10 is an endoglucanase. A phylogenetic tree analysis indicates that Cel10 belongs to the glycoside hydrolase 8 (GH8) subfamily. In order to further understanding of its substrate specificity, the structure of Cel10 was solved by molecular replacement and refined to 1.76 Å resolution. The overall fold is distinct from those of most other enzymes belonging to the GH8 subfamily. Although it forms the typical (α/α)6-barrel motif fold, like Acetobacterxylinum CMCax, one helix is missing. Structural comparisons with Clostridium thermocellum CelA (CtCelA), the best characterized GH8 endoglucanase, revealed that sugar-recognition subsite -3 is completely missing in Cel10. The absence of this subsite correlates to a more open substrate-binding cleft on the cellooligosaccharide reducing-end side.

A single heterologously expressed plant cellulose synthase isoform is sufficient for cellulose microfibril formation in vitro

Plant cell walls are a composite material of polysaccharides, proteins, and other noncarbohydrate polymers. In the majority of plant tissues, the most abundant polysaccharide is cellulose, a linear polymer of glucose molecules. As the load-bearing component of the cell wall, individual cellulose chains are frequently bundled into micro and macrofibrils and are wrapped around the cell. Cellulose is synthesized by membrane-integrated and processive glycosyltransferases that polymerize UDP-activated glucose and secrete the nascent polymer through a channel formed by their own transmembrane regions. Plants express several different cellulose synthase isoforms during primary and secondary cell wall formation; however, so far, none has been functionally reconstituted in vitro for detailed biochemical analyses. Here we report the heterologous expression, purification, and functional reconstitution of Populus tremula x tremuloides CesA8 (PttCesA8), implicated in secondary cell wall formation. The recombinant enzyme polymerizes UDP-activated glucose to cellulose, as determined by enzyme degradation, permethylation glycosyl linkage analysis, electron microscopy, and mutagenesis studies. Catalytic activity is dependent on the presence of a lipid bilayer environment and divalent manganese cations. Further, electron microscopy analyses reveal that PttCesA8 produces cellulose fibers several micrometers long that occasionally are capped by globular particles, likely representing PttCesA8 complexes. Deletion of the enzyme's N-terminal RING-finger domain almost completely abolishes fiber formation but not cellulose biosynthetic activity. Our results demonstrate that reconstituted PttCesA8 is not only sufficient for cellulose biosynthesis in vitro but also suffices to bundle individual glucan chains into cellulose microfibrils.

Adhesins Involved in Attachment to Abiotic Surfaces by Gram-Negative Bacteria

During the first step of biofilm formation, initial attachment is dictated by physicochemical and electrostatic interactions between the surface and the bacterial envelope. Depending on the nature of these interactions, attachment can be transient or permanent. To achieve irreversible attachment, bacterial cells have developed a series of surface adhesins promoting specific or nonspecific adhesion under various environmental conditions. This article reviews the recent advances in our understanding of the secretion, assembly, and regulation of the bacterial adhesins during biofilm formation, with a particular emphasis on the fimbrial, nonfimbrial, and discrete polysaccharide adhesins in Gram-negative bacteria.

In silico Identification and Taxonomic Distribution of Plant Class C GH9 Endoglucanases

The glycoside hydrolase 9 superfamily, mainly comprising the endoglucanases, is represented in all three domains of life. The current division of GH9 enzymes, into three subclasses, namely A, B, and C, is centered on parameters derived from sequence information alone. However, this classification is ambiguous, and is limited by the paralogous ancestry of classes B and C endoglucanases, and paucity of biochemical and structural data. Here, we extend this classification schema to putative GH9 endoglucanases present in green plants, with an emphasis on identifying novel members of the class C subset. These enzymes cleave the β(1 → 4) linkage between non-terminal adjacent D-glucopyranose residues, in both, amorphous and crystalline regions of cellulose. We utilized non redundant plant GH9 enzymes with characterized molecular data, as the training set to construct Hidden Markov Models (HMMs) and train an Artificial Neural Network (ANN). The parameters that were used for predicting dominant enzyme function, were derived from this training set, and subsequently refined on 147 sequences with available expression data. Our knowledge-based approach, can ascribe differential endoglucanase activity (A, B, or C) to a query sequence with high confidence, and was used to construct a local repository of class C GH9 endoglucanases (GH9C = 241) from 32 sequenced green plants.

