Philip J. Harris and Bruce A. Stone

Abbreviations used: Apif Apiofuranosyl; Araf Arabinofuranosyl; AGP, Arabinogalactan — protein; Arap, Arabinopyranosyl; pCA, p-Coumaric acid; DDFA, Dehydrodiferulic acid; DDQ, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone; FA, Ferulic acid; Fucp, Fucopyranosyl; Galp, Galactopyranosyl; Glcp, Glucopyranosyl; GlcpA, Glucopyranuronosyl; GalpA, Galac — topyranuronosyl; GAX, Glucuronoarabinoxylan; GRP, Glycine-rich protein; HCA, Hydrox — ycinnamic acid; HGA, Homogalacturonan; HRGP, Hydroxyproline-rich glycoprotein; 4-0- methyl-a-D-GlcpA, 4-0-methyl-a-D-glucopyranosyluronic acid; Rhap, Rhamnopyranosyl; RG-I, Rhamnogalacturonan I; RG-II, Rhamnogalacturonan II; Rhap, Rhamnopyranosyl; SA, Sinapic acid; XGA, Xylogalacturonan; XG, Xyloglucan; Xylp, Xylopyranosyl.

4.1 Introduction

The cell walls ofseed plants (angiosperms and gymnosperms) represent an enormous store of fermentable carbohydrate that is a potential source of ethanol. However, this carbohydrate is in the form of high molecular weight cellulose and the accompanying non-cellulosic polysaccharides, which are intimately associated both non-covalently and covalently with one another and often with non-carbohydrate polymers particularly lignins, and other polymers such as proteins, and in some situations suberin and cutin. These associations must be broken to allow access of polysaccharide degrading enzymes to their substrates during the digestion phase of bioethanol production in which fermentable sugars (monosaccharides) are released from the plant feedstocks.

Two types of cell walls are recognized, primary and secondary (1-3). Primary walls are formed while cells are still developing and enlarging and for many cell types, e. g., some parenchyma cells, the primary wall is the only wall. Primary walls are typically non-lignified and their thickness in mature cells depends on the cell type. Secondary walls, e. g., in scle — renchyma fibers, are deposited on the primary wall after the cells are fully expanded and at maturity the entire wall (primary and secondary) is typically lignified. The middle lamella, the interfacial layer between adjacent cells, which develops from the cell plate present at division, is also typically lignified. Although less common, cell types occur that have both primary and secondary walls but that are not lignified, or only slightly lignified, for example,

Biomass Recalcitrance: Deconstructing the Plant Cell Wall for Bioenergy. Edited by Michael. E. Himmel © 2008 Blackwell Publishing Ltd. ISBN: 978-1-405-16360-6

the fibers of tension wood (in hardwoods), bast (phloem) fibers from the stems of flax (Linum usitatissimum), hemp (Cannabis sativa), ramie (Boehmeria nivea) and nettle (Urtica dioica), and the hairs of cotton (Gossypium spp.) seeds (4). Many of the parenchyma cells in stems of grasses have secondary walls and the entire wall is lignified (5).

This chapter provides a background to the discussion on current methods for disasso­ciating polysaccharides from plant cell walls to make them accessible to depolymerizing enzymes (see Chapter 14) and on the specificities and action patterns of the polysaccharide depolymerizing enzymes that may be used to release monosaccharides for fermentation (see Chapter 10). In particular, we review the chemistry of cell wall polysaccharides, their associations with one another and with non-carbohydrate polymers, and models of cell wall architecture. We shall focus particularly on primary and lignified secondary cell walls of vege­tative organs of common groups of plants that have been suggested as major biomass sources for bioethanol production. Of particular importance are the herbaceous angiosperms, espe­cially the grasses, including the cereals, which form the monocotyledon family Poaceae. This family yields grain-milling residues, e. g., wheat (Triticum aestivum) bran, and crop residues, e. g., wheat straw and maize (Zea mays) stover. In addition, perennial grasses such as Miscant — hus (Miscanthus spp.), switchgrass (Panicum virgatum), giant reed (Arundo donax), and reed canarygrass (Phalaris arundinacea) have been proposed in the United States (6) or selected by the European Union (7) as being particularly promising as energy crops. The grass family forms part of a major group of monocotyledons known as commelinid monocotyledons that are characterized by the presence of ester-linked ferulic acid in their primary walls. We refer to other monocotyledons as non-commelinid monocotyledons (8).

In the rest of the angiosperms, the term eudicotyledon (or true dicotyledons) is now used to refer to most of the group previously known as the dicotyledons. Some herbaceous eudicotyledons, such as the forage legume alfalfa (Medicago sativa), have also been considered as energy crops (9). Hardwoods (woody angiosperms), e. g., poplars (Populus spp.) and willows (Salix spp.), and softwoods (coniferous gymnosperms), e. g., spruce (Picea abies), are also potential feedstocks.