The maize primary cell wall microfibril: a new model derived from direct visualization

Mutations of cellulose synthase (CESA1) phosphorylation sites modulate anisotropic cell expansion and bidirectional mobility of cellulose synthase

The CESA1 component of cellulose synthase is phosphorylated at sites clustered in two hypervariable regions of the protein. Mutations of the phosphorylated residues to Ala (A) or Glu (E) alter anisotropic cell expansion and cellulose synthesis in rapidly expanding roots and hypocotyls. Expression of T166E, S686E, or S688E mutants of CESA1 fully rescued the temperature sensitive cesA1-1 allele (rsw1) at a restrictive temperature whereas mutations to A at these positions caused defects in anisotropic cell expansion. However, mutations to E at residues surrounding T166 (i.e., S162, T165, and S167) caused opposite effects. Live-cell imaging of fluorescently labeled CESA showed close correlations between tissue or cell morphology and patterns of bidirectional motility of CESA complexes in the plasma membrane. In the WT, CESA complexes moved at similar velocities in both directions along microtubule tracks. By contrast, the rate of movement of CESA particles was directionally asymmetric in mutant lines that exhibited abnormal tissue or cell expansion, and the asymmetry was removed upon depolymerizing microtubules with oryzalin. This suggests that phosphorylation of CESA differentially affects a polar interaction with microtubules that may regulate the length or quantity of a subset of cellulose microfibrils and that this, in turn, alters microfibril structure in the primary cell wall resulting in or contributing to the observed defect in anisotropic cell expansion.

Shaping the micromechanical behavior of multi-phase composites for bone tissue engineering

Mechanical stiffness is a fundamental parameter in the rational design of composites for bone tissue engineering in that it affects both the mechanical stability and the osteo-regeneration process at the fracture site. A mathematical model is presented for predicting the effective Young's modulus (E) and shear modulus (G) of a multi-phase biocomposite as a function of the geometry, material properties and volume concentration of each individual phase. It is demonstrated that the shape of the reinforcing particles may dramatically affect the mechanical stiffness: E and G can be maximized by employing particles with large geometrical anisotropy, such as thin platelet-like or long fibrillar-like particles. For a porous poly(propylene fumarate) (60% porosity) scaffold reinforced with silicon particles (10% volume concentration) the Young's (shear) modulus could be increased by more than 10 times by just using thin platelet-like as opposed to classical spherical particles, achieving an effective modulus E approximately 8 GPa (G approximately 3.5 GPa). The mathematical model proposed provides results in good agreement with several experimental test cases and could help in identifying the proper formulation of bone scaffolds, reducing the development time and guiding the experimental testing.

Imaging cell wall architecture in single Zinnia elegans tracheary elements

The chemical and structural organization of the plant cell wall was examined in Zinnia elegans tracheary elements (TEs), which specialize by developing prominent secondary wall thickenings underlying the primary wall during xylogenesis in vitro. Three imaging platforms were used in conjunction with chemical extraction of wall components to investigate the composition and structure of single Zinnia TEs. Using fluorescence microscopy with a green fluorescent protein-tagged Clostridium thermocellum family 3 carbohydrate-binding module specific for crystalline cellulose, we found that cellulose accessibility and binding in TEs increased significantly following an acidified chlorite treatment. Examination of chemical composition by synchrotron radiation-based Fourier-transform infrared spectromicroscopy indicated a loss of lignin and a modest loss of other polysaccharides in treated TEs. Atomic force microscopy was used to extensively characterize the topography of cell wall surfaces in TEs, revealing an outer granular matrix covering the underlying meshwork of cellulose fibrils. The internal organization of TEs was determined using secondary wall fragments generated by sonication. Atomic force microscopy revealed that the resulting rings, spirals, and reticulate structures were composed of fibrils arranged in parallel. Based on these combined results, we generated an architectural model of Zinnia TEs composed of three layers: an outermost granular layer, a middle primary wall composed of a meshwork of cellulose fibrils, and inner secondary wall thickenings containing parallel cellulose fibrils. In addition to insights in plant biology, studies using Zinnia TEs could prove especially productive in assessing cell wall responses to enzymatic and microbial degradation, thus aiding current efforts in lignocellulosic biofuel production.