The expanding roles of c-di-GMP in the biosynthesis of exopolysaccharides and secondary metabolites

The cyclic dinucleotide c-di-GMP has emerged in the last decade as a prevalent intracellular messenger that orchestrates the transition between the motile and sessile lifestyles of many bacterial species. The motile-to-sessile transition is often associated with the formation of extracellular matrix-encased biofilm, an organized community of bacterial cells that often contributes to antibiotic resistance and host-pathogen interaction. It is increasingly clear that c-di-GMP controls motility, biofilm formation and bacterial pathogenicity partially through regulating the production of exopolysaccharides (EPS) and small-molecule secondary metabolites. This review summarizes our current understanding of the regulation of EPS biosynthesis by c-di-GMP in a diversity of bacterial species and highlights the emerging role of c-di-GMP in the biosynthesis of small-molecule secondary metabolites.

Plant cellulose synthesis: CESA proteins crossing kingdoms

Cellulose is a biopolymer of considerable economic importance. It is synthesised by the cellulose synthase complex (CSC) in species ranging from bacteria to higher plants. Enormous progress in our understanding of bacterial cellulose synthesis has come with the recent publication of both the crystal structure and biochemical characterisation of a purified complex able to synthesis cellulose in vitro. A model structure of a plant CESA protein suggests considerable similarity between the bacterial and plant cellulose synthesis. In this review article we will cover current knowledge of how plant CESA proteins synthesise cellulose. In particular the focus will be on the lessons learned from the recent work on the catalytic mechanism and the implications that new data on cellulose structure has for the assembly of CESA proteins into the large complex that synthesis plant cellulose microfibrils.

A molecular description of cellulose biosynthesis

Cellulose is the most abundant biopolymer on Earth, and certain organisms from bacteria to plants and animals synthesize cellulose as an extracellular polymer for various biological functions. Humans have used cellulose for millennia as a material and an energy source, and the advent of a lignocellulosic fuel industry will elevate it to the primary carbon source for the burgeoning renewable energy sector. Despite the biological and societal importance of cellulose, the molecular mechanism by which it is synthesized is now only beginning to emerge. On the basis of recent advances in structural and molecular biology on bacterial cellulose synthases, we review emerging concepts of how the enzymes polymerize glucose molecules, how the nascent polymer is transported across the plasma membrane, and how bacterial cellulose biosynthesis is regulated during biofilm formation. Additionally, we review evolutionary commonalities and differences between cellulose synthases that modulate the nature of the cellulose product formed.

Mechanisms and regulation of surface interactions and biofilm formation in Agrobacterium

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

A bifunctional glycosyltransferase from Agrobacterium tumefaciens synthesizes monoglucosyl and glucuronosyl diacylglycerol under phosphate deprivation

Glycolipids are mainly found in phototrophic organisms (like plants and cyanobacteria), in Gram-positive bacteria, and a few other bacterial phyla. Besides the function as bulk membrane lipids, they often play a role under phosphate deprivation as surrogates for phospholipids. The Gram-negative Agrobacterium tumefaciens accumulates four different glycolipids under phosphate deficiency, including digalactosyl diacylglycerol and glucosylgalactosyl diacylglycerol synthesized by a processive glycosyltransferase. The other two glycolipids have now been identified by mass spectrometry and nuclear magnetic resonance spectroscopy as monoglucosyl diacylglycerol and glucuronosyl diacylglycerol. These two lipids are synthesized by a single promiscuous glycosyltransferase encoded by the ORF atu2297, with UDP-glucose or UDP-glucuronic acid as sugar donors. The transfer of sugars differing in their chemistry is a novel feature not observed before for lipid glycosyltransferases. Furthermore, this enzyme is the first glucuronosyl diacylglycerol synthase isolated. Deletion mutants of Agrobacterium lacking monoglucosyl diacylglycerol and glucuronosyl diacylglycerol or all glycolipids are not impaired in growth or virulence during infection of tobacco leaf discs. Our data suggest that the four glycolipids and the nonphospholipid diacylglyceryl trimethylhomoserine can mutually replace each other during phosphate deprivation. This redundancy of different nonphospholipids may represent an adaptation mechanism to enhance the competitiveness in nature.

