木质素生物合成途径及其基因调控的研究进展

木质素是植物体内含量仅次于纤维素的一类高分子有机物质,在植物体的机械支持、水分运输及抵抗外界不良环境的侵袭方面起着重要的作用。然而,它的存在严重影响植物材料在造纸工业、纺织业、畜牧业生产中的应用。木质素代谢过程中存在多基因现象使得木质素的合成途径出现多样性,利用共抑制、反义抑制等转基因技术开发低木质素含量的优良新品种具有重要的意义。     

木质素的生物合成及其调控研究进展

木质素是植物体中仅次于纤维素的一种重要大分子有机物质,具有重要生物学功能,其3种主要单体的生物合成途径已经基本清楚。从木质素生物合成及基因工程在调控木质素生物合成中的作用等方面的研究进展进行了综述,并提出了存在的问题及对策。     

木质素合成研究进展

木质素是植物体内一类重要的大分子有机物,具有支持、保护和运输等生物学功能。然而,木质素的存在对植物资源的利用有一些不利影响,主要体现在导致造纸污染严重及影响牲畜对饲草的消化和吸收等。通过生物工程手段调控木质素的合成具有重要的环境保护价值和经济价值,已经成为国际上生命科学研究的热点之一。近年来,这一领域发展十分迅速。本文介绍了木质素生物合成途径,重点综述了调控木质素合成的手段及限速酶研究方面的最新进展,并提出存在的问题及展望。     

木质素生物合成中C3H/HCT的研究进展

木质素是由三种不同的木质素单体聚合而成的,其含量和单体组成与植物体的综合利用密切相关。木质素单体的生物合成途径涉及到许多酶的参与,C3H/HCT是发现较晚的两个酶,在从对-香豆酰辅酶A（p-coumaroyl CoA）到咖啡酰辅酶A（caffeoyl CoA）的羟基化过程中起作用,是控制木质素H-单体与G/S-单体相互转化的关键酶。该文主要对C3H/HCT的研究进展及在木质素生物合成中的作用进行了阐述。     

基于转录组测序的慈竹笋木质素基因筛选及其表达分析

慈竹（Bambusa emeiensis）纤维素含量丰富,是较好的造纸原料,但竹茎中木质素影响着制浆生产及纸浆质量。目前,对慈竹木质素生物合成机制所知甚少,这限制了遗传调控竹木质素的研究。本文以拟南芥、水稻等植物的已知木质素基因作为查询序列,通过BLASTp和系统进化分析,从10、50、100和150 cm慈竹笋转录组数据中筛选到351个木质素生物合成相关Unigenes,包括51个LAC,37个4CL、26个PAL、34个CCR和25个CAD相关转录子,其数量高于其他已报道的竹类植物。转录丰度和定量基因表达分析发现16个木质素基因,包括2个PAL、5个CCR、3个4CL、2个CADH2和4个LAC,随着笋发育而表达上调,表明其可能与发育性木质素积累相关。     

木质纤维素资源化主要途径及半纤维素优先资源化利用策略

木质纤维生物质是地球上最丰富的可再生资源,可转化为能源、化学品和材料,开发木质纤维生物质有利于废弃物的高值化利用和缓解目前面临的环境污染等问题。木质纤维素主要包括纤维素、半纤维素和木质素,将其主要组分进行高效分离,是实现多元化、高值化生物精炼的基础。基于此,笔者简要总结了目前主要的木质纤维素资源化途径,如基于纤维素资源化、基于半纤维素资源化、基于木质素资源化、基于碳水化合物资源化以及全组分资源化的研究策略。依据半纤维素在植物细胞壁中承担的角色,结合前期的研究基础,提出半纤维素优先原位催化转化的木质纤维素生物炼制新策略,实现半纤维素的高选择性溶出和高效转化,保留结构完整的纤维素和木质素分级转化为小分子化学品和材料,最终实现资源生物量全利用,多元化产品联产的目的。     

木质素生物合成及其基因工程研究进展

木质素是维管植物的一种主要组成成分,是植物适应陆地环境的重要特征之一.然而,它的存在严重影响植物材料在造纸工业与畜牧业生产中的应用,因此其生物合成调控的研究引起人们极大关注.随着各种分析技术和手段的提高,该领域研究取得了突破性的进展.该文重点阐述这些新进展,同时较系统地介绍利用基因工程技术调控木质素生物合成的研究成果,并提出一些关于更有效地利用生物技术手段改良造纸资源植物品质的建议.     