Giardia cyst wall protein 1 is a lectin that binds to curled fibrils of the GalNAc homopolymer

The infectious and diagnostic stage of Giardia lamblia (also known as G. intestinalis or G. duodenalis) is the cyst. The Giardia cyst wall contains fibrils of a unique beta-1,3-linked N-acetylgalactosamine (GalNAc) homopolymer and at least three cyst wall proteins (CWPs) composed of Leu-rich repeats (CWP(LRR)) and a C-terminal conserved Cys-rich region (CWP(CRR)). Our goals were to dissect the structure of the cyst wall and determine how it is disrupted during excystation. The intact Giardia cyst wall is thin (approximately 400 nm), easily fractured by sonication, and impermeable to small molecules. Curled fibrils of the GalNAc homopolymer are restricted to a narrow plane and are coated with linear arrays of oval-shaped protein complex. In contrast, cyst walls of Giardia treated with hot alkali to deproteinate fibrils of the GalNAc homopolymer are thick (approximately 1.2 microm), resistant to sonication, and permeable. The deproteinated GalNAc homopolymer, which forms a loose lattice of curled fibrils, is bound by native CWP1 and CWP2, as well as by maltose-binding protein (MBP)-fusions containing the full-length CWP1 or CWP1(LRR). In contrast, neither MBP alone nor MBP fused to CWP1(CRR) bind to the GalNAc homopolymer. Recombinant CWP1 binds to the GalNAc homopolymer within secretory vesicles of Giardia encysting in vitro. Fibrils of the GalNAc homopolymer are exposed during excystation or by treatment of heat-killed cysts with chymotrypsin, while deproteinated fibrils of the GalNAc homopolymer are degraded by extracts of Giardia cysts but not trophozoites. These results show the Leu-rich repeat domain of CWP1 is a lectin that binds to curled fibrils of the GalNAc homopolymer. During excystation, host and Giardia proteases appear to degrade bound CWPs, exposing fibrils of the GalNAc homopolymer that are digested by a stage-specific glycohydrolase.

Characterization of cellulose synthase complexes in Populus xylem differentiation

*It is generally hypothesized that the synthesis of cellulose in higher plants is mediated by cellulose synthase complexes (CSCs) localized on the plasma membrane. However, CSCs have not been investigated thoroughly through their isolation. The availability of ample Populus tissue allowed Populus CSCs to be isolated and characterized in association with xylem differentiation. *The methods used here included co-immunoprecipitation, proteomic analysis, laser microdissection, immunolocalization and others. *Western blot analysis of the immunoprecipitated CSCs led to the identification of at least two types of CSC in the membrane protein of Populus xylem tissue. Proteomic analysis further revealed that the two types of CSC were assembled from different cellulose synthase proteins. Immunolocalization confirmed that both types of CSC were involved in secondary cell wall formation. In addition, a number of noncellulose synthase proteins were also identified in association with CSC precipitation. *The results indicate that two types of CSC participate in secondary wall formation in Populus, suggesting a new mechanism of cellulose formation involved in the thickening of wood cell walls. This study also suggests that the CSC machinery may be aided by other proteins in addition to cellulose synthase proteins.

