1366 I. 1366 II. 1367 III. 1368 IV. 1368 V. 1369 VI. 1370 VII. 1372 VIII. 1372 IX. 1376 X. 1377 1377 References 1377 SUMMARY: The aim of producing sustainable liquid biofuels and chemicals from ...lignocellulosic biomass remains high on the sustainability agenda, but is challenged by the costs of producing fermentable sugars from these materials. Sugars from plant biomass can be fermented to alcohols or even alkanes, creating a liquid fuel in which carbon released on combustion is balanced by its photosynthetic capture. Large amounts of sugar are present in the woody, nonfood parts of crops and could be used for fuel production without compromising global food security. However, the sugar in woody biomass is locked up in the complex and recalcitrant lignocellulosic plant cell wall, making it difficult and expensive to extract. In this paper, we review what is known about the major polymeric components of woody plant biomass, with an emphasis on the molecular interactions that contribute to its recalcitrance to enzymatic digestion. In addition, we review the extensive research that has been carried out in order to understand and reduce lignocellulose recalcitrance and enable more cost‐effective production of fuel from woody plant biomass.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Summary
Wheat is the most widely grown crop globally, providing 20% of all human calories and protein. Achieving step changes in genetic yield potential is crucial to ensure food security, but ...efforts are thwarted by an apparent trade‐off between grain size and number. Expansins are proteins that play important roles in plant growth by enhancing stress relaxation in the cell wall, which constrains cell expansion.
Here, we describe how targeted overexpression of an α‐expansin in early developing wheat seeds leads to a significant increase in grain size without a negative effect on grain number, resulting in a yield boost under field conditions.
The best‐performing transgenic line yielded 12.3% higher average grain weight than the control, and this translated to an increase in grain yield of 11.3% in field experiments using an agronomically appropriate plant density.
This targeted transgenic approach provides an opportunity to overcome a common bottleneck to yield improvement across many crops.
See also the Commentary on this article by Cosgrove, 230: 403‐405.
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Potato pectin falls to
Phytophthora
Phytophthora infestans
is a plant oomycete pathogen that drove the potato famines of the 1800s and continues to afflict potato fields today. The polysaccharide ...pectin makes up about a third of the cell wall in potatoes. Sabbadin
et al
. identified a family of lytic polysaccharide monooxygenases (LMPOs) that cleave pectin and are upregulated in
P. infestans
during infection. Silencing the relevant LMPO gene successfully inhibited
P. infestans
infections. These findings open doors for disease intervention targets and for biotech applications. —PJH
Virulence factors secreted by the plant pathogen
Phytophthora infestans
target pectin polysaccharides in plant cell walls.
The oomycete
Phytophthora infestans
is a damaging crop pathogen and a model organism to study plant-pathogen interactions. We report the discovery of a family of copper-dependent lytic polysaccharide monooxygenases (LPMOs) in plant pathogenic oomycetes and its role in plant infection by
P. infestans
. We show that LPMO-encoding genes are up-regulated early during infection and that the secreted enzymes oxidatively cleave the backbone of pectin, a charged polysaccharide in the plant cell wall. The crystal structure of the most abundant of these LPMOs sheds light on its ability to recognize and degrade pectin, and silencing the encoding gene in
P. infestans
inhibits infection of potato, indicating a role in host penetration. The identification of LPMOs as virulence factors in pathogenic oomycetes opens up opportunities in crop protection and food security.
Although cell wall polymers play important roles in the tolerance of plants to abiotic stress, the effects of salinity on cell wall composition and metabolism in grasses remain largely unexplored. ...Here, we conducted an in‐depth study of changes in cell wall composition and phenolic metabolism induced upon salinity in maize seedlings and plants. Cell wall characterization revealed that salt stress modulated the deposition of cellulose, matrix polysaccharides and lignin in seedling roots, plant roots and stems. The extraction and analysis of arabinoxylans by size‐exclusion chromatography, 2D‐NMR spectroscopy and carbohydrate gel electrophoresis showed a reduction of arabinoxylan content in salt‐stressed roots. Saponification and mild acid hydrolysis revealed that salinity also reduced the feruloylation of arabinoxylans in roots of seedlings and plants. Determination of lignin content and composition by nitrobenzene oxidation and 2D‐NMR confirmed the increased incorporation of syringyl units in lignin of maize roots. Salt stress also induced the expression of genes and the activity of enzymes enrolled in phenylpropanoid biosynthesis. The UHPLC–MS‐based metabolite profiling confirmed the modulation of phenolic profiling by salinity and the accumulation of ferulate and its derivatives 3‐ and 4‐O‐feruloyl quinate. In conclusion, we present a model for explaining cell wall remodeling in response to salinity.