木质素生物炼制研究进展

木质素为天然的芳香族聚合物,是自然界第二大丰富的可再生碳源,占木质纤维素干重的15%~30%。因木质素富含芳香族结构,故其具有极高的应用价值。生物法转化利用木质素具有专一性强和环境友好等特点,使得木质素生物炼制成为研究热点。本文根据国内外研究进展,从木质素降解酶的研究现状、芳香族化合物胞内代谢途径及木质素生物基化学品研究进展等几个方面做了综述。     

生物质生物预处理研究进展与展望

木质纤维素生物质分布广、产量大、可再生,用于制备生物基能源、生物基材料和生物基化学品。木质纤维素生物质组成复杂,包含纤维素、半纤维素和木质素等,木质素与半纤维素通过共价键、氢键交联形成独特的“包裹结构”,纤维素含有复杂的分子内与分子间氢键,上述因素制约着其资源化利用。生物预处理以其独特优越性成为生物质研究的重要方面。系统阐述了生物预处理过程中木质素降解和基团修饰对纤维素酶解的影响,纤维素含量及结晶区变化,半纤维素五碳糖利用,微观物理结构的改变。进一步提出了以生物预处理为核心的组合预处理、基于不同功能的多酶协同催化体系、木质纤维素组分分级利用和新型高效细菌预处理工艺是生物预处理未来发展的重要趋势。     

比利时VIB研究所发现木质素生物合成途径的关键酶

2013年8月15日,比利时VIB研究所宣布,在植物细胞壁主要成分之一的木质素生物合成途径中,发现了编码caffeoyl shikimate esterase（CSE）酶的新基因。木质素是在用植物类生物质生产生物燃料过程中提取纤维素糖时的障碍,由于发现了新基因,以更高效率生产生物燃料出现了可能性。     

四川2种丛生竹理化特性及纤维形态研究

以四川不同地区两种丛生竹—慈竹和梁山慈竹为研究对象,通过测定纤维素和木质素含量以及纤维形态,对其变异规律进行研究。结果表明,同一地区不同竹种间纤维素和木质素含量差异显著,同一竹种不同地区间纤维素和木质素含量、纤维长度、宽度和纤维长宽比因地域差异而异,且与竹龄有关。从纸浆用竹和纤维原料综合考虑,竹海地区的慈竹（纤维素含量在45%左右,木质素含量低于28%）、梁山慈竹（纤维素含量在50%以上,木质素含量低于20%）比较适合竹浆造纸。     

四川2种丛生竹木质素和纤维含量的研究

以四川4个地区2种丛生竹(慈竹和硬头黄)为研究对象,对其木质素和综纤维含量进行研究.结果表明,不同地区竹种间综纤维素含量差异显著;除青神地区外,其它几个地区木质素含量差异不大.纬度对木质素积累的作用因竹种而异,对慈竹木质素含量的作用呈极明显的正效应,对硬头黄的作用则呈不明显的正效应,对综纤维含量的影响不大.竹龄影响木质素和综纤维素含量的积累,对慈竹木质素和综纤维的积累呈显著的正效应,但对硬头黄的作用却不明显.研究认为,青神、长宁和绵阳地区的硬头黄以及长宁地区的慈竹是造纸的较好原料.     

Cellulose, hemicelluloses, lignin and ash content of some organic materials and their suitability for use as paper pulp supplements

Freshwater algal biomass and orange and lemon peels were assessed as tissue paper pulp supplements. Cellulose and hemicellulose contents of algal biomass were 7.1% and 16.3%, respectively, whereas for citrus peels cellulose content ranged from 12.7% to 13.6% and hemicellulose from 5.3% to 6.1%. For all materials, lignin and ash content was 2% or lower, rendering them suitable for use as paper pulp supplements. The addition of algal biomass to paper pulp increased its mechanical strength significantly. However, brightness was adversely affected by chlorophyll. The addition of citrus peels in paper pulp had no effect on breaking length, increased bursting strength and decreased tearing resistance. Brightness was negatively affected at proportions of 10%, because citrus peel particles behave as coloured pigments. The cost of both materials is about 45% lower than that of conventional pulp, resulting in a 0.9-4.5% reduction in final paper price upon their addition to the pulp.     

