Cell-wall carbohydrates and their modification as a resource for biofuels

Plant cell walls represent the most abundant renewable resource on this planet. Despite their great abundance, only 2% of this resource is currently used by humans. Hence, research into the feasibility of using plant cell walls in the production of cost-effective biofuels is desirable. The main bottleneck for using wall materials is the recalcitrance of walls to efficient degradation into fermentable sugars. Manipulation of the wall polysaccharide biosynthetic machinery or addition of wall structure-altering agents should make it possible to tailor wall composition and architecture to enhance sugar yields upon wall digestion for biofuel fermentation. Study of the biosynthetic machinery and its regulation is still in its infancy and represents a major scientific and technical research challenge. Of course, any change in wall structure to accommodate cost-efficient biofuel production may have detrimental effects on plant growth and development due to the diverse roles of walls in the life of a plant. However, the diversity and abundance of wall structures present in the plant kingdom gives hope that this challenge can be met.     

Altered matrix polysaccharides in cell walls of pocket galls formed by an aphid on Distylium racemosum leaves

The present work reports the results of a study on the isolation and characterization of matrix polysaccharides in the cell walls of galls formed by an aphid (Neothoracaphis yanonis) on Distylium racemosum leaves. Cell walls were isolated from both healthy Distylium leaf and gall tissues and then extracted sequentially with cyclohexane‐trans‐1,2‐diaminetetra‐acetate (CDTA), Na₂CO₃, 1 m KOH, and 4 m KOH. The amount of pectin solubilized from gall cell walls was approximately 2.6‐fold higher than the pectin solubilized from leaf cell walls, whereas the amount of hemicellulose solubilized from gall cell walls was 1.4‐fold higher than that from normal leaf cell walls. When the polysaccharides were fractionated by anion‐exchange chromatography, considerable increases in arabinose and galactose were observed in CDTA‐soluble pectic polymer (fraction PI‐1) from gall cell walls, whereas the gall cell walls had less xylose in 1 m KOH‐soluble hemicellulosic polymers (fractions HI‐2, HI‐3, and HI‐4) than did the cell walls from the healthy leaf. The hemicellulosic polymers of the gall cell walls exhibited distinctly different patterns of molecular mass, compared with the healthy leaf cell walls. These results suggest that an extensive change occurs in the matrix polysaccharide structure of the cell walls of Distylium galls formed by an aphid. In addition, many glycosylhydrolase activities were detected in the protein fraction solubilized with strong saline solution from the gall cell walls, and the activities of β‐galactosidase, β‐xylosidase and α‐l ‐arabinofuranosidase were considerably increased under gall formation.     

Use of Xylanase and Rabinofuranosidase for Arabinose Removal from Unbleached Kraft Pulp

Preparations of arabinofuranosidase and xylanase, respectively from Aureobasidium pullulans and Trichoderma longibrachiatum, were used to remove selectively xylose and arabinose from kraft pulp. The equilibrium moisture content of pulps treated with both enzymes, at varying relative humidities, revealed a consistently lower percent moisture content at all humidity set points. Shorter fiber lengths indicated some deterioration when pulp was exposed to high concentrations of both enzymes.     

Experimental warming alters potential function of the fungal community in boreal forest

Fungal community composition often shifts in response to warmer temperatures, which might influence decomposition of recalcitrant carbon (C). We hypothesized that evolutionary trade‐offs would enable recalcitrant C‐using taxa to respond more positively to warming than would labile C‐using taxa. Accordingly, we performed a warming experiment in an Alaskan boreal forest and examined changes in the prevalence of fungal taxa. In a complementary field trial, we characterized the ability of fungal taxa to use labile C (glucose), intermediate C (hemicellulose or cellulose), or recalcitrant C (lignin). We also assigned taxa to functional groups (e.g., free‐living filamentous fungi, ectomycorrhizal fungi, and yeasts) based on taxonomic identity. We found that response to warming varied most among taxa at the order level, compared to other taxonomic ranks. Among orders, ability to use lignin was significantly related to increases in prevalence in response to warming. However, the relationship was weak, given that lignin use explained only 9% of the variability in warming responses. Functional groups also differed in warming responses. Specifically, free‐living filamentous fungi and ectomycorrhizal fungi responded positively to warming, on average, but yeasts responded negatively. Overall, warming‐induced shifts in fungal communities might be accompanied by an increased ability to break down recalcitrant C. This change in potential function may reduce soil C storage under global warming.     