A signaling pathway involving the diguanylate cyclase CelR and the response regulator DivK controls cellulose synthesis in Agrobacterium tumefaciens

The production of cellulose fibrils is involved in the attachment of Agrobacterium tumefaciens to its plant host. Consistent with previous studies, we reported recently that a putative diguanylate cyclase, celR, is required for synthesis of this polymer in A. tumefaciens. In this study, the effects of celR and other components of the regulatory pathway of cellulose production were explored. Mutational analysis of celR demonstrated that the cyclase requires the catalytic GGEEF motif, as well as the conserved aspartate residue of a CheY-like receiver domain, for stimulating cellulose production. Moreover, a site-directed mutation within the PilZ domain of CelA, the catalytic subunit of the cellulose synthase complex, greatly reduced cellulose production. In addition, deletion of divK, the first gene of the divK-celR operon, also reduced cellulose production. This requirement for divK was alleviated by expression of a constitutively active form of CelR, suggesting that DivK acts upstream of CelR activation. Based on bacterial two-hybrid assays, CelR homodimerizes but does not interact with DivK. The mutation in divK additionally affected cell morphology, and this effect was complementable by a wild-type copy of the gene, but not by the constitutively active allele of celR. These results support the hypothesis that CelR is a bona fide c-di-GMP synthase and that the nucleotide signal produced by this enzyme activates CelA via the PilZ domain. Our studies also suggest that the DivK/CelR signaling pathway in Agrobacterium regulates cellulose production independent of cell cycle checkpoint systems that are controlled by divK.

BcsA and BcsB form the catalytically active core of bacterial cellulose synthase sufficient for in vitro cellulose synthesis

Cellulose is a linear extracellular polysaccharide. It is synthesized by membrane-embedded glycosyltransferases that processively polymerize UDP-activated glucose. Polymer synthesis is coupled to membrane translocation through a channel formed by the cellulose synthase. Although eukaryotic cellulose synthases function in macromolecular complexes containing several different enzyme isoforms, prokaryotic synthases associate with additional subunits to bridge the periplasm and the outer membrane. In bacteria, cellulose synthesis and translocation is catalyzed by the inner membrane-associated bacterial cellulose synthase (Bcs)A and BcsB subunits. Similar to alginate and poly-β-1,6 N-acetylglucosamine, bacterial cellulose is implicated in the formation of sessile bacterial communities, termed biofilms, and its synthesis is likewise stimulated by cyclic-di-GMP. Biochemical studies of exopolysaccharide synthesis are hampered by difficulties in purifying and reconstituting functional enzymes. We demonstrate robust in vitro cellulose synthesis reconstituted from purified BcsA and BcsB proteins from Rhodobacter sphaeroides. Although BcsA is the catalytically active subunit, the membrane-anchored BcsB subunit is essential for catalysis. The purified BcsA-B complex produces cellulose chains of a degree of polymerization in the range 200-300. Catalytic activity critically depends on the presence of the allosteric activator cyclic-di-GMP, but is independent of lipid-linked reactants. Our data reveal feedback inhibition of cellulose synthase by UDP but not by the accumulating cellulose polymer and highlight the strict substrate specificity of cellulose synthase for UDP-glucose. A truncation analysis of BcsB localizes the region required for activity of BcsA within its C-terminal membrane-associated domain. The reconstituted reaction provides a foundation for the synthesis of biofilm exopolysaccharides, as well as its activation by cyclic-di-GMP.