Investigation of lignin-carbohydrate complexes in kraft pulps by selective enzymatic treatments

The occurrence of covalent bonds between residual lignin and polysaccharides in birch and pine kraft pulps was investigated by specific enzymatic treatments. Pure enzymes degrading cellulose, xylan and mannan were used both separately and in combination. Comparison of the molar masses of polysaccharides and lignin in the orginal pulps and in the residual pulps after enzymatic treatments showed that residual lignin in birch kraft pulp is linked at least to xylan. A minor portion may also be linked to cellulose. In pine kraft pulp some of the residual lignin appears to be linked to cellulose, glucomannan and xylan. The linkages between lignin and cellulose and hemicelluloses may be either native or formed during pulp processing. The results also provided new information on the synergistic action of cellulose- and hemicellulose-degrading enzymes on pulp fibres. The synergism appears to be mainly due to the structure of the pulp fibres, with different layers of cellulose sheets, hemicelluloses and lignin. On the other hand the results also provided information about fibre structure. The degradation of xylan clearly enhanced the action of enzymes on cellulose, suggesting that xylan partially covers the cellulose. A similar phenomenon was not observed in the simultaneous hydrolysis of glucomannan and cellulose. However, the results suggest that glucomannan does interact with cellulose, possibly by non-covalent linkages. Received: 8 July 1998 / Received revision: 7 October 1998 / Accepted: 11 October 1998     

Flexible biorefinery for producing fermentation sugars, lignin and pulp from corn stover

A new biorefining process is presented that embodies green processing and sustainable development. In the spirit of a true biorefinery, the objective is to convert agricultural residues and other biomass feedstocks into value-added products such as fuel ethanol, dissolving pulp, and lignin for resin production. The continuous biomass fractionation process yields a liquid stream rich in hemicellulosic sugars, a lignin-rich liquid stream, and a solid cellulose stream. This paper generally discusses potential applications of the three streams and specifically provides results on the evaluation of the cellulose stream from corn stover as a source of fermentation sugars and specialty pulp. Enzymatic hydrolysis of this relatively pure cellulose stream requires significantly lower enzyme loadings because of minimal enzyme deactivation from nonspecific binding to lignin. A correlation was shown to exist between lignin removal efficiency and enzymatic digestibility. The cellulose produced was also demonstrated to be a suitable replacement for hardwood pulp, especially in the top ply of a linerboard. Also, the relatively pure nature of the cellulose renders it suitable as raw material for making dissolving pulp. This pulping approach has significantly smaller environmental footprint compared to the industry-standard kraft process because no sulfur- or chlorine-containing compounds are used. Although this option needs some minimal post-processing, it produces a higher value commodity than ethanol and, unlike ethanol, does not need extensive processing such as hydrolysis or fermentation. Potential use of low-molecular weight lignin as a raw material for wood adhesive production is discussed as well as its use as cement and feed binder. As a baseline application the hemicellulosic sugars captured in the hydrolyzate liquor can be used to produce ethanol, but potential utilization of xylose for xylitol fermentation is also feasible. Markets and values of these applications are juxtaposed with market penetration and saturation.     

Quantitative trait locus (QTL) analysis of wood quality traits in <Emphasis Type="Italic">Eucalyptus nitens</Emphasis>

To identify the chromosomal regions affecting wood quality traits, we conducted a genome-wide quantitative trait locus (QTL) analysis of wood quality traits in Eucalyptus nitens. This information is important to exploit the full potential of the impending Eucalyptus genome sequence. A three generational mapping population consisting of 296 progeny trees was used to identify QTL associated with several wood quality traits in E. nitens. Thirty-six QTL positions for cellulose content, pulp yield, lignin content, density, and microfibril angle (MFA) were identified across different linkage groups. On linkage groups (LG)2 and 8, cellulose QTL cluster with pulp yield and extractives QTL while on LG4 and 10 cellulose and pulp yield QTLs cluster together. Similarly, on LG4, 5, and 6 QTL for lignin traits were clustered together. At two positions, QTL for MFA, a physical trait related to wood stiffness, were clustered with QTL for lignin traits. Several cell wall candidate genes were co-located to QTL positions affecting different traits. Comparative QTL analysis with Eucalyptus globulus revealed two common QTL regions for cellulose and pulp yield. The QTL positions identified in this study provide a resource for identifying wood quality genes using the impending Eucalyptus genome sequence. Candidate genes identified in this study through co-location to QTL regions may be useful in association studies.     

