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Available online at www.sciencedirect.com Plant cell wall polymers as precursors for biofuels Markus Pauly1 and Kenneth Keegstra Current Opinion in Plant Biology 2010, 13:305–312 also qualitative differences in their components. Sensitive analytical techniques can be used to identify and characterize specific wall components at the tissue or cellular level. Sensitive mass spectrometric methods have been developed to analyze wall fragments released by enzymes from laser-dissected plant tissues [3]. Not surprisingly, the vascular tissues contained very different components from those in other leaf tissues. Even differences in the substitution patterns of the hemicellulose xyloglucan between the outer wall of leaf epidermal cells and the walls derived from the entire layer were observed, indicating that there are local structural differences within xyloglucan from different parts of a single cell. More significantly, the use of molecular probes, such as specific stains [4] and antibodies, reveals an amazing diversity of patterns ([5]; Figure 1). A number of monoclonal antibodies that recognize different epitopes on xylan [6], arabinan [7,8], or xyloglucan [9,10,11] have been used to demonstrate very specific patterns of distribution among cell types and within the wall of single cells. We need to consider this compositional complexity of plant feedstocks when evaluating and assessing the robustness of technical processes converting biomass to biofuels. This review comes from a themed issue on Physiology and metabolism Edited by Uwe Sonnewald and Wolf B. Frommer Wall composition and fuel production processes The conversion of plant biomass into liquid transportation fuels is a complex process that could be simplified by altering the ratios of the cell wall polymers that constitute the main biomass components. The composition of biomass varies naturally depending upon plant species and cell type, including some highly specialized walls that consist mainly of a single component. Progress is being made in understanding the molecular basis of these natural variations in wall composition. This new knowledge will be a valuable resource that can be used during efforts to generate designer biofuel crops using either selected breeding methods or recombinant DNA techniques. Address DOE Great Lakes Bioenergy Research Center, DOE Plant Research Lab, Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI 48824, USA Corresponding author: Keegstra, Kenneth ([email protected]) 1 Current address: Energy Biosciences Institute, Department of Plant and Microbial Biology, Calvin Lab, University of California-Berkeley, Berkeley, CA, USA. Available online 22nd January 2010 1369-5266/$ – see front matter # 2010 Elsevier Ltd. All rights reserved. DOI 10.1016/j.pbi.2009.12.009 Introduction Plant cell wall polymers have received significant attention in recent years because they are the major components in the plant biomass that is under consideration as a source of reduced carbon to partially replace fossil fuels. Although plant biomass is often considered as having a uniform composition, there is in fact substantial diversity in composition that arises from two important areas. First, different species of plants have significant differences in the proportions of cellulose, hemicellulose, and lignin found in their biomass and, further, important differences in the types of hemicelluloses and/or the ratios of monomers in lignin [1,2]. In addition to these species-specific differences, the average composition of a single species, which is often quite uniform, hides a great deal of diversity. Every plant consists of many different cell types, each with a unique cell wall that contains not only different ratios of wall components, but sometimes www.sciencedirect.com Lignocellulosic biomass can be utilized as a feedstock for biofuel production in a number of ways. Depending on the process, different attributes in wall composition are required or desired. One approach, similar to that used in the corn ethanol industry, entails degradation of the polysaccharides in the lignocellulosic feedstock to monosaccharides, and their subsequent fermentation to ethanol or other advanced biofuels. The remaining lignin residue can be used to generate heat, for example, for ethanol distillation. The major challenge in this process is that plants have evolved wall structures that are recalcitrant to biological degradation [1,12]. Hence, the biomass must be subjected to such energy-intensive and cost-intensive treatments as steam, weak acid, or non-aqueous ammonia [13]. In addition, a