CelR, an ortholog of the diguanylate cyclase PleD of Caulobacter, regulates cellulose synthesis in Agrobacterium tumefaciens

Cellulose fibrils play a role in attachment of Agrobacterium tumefaciens to its plant host. While the genes for cellulose biosynthesis in the bacterium have been identified, little is known concerning the regulation of the process. The signal molecule cyclic di-GMP (c-di-GMP) has been linked to the regulation of exopolysaccharide biosynthesis in many bacterial species, including A. tumefaciens. In this study, we identified two putative diguanylate cyclase genes, celR (atu1297) and atu1060, that influence production of cellulose in A. tumefaciens. Overexpression of either gene resulted in increased cellulose production, while deletion of celR, but not atu1060, resulted in decreased cellulose biosynthesis. celR overexpression also affected other phenotypes, including biofilm formation, formation of a polar adhesion structure, plant surface attachment, and virulence, suggesting that the gene plays a role in regulating these processes. Analysis of celR and Δcel mutants allowed differentiation between phenotypes associated with cellulose production, such as biofilm formation, and phenotypes probably resulting from c-di-GMP signaling, which include polar adhesion, attachment to plant tissue, and virulence. Phylogenetic comparisons suggest that species containing both celR and celA, which encodes the catalytic subunit of cellulose synthase, adapted the CelR protein to regulate cellulose production while those that lack celA use CelR, called PleD, to regulate specific processes associated with polar localization and cell division.

Crystallographic snapshot of cellulose synthesis and membrane translocation

Cellulose, the most abundant biological macromolecule, is an extracellular, linear polymer of glucose molecules. It represents an essential component of plant cell walls but is also found in algae and bacteria. In bacteria, cellulose production frequently correlates with the formation of biofilms, a sessile, multicellular growth form. Cellulose synthesis and transport across the inner bacterial membrane is mediated by a complex of the membrane-integrated catalytic BcsA subunit and the membrane-anchored, periplasmic BcsB protein. Here we present the crystal structure of a complex of BcsA and BcsB from Rhodobacter sphaeroides containing a translocating polysaccharide. The structure of the BcsA-BcsB translocation intermediate reveals the architecture of the cellulose synthase, demonstrates how BcsA forms a cellulose-conducting channel, and suggests a model for the coupling of cellulose synthesis and translocation in which the nascent polysaccharide is extended by one glucose molecule at a time.

Crystallographic snapshot of cellulose synthesis and membrane translocation

Cellulose, the most abundant biological macromolecule, is an extracellular, linear polymer of glucose molecules. It represents an essential component of plant cell walls but is also found in algae and bacteria. In bacteria, cellulose production frequently correlates with the formation of biofilms, a sessile, multicellular growth form. Cellulose synthesis and transport across the inner bacterial membrane is mediated by a complex of the membrane-integrated catalytic BcsA subunit and the membrane-anchored, periplasmic BcsB protein. Here we present the crystal structure of a complex of BcsA and BcsB from Rhodobacter sphaeroides containing a translocating polysaccharide. The structure of the BcsA-BcsB translocation intermediate reveals the architecture of the cellulose synthase, demonstrates how BcsA forms a cellulose-conducting channel, and suggests a model for the coupling of cellulose synthesis and translocation in which the nascent polysaccharide is extended by one glucose molecule at a time.

Role of Rhizobium endoglucanase CelC2 in cellulose biosynthesis and biofilm formation on plant roots and abiotic surfaces

Background

The synthesis of cellulose is among the most important but poorly understood biochemical processes, especially in bacteria, due to its complexity and high degree of regulation. In this study, we analyzed both the production of cellulose by all known members of the Rhizobiaceae and the diversity of Rhizobium celABC operon predicted to be involved in cellulose biosynthesis. We also investigated the involvement in cellulose production and biofilm formation of celC gene encoding an endoglucanase (CelC2) that is required for canonical symbiotic root hair infection by Rhizobium leguminosarum bv. trifolii.

Results

ANU843 celC mutants lacking (ANU843ΔC2) or overproducing cellulase (ANU843C2⁺) produced greatly increased or reduced amounts of external cellulose micro fibrils, respectively. Calcofluor-stained cellulose micro fibrils were considerably longer when formed by ANU843ΔC2 bacteria rather than by the wild-type strain, in correlation with a significant increase in their flocculation in batch culture. In contrast, neither calcofluor-stained extracellular micro fibrils nor flocculation was detectable in ANU843C2⁺ cells. To clarify the role of cellulose synthesis in Rhizobium cell aggregation and attachment, we analyzed the ability of these mutants to produce biofilms on different surfaces. Alteration of wild-type CelC2 levels resulted in a reduced ability of bacteria to form biofilms both in abiotic surfaces and in planta.

