In vitro versus in vivo cellulose microfibrils from plant primary wall synthases: structural differences

How Many Glucan Chains Form Plant Cellulose Microfibrils? A Mini Review

Assessing the number of glucan chains in cellulose microfibrils (CMFs) is crucial for understanding their structure-property relationships and interactions within plant cell walls. This Review examines the conclusions and limitations of the major experimental techniques that have provided insights into this question. Small-angle X-ray and neutron scattering data predominantly support an 18-chain model, although analysis is complicated by factors such as fibril coalescence and matrix polysaccharide associations. Solid-state nuclear magnetic resonance (NMR) spectroscopy allows the estimation of the CMF width from the ratio of interior to surface glucose residues. However, there is uncertainty in the assignment of NMR spectral peaks to surface or interior chains. Freeze-fracture transmission electron microscopy images show cellulose synthase complexes to be "rosettes" of six lobes each consistent with a trimer of cellulose synthase enzymes, consistent with the synthesis of 18 parallel glucan chains in the CMF. Nevertheless, the number of chains in CMFs remains to be conclusively demonstrated.

Bottom-Up Synthesized Glucan Materials: Opportunities from Applied Biocatalysis

Linear d-glucans are natural polysaccharides of simple chemical structure. They are comprised of d-glucosyl units linked by a single type of glycosidic bond. Noncovalent interactions within, and between, the d-glucan chains give rise to a broad variety of macromolecular nanostructures that can assemble into crystalline-organized materials of tunable morphology. Structure design and functionalization of d-glucans for diverse material applications largely relies on top-down processing and chemical derivatization of naturally derived starting materials. The top-down approach encounters critical limitations in efficiency, selectivity, and flexibility. Bottom-up approaches of d-glucan synthesis offer different, and often more precise, ways of polymer structure control and provide means of functional diversification widely inaccessible to top-down routes of polysaccharide material processing. Here the natural and engineered enzymes (glycosyltransferases, glycoside hydrolases and phosphorylases, glycosynthases) for d-glucan polymerization are described and the use of applied biocatalysis for the bottom-up assembly of specific d-glucan structures is shown. Advanced material applications of the resulting polymeric products are further shown and their important role in the development of sustainable macromolecular materials in a bio-based circular economy is discussed.

Non-targeted discovery of high-value bio-products in Nicotiana glauca L: a potential renewable plant feedstock

The evaluation of plant-based feedstocks is an important aspect of biorefining. Nicotiana glauca is a solanaceous, non-food crop that produces large amounts of biomass and is well adapted to grow in suboptimal conditions. In the present article, compatible sequential solvent extractions were applied to N. glauca leaves to enable the generation of enriched extracts containing higher metabolite content comparing to direct leaf extracts. Typically, between 60 to 100 metabolite components were identified within the fractions. The occurrence of plant fatty acids, fatty acid alcohols, alkanes, sterols and terpenoids was detected by gas liquid chromatography-mass spectrometry (GC-MS) and metabolite identification was confirmed by comparison of physico-chemical properties displayed by available authentic standards. Collectively, co-products such waxes, oils, fermentable sugars, and terpenoids were all identified and quantified. The enriched fractions of N. glauca revealed a high level of readily extractable hydrocarbons, oils and high value co-products. In addition, the saccharification yield and cell wall composition analyses in the stems revealed the potential of the residue material as a promising lignocellulosic substrate for the production of fermentable sugars. In conclusion a multifractional cascade for valuable compounds/commodities has been development, that uses N. glauca biomass. These data have enabled the evaluation of N. glauca material as a potential feedstock for biorefining.

Structural Color from Cellulose Nanocrystals or Chitin Nanocrystals: Self-Assembly, Optics, and Applications

Widespread concerns over the impact of human activity on the environment have resulted in a desire to replace artificial functional materials with naturally derived alternatives. As such, polysaccharides are drawing increasing attention due to offering a renewable, biodegradable, and biocompatible feedstock for functional nanomaterials. In particular, nanocrystals of cellulose and chitin have emerged as versatile and sustainable building blocks for diverse applications, ranging from mechanical reinforcement to structural coloration. Much of this interest arises from the tendency of these colloidally stable nanoparticles to self-organize in water into a lyotropic cholesteric liquid crystal, which can be readily manipulated in terms of its periodicity, structure, and geometry. Importantly, this helicoidal ordering can be retained into the solid-state, offering an accessible route to complex nanostructured films, coatings, and particles. In this review, the process of forming iridescent, structurally colored films from suspensions of cellulose nanocrystals (CNCs) is summarized and the mechanisms underlying the chemical and physical phenomena at each stage in the process explored. Analogy is then drawn with chitin nanocrystals (ChNCs), allowing for key differences to be critically assessed and strategies toward structural coloration to be presented. Importantly, the progress toward translating this technology from academia to industry is summarized, with unresolved scientific and technical questions put forward as challenges to the community.

Controlled processivity in glycosyltransferases: A way to expand the enzymatic toolbox

Glycosyltransferases (GT) catalyse the biosynthesis of complex carbohydrates which are the most abundant group of molecules in nature. They are involved in several key mechanisms such as cell signalling, biofilm formation, host immune system invasion or cell structure and this in both prokaryotic and eukaryotic cells. As a result, research towards complete enzyme mechanisms is valuable to understand and elucidate specific structure-function relationships in this group of molecules. In a next step this knowledge could be used in GT protein engineering, not only for rational drug design but also for multiple biotechnological production processes, such as the biosynthesis of hyaluronan, cellooligosaccharides or chitooligosaccharides. Generation of these poly- and/or oligosaccharides is possible due to a common feature of several of these GTs: processivity. Enzymatic processivity has the ability to hold on to the growing polymer chain and some of these GTs can even control the number of glycosyl transfers. In a first part, recent advances in understanding the mechanism of various processive enzymes are discussed. To this end, an overview is given of possible engineering strategies for the purpose of new industrial and fundamental applications. In the second part of this review, we focused on specific chain length-controlling mechanisms, i.e., key residues or conserved regions, and this for both eukaryotic and prokaryotic enzymes.