Dark fermentative hydrogen production from enzymatic hydrolysate of xylan and pretreated rice straw by Clostridium butyricum CGS5

Xylan and rice straw were used to produce H(2) via a two-stage approach combining feedstock pretreatment/hydrolysis and dark H(2) fermentation. Acinetobacter junii F6-02 was used to produce cellulolytic enzymes (mainly xylanase) to hydrolyze xylan and pretreated rice straw. The hydrolysates were converted to H(2) by Clostridium butyricum CGS5 via dark fermentation. Investigation of kinetics of xylanase on xylan and NaOH-pretreated rice straw shows nu(max) values of 8.6 and 3.6g/L/h, and K(m) values of 10.6 and 26.9 g/L, respectively. A maximum hydrogen production rate of 62.5 and 26.8 ml/h/L was obtained from hydrolysate of xylan and pretreated rice straw, respectively, while the hydrogen yield was 0.70 and 0.76 mol H(2)/mol xylose, respectively. Simultaneous saccharification and BioH(2) fermentation from xylan was also conducted but giving a lower hydrogen production rate (35.3 ml/h/L) than that of the two-stage process.

Characterization and varied expression of a membrane-bound endo-beta-1,4-glucanase in hybrid poplar

To understand better the intricacies of secondary cell wall biosynthesis in trees, we investigated changes in cellulose chemistry and ultrastructure manifested by the mis-regulation of the poplar membrane-bound beta-1,4-endoglucanase orthologous to KORRIGAN (AtKOR). We isolated the poplar KORRIGAN gene from hybrid poplar (Populus albaxgrandidentata; designated PaxgKOR) and created a self-complementary (hairpin) RNAi suppression construct using PCR products derived from the gene. Additionally, AtKOR was employed to generate transgenic poplar over-expressing KORRIGAN. It was found that down-regulation leads to moderate to severe defects in plant growth, an irregular xylem (irx) phenotype, and significantly impacts the ultrastructure of the cellulose synthesized. The RNAi-suppressed lines deposited significantly reduced quantities of a more highly crystalline cellulose, while the hemicellulose content and, more specifically, the xylose content increased. In addition, the amount of soluble sucrose in the leaves and xylem decreased. Conversely, the AtKOR transgenics did not significantly alter cell wall development or plant growth parameters, but it did impact the ultrastructure of the cellulose produced, generating trees with less crystalline cellulose and reduced xylose content.

Dehydration-induced expression of a 31-kDa dehydrin in Polypodium polypodioides (Polypodiaceae) may enable large, reversible deformation of cell walls

Current and predicted climate changes caused by global warming compel greater understanding of the molecular mechanisms that plants use to survive drought. The desiccation-tolerant fern Polypodium polypodioides exhibits extensive cell wall folding when dried to less than 15% relative water content (RWC) and rapidly (within 24 h) rehydrates when exposed to water and high humidity. A 31-kDa putative dehydrin polypeptide expressed in partially and fully dry tissues detected via western blotting was present only during drying and rapidly dissipated (within 24 h) upon tissue rehydration. Immunostaining indicates the presence of dehydrin near the cell wall of partially and fully dried tissues. Atomic force microscopy of tracheal scalariform perforations indicates that dry vascular tissue does not undergo significant strain. Additionally, environmental scanning electron microscopy reveals differential hydrophilicity between the abaxial and adaxial leaf surfaces as well as large, reversible deformation. The ability to avoid cell wall damage in some desiccation-tolerant species may be partially attributed to cell wall localization of dehydrins enabling reversible, large cell-wall deformation. Thus, the de novo synthesis of dehydrin proteins and potential localization to the cell walls of these desiccation-tolerant species may play a role in avoiding mechanical failure during drought.

Synthesis, regulation and utilization of lignocellulosic biomass

Increasing the range of fuels and bioproducts that are derived from lignocellulosic biomass and the efficiency at which they are produced hinges on a detailed understanding of the cell wall biosynthetic process. Herein, we review the structure and biosynthesis of lignocellulosic biomass and also highlight recent breakthroughs that demonstrate a complex regulatory system of transcription factors, small interfering RNAs and phosphorylation that ultimately dictate the development of the polyalaminate cell wall. Finally, we provide an update on cases where plant biotechnology has been used to improve lignocellulosic biomass utilization as a second-generation biofuel source.

