G-lignin and hemicellulosic monosaccharides distinctively affect biomass digestibility in rapeseed

Small-scale OrganoCat processing to screen rapeseed straw for efficient lignocellulose fractionation

Agricultural residues such as rapeseed straw can be a valuable source of cellulose, sugars, and aromatic molecules like lignin. Understanding its composition is crucial in order to develop suitable processing technology for the production of biofuel or biochemicals from rapeseed straw. Here, we developed a small-scale OrganoCat system to screen multiple technical conditions and different samples at higher throughput and utilize this system to analyze straw samples from a set of 14 genetically different Brassica lines on their processability. Correlation analysis was performed to investigate the effects of cell wall polymer features on rapeseed biomass disintegration. At comparably mild reaction conditions, the differences in recalcitrance towards OrganoCat fractionation within the set were especially associated with parameters such as pectic polysaccharide content, acetylation, and hemicellulose composition. These findings can subsequently be used to optimize and scale up the pretreatment and fractionation of lignocellulose derived from rapeseed straw.

QTL Identification for Stem Fiber, Strength and Rot Resistance in a DH Population from an Alien Introgression of Brassica napus

Stem fiber, stem strength and stem-rot resistance are important agronomic traits in Brassica napus. To understand the molecular mechanism that controls the stem-related traits, we investigated the stem lignin (ADL), cellulose (Cel), hemicellulose (Hem) content, S/G monolignol ratio (SG), stem breaking force (BF), breaking strength (F) and Sclerotinia sclerotiorum resistance (SSR). Each trait was significantly positively or negatively correlated with more than three of the other six traits. QTL mapping for ADL, Cel, Hem, SG, BF, F and SSR were performed using a doubled haploid population derived from an intertribal B. napus introgression line 'Y689' crossed with B. napus cv. 'Westar'. A total of 67 additive QTL were identified and integrated into 55 consensus QTL by meta-analysis. Among the 55 consensus QTL, 23 (41.8%) QTL were co-located and were integrated into 11 unique QTL. The QTL by environment (Q × E) interactions were analyzed and 22 combined QTL were identified. In addition, candidate genes within the QTL intervals were proposed based on the known function of Arabidopsis orthologs. These results provided valuable information for improving lodging resistance, S. sclerotiorum resistance and mechanized harvesting of B. napus.

MiR396-GRF module associates with switchgrass biomass yield and feedstock quality

Improving plant biomass yield and/or feedstock quality for highly efficient lignocellulose conversion has been the main research focus in genetic modification of switchgrass (Panicum virgatum L.), a dedicated model plant for biofuel production. Here, we proved that overexpression of miR396 (OE-miR396) leads to reduced plant height and lignin content mainly by reducing G-lignin monomer content. We identified nineteen PvGRFs in switchgrass and proved thirteen of them were cleaved by miR396. MiR396-targeted PvGRF1, PvGRF9 and PvGRF3 showed significantly higher expression in stem. By separately overexpressing rPvGRF1, 3 and 9, in which synonymous mutations abolished the miR396 target sites, and suppression of PvGRF1/3/9 activity via PvGRF1/3/9-SRDX overexpression in switchgrass, we confirmed PvGRF1 and PvGRF9 played positive roles in improving plant height and G-lignin content. Overexpression of PvGRF9 was sufficient to complement the defective phenotype of OE-miR396 plants. MiR396-PvGRF9 modulates these traits partly by interfering GA and auxin biosynthesis and signalling transduction and cell wall lignin, glucose and xylan biosynthesis pathways. Moreover, by enzymatic hydrolysis analyses, we found that overexpression of rPvGRF9 significantly enhanced per plant sugar yield. Our results suggest that PvGRF9 can be utilized as a candidate molecular tool in modifying plant biomass yield and feedstock quality.

