Cellulose is the predominant polymer in both primary and secondary land plant cell walls and is thought to bare most of the load in supporting the mass of a plant (Cosgrove 2005; Carpita 2011). Higher plant cellulose consists of long chains of 500 to 15000 fi-(1-4)-covalently bonded glucose residues that hydrogen bond with approximately 36 other chains to form compact crystalline microfibrils that exclude internal water solvation and form crystalline surfaces that are inaccessible to enzymatic hydrolysis (Somerville

2006) . Primary walls have shorter, less crystalline microfibrils, while those of secondary walls are longer and more crystalline. This variation may contribute to the varied functions of the wall throughout development.

From the perspective of optimizing biological conversion to biofuels, cellulose is the most important polymer, since it is abundant and fermenting organisms readily metabolize its constituent glucose. This is also the case for the other, scarcer 6-carbon sugars present in cell walls, including mannose, galactose, and the sugar acids, glucuronic acid and galacturonic acid. However, that laboratory strains of yeast and E. coli that also make efficient use of 5-carbon sugars like xylose have been developed should eventually alleviate this need (Jeffries et al. 2004).

In contrast to cellulose, other specific polymers present in grass cell walls are different in abundance and often structure compared with dicotyledenous plants. These differences are most pronounced in the matrix polysaccharides of primary cell walls (Vogel 2008). For example, primary cell walls of dicots consist of ~30% pectin and ~20% xyloglucan (Vogel

2008) . In contrast, grass primary walls contain very little pectin (~5%) and xyloglucan (~1%), but instead consist of the grass-specific polymer, mixed linkage glucan (5-10%) and glucuronoarabinoxylan (~30%) (Scheller et al.

2010) . Mixed-linkage glucan, which consists of fi-(1-4) and fi-(1-3) linked glucose residues, hydrogen bonds to cellulose and other polymers and
plays a role in strengthening and increasing the flexibility of grass cell walls (Vega-Sanchez et al. 2012).

In grass cell walls, glucuronoarabinoxylan, or xylan for short, consists of a backbone of fi-(1-4) linked xylose residues (Fig. 2). Grass xylose is periodically linked at the O-3 position to the 1-carbon of arabinose residues. The arabinose residues are in the furanose f) form with five atoms in the sugar ring, (Araf rather than in the pyranose form (p), which has a 6-membered ring. Of apparent importance to the structure and recalcitrance of grass cell walls, some of the arabinose residues of xylan are modified at the 5-carbon by acylation with hydroxycinnamic acids, especially, ferulic acid and to a lesser extent, p-coumaric acid (Buanafina 2009; Bartley et al. 2013a). Ferulate residues are especially important as they undergo oxidative coupling reactions to form dehydrodimers, i. e., diferulates. Consistent with a role in cross-linking and rigidifying cell walls, the concentration of diferulates in grass cell walls correlates with reduced digestibility (Casler et al. 2006).

Xylan backbone: B-(l,4)-D-Xylopyranose

Arabino furanose Ую

B-(l,4)-D-Xylopyranose a-(l,3)-D-Arabinofuranose

Ferulic Acid

a-(l,2)-D-Glucuromc Acid

Reducing End: GT47, GT8

GT8

Xylan backbone: GT43 GT47

(1,2)-4-0-Methyl-a-D-Glucuromc Acid

Figure 2. Structure of glucuronoarabinoxylan in (A) grasses and (B) dicots. The glycosytransferase (GT) enzyme families implicated in the synthesis of each bond are noted when known. See text and Table 2 for references.