Opportunities for Ivory Nut Residue Valorization as a Source of Nanocellulose Colloidal Suspensions

Ivory nut seeds have been traditionally exploited in Central and South America for obtaining vegetable ivory. The residues from this industry are susceptible to valorization as a source of fatty acids (by organic extraction) and mannans (by alkaline dissolution and regeneration). Nonetheless, cellulose may also be recovered at the end of this fractionation process by acid hydrolysis and functionalization, with associated advantages over other lignocellulosic sources due to the absence of lignin in the endospermic tissue. In this work, various experimental parameters (sulfuric acid concentration, temperature, and hydrolysis time) were investigated to optimize the processing conditions for preparing stable nanocellulose suspensions after ultrasonication. The most stable nanocellulose gel (1 wt% solid content) was obtained after 4-h hydrolysis at 60 °C with 8 M H2SO4 and was characterized by using complementary tech-niques, including dynamic light scattering (DLS), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), nano-fibril sulfation measurements, vibrational and solid-state nuclear magnetic resonance (CP/MAS 13C-NMR) spectroscopies, and thermal analysis. This nanocellulose hydrogel is susceptible to further utilization in various applications and fields, e.g., in agricul-ture for controlling the release of agrochemicals, in pharmaceutics for developing new dosage forms, and in the treatment of wastewater from the textile and paper industries.

Structure of In Vitro-Synthesized Cellulose Fibrils Viewed by Cryo-Electron Tomography and 13C Natural-Abundance Dynamic Nuclear Polarization Solid-State NMR

Cellulose, the most abundant biopolymer, is a central source for renewable energy and functionalized materials. In vitro synthesis of cellulose microfibrils (CMFs) has become possible using purified cellulose synthase (CESA) isoforms from Physcomitrium patens and hybrid aspen. The exact nature of these in vitro fibrils remains unknown. Here, we characterize in vitro-synthesized fibers made by CESAs present in membrane fractions of P. patens over-expressing CESA5 by cryo-electron tomography and dynamic nuclear polarization (DNP) solid-state NMR. DNP enabled measuring two-dimensional 13C-13C correlation spectra without isotope-labeling of the fibers. Results show structural similarity between in vitro fibrils and native CMF in plant cell walls. Intensity quantifications agree with the 18-chain structural model for plant CMF and indicate limited fibrillar bundling. The in vitro system thus reveals insights into cell wall synthesis and may contribute to novel cellulosic materials. The integrated DNP and cryo-electron tomography methods are also applicable to structural studies of other carbohydrate-based biomaterials.

Recent Advances in Electron Microscopy of Carbohydrate Nanoparticles

Carbohydrate nanoparticles, both naturally derived and synthetic ones, have attracted scientific and industrial attention as high-performance renewable building blocks of functional materials. Electron microscopy (EM) has played a central role in investigations of their morphology and molecular structure, although the intrinsic radiation sensitivity of carbohydrate crystals has often hindered the in-depth characterization with EM techniques. This contribution reviews the recent advances in the electron microscopy of the carbohydrate nanoparticles. In particular, we highlight the recent efforts made to understand the three-dimensional shape and structural heterogeneity of nanoparticles using low-dose electron tomography and electron diffraction techniques coupled with cryogenic transmission electron microscopy.

Recent advances in enzymatic synthesis of β-glucan and cellulose

Bottom-up synthesis of β-glucans such as callose, fungal β-(1,3)(1,6)-glucan and cellulose, can create the defined compounds that are needed to perform fundamental studies on glucan properties and develop applications. With the importance of β-glucans and cellulose in high-profile fields such as nutrition, renewables-based biotechnology and materials science, the enzymatic synthesis of such relevant carbohydrates and their derivatives has attracted much attention. Here we review recent developments in enzymatic synthesis of β-glucans and cellulose, with a focus on progress made over the last five years. We cover the different types of biocatalysts employed, their incorporation in cascades, the exploitation of enzyme promiscuity and their engineering, and reaction conditions affecting the production as well as in situ self-assembly of (non)functionalised glucans. The recent achievements in the application of glycosyl transferases and β-1,4- and β-1,3-glucan phosphorylases demonstrate the high potential and versatility of these biocatalysts in glucan synthesis in both industrial and academic contexts.

Decreased cellulose-degrading enzyme activity causes pod hardening of okra (Abelmoschus esculentus L. Moench)

Okra (Abelmoschus esculentus L. Moench) is an important tropical and subtropical crop species, but okra pods age rapidly after they meet harvest standards. The underlying mechanisms by which okra pods harden are unclear. In this study, we determined the cellulose and lignin contents of 'Chaowuxing' okra pods from 4 to 14 days postanthesis (DPA). Based on the histochemical staining of okra fruit during the active period of cellulose accumulation, we found that the hardening of okra fruit is due to the rapid accumulation of cellulose in the cell walls of vascular cells in the pulp. We used RNA-sequencing (RNA-Seq) analyses to investigate the genes that regulate okra fruit aging. Transcriptome sequencing data showed that after 7 DPA, expression of the cellulose synthase gene (CesA) decreased with time. In addition, expression of the gene encoding the first functional enzyme involved in cellulose hydrolysis (endoglucanase) also decreased, but its rate of decrease was much faster than that of the CesA. The quantitative real-time PCR (qRT-PCR) results were consistent with the RNA-Seq data. Accordingly, we speculate that the accumulation of cellulose during okra pod hardening occurs via a reduction in cellulose hydrolysis activity. The above results suggest that thickening of the cell wall caused by a significant increase in cellulose content in the vascular bundles causes okra hardening. The accumulation of cellulose is not directly achieved via increased expression of CesA but rather indirectly via decreased hydrolysis of cellulose.