Arabidopsis mannan synthase CSLA9 and glucan synthase CSLC4 have opposite orientations in the Golgi membrane

Several proteins encoded by the cellulose synthase-like (CSL) gene family are known to be processive glycan synthases involved in the synthesis of cell-wall polysaccharides. These include CSLA proteins, which synthesize β-(1→4)-linked mannans found in the walls of many plant species, and CSLC proteins, which are thought to synthesize the β-(1→4)-linked glucan backbone of xyloglucan, an abundant polysaccharide in the primary walls of many plants. CSLA and CSLC proteins are predicted to have multiple membrane spans, and their products (mannan and xyloglucan) accumulate in the Golgi lumen. Knowing where these proteins are located in the cell and how they are orientated in the membrane is important for understanding many aspects of mannan and xyloglucan biosynthesis. In this study, we investigate the subcellular localization and membrane protein topology of CSLA9 and CSLC4, the members of these two families that are most highly expressed in Arabidopsis. CSLA9 and CSLC4 are found predominantly in Golgi membranes, based on co-localization with the known ER/Golgi marker ERD2-YFP. The topology of epitope-tagged proteins was examined using protease protection experiments. Experiments were designed to determine the positions of both the protein termini and the active loop of the CSL proteins investigated. The topology of CSLA9 is characterized by an odd number of transmembrane domains (probably five) and an active site that faces the Golgi lumen. In contrast, CSLC4 has an even number of transmembrane domains (probably six) and an active site that faces the cytosol. The implications of these topologies on various aspects of hemicellulose biosynthesis are discussed.     

The Arabidopsis IRX10 and IRX10-LIKE glycosyltransferases are critical for glucuronoxylan biosynthesis during secondary cell wall formation

Arabidopsis IRX10 and IRX10-LIKE (IRX10-L) proteins are closely related members of the GT47 glycosyltransferase family. Single gene knock-outs of IRX10 or IRX10-L result in plants with either a weak or no mutant phenotype. However irx10 irx10-L double mutants are severely affected in their development, with a reduced rosette size and infrequent formation of a small infertile inflorescence. Plants homozygous for irx10 and heterozygous for irx10-L have an intermediate phenotype exhibiting a short inflorescence compared with the wild type, and an almost complete loss of fertility. Stem sections of the irx10 homozygous irx10-L heterozygous or irx10 irx10-L double mutants show decreased secondary cell-wall formation. NMR analysis shows that signals derived from the reducing end structure of glucuronoxylan were detected in the irx10 single mutant, and in the irx10 homozygous irx10-L heterozygous combination, but that the degree of polymerization of the xylan backbone was reduced compared with the wild type. Additionally, xylans from irx10 stem tissues have an almost complete loss of the GlcUA side chain, whereas the level of 4- O -Me-GlcUA was similar to that in wild type. Deletion of the predicted signal peptide from the N terminus of IRX10 or IRX10-L results in an inability to rescue the irx10 irx10-L double mutant phenotype. These findings demonstrate that IRX10 and IRX10-L perform a critical function in the synthesis of glucuronoxylan during secondary cell-wall formation, and that this activity is associated with the formation of the xylan backbone structure. This contrasts with the proposed function of the tobacco NpGUT1, which is closely related to the Arabidopsis IRX10 and IRX10-L proteins, in rhamnogalacturonan II biosynthesis.     

纤维素生物质新型水解技术的研究进展

纤维素生物质水解技术是生物质资源转化的关键技术之一,在传统的酸水解和酶水解技术基础上,近年来出现了一些新型的水解技术,它们一般都具有绿色高效、对环境友好等特点;回顾并综述了纤维素生物质水解技术的最新进展,并对纤维素生物质水解技术的发展研究方向提出了设想.     

木糖发酵生产乙醇的研究

选育出一株优良的木糖发酵菌株树干毕赤酵母菌7124,并利用纯木糖优化了木糖发酵条件,利用海藻酸钠固定化树干毕赤酵母菌增殖细胞,不仅能较好满足限氧发酵条件,而且能耐较高糖浓度,使乙醇发酵浓度提高到20g/L。利用半纤维素水解液进行了乙醇发酵的初步研究,基本达到了纯木糖发酵的效果。     

Ethanol production from dilute acid hydrolysate of rice hulls using genetically engineered Escherichia coli

Approximately 30% of rice hulls, which represent an abundant residue with little commercial value, was solubilized with 0.4 M H2SO4 acid to produce a syrup containing over 100 g monomer sugar/l. Toxins generated during hydrolysis were mitigated with Ca(OH)2. Treated hydrolysate plus additional nutrients was fermented with Escherichia coli KO11 to produce over 46 g ethanol/L in 72 h (92% of theoretical yield). © Rapid Science Ltd. 1998