The lignin content of biomass can impact the ease and cost of biomass processing. Lignin reduction through breeding and genetic modification therefore has potential to reduce costs in biomass-processing industries (e.g. pulp and paper, forage, and lignocellulosic ethanol). We investigated compositional changes in two low-lignin alfalfa (Medicago sativa) lines with antisense down-regulation of p-coumarate 3-hydroxylase (C3H) or hydroxycinnamoyl-CoA:shikimate hydroxycinnamoyltransferase (HCT). We investigated whether the difference in reactivity during lignification of 4-coumaryl alcohol (H) monomers versus the naturally dominant sinapyl alcohol and coniferyl alcohol lignin monomers alters the lignin structure. Sequential base extraction readily reduced the H monomer content of the transgenic lines, leaving a residual lignin greatly enriched in H subunits; the extraction profile highlighted the difference between the control and transgenic lines. Gel permeation chromatography of isolated ball-milled lignin indicated significant changes in the weight average molecular weight distribution of the control versus transgenic lines (CTR1a, 6000; C3H4a, 5500; C3H9a, 4000; and HCT30a, 4000).The advent of large-scale liquid fuel production from biomass has served to highlight how difficult it is to commercially process biomass effectively and efficiently. Much of the difficulty is due to the recalcitrant nature of lignocelluloses (1), a complex interlinking structure composed of cellulose, hemicelluloses, and lignin that makes up the bulk of terrestrial biomass. Accessibility to the cell wall is influenced by lignin, which provides structural integrity to the cell wall. Both total lignin content and lignin monomer composition may impact the ease with which biomass is processed. This study examines whether lignin molecular weight is altered by changing the lignin monomer composition and if these changes affect the ease with which lignin can be removed by chemical processing.Three monomers (Fig. 1), 4-coumaryl alcohol (H), 2 coniferyl alcohol (G), and sinapyl alcohol (S), polymerize in what is thought to be a combinatorial fashion to form the bulk of the lignin polymer (2, 3). The amount of each unit depends on the species, age, cell type, and tissue type (4, 5). The presence of each additional methoxy group on a lignin unit results in one less reactive site (S Ͻ G Ͻ H) and therefore fewer possible combinations during the polymerization reaction. For example, the S unit has no vacant 5-position; therefore 5,5Ј-cross-linking is unavailable for lignin S subunits. As a result, lignin rich in S subunits is more easily depolymerized than lignin rich in G subunits.The relative level of S and G lignin subunits is expressed as the S/G ratio, an important measurement used in the assessment of biomass. H lignin subunits are present in low levels in natural materials (4.9% of lignin in wild-type alfalfa (1) and 0.8% in Norway Spruce (6)); consequently, less is known about lignin high in H subunits and its impact on biomass processing. I...

Sugarcane bagasse is a large-volume agriculture residue that is generated on a ~540 million metric tons per year basis globally 1,2 with the top-three producing countries in Latin America being Brazil (~181 million metric ton yr −1 ), 3 Mexico (~15 million metric ton yr −1 ), 4 and Colombia (~7 million metric ton yr −1 ), 5 respectively. 6 Given sustainability concerns and the need to maximize the utilization of bioresources, the use of sugarcane bagasse is receiving signifi cant attention in biorefi ning applications, as it is a promising resource for the conversion to biofuels and biopower. This review provides a comprehensive review of bagasse and its chemical constituents and on-going research into its utilization as a feedstock for cellulosic ethanol and electricity generation.

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to analyze the molecular constituents on cross sections of juvenile poplar (Populus deltoids) stems before and after dilute acid pretreatment (DAP). Bulk analysis of milled and 50 μm thick cross sections of poplar before and after DAP was shown to be chemically equivalent by FT-IR and carbohydrate analysis. ToF-SIMS analysis of dilute acid pretreated material indicated significant changes in relative contents of cellulose, xylan, and lignin occurred upon pretreatment. The relative content of xylan after DAP increased by 30% on the surface of the poplar stem by ToF-SIMS, while bulk carbohydrate analysis showed that the relative concentration of xylose decreased 10-fold in comparison with untreated poplar wood. The relative content of cellulose and G-lignin units doubled on the surface of the poplar stem sections, while the bulk glucose concentration and Klason lignin increased 40% and 5%, respectively, as determined by bulk carbohydrate and Klason lignin analysis. The spatial distributions of the major lignocellulosic components on the surface of juvenile poplar stem before and after DAP were examined by SIMS and this data was processed into mapping images. Scanning electron microscopy (SEM) was used to evaluate the morphological changes of the cell wall layers before and after DAP, which was also correlated with the results of ToF-SIMS analysis.

The three major components of plant biomass, cellulose, hemicellulose and lignin, are highly recalcitrant and deconstruction involves thermal and chemical pretreatment. Microbial conversion is a possible solution, but few anaerobic microbes utilize both cellulose and hemicellulose and none are known to solubilize lignin.Herein, we show that the majority (85%) of insoluble switchgrass biomass that had not been previously chemically treated was degraded at 78 C by the anaerobic bacterium Caldicellulosiruptor bescii.Remarkably, the glucose/xylose/lignin ratio and physical and spectroscopic properties of the remaining insoluble switchgrass were not significantly different than those of the untreated plant material. C. bescii is therefore able to solubilize lignin as well as the carbohydrates and, accordingly, lignin-derived aromatics were detected in the culture supernatants. From mass balance analyses, the carbohydrate in the solubilized switchgrass quantitatively accounted for the growth of C. bescii and its fermentation products, indicating that the lignin was not assimilated by the microorganism. Immunoanalyses of biomass and transcriptional analyses of C. bescii showed that the microorganism when grown on switchgrass produces enzymes directed at key plant cell wall moieties such as pectin, xyloglucans and rhamnogalacturonans, and that these and as yet uncharacterized enzymes enable the degradation of cellulose, hemicellulose and lignin at comparable rates. This unexpected finding of simultaneous lignin and carbohydrate solubilization bodes well for industrial conversion by extremely thermophilic microbes of biomass that requires limited or, more importantly, no chemical pretreatment. Broader contextThe three major components of plant biomass are cellulose (a glucose polymer), hemicellulose (a polymer of xylose and a variety of other sugars) and lignin (a complex polymer of aromatic units). The sugar polymers are potential feedstocks for the production of biofuels by anaerobic microorganisms. However, plant biomass is highly recalcitrant and harsh and inefficient chemical treatments are required to solubilize the biomass and release the sugars. Moreover, no anaerobic microorganism is known that can degrade the highly recalcitrant lignin. Herein it is shown that switchgrass, a model plant for bioenergy production, can be degraded at moderate temperatures (78 C) by an anaerobic bacterium that solubilizes lignin as well as cellulose and hemicellulose. The microorganism produces a range of both known and as yet uncharacterized enzymes that degrade at comparable rates all of the major components of the plant cell wall. Such thermophilic microbes could potentially be developed to enable the direct conversion of plant biomass to biofuels without the need for any chemical pretreatment.