As mentioned above, cellulose composes about 40% of switchgrass cell wall and is the primary target for bioconversion to biofuel. Cellulose microfibrils are synthesized at the plasma membrane by cellulose synthase A (CESA) complexes with glycosyltransferase (GT) activity (Lei et al. 2012). Microfibril orientation determines cell growth direction and cell wall mechanical properties (Saxena et al. 2005; Crowell et al. 2010). In higher plants, the complex consists of a hexagonal formation of six rosettes; each rosette consists of six CESA polypeptides (Doblin et al. 2002). Many reviews have covered recent progress in cellulose synthase machinery and cellulose

Table 2. Enzymes that synthesize and modify xylan. Protein Name Locus ID Mutant CAZy* or DUF+ Family Activity Comments Reference IRX9 At2g37090 irx9 GT43 p(1^4)-xylan synthesis Backbone elongation (Brown et al. 2007; Pena et al. 2007) IRX14 At4g36890 irxl4 GT43 Backbone elongation (Brown et al. 2007) TaGT43-4 wheat GT43 (Zeng et al. 2010) IRX10/GUT1; IRX10-LIKE/GUT2; TaGT47-13 Atlg27440 At5g61840 irxlO irxlO irxlO-L GT47 p(1^4)-xylan synthesis Backbone elongation (Brown et al. 2009) (Zeng et al. 2010) IRX7/FRA8; IRX7L/ F8H At2g28110 At5g22940 irx7/fra8 GT47 Reducing end (Rennie et al. 2012) IRX8/GAUT12 At5g54690 irx8 GT8 p(1^4)-xylan synthesis Reducing end (Pena et al. 2007) PARVUS/GLZ1 Atlgl9300 parvus GT8 Reducing end (Lee et al. 2007) GUX1 GUX2 At3gl8660 At4g33330 GT8 Side-chain glucuronic acid and 4-O-methylglucuronic acid branches to xylan (Mortimer et al. 2010) IRX15 IRX15L At3g50220 At5g67210 DUF579 unknown Mutant has methylglucuronic acid side chains instead of glucuronic acid side chains (Brown et al. 2011) XAX1 Os02g22380 GT61 p-Xylp-(l—»2)-a-Ara/- (1-3) (Chiniquy et al. 2012) TaXATl wheat GT61 a(1^3)-Ara/Transferase (Anders et al. 2012)

microfibril synthesis (Somerville 2006; Carpita 2011; Endler et al. 2011; Domon et al. 2012; Lei et al. 2012). As revealed by genomic sequencing, Arabidopsis, rice, and sorghum each possesses 10 CesA genes; whereas, maize possesses 20 CesA genes (http://cellwall. genomics. purdue. edu/) (Penning et al. 2009). The Arabidopsis CESA proteins as well as CESAs that have been studied in grasses are listed in Table 1. Recent results highlight the utility of understanding cellulose synthesis for enhancing biofuel production. DeBolt and colleagues reported that mutations in the C-terminal transmembrane domain region of Arabidopsis CESA1 and CESA3 proteins decrease microfibril crystallinity and increase saccharification efficiency (Harris et al. 2012).

Mutant studies, mostly in Arabidopsis, have greatly enhanced our understanding of physiological functions of CESA family members. Arabidopsis AtCESA1, AtCESA3 and AtCESA6 are mainly responsible for cellulose synthesis in primary walls. Mutations in the genes that synthesize these proteins present dwarfism, sterility, swollen etiolated hypocotyls, ectopic accumulation of lignin and reduced root elongation phenotypes (Endler et al. 2011). On the other hand, mutations in AtCesA4, 7 and 8 genes affect cellulose synthesis in secondary walls and are accompanied by collapsed xylem vessels and significant decreases in cellulose contents (Endler et al. 2011). Researchers have hypothesized that the longer and more crystalline microfibrils of secondary walls might be due to different CESA components in the cellulose synthase complexes. However, recent studies have detected expression of the "primary wall" cellulose synthase genes, AtCesA1, 3 and 6, in both primary and secondary cell walls of Arabidopsis (Betancur et al. 2010). Furthermore, AtCESA2,5 and 9, which have greatest homology to CESA6, have been found to be involved in the biosynthesis of secondary cell wall in the epidermal seed coat (Stork et al. 2010; Mendu et al. 2011; Sullivan et al. 2011). These findings suggest that much more remains to be learned about the mechanism of the cellulose synthase complex.

Though phylogenetic analyses are common, direct studies of cellulose synthesis in grasses are less so. Still, mutations in the rice secondary wall cellulose synthases, OsCesA7, OsCesA4 and OsCesA9, respectively, lead to a brittle culm phenotype caused by thinner cortical fiber cell walls (Tanaka et al. 2003). Also, prior to the availability of the maize genome and a more complete description of the maize CesA family, Appenzeller and colleagues isolated 12 CesA genes from maize. Among them, ZmCesA10, 11, 12 are the probable orthologs of the Arabidopsis secondary cell wall CesA genes (Appenzeller et al. 2004). As expected, numerous expressed sequence tags for switchgrass cellulose syntheses appear in databases (Tobias et al. 2008), though a detailed naming scheme and apportionment into families has not yet been conducted, and may be premature at the time of this writing due to the draft nature of the genome.