high enzyme loading is required to release fermentable glucose monomers from cellulose because of its tight linkage with hemicelluloses and lignin in native walls. Therefore, one major research objective of plant scientists is to make walls more open and accessible to enzymatic degradation. One way to enhance enzyme accessibility would be to increase the water solubility of polysaccharides. This shift in the ratio of less soluble to more soluble polysaccharides could be achieved by an increase in the abundance of amorphous glucan chains rather than crystalline microfibrils of cellulose; addition of side chain substitutions to the backbone of hemicelluloses, Current Opinion in Plant Biology 2010, 13:305–312 306 Physiology and metabolism Figure 1 Another issue is the fermentability of the released monosaccharides. Currently, yeasts used in the ethanol fermentation process utilize only hexoses such as glucose and mannose. However, the most common hemicellulosic polysaccharides consist mainly of xylose and arabinose, pentoses that do not ferment so readily. Progress has been made in developing specialized yeast and bacterial strains that can ferment these pentoses, but they are not yet very efficient [16]. Therefore, one desirable change in biomass is an increased abundance of hexose-containing polymers such as cellulose or mannans rather than xylans. Another process used to convert lignocellulosic biomass to fuel is a catalyst-based chemical process [17,18]. However, this process is hampered by inhibiting components, such as phenolic compounds or aliphatic acids, present to varying degrees in the degraded biomass [19]. One plant breeding goal should thus be to reduce the abundance of such compounds to a minimum. An alternative way to produce fuels is by combustion and gasification of lignocellulosics to syngas (carbon-monoxide, carbon-dioxide, and hydrogen gas) that can be transformed to ethanol via microbes [20] or hydrocarbons by the Fischer–Tropsch process. The advantage of the gasification process is that not only the polysaccharides but also the carbon present in lignin is available for fuel production. For these processes, the composition and aggregation status of the various polymers within the lignocellulosic material does not play a role, but low water and ash content are desirable [21]. Native structural diversity of plant cell walls The wall composition and structure of differentiated cells in plants are diverse. Sections of plant tissues stained for lignin [69] and viewed with an inverted microscope (panel a) or with fluorescent-tagged antibodies directed against specific polysaccharide epitopes [7,8] and viewed by fluorescence microscopy (panels b–f) reveals that the wall structural diversity can be observed not only in different cell types (a, b, f), but also in different layers or areas of a single cell (c, d, e). a: Section of the first internode of a maize stem stained with phloroglucinol (courtesy of Debra Goffner, CNRS Castanet Tolosan). b: Section from the first internode of a maize stem stained with Mirande’s reagent (courtesy of Debra Goffner, CNRS Castanet Tolosan). c/d: Immunofluorescence detection of pectic arabinan epitopes present in the parenchyma cells of an Arabidopsis inflorescence stem. Two antibodies recognising different arabinan epitopes (c-LM13; d-LM16) label distinct different regions of those cells (courtesy of Paul Knox, University of Leeds). e: In tobacco stem sections a certain pectic arabinan epitope (LM16) is observed only in xylem fiber cells and phloem cells. f: Mature fibers of hemp: localization of an arabinogalactan-protein epitope (visulaized by JIM14) at the inner side of secondary walls (courtesy of Paul Knox, University of Leeds). thereby decreasing hydrogen bonding with cellulose microfibrils; the creation of a more easily degradable lignin through the introduction of specific easily cleavable monolignols [2,14]; and/or a reduction of lignin–hemicellulose linkages [15]. Current Opinion in Plant Biology 2010, 13:305–312 When thinking about strategies for changing the composition of plant cell walls, whether in terms of abundance of certain polymers or substitution patterns of specific polymers, one can gain significant insights by looking into nature’s ‘laboratory’. Certain plant species have evolved specialized tissues that have unusual wall compositions, that is, they contain elevated levels of cellulose, hemicellulose, or lignin (for overview see Figure 2). Specialized cells that make cellulose One of the best-studied examples is the seed trichome of cotton, in which the fiber cells have secondary walls containing almost pure cellulose [22]. Studies of this system have revealed many important features of cellulose biosynthesis, including the first identification of the cellulose synthase genes in plants [23]. In recent years, progress has been made in identifying the regulatory events that allow the deposition of almost pure cellulose in cotton fibers [24]. Cotton fiber development is a complex process that involves many events, including the action of various hormones [25], but some of the key transcription factors have been identified [26,27]. A detailed understanding of the metabolic and regulatory events needed for a single cell to convert almost all of its www.sciencedirect.com Cell Walls and Biofuels Pauly and Keegstra 307 Figure 2 Examples of specialized tissues in plants that produce predominantly a single cell wall polymer. Cellulose—Mature cotton plants (upper left) produce cotton fibers (lower left), whose secondary walls that are nearly pure cellulose (courtesy of Candice Haigler, E. Roberts, and E. Hequet, NC State University). Poplar trees (upper right) produce tension wood (lower right) with walls that are also nearly pure cellulose (courtesy of Frank Telewski and Jameel Al-Haddad, Michigan State University). Hemicelluloses—Fenugreek plants (upper left) produce seeds containing endosperm walls (lower left) that are largely galactomannan. Psyllium plants (upper center) produce seeds surrounded by a mucilaginous layer (lower center) with walls rich in arabinoxylan. Nasturtium plants (upper right) produce seeds where the cotyledon cells (lower right) have a wall that is largely xyloglucan (courtesy of Marlene Cameron, Curtis Wilkerson, Michigan State University). Lignin—Pine trees (upper panel) produce compression wood (lower panel) that is enriched in lignin with a different composition from normal wood (courtesy of Frank Telewski and Jameel Al-Haddad, Michigan State University). carbon resources into a single polysaccharide, cellulose, should be useful in efforts to manipulate and enhance cellulose deposition, thereby increasing C6 sugar abundance in biofuel crops. the work of Cocuron et al. [35], who used studies of developing nasturtium seeds, which store xyloglucan, to provide evidence that the xyloglucan glucan synthase is encoded by a CslC gene. Specialized cells that make a single hemicellulose The fact that these seeds are capable of making a single hemicellulosic polysaccharide, while not making other parts of the wall, indicates that these biosynthetic pathways can be regulated independently from other wall components. Furthermore, the use of cell wall polymers as reserve carbohydrates occurs in several unrelated plants [28], indicating that this trait has arisen independently many times during the evolution of land plants. If correct, this logic would lead to the conclusion that only a few changes are needed to accomplish this change in regulation and that it could be done by design to modify the walls of biofuel crops. Many plant species create a special cell wall in the endosperm or cotyledon cells of developing seeds [28]. These cell wall polysaccharides serve as reserve carbohydrates, being synthesized during seed development and later mobilized during seedling germination. Seeds that store galactomannan or xyloglucan are abundant in nature [28], but there are seeds or other tissues that produce other polymers in specialized cells, such as psyllium seeds, which make arabinoxylan in a special layer of epidermal cells [29]. Studies of developing seed systems have led to the identification of genes and enzymes involved in hemicellulose biosynthesis. For example, studies of developing guar seeds led to the conclusion that mannan synthase is encoded by a CslA gene [30], and more recent studies have confirmed that this conclusion is valid for most plant species [31,32], including coffee [33]. Recently, Goubet et al. [34] used reverse genetics to provide convincing in vivo evidence to further support the earlier conclusions. Another example comes from www.sciencedirect.com Changes in wall composition due to environmental responses Plants have the capacity to react to environmental stimuli by changing their metabolism. Reprogramming cell wall polymer biosynthesis is no exception. For example, as a response to mechanical stresses, such as wind or changes in gravitational stimuli, trees can develop specialized tissues known as reaction wood [36]. This type of wood enables the return of stems back to a vertical orientation. Current Opinion in Plant Biology 2010, 13:305–312 308 Physiology and metabolism Reaction wood of gymnosperms, known as compression wood, develops on the lower side of branches. Regular