We demonstrate the modulations induced by salt stress in the amounts of crystalline cellulose, matrix polysaccharides and lignin in roots of maize seedlings and roots, stems and leaves of maize plants. Salt stress reduced the feruloylation of arabinoxylan, increased the incorporation of syringyl lignin, and induced the biosynthesis and accumulation of ferulic acid and its derivatives. Based on these findings, we propose a model of grass cell wall remodeling in response to salinity.
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Thermobia domestica belongs to an ancient group of insects and has a remarkable ability to digest crystalline cellulose without microbial assistance. By investigating the digestive proteome of ...Thermobia, we have identified over 20 members of an uncharacterized family of lytic polysaccharide monooxygenases (LPMOs). We show that this LPMO family spans across several clades of the Tree of Life, is of ancient origin, and was recruited by early arthropods with possible roles in remodeling endogenous chitin scaffolds during development and metamorphosis. Based on our in-depth characterization of Thermobia's LPMOs, we propose that diversification of these enzymes toward cellulose digestion might have endowed ancestral insects with an effective biochemical apparatus for biomass degradation, allowing the early colonization of land during the Paleozoic Era. The vital role of LPMOs in modern agricultural pests and disease vectors offers new opportunities to help tackle global challenges in food security and the control of infectious diseases.
SUMMARY
Cell walls are dynamic and multi‐component materials that play important roles in many areas of plant biology. The composition and interactions of the structural elements give rise to ...material properties, which are modulated by the activity of wall‐related enzymes. Studies of the genes and enzymes that determine wall composition and function have made great progress, but rarely take account of potential compensatory changes in wall polymers that may accompany and accommodate changes in other components, particularly for specific polysaccharides. Here, we present a method that allows the simultaneous examination of the mass distributions and quantities of specific cell wall matrix components, allowing insight into direct and indirect consequences of cell wall manipulations. The method employs gel‐permeation chromatography fractionation of cell wall polymers followed by enzyme‐linked immunosorbent assay to identify polymer types. We demonstrate the potential of this method using glycan‐directed monoclonal antibodies to detect epitopes representing xyloglucans, heteromannans, glucuronoxylans, homogalacturonans (HGs) and methyl‐esterified HGs. The method was used to explore compositional diversity in different Arabidopsis organs and to examine the impacts of changing wall composition in a number of previously characterized cell wall mutants. As demonstrated in this article, this methodology allows a much deeper understanding of wall composition, its dynamism and plasticity to be obtained, furthering our knowledge of cell wall biology.
Significance Statement
Gel‐permeation chromatography fractionation of cell wall polymers followed by enzyme‐linked immunosorbent assay using glycan‐directed monoclonal antibodies is a new mass distribution profiling method providing specific polysaccharide detection, relative quantification and simultaneous analysis of different cell wall matrix polysaccharides. This method allows deeper analysis of previously characterized Arabidopsis cell wall biosynthesis mutants revealing further compositional and mass distribution changes and potential compensatory effects in the wall polysaccharides, underlying the dynamic plasticity of the plant cell wall.
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Domination of the global biosphere by human beings is unprecedented in the history of the planet, and our impact is such that substantive changes in ecosystems, and the global environment as a whole, ...are now becoming apparent. Our activity drives the steady increase in global temperature observed in recent decades. The realization of the adverse effects of greenhouse gas emissions on the environment, together with declining petroleum reserves, has ensured that the quest for sustainable and environmentally benign sources of energy for our industrial economies and consumer societies has become urgent in recent years. Consequently, there is renewed interest in the production and use of fuels from plants. The 'first-generation' biofuels made from starch and sugar appear unsustainable because of the potential stress that their production places on food commodities. Second-generation biofuels, produced from cheap and abundant plant biomass, are seen as the most attractive solution to this problem, but a number of technical hurdles must be overcome before their potential is realized. This review will focus on the underpinning research necessary to enable the cost-effective production of liquid fuels from plant biomass, with a particular focus on aspects related to plant cell walls and their bioconversion.