Phosphorylase-catalyzed bottom-up synthesis of short-chain soluble cello-oligosaccharides and property-tunable cellulosic materials

Cellulose-based materials are produced industrially in countless varieties via top-down processing of natural lignocellulose substrates. By contrast, cellulosic materials are only rarely prepared via bottom up synthesis and oligomerization-induced self-assembly of cellulose chains. Building up a cellulose chain via precision polymerization is promising, however, for it offers tunability and control of the final chemical structure. Synthetic cellulose derivatives with programmable material properties might thus be obtained. Cellodextrin phosphorylase (CdP; EC 2.4.1.49) catalyzes iterative β-1,4-glycosylation from α-d-glucose 1-phosphate, with the ability to elongate a diversity of acceptor substrates, including cellobiose, d-glucose and a range of synthetic glycosides having non-sugar aglycons. Depending on the reaction conditions leading to different degrees of polymerization (DP), short-chain soluble cello-oligosaccharides (COS) or insoluble cellulosic materials are formed. Here, we review the characteristics of CdP as bio-catalyst for synthetic applications and show advances in the enzymatic production of COS and reducing end-modified, tailored cellulose materials. Recent studies reveal COS as interesting dietary fibers that could provide a selective prebiotic effect. The bottom-up synthesized celluloses involve chains of DP ≥ 9, as precipitated in solution, and they form ~5 nm thick sheet-like crystalline structures of cellulose allomorph II. Solvent conditions and aglycon structures can direct the cellulose chain self-assembly towards a range of material architectures, including hierarchically organized networks of nanoribbons, or nanorods as well as distorted nanosheets. Composite materials are also formed. The resulting materials can be useful as property-tunable hydrogels and feature site-specific introduction of functional and chemically reactive groups. Therefore, COS and cellulose obtained via bottom-up synthesis can expand cellulose applications towards product classes that are difficult to access via top-down processing of natural materials.

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.

Review: Plant cell wall biochemical omics: The high-throughput biochemistry for polysaccharide biosynthesis

Progress in the functional biochemical analysis of plant glycosyltransferases (GTs) has been slow because plant GTs are generally membrane proteins, operate as part of larger, multimeric complexes, and utilize a vast complexity of substrate acceptors. Therefore, the field would benefit from development of adequate high throughput expression as well as product detection and characterization techniques. Here we review current approaches to tackle such obstacles and suggest a new path forward: nucleic acid programmable protein arrays (NAPPA) with liquid sample desorption ionization (LS-DESI-MS) mass spectrometry. NAPPA utilizes in vitro transcription and translation to produce epitope-tagged fusion proteins from cloned GT cDNAs. LS-DESI is a soft ionization technique that allows rapid and sensitive MS-based product characterization in situ. Coupling both approaches provides the opportunity to examine individual GT functions as well as protein-protein interactions. Furthermore, advances in automated oligosaccharide synthesis and lipid nanodisc technology should allow testing of plant GT activity in presence of numerous substrate acceptors and lipid environments in a high throughput fashion. Thus, NAPPA-DESI-MS has great potential to make headway in biochemical characterization of the large number of plant GTs.

Observation of in vitro cellulose synthesis by bacterial cellulose synthase with time-resolved small angle X-ray scattering

Cellulose synthase is the enzyme that produces cellulose in the living organisms like plant, and has two functions: polymerizing glucose residues (polymerization) and assembling these polymerized molecules into a crystalline microfibril with a "cellulose I" crystallographic structure (crystallization). Many studies, however, have shown that an in vitro reaction of cellulose synthase produces aggregates of a non-native crystallographic structure "cellulose II", despite the remaining polymerizing activity. This is partial denaturation or loss of crystallization function in cellulose synthase, which needs to be resolved to reconstitute its native activity. To this end, we aimed to clarify the process of cellulose II formation by bacterial cellulose synthase in vitro, using in situ small angle X-ray scattering (SAXS). An increase in scattering specific to synthesis was observed around two distinct regions of q (0.2-0.4 nm^-1 and <0.1 nm^-1) by time-resolved SAXS measurement. The scattering at higher q-region appears prior to lower-q scattering at beginning of the reaction, indicating the existence of smaller primitive aggregations at the initiation stage. This study demonstrates the use of in situ SAXS measurement to decipher the dynamics of biosynthesized cellulose chains, which is a remarkable example of polymer assembly in ambient conditions.