Conclusions

Our results support a key role of the CelC2 cellulase in cellulose biosynthesis by modulating the length of the cellulose fibrils that mediate firm adhesion among Rhizobium bacteria leading to biofilm formation. Rhizobium cellulose is an essential component of the biofilm polysaccharidic matrix architecture and either an excess or a defect of this “building material” seem to collapse the biofilm structure. These results position cellulose hydrolytic enzymes as excellent anti-biofilm candidates.

Genome analyses highlight the different biological roles of cellulases

Cellulolytic enzymes have been the subject of renewed interest owing to their potential role in the conversion of plant lignocellulose to sustainable biofuels. An analysis of ∼1,500 complete bacterial genomes, presented here, reveals that ∼40% of the genomes of sequenced bacteria encode at least one cellulase gene. Most of the bacteria that encode cellulases are soil and marine saprophytes, many of which encode a range of enzymes for cellulose hydrolysis and also for the breakdown of the other constituents of plant cell walls (hemicelluloses and pectins). Intriguingly, cellulases are present in organisms that are usually considered as non-saprophytic, such as Mycobacterium tuberculosis, Legionella pneumophila, Yersinia pestis and even Escherichia coli. We also discuss newly emerging roles of cellulases in such non-saprophytic organisms.

Deciphering the molecular functions of sterols in cellulose biosynthesis

Sterols play vital roles in plant growth and development, as components of membranes and as precursors to steroid hormones. Analysis of Arabidopsis mutants indicates that sterol composition is crucial for cellulose biosynthesis. Sterols are widespread in the plasma membrane (PM), suggesting a possible link between sterols and the multimeric cellulose synthase complex. In one possible scenario, molecular interactions in sterol-rich PM microdomains or another form of sterol-dependent membrane scaffolding may be critical for maintaining the correct subcellular localization, structural integrity and/or activity of the cellulose synthase machinery. Another possible link may be through steryl glucosides, which could act as primers for the attachment of glucose monomers during the synthesis of β-(1 → 4) glucan chains that form the cellulose microfibrils. This mini-review examines genetic and biochemical data supporting the link between sterols and cellulose biosynthesis in cell wall formation and explores potential approaches to elucidate the mechanism of this association.

Apo- and cellopentaose-bound structures of the bacterial cellulose synthase subunit BcsZ

Cellulose, a very abundant extracellular polysaccharide, is synthesized in a finely tuned process that involves the activity of glycosyl-transferases and hydrolases. The cellulose microfibril consists of bundles of linear β-1,4-glucan chains that are synthesized inside the cell; however, the mechanism by which these polymers traverse the cell membrane is currently unknown. In Gram-negative bacteria, the cellulose synthase complex forms a trans-envelope complex consisting of at least four subunits. Although three of these subunits account for the synthesis and translocation of the polysaccharide, the fourth subunit, BcsZ, is a periplasmic protein with endo-β-1,4-glucanase activity. BcsZ belongs to family eight of glycosyl-hydrolases, and its activity is required for optimal synthesis and membrane translocation of cellulose. In this study we report two crystal structures of BcsZ from Escherichia coli. One structure shows the wild-type enzyme in its apo form, and the second structure is for a catalytically inactive mutant of BcsZ in complex with the substrate cellopentaose. The structures demonstrate that BcsZ adopts an (α/α)(6)-barrel fold and that it binds four glucan moieties of cellopentaose via highly conserved residues exclusively on the nonreducing side of its catalytic center. Thus, the BcsZ-cellopentaose structure most likely represents a posthydrolysis state in which the newly formed nonreducing end has already left the substrate binding pocket while the enzyme remains attached to the truncated polysaccharide chain. We further show that BcsZ efficiently degrades β-1,4-glucans in in vitro cellulase assays with carboxymethyl-cellulose as substrate.

What do we really know about cellulose biosynthesis in higher plants?