gymnosperm wood contains lignin that lacks syringyl units and consists almost entirely of guaiacyl units, whereas compression wood contains higher amounts of lignin that is particularly enriched in p-hydroxyphenyl units [37]. The higher abundance is probably due to elevated expression of enzymes in the lignin metabolic pathway [38]. Consequently, silencing one of those genes (4-coumarate-ligase) resulted in an up to 50% reduction of lignin in the tracheary elements [39], severely dwarfing the plant and altering bark and wood anatomy. Compression wood also has alterations in polysaccharides with reduced levels of cellulose and glucomannans compared to regular wood. By contrast, the reaction wood of angiosperms, such as poplar, develops on the upper side of branches and is termed tension wood. Tension wood fibers form a distinct additional inner gelatinous layer, the G-layer, with highly elevated cellulose content (95% vs. 45% in regular vessel elements; [40]). A detailed analysis of the transcriptome and metabolome of the G-layer in poplar demonstrated a higher transcript level of sucrose synthase, suggesting an increased carbon flux into cellulose production [41]. Concomitantly, pathways for the production of the hemicelluloses and lignin were downregulated. Also, specific cellulose synthase genes are upregulated in the G-layer, probably because they contain mechanical stress-responsive elements in their promoters [42]. Despite its reduced abundance, xyloglucan is present in the G-layer and its metabolism by xyloglucan endotransglycosylases has been proposed to play a major role in conferring the mechanical properties of the G-layer and its connection to the adjacent S2 layer [40,43]. Another example of a distinct wall structure made by a plant cell is the papilla formed at the site where a pathogenic fungus attempts to penetrate a plant cell. A papilla, a local apposition of a multi-layered wall structure, contains callose, structural proteins, and lignin [44]. All of these components are produced as a result of the plant cell’s sensing mechanism triggered by invading pathogens and are necessary to form a first line of defense against this pathogen [45]. Genetic evidence has been presented that components of the secretory system are responsible for the distinct location of the papilla [46]. The lesson that one can learn from these various wall systems is that plant cells are able to undergo reprogramming of their wall biosynthetic machinery, including but not limited to diverting the flow of assimilated carbon, altered regulation of glycan synthases, glycosyltransferases, monolignol synthetic enzymes, and the secretory system. This reprogramming results in fundamentally different and unique Current Opinion in Plant Biology 2010, 13:305–312 wall materials that have an altered ratio of components or even different components. Understanding how the cell accomplishes this task will provide valuable knowledge that will enable rational changes of cell wall composition that will allow enhanced biofuel production. Strategies for manipulating wall composition Knowledge regarding the regulation of wall polymer biosynthesis remains largely elusive. The mechanisms by which plant cells regulate the flow of carbon into wall polymers and balance the quantities of the various wall polymers are hot topics for research. Photosynthetic assimilates are used as the building blocks for wall polysaccharide biosynthesis. One of the first enzymes that utilizes the newly fixed carbon is sucrose synthase (SuSy), which converts sucrose into UDP-glucose and fructose. Recently, it has been shown that overexpressing SuSy in poplar leads to a higher proportion and absolute amount (2–6%) of cellulose in fiber cells [47], demonstrating that this enzyme might be a key determinant regulating cellulose biosynthesis. Interestingly, the increase in wall cellulose content had very little detrimental effect on the plant, as growth and biomass remained similar to non-transformed plants when grown in the greenhouse. UDP-glucose or other hexose-phosphates are used as a substrate for the nucleotide sugar conversion pathway that results in the synthesis of the 14 different nucleotide sugars necessary for the various wall polymers [48]. Hence, manipulating these enzymes could lead to alteration in cell wall composition. Although most of the genes encoding the nucleotide sugar conversion enzymes are known and their activities have been demonstrated in vitro [48], very few studies have demonstrated that manipulation of their expression leads to significant cell wall changes. Overexpression of these enzymes has so far not yielded