Imaging cellulose synthase motility during primary cell wall synthesis in the grass Brachypodium distachyon

The mechanism of cellulose synthesis has been studied by characterizing the motility of cellulose synthase complexes tagged with a fluorescent protein; however, this approach has been used exclusively on the hypocotyl of Arabidopsis thaliana. Here we characterize cellulose synthase motility in the model grass, Brachypodium distachyon. We generated lines in which mEGFP is fused N-terminal to BdCESA3 or BdCESA6 and which grew indistinguishably from the wild type (Bd21-3) and had dense fluorescent puncta at or near the plasma membrane. Measured with a particle tracking algorithm, the average speed of GFP-BdCESA3 particles in the mesocotyl was 164 ± 78 nm min⁻¹ (error gives standard deviation [SD], n = 1451 particles). Mean speed in the root appeared similar. For comparison, average speed in the A. thaliana hypocotyl expressing GFP-AtCESA6 was 184 ± 86 nm min⁻¹ (n = 2755). For B. distachyon, we quantified root diameter and elongation rate in response to inhibitors of cellulose (dichlorobenylnitrile; DCB), microtubules (oryzalin), or actin (latrunculin B). Neither oryzalin nor latrunculin affected the speed of CESA complexes; whereas, DCB reduced average speed by about 50% in B. distachyon and by about 35% in A. thaliana. Evidently, between these species, CESA motility is well conserved.

How the Depletion in Mineral Major Elements Affects Grapevine (Vitis vinifera L.) Primary Cell Wall

The noteworthy fine remodeling that plant cell walls (CWs) undergo to adapt to developmental, physiological and environmental cues and the observation that its composition and dynamics differ between species represents an opportunity to couple crop species agronomic studies with research on CW modifications. Vitis vinifera is one of the most important crops from an economic point-of-view due to the high value of the fruit, predominantly for winemaking. The availability of some information related to this species' CWs allows researching its responses to imposed conditions that affect the plant's development. Mineral deficiency, in particular nitrogen, phosphorus, potassium and sulfur, strongly affects plant metabolism, reducing both growth and crop yield. Despite the importance of mineral nutrition in development, its influence on CW synthesis and modifications is still insufficiently documented. Addressing this knowledge gap, V. vinifera experimental models were used to study CW responses to imposed mineral depletion in unorganized (callus) and organized (shoots) tissues. The discussion of the obtained results is the main focus of this review. Callus and shoots submitted to mineral restriction are impaired in specific CW components, predominantly cellulose. Reorganization on structure and deposition of several other polymers, in particular the degree and pattern of pectin methyl-esterification and the amount of xyloglucan (XyG), arabinan and extensin, is also observed. In view of recently proposed CW models that consider biomechanical hotspots and direct linkages between pectins and XyG/cellulose, the outcome of these modifications in explaining maintenance of CW integrity through compensatory stiffening can be debated. Nutrient stresses do not affect evenly all tissues with undifferentiated callus tissues showing more pronounced responses, followed by shoot mature internodes, and then newly formed internodes. The impact of nitrogen depletion leads to more noticeable responses, supporting this nutrient's primary role in plant development and metabolism. The consequential compensatory mechanisms highlight the pivotal role of CW in rearranging under environmental stresses.

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.

Regulation of cell wall remodeling in grapevine (Vitis vinifera L.) callus under individual mineral stress deficiency

Cell wall (CW) is a dynamic structure that determines the plant form, growth and response to environmental conditions. Vitis vinifera callus grown under nitrogen (-N), phosphorous (-P) and sulfur (-S) deficiency were used as a model system to address the influence of mineral stress in CW remodeling. Callus cells morphology was altered, mostly under -N, resulting in changes in cell length and width compared with the control. CW composition ascertained with specific staining and immuno-detection showed a decrease in cellulose and altered pattern of pectin methylesterification. Under mineral stress genes expression from candidate families disclosed mainly a downregulation of a glycosyl hydrolase family 9C (GH9C), xyloglucan transglycosylase/hydrolases (XTHs) with predicted hydrolytic activity and pectin methylesterases (PMEs). Conversely, upregulation of PMEs inhibitors (PMEIs) was observed. While methylesterification patterns can be associated to PME/PMEI gene expression, the lower cellulose content cannot be attributed to altered cellulose synthase (CesA) gene expression suggesting the involvement of other gene families. Salt extracts from -N and -P callus tissues increased plastic deformation in cucumber hypocotyls while no effect was observed with -S extracts. The lower endo-acting glycosyl hydrolase activity of -N callus extracts pinpoints a more expressive impact of -N on CW-remodeling.

In vitro synthesis of cellulose microfibrils by a membrane protein from protoplasts of the non-vascular plant Physcomitrella patens

Plant cellulose synthases (CesAs) form a family of membrane proteins that are associated with hexagonal structures in the plasma membrane called CesA complexes (CSCs). It has been difficult to purify plant CesA proteins for biochemical and structural studies. We describe CesA activity in a membrane protein preparation isolated from protoplasts of Physcomitrella patens overexpressing haemagglutinin (HA)-tagged PpCesA5. Incubating the membrane preparation with UDP-glucose predominantly produced cellulose. Negative-stain EM revealed microfibrils. Cellulase bound to and degraded these microfibrils. Vibrational sum frequency generation (SFG) spectroscopic analysis detected the presence of crystalline cellulose in the microfibrils. Putative CesA proteins were frequently observed attached to the microfibril ends. Combined cross-linking and gradient centrifugation showed bundles of cellulose microfibrils with larger particle aggregates, possibly CSCs. These results suggest that P. patens is a useful model system for biochemical and structural characterization of plant CSCs and their components.

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.