Cellulose biosynthesis is one of the most important biochemical processes in plant biology. Despite the considerable progress made during the last decade, numerous fundamental questions related to this key process in plant development are outstanding. Numerous models have been proposed through the years to explain the detailed molecular events of cellulose biosynthesis. Almost all models integrate solid experimental data with hypotheses on several of the steps involved in the process. Speculative models are most useful to stimulate further research investigations and bring new exciting ideas to the field. However, it is important to keep their hypothetical nature in mind and be aware of the risk that some undemonstrated hypotheses may progressively become admitted. In this review, we discuss the different steps required for cellulose formation and crystallization, and highlight the most important specific aspects that are supported by solid experimental data.

Insights into bacterial cellulose biosynthesis by functional metagenomics on Antarctic soil samples

In this study, the mining of an Antarctic soil sample by functional metagenomics allowed the isolation of a cold-adapted protein (RBcel1) that hydrolyzes only carboxymethyl cellulose. The new enzyme is related to family 5 of the glycosyl hydrolase (GH5) protein from Pseudomonas stutzeri (Pst_2494) and does not possess a carbohydrate-binding domain. The protein was produced and purified to homogeneity. RBcel1 displayed an endoglucanase activity, producing cellobiose and cellotriose, using carboxymethyl cellulose as a substrate. Moreover, the study of pH and the thermal dependence of the hydrolytic activity shows that RBcel1 was active from pH 6 to pH 9 and remained significantly active when temperature decreased (18% of activity at 10 degrees C). It is interesting that RBcel1 was able to synthetize non-reticulated cellulose using cellobiose as a substrate. Moreover, by a combination of bioinformatics and enzyme analysis, the physiological relevance of the RBcel1 protein and its mesophilic homologous Pst_2494 protein from P. stutzeri, A1501, was established as the key enzymes involved in the production of cellulose by bacteria. In addition, RBcel1 and Pst_2494 are the two primary enzymes belonging to the GH5 family involved in this process.

Complex regulatory pathways coordinate cell-cycle progression and development in Caulobacter crescentus

Caulobacter crescentus has become the predominant bacterial model system to study the regulation of cell-cycle progression. Stage-specific processes such as chromosome replication and segregation, and cell division are coordinated with the development of four polar structures: the flagellum, pili, stalk, and holdfast. The production, activation, localization, and proteolysis of specific regulatory proteins at precise times during the cell cycle culminate in the ability of the cell to produce two physiologically distinct daughter cells. We examine the recent advances that have enhanced our understanding of the mechanisms of temporal and spatial regulation that occur during cell-cycle progression.

Rhizobium cellulase CelC2 is essential for primary symbiotic infection of legume host roots

The rhizobia-legume, root-nodule symbiosis provides the most efficient source of biologically fixed ammonia fertilizer for agricultural crops. Its development involves pathways of specificity, infectivity, and effectivity resulting from expressed traits of the bacterium and host plant. A key event of the infection process required for development of this root-nodule symbiosis is a highly localized, complete erosion of the plant cell wall through which the bacterial symbiont penetrates to establish a nitrogen-fixing, intracellular endosymbiotic state within the host. This process of wall degradation must be delicately balanced to avoid lysis and destruction of the host cell. Here, we describe the purification, biochemical characterization, molecular genetic analysis, biological activity, and symbiotic function of a cell-bound bacterial cellulase (CelC2) enzyme from Rhizobium leguminosarum bv. trifolii, the clover-nodulating endosymbiont. The purified enzyme can erode the noncrystalline tip of the white clover host root hair wall, making a localized hole of sufficient size to allow wild-type microsymbiont penetration. This CelC2 enzyme is not active on root hairs of the nonhost legume alfalfa. Microscopy analysis of the symbiotic phenotypes of the ANU843 wild type and CelC2 knockout mutant derivative revealed that this enzyme fulfils an essential role in the primary infection process required for development of the canonical nitrogen-fixing R. leguminosarum bv. trifolii-white clover symbiosis.