Atomic force microscopy characterization of cellulose nanocrystals

Cellulose nanocrystals (CNCs) are gaining interest as a "green" nanomaterial with superior mechanical and chemical properties for high-performance nanocomposite materials; however, there is a lack of accurate material property characterization of individual CNCs. Here, a detailed study of the topography, elastic and adhesive properties of individual wood-derived CNCs is performed using atomic force microscopy (AFM). AFM experiments involving high-resolution dynamic mode imaging and jump-mode measurements were performed on individual CNCs under ambient conditions with 30% relative humidity (RH) and under a N(2) atmosphere with 0.1% RH. A procedure was also developed to calculate the CNC transverse elastic modulus (E(T)) by comparing the experimental force-distance curves measured on the CNCs with 3D finite element calculations of tip indentation on the CNC. The E(T) of an isolated CNC was estimated to be between 18 and 50 GPa at 0.1% RH; however, the associated crystallographic orientation of the CNC could not be determined. CNC properties were reasonably uniform along the entire CNC length, despite variations along the axis of 3-8 nm in CNC height. The range of RH used in this study was found to have a minimal effect on the CNC geometry, confirming the resistance of the cellulose crystals to water penetration. CNC flexibility was also investigated by using the AFM tip as a nanomanipulator.

Access to cellulose limits the efficiency of enzymatic hydrolysis: the role of amorphogenesis

The efficient enzymatic saccharification of cellulose at low cellulase (protein) loadings continues to be a challenge for commercialization of a process for bioconversion of lignocellulose to ethanol. Currently, effective pretreatment followed by high enzyme loading is needed to overcome several substrate and enzyme factors that limit rapid and complete hydrolysis of the cellulosic fraction of biomass substrates. One of the major barriers faced by cellulase enzymes is their limited access to much of the cellulose that is buried within the highly ordered and tightly packed fibrillar architecture of the cellulose microfibrils. Rather than a sequential 'shaving' or 'planing' of the cellulose fibrils from the outside, it has been suggested that these inaccessible regions are disrupted or loosened by non-hydrolytic proteins, thereby increasing the cellulose surface area and making it more accessible to the cellulase enzyme complex. This initial stage in enzymatic saccharification of cellulose has been termed amorphogenesis. In this review, we describe the various amorphogenesis-inducing agents that have been suggested, and their possible role in enhancing the enzymatic hydrolysis of cellulose.

Dynamic coordination of cytoskeletal and cell wall systems during plant cell morphogenesis

Underlying the architectural complexity of plants are diverse cell types that, under the microscope, easily reveal relationships between cell structure and specialized functions. Much less obvious are the mechanisms by which the cellular growth machinery and mechanical properties of the cell interact to dictate cell shape. The recent combined use of mutants, genomic analyses, sophisticated spectroscopies, and live cell imaging is providing new insight into how cytoskeletal systems and cell wall biosynthetic activities are integrated during morphogenesis. The purpose of this review is to discuss the unique geometric properties and physical processes that regulate plant cell expansion, then to overlay on this mechanical system some of the recent discoveries about the protein machines and cellular polymers that regulate cell shape. In the end, we hope to make clear that there are many interesting opportunities to develop testable mathematical models that improve our understanding of how subcellular structures, protein motors, and extracellular polymers can exert effects at spatial scales that span cells, tissues, and organs.

Real-time imaging of cellulose reorientation during cell wall expansion in Arabidopsis roots

Cellulose forms the major load-bearing network of the plant cell wall, which simultaneously protects the cell and directs its growth. Although the process of cellulose synthesis has been observed, little is known about the behavior of cellulose in the wall after synthesis. Using Pontamine Fast Scarlet 4B, a dye that fluoresces preferentially in the presence of cellulose and has excitation and emission wavelengths suitable for confocal microscopy, we imaged the architecture and dynamics of cellulose in the cell walls of expanding root cells. We found that cellulose exists in Arabidopsis (Arabidopsis thaliana) cell walls in large fibrillar bundles that vary in orientation. During anisotropic wall expansion in wild-type plants, we observed that these cellulose bundles rotate in a transverse to longitudinal direction. We also found that cellulose organization is significantly altered in mutants lacking either a cellulose synthase subunit or two xyloglucan xylosyltransferase isoforms. Our results support a model in which cellulose is deposited transversely to accommodate longitudinal cell expansion and reoriented during expansion to generate a cell wall that is fortified against strain from any direction.