Endo-β-1,4-glucanases impact plant cell wall development by influencing cellulose crystallization

Cell walls are vital to the normal growth and development of plants as they protect the protoplast and provide rigidity to the stem. Here, two poplar and Arabidopsis orthologous endoglucanases, which have been proposed to play a role in secondary cell wall development, were examined. The class B endoglucanases, PtGH9B5 and AtGH9B5, are secreted enzymes that have a predicted glycosylphosphatidylinositol anchor, while the class C endoglucanases, PtGH9C2 and AtGH9C2, are also predicted to be secreted but instead contain a carbohydrate-binding module. The poplar endoglucanases were expressed in Arabidopsis using both a 35S promoter and the Arabidopsis secondary cell wall-specific CesA8 promoter. Additionally, Arabidopsis t-DNA insertion lines and an RNAi construct was created to downregulate AtGH9C2 in Arabidopsis. All of the plant lines were examined for changes in cell morphology and patterning, growth and development, cell wall crystallinity, microfibril angle, and proportion of cell wall carbohydrates. Misregulation of PtGH9B5/AtGH9B5 resulted in changes in xylose content, while misregulation of PtGH9C2/AtGH9C2 resulted in changes in crystallinity, which was inversely correlated with changes in plant height and rosette diameter. Together, these results suggest that these endoglucanases affect secondary cell wall development by contributing to the cell wall crystallization process.

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.

Indaziflam herbicidal action: a potent cellulose biosynthesis inhibitor

Cellulose biosynthesis is a common feature of land plants. Therefore, cellulose biosynthesis inhibitors (CBIs) have a potentially broad-acting herbicidal mode of action and are also useful tools in decoding fundamental aspects of cellulose biosynthesis. Here, we characterize the herbicide indaziflam as a CBI and provide insight into its inhibitory mechanism. Indaziflam-treated seedlings exhibited the CBI-like symptomologies of radial swelling and ectopic lignification. Furthermore, indaziflam inhibited the production of cellulose within <1 h of treatment and in a dose-dependent manner. Unlike the CBI isoxaben, indaziflam had strong CBI activity in both a monocotylonous plant (Poa annua) and a dicotyledonous plant (Arabidopsis [Arabidopsis thaliana]). Arabidopsis mutants resistant to known CBIs isoxaben or quinoxyphen were not cross resistant to indaziflam, suggesting a different molecular target for indaziflam. To explore this further, we monitored the distribution and mobility of fluorescently labeled CELLULOSE SYNTHASE A (CESA) proteins in living cells of Arabidopsis during indaziflam exposure. Indaziflam caused a reduction in the velocity of YELLOW FLUORESCENT PROTEIN:CESA6 particles at the plasma membrane focal plane compared with controls. Microtubule morphology and motility were not altered after indaziflam treatment. In the hypocotyl expansion zone, indaziflam caused an atypical increase in the density of plasma membrane-localized CESA particles. Interestingly, this was accompanied by a cellulose synthase interacting1-independent reduction in the normal coincidence rate between microtubules and CESA particles. As a CBI, for which there is little evidence of evolved weed resistance, indaziflam represents an important addition to the action mechanisms available for weed management.

Molecular counting by photobleaching in protein complexes with many subunits: best practices and application to the cellulose synthesis complex

The constituents of large, multisubunit protein complexes dictate their functions in cells, but determining their precise molecular makeup in vivo is challenging. One example of such a complex is the cellulose synthesis complex (CSC), which in plants synthesizes cellulose, the most abundant biopolymer on Earth. In growing plant cells, CSCs exist in the plasma membrane as six-lobed rosettes that contain at least three different cellulose synthase (CESA) isoforms, but the number and stoichiometry of CESAs in each CSC are unknown. To begin to address this question, we performed quantitative photobleaching of GFP-tagged AtCESA3-containing particles in living Arabidopsis thaliana cells using variable-angle epifluorescence microscopy and developed a set of information-based step detection procedures to estimate the number of GFP molecules in each particle. The step detection algorithms account for changes in signal variance due to changing numbers of fluorophores, and the subsequent analysis avoids common problems associated with fitting multiple Gaussian functions to binned histogram data. The analysis indicates that at least 10 GFP-AtCESA3 molecules can exist in each particle. These procedures can be applied to photobleaching data for any protein complex with large numbers of fluorescently tagged subunits, providing a new analytical tool with which to probe complex composition and stoichiometry.

Spatial structure of plant cell wall polysaccharides and its functional significance

Plant polysaccharides comprise the major portion of organic matter in the biosphere. The cell wall built on the basis of polysaccharides is the key feature of a plant organism largely determining its biology. All together, around 10 types of polysaccharide backbones, which can be decorated by different substituents giving rise to endless diversity of carbohydrate structures, are present in cell walls of higher plants. Each of the numerous cell types present in plants has cell wall with specific parameters, the features of which mostly arise from the structure of polymeric components. The structure of polysaccharides is not directly encoded by the genome and has variability in many parameters (molecular weight, length, and location of side chains, presence of modifying groups, etc.). The extent of such variability is limited by the "functional fitting" of the polymer, which is largely based on spatial organization of the polysaccharide and its ability to form supramolecular complexes of an appropriate type. Consequently, the carrier of the functional specificity is not the certain molecular structure but the certain type of the molecules having a certain degree of heterogeneity. This review summarizes the data on structural features of plant cell wall polysaccharides, considers formation of supramolecular complexes, gives examples of tissue- and stage-specific polysaccharides and functionally significant carbohydrate-carbohydrate interactions in plant cell wall, and presents approaches to analyze the spatial structure of polysaccharides and their complexes.