plants with altered wall compositions, possibly because of the redundancy of genes encoding many of the sugar nucleotide metabolizing enzymes. For example, a concomitant reduction of several UDP-glucose epimerase transcripts led to not only a decrease in wall galactose levels [49], but also severe changes in root growth. Hence, such changes would presumably lead to less biomass in the plant. Another strategy for modifying the composition of cell walls is to upregulate or downregulate the levels of glycosyltransferases and glycan synthases that polymerize the sugars from nucleotide sugars into polysaccharides. Cavalier et al. [10] created an Arabidopsis plant lacking two genes encoding xyloglucan xylosyltransferases and showed that xyloglucan could not be detected in mutant plants. Interestingly, the mutant plants grew and developed relatively normally, although they were slightly smaller than wild-type plants and had www.sciencedirect.com Cell Walls and Biofuels Pauly and Keegstra 309 abnormal root hairs. It remains to be determined what polymers are substituting for xyloglucans in the primary wall of these mutant plants, but one important conclusion from these studies is that it is possible to make dramatic alterations in the hemicellulose composition of a plant without serious consequences for its ability to grow and reproduce. Reid et al. [50] were the first to successfully modify the composition of a cell wall polysaccharide by overexpressing a gene encoding a glycosyltransferase. They used a constitutive promoter to express the galactomannan galactosyltransferase from fenugreek in tobacco plants. The higher levels of galactosyltransferase activity led to an increase in the galactosyl substitution of the galactomannan that is normally found in the walls of seed endosperm cells. Efforts to modify the mannan content of seeds by overexpression of a mannan synthase gene led to a more complex result. Naoumkina et al. [51] used a seed-specific promoter to overexpress the mannan synthase gene from guar in Medicago truncatula plants. The transgenic Medicago seeds had a lower rather than a higher level of galactomannan, but the molecular weight and viscosity of the polymers were significantly increased. In addition, the authors found that overexpression of the mannan synthase gene caused large changes in the levels of various sugars and sugar alcohols as well as significant changes in the expression of more than 900 genes, with more than 300 genes upregulated and almost 600 genes expressed at lower levels. The major conclusion from this study is that overproduction of a cell wall polysaccharide cannot be accomplished simply by upregulation of the gene responsible for polymer backbone synthesis. One system where considerable progress has been made in understanding the regulation of wall polymer biosynthesis on a molecular level is differentiating vascular elements, especially the deposition of secondary walls in xylem cells (see the review by Demura and Ye [52] in this issue). These unique cell walls are the most abundant component of harvested biomass crops and thus deserve special mention. Many of the genes involved in cellulose and lignin biosynthesis have been identified and progress is being made on identifying the genes required for the biosynthesis of xylan [53–55], the hemicellulose found in the vascular tissues of most potential bioenergy crops. More importantly, key regulatory genes that control the formation of vascular elements [56,57] and many of the components in the transcriptional network that controls secondary wall biosynthesis have been identified in recent years [58–61], including two transcriptional activators of the lignin biosynthetic pathway [62]. It is interesting to consider the possibility that discrete transcription factors independently regulate the cellulose, hemicellulose, and lignin pathways. The deposition of these three components is www.sciencedirect.com normally coordinated during wall biosynthesis [63]. If it is possible to control the deposition of each separately, then it should be possible to modify secondary walls in ways that will allow the production of designer walls in biomass crops. It should not be surprising that the flow of carbon into a major reserve polysaccharide requires the coordinated expression of many genes involved in the synthesis of that polymer. Combining the information of key regulatory genes with the promoters specific for various cell types of the vascular system [56,57] should make it possible to modify the composition of the secondary cell walls of various cell types in stems or other vegetative tissues. Investigating the