Plant cell wall characterization using scanning probe microscopy techniques

Lignocellulosic biomass is today considered a promising renewable resource for bioenergy production. A combined chemical and biological process is currently under consideration for the conversion of polysaccharides from plant cell wall materials, mainly cellulose and hemicelluloses, to simple sugars that can be fermented to biofuels. Native plant cellulose forms nanometer-scale microfibrils that are embedded in a polymeric network of hemicelluloses, pectins, and lignins; this explains, in part, the recalcitrance of biomass to deconstruction. The chemical and structural characteristics of these plant cell wall constituents remain largely unknown today. Scanning probe microscopy techniques, particularly atomic force microscopy and its application in characterizing plant cell wall structure, are reviewed here. We also further discuss future developments based on scanning probe microscopy techniques that combine linear and nonlinear optical techniques to characterize plant cell wall nanometer-scale structures, specifically apertureless near-field scanning optical microscopy and coherent anti-Stokes Raman scattering microscopy.

Natural paradigms of plant cell wall degradation

Natural processes of recycling carbon from plant cell walls are slow but very efficient, generally involving microbial communities and their secreted enzymes. Efficient combinations of microbial communities and enzymes act in a sequential and synergistic manner to degrade plant cell walls. Recent understanding of plant cell wall ultra-structure, as well as the carbon metabolism, ATP production, and ecology of participating microbial communities, and the biochemical properties of their cellulolytic enzymes have led to new perspectives on saccharification of biomass. Microbial communities are dynamic functions of the chemical and structural compositions of plant cell wall components. The primitive 'multicellularity' exhibited by certain cellulolytic microorganisms may play a role in facilitating cell-cell communication and cell-plant cell wall-substrate interaction.

Characterization of carbons derived from cellulose and lignin and their oxidative behavior

In this study the oxidative behavior of carbons derived from cellulose and lignin were compared using thermogravimetric analysis (TGA). Specific surface area and chemical composition of the two types of carbon were analyzed using nitrogen adsorption at 77K and infrared spectroscopy respectively. The results demonstrate that cellulose carbon has a higher reaction order and lower activation energy than lignin carbon under identical experimental conditions when they were prepared at temperatures lower than 500 degrees C. However, such differences were considerably reduced for the carbon samples prepared at temperatures greater than 700 degrees C. It was verified that lignin carbon is more stable than cellulose carbon due to its higher content of aromatic structures when they are prepared at lower temperature. The specific surface area and porosity have a more limited contribution to the differential oxidative behaviors of the two types of carbon. This research has significance related to the formation of carbon nanotubes from plant materials during low temperature carbonization.

Plant cell walls throughout evolution: towards a molecular understanding of their design principles

Throughout their life, plants typically remain in one location utilizing sunlight for the synthesis of carbohydrates, which serve as their sole source of energy as well as building blocks of a protective extracellular matrix, called the cell wall. During the course of evolution, plants have repeatedly adapted to their respective niche, which is reflected in the changes of their body plan and the specific design of cell walls. Cell walls not only changed throughout evolution but also are constantly remodelled and reconstructed during the development of an individual plant, and in response to environmental stress or pathogen attacks. Carbohydrate-rich cell walls display complex designs, which together with the presence of phenolic polymers constitutes a barrier for microbes, fungi, and animals. Throughout evolution microbes have co-evolved strategies for efficient breakdown of cell walls. Our current understanding of cell walls and their evolutionary changes are limited as our knowledge is mainly derived from biochemical and genetic studies, complemented by a few targeted yet very informative imaging studies. Comprehensive plant cell wall models will aid in the re-design of plant cell walls for the purpose of commercially viable lignocellulosic biofuel production as well as for the timber, textile, and paper industries. Such knowledge will also be of great interest in the context of agriculture and to plant biologists in general. It is expected that detailed plant cell wall models will require integrated correlative multimodal, multiscale imaging and modelling approaches, which are currently underway.