Cellulose synthases: new insights from crystallography and modeling

Detailed information about the structure and biochemical mechanisms of cellulose synthase (CelS) proteins remained elusive until a complex containing the catalytic subunit (BcsA) of CelS from Rhodobacter sphaeroides was crystalized. Additionally, a 3D structure of most of the cytosolic domain of a plant CelS (GhCESA1 from cotton, Gossypium hirsutum) was produced by computational modeling. This predicted structure contributes to our understanding of how plant CelS proteins may be similar and different as compared with BcsA. In this review, we highlight how these structures impact our understanding of the synthesis of cellulose and other extracellular polysaccharides. We show how the structures can be used to generate hypotheses for experiments testing mechanisms of glucan synthesis and translocation in plant CelS.

Cellulose synthesis and its regulation

Cellulose, the most abundant biopolymer synthesized on land, is made of linear chains of ß (1-4) linked D-glucose. As a major structural component of the cell wall, cellulose is important not only for industrial use but also for plant growth and development. Cellulose microfibrils are tethered by other cell wall polysaccharides such as hemicellulose, pectin, and lignin. In higher plants, cellulose is synthesized by plasma membrane-localized rosette cellulose synthase complexes. Despite the recent advances using a combination of molecular genetics, live cell imaging, and spectroscopic tools, many aspects of the cellulose synthesis remain a mystery. In this chapter, we highlight recent research progress towards understanding the mechanism of cellulose synthesis in Arabidopsis.

The cell biology of cellulose synthesis

Plant stature and development are governed by cell proliferation and directed cell growth. These parameters are determined largely by cell wall characteristics. Cellulose microfibrils, composed of hydrogen-bonded β-1,4 glucans, are key components for anisotropic growth in plants. Cellulose is synthesized by plasma membrane-localized cellulose synthase complexes. In higher plants, these complexes are assembled into hexameric rosettes in intracellular compartments and secreted to the plasma membrane. Here, the complexes typically track along cortical microtubules, which may guide cellulose synthesis, until the complexes are inactivated and/or internalized. Determining the regulatory aspects that control the behavior of cellulose synthase complexes is vital to understanding directed cell and plant growth and to tailoring cell wall content for industrial products, including paper, textiles, and fuel. In this review, we summarize and discuss cellulose synthesis and regulatory aspects of the cellulose synthase complex, focusing on Arabidopsis thaliana.

Identification and characterization of UDP-glucose pyrophosphorylase in cyanobacteria Anabaena sp. PCC 7120

Exopolysaccharides produced by photosynthetic cyanobacteria have received considerable attention in recent years for their potential applications in the production of renewable biofuels. Particularly, cyanobacterial cellulose is one of the most promising products because it is extracellularly secreted as a non-crystalline form, which can be easily harvested from the media and converted into glucose units. In cyanobacteria, the production of UDP-glucose, the cellulose precursor, is a key step in the cellulose synthesis pathway. UDP-glucose is synthesized from UTP and glucose-1-phosphate (Glc-1P) by UDP-glucose pyrophosphorylase (UGPase), but this pathway in cyanobacteria has not been well characterized. Therefore, to elucidate the overall cellulose biosynthesis pathway in cyanobacteria, we studied the putative UGPase All3274 and seven other putative NDP-sugar pyrophosphorylases (NSPases), All4645, Alr2825, Alr4491, Alr0188, Alr3400, Alr2361, and Alr3921 of Anabaena sp. PCC 7120. Assays using the purified recombinant proteins revealed that All3274 exhibited UGPase activity, All4645, Alr2825, Alr4491, Alr0188, and Alr3921 exhibited pyrophosphorylase activities on ADP-glucose, CDP-glucose, dTDP-glucose, GDP-mannose, and UDP-N-acetylglucosamine, respectively. Further characterization of All3274 revealed that the kcat for UDP-glucose formation was one or two orders lower than those of other known UGPases. The activity and dimerization tendency of All3274 increased at higher enzyme concentrations, implying catalytic activation by dimerization. However, most interestingly, All3274 dimerization was inhibited by UTP and Glc-1P, but not by UDP-glucose. This study presents the first in vitro characterization of a cyanobacterial UGPase, and provides insights into biotechnological attempts to utilize the photosynthetic production of cellulose from cyanobacteria.

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.

Accumulation of noncrystalline cellulose in Physarum microplasmodia

Physarum plasmodium lives as a slimy mass of protoplast in the dark fragments into small multinucleated microplasmodia (mPL) in a liquid medium. When mPL are exposed to several unfavorable environments, they transform into "spherules" with a cell wall. Using a synchronous spherule-induction system for mPL, we examined the effect of 2,6-dichlorobenzonitrile on the synthesis of cellulose in mPL, by observing mPL under a fluorescence microscope, and isolated cellulose from mPL to identify them morphologically under scanning electron microscopy. Moreover, we examined in vivo labeling to determine when cellulose synthesis is activated in step 2. We found that the nourishment medium in step 2 was essential for mPL prior to spherulation and that the conversion starts at 48 h in step 2 of our system. From the experiments using Updegraff reagent for the sedimentation of cellulose in the cell wall fraction from mPL, we propose that cellulose produced in mPL is likely noncrystalline cellulose. We conclude that mPL of multinucleated protoplasts without the cell wall structure synthesize cellulose under constitutive condition and accumulate abundantly noncrystalline cellulose, in preparation for unfavorable environments that may occur in the future in which mPL must initiate the program to form the cell wall of spherules.

