Maize - Bioenergy Crop Plants - page 414

CesA was first isolated from the bacterium Acetobacter xylinum by enzyme purification and peptide
sequencing (Saxena et al. 1990). Similar attempts at identifying plant polysaccharide synthases
achieved little success, although a solubilized callose synthase was purified (Meikle et al. 1991;
Dhugga and Ray 1994). The first plant CesA gene was isolated from developing cotton fibers by
screening a few hundred expressed sequence tags (Pear et al. 1996). The CesA gene family, which
is represented by at least 10 members in various plant species, has at least 13 members in maize
(Appenzeller et al. 2004; Djerbi et al. 2005; Somerville 2006). Arabidopsis , maize, and rice all have
a group of three different CesA genes, that are co-expressed in secondary cell wall-forming cells
(Tanaka et al. 2003; Taylor et al. 2003; Appenzeller et al. 2004).
The CesA genes are believed to encode plasma- membrane-localized catalytic subunits of
the rosette complex. These transmembrane complexes are assembled as hexamers, presumably
consisting of 36 individual cellulose synthase proteins. After assembly in the Golgi apparatus, the
cellulose synthase complexes are exported for integration into the plasma membrane (Somerville
2006). Cellulose is synthesized on the cytosolic side and is subsequently extruded out to the plasma
membrane through the pore made by the predicted transmembrane helices of the CesA protein. CesA
proteins consist of two transmembrane domains (TMDs) near the amino-terminal (N-terminal) end
and six TMDs toward the carboxy-terminus. The zinc-binding N-terminal domain is putatively
involved in CesA-CesA interactions (Kurek et al. 2002). The cytoplasmic catalytic domain between
TMD2 and TMD3 contains the signature of β-glycosyltransferases, D,D,D,QXXRW (Saxena et al.
1995; Vergara and Carpita 2001).
Several other proteins are known to affect cellulose synthesis: a membrane-associated protein,
Kobito, of unknown function; Korrigan, a membrane-anchored β-glucanase; and Cobra, a protein
attached to the membrane through a glycophosphatidylinositol (GPI) anchor (Somerville 2006). A
member of the Cobra gene family, Brittle culm-1 ( Bc-1 ), was found to specifically affect secondary
wall formation in rice (Li et al. 2003). Its orthologs from Arabidopsis [ Cobra-like-4 ( CobL4 )] and
maize [ Brittle stalk-2 ( Bk2 )] performed orthologous function in respective species (Brown et al.
2005; Ching et al. 2006; Sindhu 2007). Two other enzymes or classes of enzymes that are also known
to affect cellulose formation are GDP-mannose pyrophosphorylase, that forms GDP-mannose,
and glycosydases. GDP-mannose contributes mannose for the formation of the GPI anchor of the
Cobra-like proteins, glycosylation of other proteins, and glucomannan formation. Glycosidases are
involved in the glycosylation processing of proteins. Both of these enzymes can thus potentially
affect cellulose formation through multiple mechanisms.
Secondary wall-forming genes are in general expressed at significantly higher levels than
the primary wall-forming genes, which could explain the rapid deposition of secondary wall on
cessation of cell expansion, assuming that there is a correspondence between the levels of CesA
proteins and their respective transcripts (Appenzeller et al. 2004).
16.4.2 h EmicElluloSE S ynthESiS
The hemicellulosic fraction, unlike pectin, is tightly associated with cellulose and requires harsh
chemical treatment to be separated from the cellulosic fraction. Hemicellulosic polysaccharides
are divided into four major categories: xyloglucans (XG), (gluco)mannans, glucuronoarabinoxylans
(GAX), and mixed-linkage glucans (MLG) (Carpita 2000; Somerville et al. 2004). Corn stover
biomass is made of cellulose microfibrils embedded in a hemicellulosic matrix consisting mainly of
GAX, a β-1,4-linked xylan backbone that is substituted with glucuronosyl and arabinosyl residues
with α-1,2 and α-1,3 linkages, respectively (Figure 16.9) (Carpita 1996). GAX also contains acetate
that is esterified to the xylosyl residues at the second and third positions. In addition to GAX, grass
cell walls contain MLG, which consists of β-1,4-linked oligosaccharides of varying lengths coupled
through β-1,3-linkages. MLG is expressed in expanding cells and is recycled as expansion ceases,
whereas it serves as a structural polysaccharide in the walls of the seed endosperm of certain grasses,
such as barley, oats, and wheat (Genc et al. 2001). Because the β-1,4 linkage of cellulose is similar