Xyloglucan: the molecular muscle of trees

BACKGROUND

Tension wood evolved in woody angiosperms to allow stems with secondary thickening to bend and thus maintain an optimal orientation. Stem bending is the result of longitudinal tensile stress that develops in tension wood tissues. In many species, a specialized secondary cell wall layer, the so-called gelatinous (G)-layer, develops, containing longitudinally orientated crystalline cellulose fibrils; these have been recently shown to generate the tensile stress by an unknown mechanism. The cellulose fibrils cannot, however, work in isolation. Both coherence between the fibrils and adherence of the G-layer to the adjacent cell wall layers are required to transfer the tensile stresses of the cellulose fibrils to the tissue. Previous work had not identified hemicelluloses within the G-layer.

RECENT PROGRESS

Sugar composition and polysaccharide linkage analyses of pure G-layers isolated by sonication have recently identified xyloglucan as the main non-cellulosic component of the G-layer. Xyloglucan has been detected by immunolabelling with the CCRC-M1 monoclonal antibody and by in-situ activity assays using XXXG-sulforhodamine substrate in the developing G-layers but not in the mature ones. However, xyloglucan endotransglucosylase/hydrolase (XTH) proteins persist in the G-layer for several years and the corresponding xyloglucan endotransglucosylase (XET) activity (EC 2.4.1.207) occurs in the adjacent layers. Correspondingly, several XTH-encoding transcripts were found to be up-regulated in developing tension wood compared with normal wood.

SCOPE

We propose that, during cellulose crystallization, a part of the xyloglucan is trapped inside the crystal, inducing longitudinal tensile stress within it; another part of it is accessible and present between the G-layer and the outer wall layers. XET activity that occurs persistently in the G-fibres maintains coherence between the G-layer and the adjacent secondary wall layers. It is postulated that these activities are essential for generation of tensile stress during fibre maturation in tension wood.

Agricultural chemistry and bioenergy

Renewed interest in converting biomass to biofuels such as ethanol, other forms of bioenergy, and bioenergy byproducts or coproducts of commercial value opens opportunities for chemists, including agricultural chemists and related disciplines. Applications include feedstock characterization and quantification of structural changes resulting from genetic modification and of the intermediates formed during enzymatic and chemical processing; development of improved processes for utilizing chemical coproducts such as lactic acid and glycerol; development of alternative biofuels such as methanol, butanol, and hydrogen; and ways to reduce greenhouse gas emission and/or use carbon dioxide beneficially. Chemists will also be heavily involved in detailing the phytochemical composition of alternative energy crops and genetically improved crops. A resurgence of demand for agricultural chemistry and related disciplines argues for increasing output through targeted programs and on-the-job training.

A biophysical perspective on the cellulosome: new opportunities for biomass conversion

The cellulosome is a multiprotein complex, produced primarily by anaerobic microorganisms, which functions to degrade lignocellulosic materials. An important topic of current debate is whether cellulosomal systems display greater ability to deconstruct complex biomass materials (e.g. plant cell walls) than nonaggregated enzymes, and in so doing would be appropriate for improved, commercial bioconversion processes. To sufficiently understand the complex macromolecular processes between plant cell wall polymers, cellulolytic microbes, and their secreted enzymes, a highly concerted research approach is required. Adaptation of existing biophysical techniques and development of new science tools must be applied to this system. This review focuses on strategies likely to permit improved understanding of the bacterial cellulosome using biophysical approaches, with emphasis on advanced imaging and computational techniques.