Brittle Culm1, a COBRA-like protein, functions in cellulose assembly through binding cellulose microfibrils

Cellulose represents the most abundant biopolymer in nature and has great economic importance. Cellulose chains pack laterally into crystalline forms, stacking into a complicated crystallographic structure. However, the mechanism of cellulose crystallization is poorly understood. Here, via functional characterization, we report that Brittle Culm1 (BC1), a COBRA-like protein in rice, modifies cellulose crystallinity. BC1 was demonstrated to be a glycosylphosphatidylinositol (GPI) anchored protein and can be released into cell walls by removal of the GPI anchor. BC1 possesses a carbohydrate-binding module (CBM) at its N-terminus. In vitro binding assays showed that this CBM interacts specifically with crystalline cellulose, and several aromatic residues in this domain are essential for binding. It was further demonstrated that cell wall-localized BC1 via the CBM and GPI anchor is one functional form of BC1. X-ray diffraction (XRD) assays revealed that mutations in BC1 and knockdown of BC1 expression decrease the crystallite width of cellulose; overexpression of BC1 and the CBM-mutated BC1s caused varied crystallinity with results that were consistent with the in vitro binding assay. Moreover, interaction between the CBM and cellulose microfibrils was largely repressed when the cell wall residues were pre-stained with two cellulose dyes. Treating wild-type and bc1 seedlings with the dyes resulted in insensitive root growth responses in bc1 plants. Combined with the evidence that BC1 and three secondary wall cellulose synthases (CESAs) function in different steps of cellulose production as revealed by genetic analysis, we conclude that BC1 modulates cellulose assembly by interacting with cellulose and affecting microfibril crystallinity.

Cellulose is an important natural resource with great economic value. Plant cellulose packs laterally into a complicated crystallographic structure, which determines cellulose quality and commercial uses. However, the mechanism of cellulose crystallization is poorly understood. Here we report that Brittle Culm1 (BC1), a COBRA-like (COBL) protein of rice, modifies cellulose crystallinity. Although previous studies have indicated the involvement of COB and COBL proteins in cellulose biosynthesis, the underlying molecular basis for this remains elusive. We demonstrate that BC1 localizes to the cell-wall and functions in a process that is distinct from that of the three secondary wall cellulose synthases (CESAs). A carbohydrate-binding module (CBM) at the N-terminus of BC1 interacts specifically with crystalline cellulose and regulates microfibril crystallite size. We conclude that BC1 modulates cellulose structure by binding to cellulose and affecting microfibril crystallinity. These findings provide new insights into the mechanism of cellulose assembly and further our understanding of the roles of COB and COBLs in cell wall biogenesis.

Purification and characterization of a soluble β-1,4-glucan from bean (Phaseolus vulgaris L.)-cultured cells dehabituated to dichlobenil

Bean cells habituated to grow in the presence of dichlobenil exhibited reduced cellulose and hemicellulose content and an increase in pectic polysaccharides. Furthermore, following the extraction of pectins and hemicelluloses, a large amount of neutral sugars was released. These sugars were found to be part of a soluble β-1,4-glucan in a preliminary characterization, as reported by Encina et al. (Physiol Plant 114:182-191, 2002). When habituated cells were subcultured in the absence of the herbicide (dehabituated cells), the release of neutral sugars after the extraction of pectins and hemicelluloses was maintained. In this study, we have isolated a soluble β-1,4-glucan from dehabituated cells by sonication of the wall residue (cellulose fraction) remaining after fractionation. Gel filtration chromatography revealed that its average molecular size was 14 kDa. Digestion of the sample with endocellulase revealed the presence of cellobiose, cellotriose, and cellotetraose. Methylation analysis showed that 4-linked glucose was the most abundant sugar residue, but 4,6-linked glucose, terminal arabinose and 4-linked galactose for xyloglucan, and arabinogalactan were also identified. NMR analysis showed that this 1,4-glucan may be composed of various kinds of substitutions along the glucan backbone together with acetyl groups linked to the OH group of sugar residues. Thus, despite its relatively high molecular mass, the β-glucan remains soluble because of its unique configuration. This is the first time that a glucan with such characteristics has been isolated and described. The discovery of new molecules, as this β-glucan with unique features, may help understand the composition and arrangement of the polymers within plant cell walls, contributing to a better understanding of this complex structure.

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.

Crystalline and amorphous cellulose in the secondary walls of Arabidopsis

In the cell walls of higher plants, cellulose chains are present in crystalline microfibril, with an amorphous part at the surface, or present as amorphous material. To assess the distribution and relative occurrence of the two forms of cellulose in the inflorescence stem of Arabidopsis, we used two carbohydrate-binding modules, CBM3a and CBM28, specific for crystalline and amorphous cellulose, respectively, with immunogold detection in TEM. The binding of the two CBMs displayed specific patterns suggesting that the synthesis of cellulose leads to variable nanodomains of cellulose structures according to cell type. In developing cell walls, only CBM3a bound significantly to the incipient primary walls, indicating that at the onset of its deposition cellulose is in a crystalline structure. As the secondary wall develops, the labeling with both CBMs becomes more intense. The variation of the labeling pattern by CBM3a between transverse and longitudinal sections appeared related to microfibril orientation and differed between fibers and vessels. Although the two CBMs do not allow the description of the complete status of cellulose microstructures, they revealed the dynamics of the deposition of crystalline and amorphous forms of cellulose during wall formation and between cell types adapting cellulose microstructures to the cell function.