Structural characterization of the Acetobacter xylinum endo-beta-1,4-glucanase CMCax required for cellulose biosynthesis

Previous studies have demonstrated that endoglucanase is required for cellulose biosynthesis both in bacteria and plants. However, it has yet to be elucidated how the endoglucanases function in the mechanism of cellulose biosynthesis. Here we describe the crystal structure of the cellulose biosynthesis-related endo-beta-1,47-glucanase (CMCax; EC 3.2.1.4) from the cellulose-producing Gramnegative bacterium, Acetobacter xylinum (= Gluconacetobacter xylinus), determined at 1.65-A resolution. CMCax falls into the glycoside hydrolase family 8 (GH-8), and the structure showed that the overall fold of the CMCax is similar to those of other glycoside hydrolases belonging to GH-8. Structure comparison with Clostridium thermocellum CelA, the best characterized GH-8 endoglucanase, revealed that sugar recognition subsite +3 is completely missing in CMCax. The absence of the subsite +3 leads to significant broadness of the cleft at the cellooligosaccharide reducing-end side. CMCax is known to be a secreted enzyme and is present in the culture medium. However, electron microscopic analysis using immunostaining clearly demonstrated that a portion of CMCax is localized to the cell surface, suggesting a link with other known membrane-anchored endoglucanases that are required for cellulose biosynthesis.

The effect of cellulose overproduction on binding and biofilm formation on roots by Agrobacterium tumefaciens

Agrobacterium tumefaciens growing in liquid attaches to the surface of tomato and Arabidopsis thaliana roots, forming a biofilm. The bacteria also colonize roots grown in sterile quartz sand. Attachment, root colonization, and biofilm formation all were markedly reduced in celA and chvB mutants, deficient in production of cellulose and cyclic beta-(1,2)-D-glucans, respectively. We have identified two genes (celG and cell) in which mutations result in the overproduction of cellulose as judged by chemical fractionation and methylation analysis. Wild-type and chvB mutant strains carrying a cDNA clone of a cellulose synthase gene from the marine urochordate Ciona savignyi also overproduced cellulose. The overproduction in a wild-type strain resulted in increased biofilm formation on roots, as evaluated by light microscopy, and levels of root colonization intermediate between those of cellulose-minus mutants and the wild type. Overproduction of cellulose by a nonattaching chvB mutant restored biofilm formation and bacterial attachment in microscopic and viable cell count assays and partially restored root colonization. Although attachment to plant surfaces was restored, overproduction of cellulose did not restore virulence in the chvB mutant strain, suggesting that simple bacterial binding to plant surfaces is not sufficient for pathogenesis.

Cellulose biosynthesis: current views and evolving concepts

AIMS

To outline the current state of knowledge and discuss the evolution of various viewpoints put forth to explain the mechanism of cellulose biosynthesis. *

SCOPE

Understanding the mechanism of cellulose biosynthesis is one of the major challenges in plant biology. The simplicity in the chemical structure of cellulose belies the complexities that are associated with the synthesis and assembly of this polysaccharide. Assembly of cellulose microfibrils in most organisms is visualized as a multi-step process involving a number of proteins with the key protein being the cellulose synthase catalytic sub-unit. Although genes encoding this protein have been identified in almost all cellulose synthesizing organisms, it has been a challenge in general, and more specifically in vascular plants, to demonstrate cellulose synthase activity in vitro. The assembly of glucan chains into cellulose microfibrils of specific dimensions, viewed as a spontaneous process, necessitates the assembly of synthesizing sites unique to most groups of organisms. The steps of polymerization (requiring the specific arrangement and activity of the cellulose synthase catalytic sub-units) and crystallization (directed self-assembly of glucan chains) are certainly interlinked in the formation of cellulose microfibrils. Mutants affected in cellulose biosynthesis have been identified in vascular plants. Studies on these mutants and herbicide-treated plants suggest an interesting link between the steps of polymerization and crystallization during cellulose biosynthesis. *

CONCLUSIONS

With the identification of a large number of genes encoding cellulose synthases and cellulose synthase-like proteins in vascular plants and the supposed role of a number of other proteins in cellulose biosynthesis, a complete understanding of this process will necessitate a wider variety of research tools and approaches than was thought to be required a few years back.

