Yanbin Yin

Department of Biological Sciences, Northern Illinois University, DeKalb, IL, USA

email: yyin@niu. edu

The major components of plant biomass are carbohydrate-rich cell walls, composed of different bio­polymers such as polysaccharides and lignins as well as some minor wall structural glycoproteins (Somerville et al., 2004). Biomass used for biofuel production is pri­marily derived from secondary cell walls. For example, wood cells from poplar trees contain a thin layer of pri­mary cell walls and multiple layers of much thicker and tougher secondary cell walls. All plant cells of different tissues have primary cell walls while only in developed cells (stopped growing) secondary cell walls appear (Cosgrove, 2005). The chemical compositions in primary and secondary cell walls differ significantly (Mohnen et al., 2008). The primary cell wall contains no lignins and the polysaccharides include celluloses, hemicellu — loses (primarily xyloglucans and mannans in dicots and xylans in monocots) and pectins. However, in the secondary cell walls, there are higher percentage of cellu­loses, different hemicellulosic polysaccharides (primar­ily xylans in both dicots and monocots) and lignins. For example, wood secondary cell walls contain 35—50% celluloses, 25—30% hemicelluloses (mostly xylans) and 15—30% lignins (Himmel et al., 2007).

pectins both refer to a collection of complex polysaccha­rides mostly with side chains. Hemicelluloses contain four major groups: xyloglucans, mannans, xylans and mixed-linkage glucans, while pectins contain three ma­jor groups: galacturonans, rhamnogalacturonan I and rhamnogalaturonan II. Each of the groups of hemicellu — loses and pectins do not refer to a single type of polysac­charides; they often refer to polysaccharides with the same backbone structure (sugars and linkages) while with different side chains. Due to this reason, all these biopolymers are cross-linked and interwoven (Somerville et al., 2004; Himmel et al., 2007) to form very complex and heterogeneous cell wall structures. Particularly celluloses are wrapped by hemicelluloses and buried in a lignin network and not accessible to enzymes so that the degradation efficiency is very low if no costly chemical pretreatment is applied.

Although celluloses are simple polymer of glucose linked by beta-1,4,-glucosidic bond, the complexity of chemical compositions of hemicelluloses and pectins is remarkably high (Somerville et al., 2004). The reasons are as follows: (1) there are 14 different monosacchar — aides (sugar units) found in hemicelluloses and pectins (Pauly and Keegstra, 2008b); (2) the possible glycosidic

Bioenergy Research: Advances and Applications http://dx. doi. org/10.1016/B978-0-444-59561-4.00006-1

linkages formed between two sugars are extremely diverse as theoretically they can be connected between any hydroxyl group of two sugars and (3) sugars in the polysaccharides can be further modified by, e. g. methylation, acetylation or esterification.

Lignins, however, are complex heterogeneous phenolic polymers and chemically very distinct from polysaccharides. They are formed by three major monomers: hydroxyphenyl (H), guaiacyl (G) and syringyl (S) units, which are derived from coumaryl alcohol, coniferyl alcohol and sinapyl alcohol, res­pectively (Boerjan et al., 2003). All the biopolymers in plant cell walls are cross-linked and interwoven (Somerville et al., 2004; Himmel et al., 2007) to form very complex and heterogeneous structures, which is believed to enable cell walls recalcitrant to enzymatic degradation.

Besides, cell wall compositions and structures also vary from tissue to tissue. The reason is because the cell wall biosynthetic enzymes are differentially regu­lated and expressed in different tissues. Furthermore, different plants, especially those of distant evolutionary clades, have very distinct cell wall biopolymer composi­tions. For example, grasses generally have significantly higher percentage of xylans than trees (Pauly and Keegstra, 2008a).

For biofuel production, polysaccharides especially celluloses are favored as their degradation releases fermentable sugars. Lignins, however, are phenolic polymers and chemically distinct from polysaccharides, not giving rise to sugars and meant to be removed in the biofuel production. In order to develop transgenic plants with modified cell wall compositions (i. e. higher cellulose and lower lignin content), we need a better un­derstanding of how plant cell wall polysaccharides and lignins are synthesized.