Integrated NIRS and QTL assays reveal minor mannose and galactose as contrast lignocellulose factors for biomass enzymatic saccharification in rice

BACKGROUND

Identifying lignocellulose recalcitrant factors and exploring their genetic properties are essential for enhanced biomass enzymatic saccharification in bioenergy crops. Despite genetic modification of major wall polymers has been implemented for reduced recalcitrance in engineered crops, it could most cause a penalty of plant growth and biomass yield. Alternatively, it is increasingly considered to improve minor wall components, but an applicable approach is required for efficient assay of large population of biomass samples. Hence, this study collected total of 100 rice straw samples and characterized all minor wall monosaccharides and biomass enzymatic saccharification by integrating NIRS modeling and QTL profiling.

RESULTS

By performing classic chemical analyses and establishing optimal NIRS equations, this study examined four minor wall monosaccharides and major wall polymers (acid-soluble lignin/ASL, acid-insoluble lignin/AIL, three lignin monomers, crystalline cellulose), which led to largely varied hexoses yields achieved from enzymatic hydrolyses after two alkali pretreatments were conducted with large population of rice straws. Correlation analyses indicated that mannose and galactose can play a contrast role for biomass enzymatic saccharification at P < 0.0 l level (n = 100). Meanwhile, we found that the QTLs controlling mannose, galactose, lignin-related traits, and biomass saccharification were co-located. By combining NIRS assay with QTLs maps, this study further interpreted that the mannose-rich hemicellulose may assist AIL disassociation for enhanced biomass enzymatic saccharification, whereas the galactose-rich polysaccharides should be effectively extracted with ASL from the alkali pretreatment for condensed AIL association with cellulose microfibrils.

CONCLUSIONS

By integrating NIRS assay with QTL profiling for large population of rice straw samples, this study has identified that the mannose content of wall polysaccharides could positively affect biomass enzymatic saccharification, while the galactose had a significantly negative impact. It has also sorted out that two minor monosaccharides could distinctively associate with lignin deposition for wall network construction. Hence, this study demonstrates an applicable approach for fast assessments of minor lignocellulose recalcitrant factors and biomass enzymatic saccharification in rice, providing a potential strategy for bioenergy crop breeding and biomass processing.

Diverse Banana Pseudostems and Rachis Are Distinctive for Edible Carbohydrates and Lignocellulose Saccharification towards High Bioethanol Production under Chemical and Liquid Hot Water Pretreatments

Banana is a major fruit crop throughout the world with abundant lignocellulose in the pseudostem and rachis residues for biofuel production. In this study, we collected a total of 11 pseudostems and rachis samples that were originally derived from different genetic types and ecological locations of banana crops and then examined largely varied edible carbohydrates (soluble sugars, starch) and lignocellulose compositions. By performing chemical (H₂SO₄, NaOH) and liquid hot water (LHW) pretreatments, we also found a remarkable variation in biomass enzymatic saccharification and bioethanol production among all banana samples examined. Consequently, this study identified a desirable banana (Refen1, subgroup Pisang Awak) crop containing large amounts of edible carbohydrates and completely digestible lignocellulose, which could be combined to achieve the highest bioethanol yields of 31–38% (% dry matter), compared with previously reported ones in other bioenergy crops. Chemical analysis further indicated that the cellulose CrI and lignin G-monomer should be two major recalcitrant factors affecting biomass enzymatic saccharification in banana pseudostems and rachis. Therefore, this study not only examined rich edible carbohydrates for food in the banana pseudostems but also detected digestible lignocellulose for bioethanol production in rachis tissue, providing a strategy applicable for genetic breeding and biomass processing in banana crops.