biological consequences Any strategy to improve the composition of lignocellulosics as feedstocks for the production of biofuels needs to assess the performance of the modified plants. Owing to the multiple functions of the wall during the life cycle of a plant, there could be numerous problems, including a reduction in plant growth and a concomitant reduction in lignocellulosic biomass. Biomass yield concessions such as dwarfism have been shown in a number of mutants [64], whereas in others the yield under greenhouse conditions does not seem to be affected [65]. Altering the walls can lead to changes in cell morphology. One detrimental example of changes in secondary wall structure are irregular xylem cells [66] leading to a loss in watertransporting capacity resulting in a reduction in biomass. Another function of the wall is to ward off plant pathogens such as bacteria and fungi [45]; structural changes in the wall might lead to increased susceptibility. However, in a few cases where pathogen infection has been tested on wall mutants, resistance was not reduced [67], and in some cases was even increased [68]. Concluding remarks Plant biomass is being considered as a feedstock for the production of biofuels and other chemicals. Because the traits that are desirable for these uses vary depending upon the method utilized for biomass processing, and because the desirable traits differ from those needed for more traditional uses of plants for food and fiber production, much discussion has focused on modifying the composition and cell wall properties in ways that will improve the biomass for these new applications. As such modifications are considered, it is important to be aware that nature has already created plant cells with diverse walls, not only those surrounding the differentiated cells found in every plant, but also those in various specialized cells that occur in many plants. Understanding how the walls of these differentiated and specialized cells are synthesized will provide valuable clues on how to modify the cell walls of plants that are grown as designer biofuel crops. Current Opinion in Plant Biology 2010, 13:305–312 310 Physiology and metabolism Acknowledgements The authors would like to thank Paul Knox, University of Leeds, Debra Goffner, CNRS Castanet Tolosan, Candice Haigler, North Carolina State University, and Frank Telewski, Jameel Al-Haddad, Curtis Wilkerson, Marlene Cameron, Michigan State University for providing photographs depicted in Figures 1 and 2. The authors would like to express their appreciation of research funding by the US Department of Energy (DOE) Great Lakes Bioenergy Research Center (DOE BER Office of Science DEFC02-07ER64494) and by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy (award no. DE-FG02-91ER20021). References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: of special interest of outstanding interest 1. Pauly M, Keegstra K: Cell-wall carbohydrates and their modification as a resource for biofuels. Plant J 2008, 54:559-568. 2. Vanholme R, Morreel K, Ralph J, Boerjan W: Lignin engineering. Curr Opin Plant Biol 2008, 11:278-285. Obel N, Erben V, Schwarz T, Kuhnel S, Fodor A, Pauly M: Microanalysis of plant cell wall polysaccharides. Mol Plant 2009, 2:922-932. The authors push the envelope on the microscale of chemical analyses of wall materials using distinct wall collection techniques such as laserdissection catapulting or organelle fractionation in combination with mass spectrometry. As a result clear structural differences in a wall polymer of different areas of a single cell was demonstrated. 3. 4. Anderson CT, Carroll A, Akhmentova L, Somerville C: Real time imaging of cellulose reorientation during cell wall expansion in Arabidopsis roots. Plant Physiol 2009 doi: 10.1104/ pp.109.150128. 5. Knox JP: Revealing the structural and functional diversity of plant cell walls. Curr Opin Plant Biol 2008, 11:308-313. 6. Herve C, Rogowski A, Gilbert HJ, Knox JP: Enzymatic treatments reveal differential capacities for xylan recognition and degradation in primary and secondary plant cell walls. Plant J 2009, 58:413-422. 7. Verhertbruggen Y, Marcus SE, Haeger A, Verhoef R, Schols HA, MCCLEARY BV, Mckee L, Gilbert HJ, Knox JP: Developmental complexity of arabinan polysaccharides and their processing in plant cell walls. Plant J 2009, 59:413-425. The authors characterized three new monoclonal antibodies specific for different epitopes present on arabinan and other polysaccharides that contain arabinose. They used the new antibodies to evaluation the distribution of the arabinan epitopes. 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