Role of cellulose fibrils and exopolysaccharides of Rhizobium leguminosarum in attachment to and infection of Vicia sativa root hairs

Infection and subsequent nodulation of legume host plants by the root nodule symbiote Rhizobium leguminosarum usually require attachment of the bacteria to root-hair tips. Bacterial cellulose fibrils have been shown to be involved in this attachment process but appeared not to be essential for successful nodulation. Detailed analysis of Vicia sativa root-hair infection by wild-type Rhizobium leguminosarum RBL5523 and its cellulose fibril-deficient celE mutant showed that wild-type bacteria infected elongated growing root hairs, whereas cellulose-deficient bacteria infected young emerging root hairs. Exopolysaccharide-deficient strains that retained the ability to produce cellulose fibrils could also infect elongated root hairs but infection thread colonization was defective. Cellulose-mediated agglutination of these bacteria in the root-hair curl appeared to prevent entry into the induced infection thread. Infection experiments with V sativa roots and an extracellular polysaccharide (EPS)- and cellulose-deficient double mutant showed that cellulose-mediated agglutination of the EPS-deficient bacteria in the infection thread was now abolished and that infection thread colonization was partially restored. Interestingly, in this case, infection threads were initiated in root hairs that originated from the cortical cell layers of the root and not in epidermal root hairs. Apparently, surface polysaccharides of R. leguminosarum, such as cellulose fibrils, are determining factors for infection of different developmental stages of root hairs.

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.

Characterization of the Arabidopsis lysine-rich arabinogalactan-protein AtAGP17 mutant (rat1) that results in a decreased efficiency of agrobacterium transformation

Arabinogalactan-proteins (AGPs) are a family of complex proteoglycans widely distributed in plants. The Arabidopsis rat1 mutant, previously characterized as resistant to Agrobacterium tumefaciens root transformation, is due to a mutation in the gene for the Lys-rich AGP, AtAGP17. We show that the phenotype of rat1 correlates with down-regulation of AGP17 in the root as a result of a T-DNA insertion into the promoter of AGP17. Complementation of rat1 plants by a floral dip method with either the wild-type AGP17 gene or cDNA can restore the plant to a wild-type phenotype in several independent transformants. Based on changes in PR1 gene expression and a decrease in free salicylic acid levels upon Agrobacterium infection, we suggest mechanisms by which AGP17 allows Agrobacterium rapidly to reduce the systemic acquired resistance response during the infection process.

Glycosyltransferases and cell wall biosynthesis: novel players and insights

Plants need an enormous biosynthetic machinery to synthesize the complex polysaccharides that are present in the plant cell wall. The isolation, characterization and mapping of wall mutants, together with biochemical approaches, have led to significant advances in our understanding of both wall polysaccharide synthesis at a molecular level and the function of polysaccharides in plant growth and development. Moreover, potential regulation mechanisms and associated protein factors are emerging from recent data.

A functional cellulose synthase from ascidian epidermis

Among animals, urochordates (e.g., ascidians) are unique in their ability to biosynthesize cellulose. In ascidians cellulose is synthesized in the epidermis and incorporated into a protective coat know as the tunic. A putative cellulose synthase-like gene was first identified in the genome sequences of the ascidian Ciona intestinalis. We describe here a cellulose synthase gene from the ascidian Ciona savignyi that is expressed in the epidermis. The predicted C. savignyi cellulose synthase amino acid sequence showed conserved features found in all cellulose synthases, including plants, but was most similar to cellulose synthases from bacteria, fungi, and Dictyostelium discoidium. However, unlike other known cellulose synthases, the predicted C. savignyi polypeptide has a degenerate cellulase-like region near the carboxyl-terminal end. An expression construct carrying the C. savignyi cDNA was found to restore cellulose biosynthesis to a cellulose synthase (CelA) minus mutant of Agrobacterium tumefaciens, showing that the predicted protein has cellulose synthase activity. The lack of cellulose biosynthesis in all other groups of metazoans and the similarity of the C. savignyi cellulose synthase to enzymes from cellulose-producing organisms support the hypothesis that the urochordates acquired the cellulose biosynthetic pathway by horizontal transfer.