Plant celluloses, hemicelluloses, lignins, and volatile oils for the synthesis of nanoparticles and nanostructured materials

A huge variety of plants are harvested worldwide and their different constituents can be converted into a broad range of bionanomaterials. In parallel, much research effort in materials science and engineering is focused on the formation of nanoparticles and nanostructured materials originating from agricultural residues. Cellulose (40-50%), hemicellulose (20-40%), and lignin (20-30%) represent major plant ingredients and many techniques have been described that separate the main plant components for the synthesis of nanocelluloses, nano-hemicelluloses, and nanolignins with divergent and controllable properties. The minor components, such as essential oils, could also be used to produce non-toxic metal and metal oxide nanoparticles with high bioavailability, biocompatibility, and/or bioactivity. This review describes the chemical structure, the physical and chemical properties of plant cell constituents, different techniques for the synthesis of nanocelluloses, nanohemicelluloses, and nanolignins from various lignocellulose sources and agricultural residues, and the extraction of volatile oils from plants as well as their use in metal and metal oxide nanoparticle production and emulsion preparation. Furthermore, details about the formation of activated carbon nanomaterials by thermal treatment of lignocellulose materials, a few examples of mineral extraction from agriculture waste for nanoparticle fabrication, and the emerging applications of plant-based nanomaterials in different fields, such as biotechnology and medicine, environment protection, environmental remediation, or energy production and storage, are also included. This review also briefly discusses the recent developments and challenges of obtaining nanomaterials from plant residues, and the issues surrounding toxicity and regulation.

A mechanism for efficient cadmium phytoremediation and high bioethanol production by combined mild chemical pretreatments with desirable rapeseed stalks

Cadmium (Cd) is one of the most hazardous trace metals, and rapeseed is a major oil crop over the world with considerable lignocellulose residues applicable for trace metal phytoremediation and cellulosic ethanol co-production. In this study, we examined that two distinct rapeseed cultivars could accumulate Cd at 72.48 and 43.70 ug/g dry stalk, being the highest Cd accumulation among all major agricultural food crops as previously reported. The Cd accumulation significantly increased pectin deposition as a major factor for trace metal association with lignocellulose. Meanwhile, the Cd-accumulated rapeseed stalks contained much reduced wall polymers (hemicellulose, lignin) and cellulose degree of polymerization, leading to improved lignocellulose enzymatic hydrolysis. Notably, three optimal chemical pretreatments were performed for enhanced biomass enzymatic saccharification and bioethanol production by significantly increasing cellulose accessibility and lignocellulose porosity, along with a complete Cd release for collection and recycling. Hence, this study proposed a mechanism model interpreting why rapeseed stalks are able to accumulate much Cd and how the Cd-accumulated stalks are of enhanced biomass saccharification. It has also provided a powerful technology for both cost-effective Cd phytoremediation and value-added bioethanol co-production with minimum waste release.

Improvement of Saccharification and Delignification Efficiency of Trichoderma reesei Rut-C30 by Genetic Bioengineering

Trichoderma reesei produces various saccharification enzymes required for biomass degradation. However, the lack of an effective lignin-degrading enzyme system reduces the species’ efficiency in producing fermentable sugars and increases the pre-treatment costs for biofuel production. In this study, we heterologously expressed the Ganoderma lucidum RMK1 versatile peroxidase gene (vp1) in the Rut-C30 strain of T. reesei. The expression of purified 6×His-tag–containing recombinant G. lucidum-derived protein (rVP1) was confirmed through western blot, which exhibited a single band with a relative molecular weight of 39 kDa. In saccharification and delignification studies using rice straw, the transformant (tVP7, T. reesei Rut-C30 expressing G. lucidum-derived rVP1) showed significant improvement in the yield of total reducing sugar and delignification, compared with that of the parent T. reesei Rut-C30 strain. Scanning electron microscopy (SEM) of tVP7-treated paddy straw showed extensive degradation of several layers of its surface compared with the parent strain due to the presence of G. lucidum-derived rVP1. Our results suggest that the expression of ligninolytic enzymes in cellulase hyperproducing systems helps to integrate the pre-treatment and saccharification steps that may ultimately reduce the costs of bioethanol production.