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Summary
Brachypodium distachyon (Brachypodium) has been proposed as a model for grasses, but there is limited knowledge regarding its lignins and no data on lignin‐related mutants. The cinnamyl ...alcohol dehydrogenase (CAD) genes involved in lignification are promising targets to improve the cellulose‐to‐ethanol conversion process. Down‐regulation of CAD often induces a reddish coloration of lignified tissues. Based on this observation, we screened a chemically induced population of Brachypodium mutants (Bd21–3 background) for red culm coloration. We identified two mutants (Bd4179 and Bd7591), with mutations in the BdCAD1 gene. The mature stems of these mutants displayed reduced CAD activity and lower lignin content. Their lignins were enriched in 8–O–4‐ and 4–O–5‐coupled sinapaldehyde units, as well as resistant inter‐unit bonds and free phenolic groups. By contrast, there was no increase in coniferaldehyde end groups. Moreover, the amount of sinapic acid ester‐linked to cell walls was measured for the first time in a lignin‐related CAD grass mutant. Functional complementation of the Bd4179 mutant with the wild‐type BdCAD1 allele restored the wild‐type phenotype and lignification. Saccharification assays revealed that Bd4179 and Bd7591 lines were more susceptible to enzymatic hydrolysis than wild‐type plants. Here, we have demonstrated that BdCAD1 is involved in lignification of Brachypodium. We have shown that a single nucleotide change in BdCAD1 reduces the lignin level and increases the degree of branching of lignins through incorporation of sinapaldehyde. These changes make saccharification of cells walls pre‐treated with alkaline easier without compromising plant growth.
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Grass lignocellulosic material consists mostly of secondary cell walls and is composed mainly of cellulose (25%–55%), hemicellulose xylan (20%–50%), lignin (10%–35%) and small amount of pectin, ...depending on plant species, organ, cell types and developmental stage of the tissue. ...plants of hybrid aspen (Populus tremula × tremuloides) down‐regulated simultaneously for PtGT43B and PtGT43C, the orthologues of IRX9 and IRX14, respectively, present reduced xylose content relative to the reducing end sequence in xylan, with slight alteration in the chemical composition of wood, a small decrease in S and H lignin, accompanied by a higher lignocellulose saccharification efficiency (Ratke et al., ). Silencing the SvBAHD01 gene by RNAi in Setaria viridis reduced FA content by 60% and increased stem saccharification efficiency (from 40 to 60%), without changing biomass productivity. ...the increase in stem saccharification obtained by Whitehead et al. ( ) reflects a synergic effect of the overall decrease in feruloylation of arabinosyl moieties linked to AXs.
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Sugarcane bagasse represents one of the best potential feedstocks for the production of second generation bioethanol. The most efficient method to produce fermentable sugars is by enzymatic ...hydrolysis, assisted by thermochemical pretreatments. Previous research was focused on conventional heating pretreatment and the pretreated biomass residue characteristics. In this work, microwave energy is applied to facilitate sodium hydroxide (NaOH) and sulphuric acid (H2SO4) pretreatments on sugarcane bagasse and the efficiency of sugar production was evaluated on the soluble sugars released during pretreatment. The results show that microwave assisted pretreatment was more efficient than conventional heating pretreatment and it gave rise to 4 times higher reducing sugar release by using 5.7 times less pretreatment time. It is highlighted that enrichment of xylose and glucose can be tuned by changing pretreatment media (NaOH/H2SO4) and holding time. SEM study shows significant delignification effect of NaOH pretreatment, suggesting a possible improved enzymatic hydrolysis process. However, severe acid conditions should be avoided (long holding time or high acid concentration) under microwave heating conditions. It led to biomass carbonization, reducing sugar production and forming ‘humins’. Overall, in comparison with conventional pretreatment, microwave assisted pretreatment removed significant amount of hemicellulose and lignin and led to high amount of sugar production during pretreatment process, suggesting microwave heating pretreatment is an effective and efficient pretreatment method.
•Production of sugar was achieved in an efficient manner; maximum sugar yield (86%) in the pretreatment media was achieved in 7 min.•Selective production of xylose and glucose was obtained by changing pretreatment media (NaOH/H2SO4); yields are up to 22% and 64% respectively.•Voids, holes and dismantled fibre bundles appeared on bagasse samples pretreated with NaOH under microwave condition, suggesting improved biomass digestibility.•Severe acid conditions should be avoided, due to biomass carbonization, and reduced sugar release.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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