Cell-wall structural changes in wheat straw pretreated for bioethanol production

BACKGROUND

Pretreatment is an essential step in the enzymatic hydrolysis of biomass and subsequent production of bioethanol. Recent results indicate that only a mild pretreatment is necessary in an industrial, economically feasible system. The Integrated Biomass Utilisation System hydrothermal pretreatment process has previously been shown to be effective in preparing wheat straw for these processes without the application of additional chemicals. In the current work, the effect of the pretreatment on the straw cell-wall matrix and its components are characterised microscopically (atomic force microscopy and scanning electron microscopy) and spectroscopically (attenuated total reflectance Fourier transform infrared spectroscopy) in order to understand this increase in digestibility.

RESULTS

The hydrothermal pretreatment does not degrade the fibrillar structure of cellulose but causes profound lignin re-localisation. Results from the current work indicate that wax has been removed and hemicellulose has been partially removed. Similar changes were found in wheat straw pretreated by steam explosion.

CONCLUSION

Results indicate that hydrothermal pretreatment increases the digestibility by increasing the accessibility of the cellulose through a re-localisation of lignin and a partial removal of hemicellulose, rather than by disruption of the cell wall.

Reviving the carbohydrate economy via multi-product lignocellulose biorefineries

Before the industrial revolution, the global economy was largely based on living carbon from plants. Now the economy is mainly dependent on fossil fuels (dead carbon). Biomass is the only sustainable bioresource that can provide sufficient transportation fuels and renewable materials at the same time. Cellulosic ethanol production from less costly and most abundant lignocellulose is confronted with three main obstacles: (1) high processing costs (dollars /gallon of ethanol), (2) huge capital investment (dollars approximately 4-10/gallon of annual ethanol production capacity), and (3) a narrow margin between feedstock and product prices. Both lignocellulose fractionation technology and effective co-utilization of acetic acid, lignin and hemicellulose will be vital to the realization of profitable lignocellulose biorefineries, since co-product revenues would increase the margin up to 6.2-fold, where all purified lignocellulose co-components have higher selling prices (> approximately 1.0/kg) than ethanol ( approximately 0.5/kg of ethanol). Isolation of large amounts of lignocellulose components through lignocellulose fractionation would stimulate R&D in lignin and hemicellulose applications, as well as promote new markets for lignin- and hemicellulose-derivative products. Lignocellulose resource would be sufficient to replace significant fractionations (e.g., 30%) of transportation fuels through liquid biofuels, internal combustion engines in the short term, and would provide 100% transportation fuels by sugar-hydrogen-fuel cell systems in the long term.

Liquefaction of lignocellulose at high-solids concentrations

To improve process economics of the lignocellulose to ethanol process a reactor system for enzymatic liquefaction and saccharification at high-solids concentrations was developed. The technology is based on free fall mixing employing a horizontally placed drum with a horizontal rotating shaft mounted with paddlers for mixing. Enzymatic liquefaction and saccharification of pretreated wheat straw was tested with up to 40% (w/w) initial DM. In less than 10 h, the structure of the material was changed from intact straw particles (length 1-5 cm) into a paste/liquid that could be pumped. Tests revealed no significant effect of mixing speed in the range 3.3-11.5 rpm on the glucose conversion after 24 h and ethanol yield after subsequent fermentation for 48 h. Low-power inputs for mixing are therefore possible. Liquefaction and saccharification for 96 h using an enzyme loading of 7 FPU/g.DM and 40% DM resulted in a glucose concentration of 86 g/kg. Experiments conducted at 2%-40% (w/w) initial DM revealed that cellulose and hemicellulose conversion decreased almost linearly with increasing DM. Performing the experiments as simultaneous saccharification and fermentation also revealed a decrease in ethanol yield at increasing initial DM. Saccharomyces cerevisiae was capable of fermenting hydrolysates up to 40% DM. The highest ethanol concentration, 48 g/kg, was obtained using 35% (w/w) DM. Liquefaction of biomass with this reactor system unlocks the possibility of 10% (w/w) ethanol in the fermentation broth in future lignocellulose to ethanol plants.