Overproduction of native endo-β-1,4-glucanases leads to largely enhanced biomass saccharification and bioethanol production by specific modification of cellulose features in transgenic rice

BACKGROUND

Genetic modification of plant cell walls has been implemented to reduce lignocellulosic recalcitrance for biofuel production. Plant glycoside hydrolase family 9 (GH9) comprises endo-β-1,4-glucanase in plants. Few studies have examined the roles of GH9 in cell wall modification. In this study, we independently overexpressed two genes from GH9B subclasses (OsGH9B1 and OsGH9B3) and examined cell wall features and biomass saccharification in transgenic rice plants.

RESULTS

Compared with the wild type (WT, Nipponbare), the OsGH9B1 and OsGH9B3 transgenic rice plants, respectively, contained much higher OsGH9B1 and OsGH9B3 protein levels and both proteins were observed in situ with nonspecific distribution in the plant cells. The transgenic lines exhibited significantly increased cellulase activity in vitro than the WT. The OsGH9B1 and OsGH9B3 transgenic plants showed a slight alteration in three wall polymer compositions (cellulose, hemicelluloses, and lignin), in their stem mechanical strength and biomass yield, but were significantly decreased in the cellulose degree of polymerization (DP) and lignocellulose crystalline index (CrI) by 21-22%. Notably, the crude cellulose substrates of the transgenic lines were more efficiently digested by cellobiohydrolase (CBHI) than those of the WT, indicating the significantly increased amounts of reducing ends of β-1,4-glucans in cellulose microfibrils. Finally, the engineered lines generated high sugar yields after mild alkali pretreatments and subsequent enzymatic hydrolysis, resulting in the high bioethanol yields obtained at 22.5% of dry matter.

CONCLUSIONS

Overproduction of OsGH9B1/B3 enzymes should have specific activity in the postmodification of cellulose microfibrils. The increased reducing ends of β-1,4-glucan chains for reduced cellulose DP and CrI positively affected biomass enzymatic saccharification. Our results demonstrate a potential strategy for genetic modification of cellulose microfibrils in bioenergy crops.

Neural Network Prediction of Corn Stover Saccharification Based on Its Structural Features

The classic assay for a large population biomass is time-consuming, labor intensive, and chemically expensive. This paper would find out a rapid assay for predicting biomass digestibility from biomass structural features without hydrolysis. We examined the 62 representative corn stover accessions that displayed a diverse cell-wall composition and varied biomass digestibility. Correlation analysis was firstly to detect effects of cell-wall compositions and wall polymer features on corn stover digestibility. Based on the dependable relationship of structural features and digestibility, a neural networks model has been developed and successfully predicted the corn stover saccharification based on the features without enzymatic hydrolysis. The actual measured and net-simulated predicted corn stover saccharification had good results as mean square error of 1.80E-05, coefficient of determination of 0.942 and average relative deviation of 3.95. The trained networks satisfactorily predicted the saccharification results based on the features of corn stover. Predicting the corn stover saccharification without hydrolysis will reduce capital and operational costs for corn stover purchasing and storage.

Distinct wall polymer deconstruction for high biomass digestibility under chemical pretreatment in Miscanthus and rice

Miscanthus is a leading bioenergy crop and rice provides enormous biomass for biofuels. Using Calcofluor White staining, this work in situ observed an initial lignocellulose hydrolysis in two distinct Miscanthus accessions, rice cultivar (NPB), and Osfc16 mutant after mild chemical pretreatments. In comparison, the M. sin and Osfc16 respectively exhibited weak Calcofluor fluorescence compared to the M. sac and NPB during enzymatic hydrolysis, consistent with the high biomass saccharification detected in vitro. Using xyloglucan-directed monoclonal antibodies (mAbs), xyloglucan deconstruction was observed from initial cellulose hydrolysis, whereas the M. sin and Osfc16 exhibited relatively strong immunolabeling using xylan-directed mAb, confirming previous findings of xylan positive impacts on biomass saccharification. Furthermore, the M. sin showed quick disappearance of RG-I immunolabeling with varied HG labelings between acid and alkali pretreatments. Hence, this study demonstrated a quick approach to explore wall polymer distinct deconstruction for enhanced biomass saccharification under chemical pretreatment in bioenergy crops.