Radiometric and spectrophotometric in vitro assays of glycosyltransferases involved in plant cell wall carbohydrate biosynthesis

Most of the glycosyltransferases (GTs) that catalyze the formation of plant cell wall carbohydrates remain to be biochemically characterized. This can be achieved only if specific assays are available for these enzymes. Here we present a protocol for in vitro assays of processive and nonprocessive membrane-bound GTs. The assays are either based on the use of radioactive nucleotide sugars (NDP sugars; e.g., UDP-[U-(14)C]glucose) and the quantification of the radiolabeled monosaccharides incorporated into soluble or insoluble carbohydrates, or on the coupling of the GT reaction with that of pyruvate kinase (PK) and the oxidation of NADH by lactate dehydrogenase (LDH). The radiometric assays are more suitable for exploratory work on poorly characterized enzymes, whereas the spectrophotometric assays require the availability of highly enriched GTs. Both assays can be performed within 1 d, depending on the number of fractions to be assayed or reaction mixtures to be tested.

Enzymatic synthesis of cellulose II-like substance via cellulolytic enzyme-mediated transglycosylation in an aqueous medium

The enzymatic synthesis of cellulose-like substance via a non-biosynthetic pathway has been achieved by transglycosylation in an aqueous system of the corresponding substrate, cellotriose for cellulolytic enzyme endo-acting endoglucanase I (EG I) from Hypocrea jecorina. A significant amount of water-insoluble product precipitated out from the reaction system. MALDI-TOF mass analysis showed that the resulting precipitate had a degree of polymerization (DP) of up to 16 from cellotriose. Solid-state (13)C NMR spectrum of the resulting water-insoluble product revealed that all carbon resonance lines were assigned to two kinds of anhydroglucose residues in the corresponding structure of cellulose II. X-ray diffraction (XRD) measurement as well as (13)C NMR analysis showed that the crystal structure corresponds to cellulose II with a high degree of crystallinity. We propose the multiple oligomers form highly crystalline cellulose II as a result of self-assembly via oligomer-oligomer interaction when they precipitate.

The missing link: do cortical microtubules define plasma membrane nanodomains that modulate cellulose biosynthesis?

Cellulose production is a crucial aspect of plant growth and development. It is functionally linked to cortical microtubules, which self-organize into highly ordered arrays often situated in close proximity to plasma membrane-bound cellulose synthase complexes (CSCs). Although most models put forward to explain the microtubule-cellulose relationship have considered mechanisms by which cortical microtubule arrays influence the orientation of cellulose microfibrils, little attention has been paid to how microtubules affect the physicochemical properties of cellulose. A recent study using the model system Arabidopsis, however, indicates that microtubules can modulate the crystalline and amorphous content of cellulose microfibrils. Microtubules are required during rapid growth for reducing crystalline content, which is predicted to increase the degree to which cellulose is tethered by hemicellulosic polysaccharides. Such tethering is, in turn, critical for maintaining unidirectional cell expansion. In this article, we hypothesize that cortical microtubules influence the crystalline content of cellulose either by controlling plasma membrane fluidity or by modulating the deposition of noncellulosic wall components in the vicinity of the CSCs. We discuss the current limitations of imaging technology to address these hypotheses and identify the image acquisition and processing strategies that will integrate live imaging with super resolution three-dimensional information.

Biosynthesis of non-cellulosic polysaccharides of plant cell walls

Enzymes that make the polymer backbones of plant cell wall polysaccharides have proven to be recalcitrant to biochemical purification. Availability of mutational genetics and genomic tools paved the way for rapid progress in identifying genes encoding various cell wall glycan synthases. Mutational genetics, the primary tool used in unraveling cellulose biosynthesis, was ineffective in assigning function to any of the hemicellulosic, polymerizing glycan synthases. A combination of comparative genomics and functional expression in a heterologous system allowed identification of various cellulose synthase-like (Csl) sequences as being involved in the formation of β-1,4-mannan, β-1,4-glucan, and mixed-linked glucan. A number of xylose-deficient mutants have led to a variety of genes, none of which thus far possesses the motifs known to be conserved among polymerizing β-glycan synthases. Except for xylan synthase, which appears to be an agglomerate of proteins just like cellulose synthase, Golgi glycan synthases already identified suggest that the catalytic polypeptide by itself is sufficient for enzyme activity, most likely as a homodimer. Several of the Csl genes remain to be assigned a function. The possibility of the involvement of various Csl genes in making more than one product remains.

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.

Photomorphogenesis

As photoautotrophs, plants are exquisitely sensitive to their light environment. Light affects many developmental and physiological responses throughout plants' life histories. The focus of this chapter is on light effects during the crucial period of time between seed germination and the development of the first true leaves. During this time, the seedling must determine the appropriate mode of action to best achieve photosynthetic and eventual reproductive success. Light exposure triggers several major developmental and physiological events. These include: growth inhibition and differentiation of the embryonic stem (hypocotyl); maturation of the embryonic leaves (cotyledons); and establishment and activation of the stem cell population in the shoot and root apical meristems. Recent studies have linked a number of photoreceptors, transcription factors, and phytohormones to each of these events.

Extraction of cellulose-synthesizing activity of Gluconacetobacter xylinus by alkylmaltoside

This study reinvestigated the synthesis of cellulose in vitro with a well-known cellulose-producing bacterium, Gluconacetobacter xylinus. Alkylmaltoside detergents, which are more frequently used in recent structural biological researches, are uniquely used in this study to solubilize cellulose-synthesizing activity from the cell membrane of G. xylinus. Activity comparable to that previously reported is obtained, while the synthesized cellulose is crystallized into a non-native polymorph of cellulose (cellulose II) as well as the previous studies. In spite of this failure to recover the native activity to synthesize cellulose I microfibril in vitro, the product is a polymer with a degree of polymerization greater than 45 as determined by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). It was thus concluded that the established protocol can solubilize cellulose-synthesizing activity of G. xylinus with polymerizing activity.