Maize - Bioenergy Crop Plants - page 416

often referred to as glucan synthase-I (GS-I) (Ray 1979). It is possible that CslC is actually GS-I
(Sandhu 2009). Adjacent glycosyl residues of the surface chains in cellulose microfibrils are
twisted with respect to each other in comparison to the generally linear arrangement of β-1,4 -
linked residues (Vietor et al. 2002). The CslC product upon deposition after microfibril assembly,
which happens at the plasma membrane, could potentially acquire the altered conformation.
A different route was taken to determine the function of two groups of genes specific to grasses—
CslF and H . Both were found to make MLG (Burton et al. 2006; Doblin et al. 2009). MLG was
mapped in barley, and a syntenic relationship between rice and barley helped to identify a cluster of
CslF genes (Burton et al. 2006). Heterologous expression of a CslF gene in Arabidopsis , a species
with no CslF and no MLG, led to the accumulation of MLG in the cell wall. Following a similar
approach, expression of a CslH gene from barley in Arabidopsis also resulted in the accumulation
of MLG (Doblin et al. 2009).
Molecular components for xylan synthase, as well as arabinosyl and glucuronosyl transferases
that add respective residues onto the xylan backbone of GAX, remain unknown. Maize stover
contains 3-5% acetate and approximately 20% xylose on a mass basis (Wooley et al. 1999;
McAloon et al. 2000). Molecular components for acetylation of GAX also remain to be identified.
Apparently, acetylation occurs in the Golgi compartment and the transferase that acetylates GAX
may use acetyl-CoA as a substrate. An acetyl transferase that acetylates rhamnogalacturonan has
been assayed in cultured potato ( Solanum tuberosum L .) cells, but the corresponding protein(s) and
gene(s) remain to be isolated (Pauly and Scheller 2000).
16.4.3 l lignin S ynthESiS
Lignin, a major component of the secondary cell wall of vascular plants, is the second-most abundant
macromolecule on earth after cellulose (Boerjan et al. 2003). Being hydrophobic, it most likely
contributes to the mechanical strength of the tissues by excluding water from the vicinity of cellulose,
the strength of which varies with moisture content (drier the stronger). Lignin has high resilience to
degradation, a characteristic that is desirable for defense against insects and pests but poses a problem
in the production of cellulosic biofuels, forage digestibility, and chemical pulping. Its distinctive
polymeric structure is the reason for its non-degradable nature. In addition, because of its covalent
linkages in the wall and its role as a matrix for the adsorption of enzymes and proteins, lignin hinders
accessibility of the hydrolytic enzymes to cellulose and hemicellulose in the cell wall matrix (Iiyama
et al. 1994). Pretreatment of lignocellulosic biomass to remove lignin from cellulose and hemicellulose
for biofuel production and paper pulping is a costly step that requires noxious chemicals and results in
the formation of compounds that inhibit downstream processes in ethanol production.
Lignin is also of significant interest for silage and forage digestibility in dairy cattle. The
high content of cell wall in forages, together with the restriction imposed by lignin on cell wall
digestibility, creates a disparity between the available energy of grain and forage although the gross
amount of energy per unit dry matter is comparable between the two (Ralph et al. 2004). Although
70% of the energy value of silage maize is derived from the grain, there exists significant genetic
variation in the extent of digestibility in maize and other forage grasses, and even among different
maize genotypes. These genetic differences are exemplified in the brown midrib ( bmr ) mutants of
maize and sorghum, in which natural or chemically induced mutations affect lignin content and
composition and ultimately cell wall digestibility.
Lignin is a heteropolymer derived from monomeric units of p -coumaryl, coniferyl, and
sinapyl alcohols that give rise, respectively, to p -hydroxyphenyl (H), guaiacyl (G), and syringyl
(S) phenylpropanoid units upon incorporation into the lignin polmer (Boerjan et al. 2003). The
monolignols are transported to the cell walls, where dehydrogenative polymerization results in
lignification. The three monolignols are generally incorporated during specific stages of cell-wall
formation, with H units being deposited first, followed by G and finally S units. In maize, young
tissues preferentially accumulate G lignin, whereas the content of S lignin increases with maturity.