Genetic loci simultaneously controlling lignin monomers and biomass digestibility of rice straw

Lignin content and composition are crucial factors affecting biomass digestibility. Exploring the genetic loci simultaneously affecting lignin-relevant traits and biomass digestibility is a precondition for lignin genetic manipulation towards energy crop breeding. In this study, a high-throughput platform was employed to assay the lignin content, lignin composition and biomass enzymatic digestibility of a rice recombinant inbred line population. Correlation analysis indicated that the absolute content of lignin monomers rather than lignin content had negative effects on biomass saccharification, whereas the relative content of p-hydroxyphenyl unit and the molar ratio of p-hydroxyphenyl unit to guaiacyl unit exhibited positive roles. Eight QTL clusters were identified and four of them affecting both lignin composition and biomass digestibility. The additive effects of clustered QTL revealed consistent relationships between lignin-relevant traits and biomass digestibility. Pyramiding rice lines containing the above four positive alleles for increasing biomass digestibility were selected and showed comparable lignin content, decreased syringyl or guaiacyl unit and increased molar percentage of p-hydroxyphenyl unit, the molar ratio of p-hydroxyphenyl unit to guaiacyl unit and sugar releases. More importantly, the lodging resistance and eating/cooking quality of pyramiding lines were not sacrificed, indicating the QTL information could be applied to select desirable energy rice lines.

Pretreatment of wheat straw leads to structural changes and improved enzymatic hydrolysis

Wheat straw (WS) is a potential biomass for production of monomeric sugars. However, the enzymatic hydrolysis ratio of cellulose in WS is relatively low due to the presence of lignin and hemicellulose. To enhance the enzymatic conversion of WS, we tested the impact of three different pretreatments, e.g. sulfuric acid (H₂SO₄), sodium hydroxide (NaOH), and hot water pretreatments to the enzymatic digestions. Among the three pretreatments, the highest cellulose conversion rate was obtained with the 4% NaOH pretreatment at 121 °C (87.2%). In addition, NaOH pretreatment was mainly effective in removing lignin, whereas the H₂SO₄ pretreatment efficiently removed hemicellulose. To investigate results of pretreated process for enhancement of enzyme-hydolysis to the WS, we used scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy to analyze structural changes of raw and treated materials. The structural analysis indicated that after H₂SO₄ and NaOH pretreatments, most of the amorphous cellulose and partial crystalline cellulose were hydrolyzed during enzymatic hydrolysis. The findings of the present study indicate that WS could be ideal materials for production of monomeric sugars with proper pretreatments and effective enzymatic base hydrolysis.

Extended light exposure increases stem digestibility and biomass production of switchgrass