Structure, organization, and functions of cellulose synthase complexes in higher plants

Annually, plants produce about 180 billion tons of cellulose making it the largest reservoir of organic carbon on Earth. Cellulose is a linear homopolymer of beta(1-4)-linked glucose residues. The coordinated synthesis of glucose chains is orchestrated by specific plasma membrane-bound cellulose synthase complexes (CelS). The CelS is postulated to be composed of approximately 36 cellulose synthase (CESA) subunits. The CelS synthesizes 36 glucose chains in close proximity before they are further organized into microfibrils that are further associated with other cell wall polymers. The 36 glucose chains in a microfibril are stabilized by intra- and inter-hydrogen bonding which confer great stability on microfibrils. Several elementary microfibrils come together to form macrofibrils. Many CESA isoforms appear to be involved in the cellulose biosynthetic process and at least three types of CESA isoforms appear to be necessary for the functional organization of CelS in higher plants.

Biomass recalcitrance: engineering plants and enzymes for biofuels production

Lignocellulosic biomass has long been recognized as a potential sustainable source of mixed sugars for fermentation to biofuels and other biomaterials. Several technologies have been developed during the past 80 years that allow this conversion process to occur, and the clear objective now is to make this process cost-competitive in today's markets. Here, we consider the natural resistance of plant cell walls to microbial and enzymatic deconstruction, collectively known as "biomass recalcitrance." It is this property of plants that is largely responsible for the high cost of lignocellulose conversion. To achieve sustainable energy production, it will be necessary to overcome the chemical and structural properties that have evolved in biomass to prevent its disassembly.

Fractionating recalcitrant lignocellulose at modest reaction conditions

Effectively releasing the locked polysaccharides from recalcitrant lignocellulose to fermentable sugars is among the greatest technical and economic barriers to the realization of lignocellulose biorefineries because leading lignocellulose pre-treatment technologies suffer from low sugar yields, and/or severe reaction conditions, and/or high cellulase use, narrow substrate applicability, and high capital investment, etc. A new lignocellulose pre-treatment featuring modest reaction conditions (50 degrees C and atmospheric pressure) was demonstrated to fractionate lignocellulose to amorphous cellulose, hemicellulose, lignin, and acetic acid by using a non-volatile cellulose solvent (concentrated phosphoric acid), a highly volatile organic solvent (acetone), and water. The highest sugar yields after enzymatic hydrolysis were attributed to no sugar degradation during the fractionation and the highest enzymatic cellulose digestibility ( approximately 97% in 24 h) during the hydrolysis step at the enzyme loading of 15 filter paper units of cellulase and 60 IU of beta-glucosidase per gram of glucan. Isolation of high-value lignocellulose components (lignin, acetic acid, and hemicellulose) would greatly increase potential revenues of a lignocellulose biorefinery.

Versatile derivatives of carbohydrate-binding modules for imaging of complex carbohydrates approaching the molecular level of resolution

The innate binding specificity of different carbohydrate-binding modules (CBMs) offers a versatile approach for mapping the chemistry and structure of surfaces that contain complex carbohydrates. We have employed the distinct recognition properties of a double His-tagged recombinant CBM tagged with semiconductor quantum dots for direct imaging of crystalline cellulose at the molecular level of resolution, using transmission and scanning transmission electron microscopy. In addition, three different types of CBMs from families 3, 6, and 20 that exhibit different carbohydrate specificities were each fused with either green fluorescent protein (GFP) or red fluorescent protein (RFP) and employed for double-labeling fluorescence microscopy studies of primary cell walls and various mixtures of complex carbohydrate target molecules. CBM probes can be used for characterizing both native complex carbohydrates and engineered biomaterials.