The role of xylanases and laccases on hexenuronic acid and lignin removal

The variation of the contents in hexenuronic acids (HexA) and lignin in Eucalyptus kraft pulp during sequences with the laccase–mediator treatment with or without xylanase pretreatment was studied. The laccase–HBT system (HBT: 1-hydroxybenzotriazole) initially oxidized lignin alone but altered cellulose in the pulp as well after some time. Once all accessible lignin was removed, the system acted on HexA. As a result, the laccase–mediator treatment reduced the HexA content of the pulp, especially if a xylanase pretreatment was applied before. A previously unseen effect was observed here: HexA removal was found to depend on the laccase and HBT doses, but not on the reaction time. In addition, the xylanase pretreatment was found to strongly boost the effects of the laccase–HBT system by facilitating their access to HexA without affecting the lignin content.     

Thermogravimetry study of xylanase- and laccase/mediator-treated eucalyptus pulp fibres

Thermogravimetric analyses (TGA) was applied to study the effects of enzymatic bleaching of eucalyptus pulp with xylanase and a laccase-mediator system. The thermal degradation profile of the pulps was sensitive to the enzymatic treatments. Xylanase treatment produced an ordered and clean microfibril, whereas laccase oxidized surface cellulose chains and increased the amorphous (paracrystalline) cellulose content. In this case, pulp viscosity decreased from 972 to 859 mL/g and apparent pulp crystallinity calculated from TGA data decreased almost 50%. Alkaline extraction was necessary to recover pulp crystallinity and to remove oxidized lignin in the laccase-treated samples. TGA data allowed differentiating and quantifying crystalline and amorphous cellulose. This thermogravimetric approach is a simple method in order to monitor superficial changes in cellulosic microfibrils.     

Pulp properties and their influence on enzymatic degradability

Endoglucanase treatment of pulp for the adjustment of viscosity and the increase in pulp reactivity is a promising step in the concept for the beneficial production of dissolving pulps from paper grade pulps. To promote the commercial applicability of these enzymes, the influence of pulp properties such as carbohydrate composition, pulp type and cellulose morphology on the enzymatic degradability of a pulp was examined. High contents of hemicelluloses and lignin were shown to impair the accessibility of the cellulose to the enzymes. Due to the elevated swelling capacity of cellulose II, conversion of the cellulose morphology from I to II upon alkaline treatments showed a large increasing effect on the cellulose accessibility, and enzymatic degradability. Reactivity measurements of softwood sulfite pulps after enzymatic degradation and acid-catalyzed hydrolysis, respectively, revealed elevated reactivity for the pulp after acid treatment. This is in contrast to effects of enzyme treatments reported for CCE treated kraft pulps.     

Biorefining of softwoods using ethanol organosolv pulping: preliminary evaluation of process streams for manufacture of fuel-grade ethanol and co-products

Pulps with residual lignin ranging from 6.4-27.4% (w/w) were prepared from mixed softwoods using a proprietary biorefining technology (the Lignol process) based on aqueous ethanol organosolv extraction. The pulps were evaluated for bioconversion using enzymatic hydrolysis of the cellulose fraction to glucose and subsequent fermentation to ethanol. All pulps were readily hydrolyzed without further delignification. More than 90% of the cellulose in low lignin pulps (< or =18.4% residual lignin) was hydrolyzed to glucose in 48 h using an enzyme loading of 20 filter paper units/g cellulose. Cellulose in a high lignin pulp (27.4% residual lignin) was hydrolyzed to >90% conversion within 48 h using 40 filter paper units/g. The pulps performed well in both sequential and simultaneous saccharification and fermentation trials indicating an absence of metabolic inhibitors. Chemical and physical analyses showed that lignin extracted during organosolv pulping of softwood is a suitable feedstock for production of lignin-based adhesives and other products due to its high purity, low molecular weight, and abundance of reactive groups. Additional co-products may be derived from the hemicellulose sugars and furfural recovered from the water-soluble stream.