Switchgrass is a photoperiod-sensitive energy grass suitable for growing in the marginal lands of China. We explored the effects of extended photoperiods of low-irradiance light (7 μmol·m-2·s-1, no effective photosynthesis) on the growth, the biomass dry weight, the biomass allocation, and, especially, the stem digestibility and cell wall characteristics of switchgrass. Two extended photoperiods (i.e., 18 and 24 h) were applied over Alamo. Extended light exposure (18 and 24 h) resulted in delayed heading and higher dry weights of vegetative organs (by 32.87 and 35.94%, respectively) at the expense of reducing the amount of sexual organs (by 40.05 and 50.87%, respectively). Compared to the control group (i.e., natural photoperiod), the yield of hexoses (% dry matter) in the stems after a direct enzymatic hydrolysis (DEH) treatment significantly increased (by 44.02 and 46.10%) for those groups irradiated during 18 and 24 h, respectively. Moreover, the yield of hexoses obtained via enzymatic hydrolysis increased after both basic (1% NaOH) and acid (1% H2SO4) pretreatments for the groups irradiated during 18 and 24 h. Additionally, low-irradiance light extension (LILE) significantly increased the content of non-structural carbohydrates (NSCs) while notably reducing the lignin content and the syringyl to guaiacyl (S/G) ratio. These structural changes were in part responsible for the observed improved stem digestibility. Remarkably, LILE significantly decreased the cellulose crystallinity index (CrI) of switchgrass by significantly increasing both the arabinose substitution degree in xylan and the content of ammonium oxalate-extractable uronic acids, both favoring cellulose digestibility. Despite this LILE technology is not applied to the cultivation of switchgrass on a large scale yet, we believe that the present work is important in that it reveals important relationships between extended day length irradiations and biomass production and quality. Additionally, this study paves the way for improving biomass production and digestibility via genetic modification of day length sensitive transcription factors or key structural genes in switchgrass leaves.

Biomass saccharification is largely enhanced by altering wall polymer features and reducing silicon accumulation in rice cultivars harvested from nitrogen fertilizer supply

In this study, two rice cultivars were collected from experimental fields with seven nitrogen fertilizer treatments. All biomass samples contained significantly increased cellulose contents and reduced silica levels, with variable amounts of hemicellulose and lignin from different nitrogen treatments. Under chemical (NaOH, CaO, H₂SO₄) and physical (hot water) pretreatments, biomass samples exhibited much enhanced hexoses yields from enzymatic hydrolysis, with high bioethanol production from yeast fermentation. Notably, both degree of polymerization (DP) of cellulose and xylose/arabinose (Xyl/Ara) ratio of hemicellulose were reduced in biomass residues, whereas other wall polymer features (cellulose crystallinity and monolignol proportion) were variable. Integrative analysis indicated that cellulose DP, hemicellulosic Xyl/Ara and silica are the major factors that significantly affect cellulose crystallinity and biomass saccharification. Hence, this study has demonstrated that nitrogen fertilizer supply could largely enhance biomass saccharification in rice cultivars, mainly by reducing cellulose DP, hemicellulosic Xyl/Ara and silica in cell walls.

Mild chemical pretreatments are sufficient for complete saccharification of steam-exploded residues and high ethanol production in desirable wheat accessions

In this study, a combined pretreatment was performed in four wheat accessions using steam explosion followed with different concentrations of H₂SO₄ or NaOH, leading to increased hexoses yields by 3-6 folds from enzymatic hydrolysis. Further co-supplied with 1% Tween-80, Talq90 and Talq16 accessions exhibited an almost complete enzymatic saccharification of steam-exploded (SE) residues after 0.5% H₂SO₄ or 1% NaOH pretreatment, with the highest bioethanol yields at 18.5%-19.4%, compared with previous reports about wheat bioethanol yields at 11%-17% obtained under relatively strong pretreatment conditions. Furthermore, chemical analysis indicated that much enhanced saccharification in Talq90 and Talq16 may be partially due to their relatively low cellulose CrI and DP values and high hemicellulose Ara and H-monomer levels in raw materials and SE residues. Hence, this study has not only demonstrated a mild pretreatment technology for a complete saccharification, but it has also obtained the high ethanol production in desirable wheat accessions.

Genetic and transcriptomic analyses of lignin- and lodging-related traits in Brassica napus

Candidate genes associated with lignin and lodging traits were identified by combining phenotypic, genotypic, and gene expression data in B. napus. Brassica napus is one of the world's most important oilseed crops, but its yield can be dramatically reduced by lodging, bending, and falling of its vertical stems. Lignin has been shown to contribute to stem mechanical strength. In this study, we found that the syringyl/guaiacyl (S/G) monolignol ratio exhibits a significant negative correlation with disease and lodging resistance. A total of 92 and 50 SNP and SSR loci, respectively, were found to be significantly associated with five traits, breaking force, breaking strength, lodging coefficient, acid detergent lignin content, and the S/G monolignol ratio using GWAS. To identify novel genes involved in lignin biosynthesis, transcriptome sequencing of high- (H) and low (L)-ADL content accessions was performed. The up-regulated genes were mainly involved in glycoside catabolic processes (especially glucosinolate catabolism) and cell wall biogenesis, while down-regulated genes were involved in glucosinolate biosynthesis, indicating that crosstalk exists between glucosinolate metabolic processes and lignin biosynthesis. Integrating this differential expression with the GWAS analysis, we identified four candidate genes regulating lignin, including glycosyl hydrolase (BnaA01g00480D), CYT1 (BnaA04g22820D), and two encoding transcription factors, SHINE1 (ERF family) and DAR6 (LIM family). This study provides insight into the genetic control of lodging and lignin in B. napus.

Steam-exploded biomass saccharification is predominately affected by lignocellulose porosity and largely enhanced by Tween-80 in Miscanthus

In this study, total ten Miscanthus accessions exhibited diverse cell wall compositions, leading to largely varied hexoses yields at 17%-40% (% cellulose) released from direct enzymatic hydrolysis of steam-exploded (SE) residues. Further supplied with 2% Tween-80 into the enzymatic digestion, the Mis7 accession showed the higher hexose yield by 14.8-fold than that of raw material, whereas the Mis10 had the highest hexoses yield at 77% among ten Miscanthus accessions. Significantly, this study identified four wall polymer features that negatively affect biomass saccharification as p<0.05 or 0.01 in the SE residues, including cellulose DP, Xyl and Ara of hemicellulose, and S-monomer of lignin. Based on Simons' stain, the SE porosity (defined by DY/DB) was examined to be the unique positive factor on biomass enzymatic digestion. Hence, this study provides the potential strategy to enhance biomass saccharification using optimal biomass process technology and related genetic breeding in Miscanthus and beyond.

Fractionation of rapeseed straw by hydrothermal/dilute acid pretreatment combined with alkali post-treatment for improving its enzymatic hydrolysis

The aim of the research was to evaluate the effect of combined treatments on fermentable sugar production from rapeseed straw. An optimum condition was found to be the combination of hydrothermal pretreatment at 180°C for 45min and post-treatment by 2% NaOH at 100°C for 2h, which was based on the quantity of monosaccharides released during enzymatic hydrolysis. As compared with the raw material without treatment, the combination of hydrothermal pretreatment and alkali post-treatment resulted in a significant increase of the saccharification rate by 5.9times. This process potentially turned rapeseed straw into value added products in accordance with the biorefinery concept.

Genetic modification of plant cell walls to enhance biomass yield and biofuel production in bioenergy crops

Plant cell walls represent an enormous biomass resource for the generation of biofuels and chemicals. As lignocellulose property principally determines biomass recalcitrance, the genetic modification of plant cell walls has been posed as a powerful solution. Here, we review recent progress in understanding the effects of distinct cell wall polymers (cellulose, hemicelluloses, lignin, pectin, wall proteins) on the enzymatic digestibility of biomass under various physical and chemical pretreatments in herbaceous grasses, major agronomic crops and fast-growing trees. We also compare the main factors of wall polymer features, including cellulose crystallinity (CrI), hemicellulosic Xyl/Ara ratio, monolignol proportion and uronic acid level. Furthermore, the review presents the main gene candidates, such as CesA, GH9, GH10, GT61, GT43 etc., for potential genetic cell wall modification towards enhancing both biomass yield and enzymatic saccharification in genetic mutants and transgenic plants. Regarding cell wall modification, it proposes a novel groove-like cell wall model that highlights to increase amorphous regions (density and depth) of the native cellulose microfibrils, providing a general strategy for bioenergy crop breeding and biofuel processing technology.