Maize - Bioenergy Crop Plants - page 417

On the basis of thioacidolysis, the most lignified organs in maize at silking are the basal internode
and roots and the least lignified tissues are leaves (Guillaumie et al. 2007). In the basal internode,
with the exception of a few parenchyma layers inside peripheral sclerenchyma, all cell types are
lignified, and the predominant lignin is of the S type. Detailed histochemical analyses of different
maize tissues in this study have shown that roots of plants at the four- to five-leaf stage contain a wide
variety of lignified cells, except for the single layer of hypodermal cells just below the epidermal
layer, and that these lignified cells are rich in S units. At the same growth stage, leaf tissue was
found to be characterized by a large proportion of sclerenchyma cells that likely synthesize H and/or
G units, whereas the parechyma cells associated with the xylem vessels are rich in S units. In maize
leaf parenchyma cells, deposition of G lignin has been shown to precede that of S lignin, whereas
in vascular bundles S lignin is deposited at the earliest stages of lignification (Wen et al. 2008). In
general, grasses have been shown to deposit large amounts of ferulate-polysaccharide esters during
the early stages of lignification, and the ferulate esters likely serve as nucleation sites for lignin
polymerization (Zhong et al. 2008; Zhou et al. 2009).
In maize, as in dicots, most of the lignin biosynthetic enzymes are encoded by multigene families
(Guillaumie et al. 2007). So far, five sequences have been identified for the phenyl ammonia lyase
(PAL) enzyme in maize, which has activity toward tyrosine and phenylalanine. These can be
grouped into classes I, II, and III, with two sequences each within classes I and III and one in class
II. All are expressed at higher levels in stems and roots, with relatively low expression in leaves, and
the highest expression being in the sixth internode at silking. The expression pattern agrees with the
role of PAL in secondary wall formation. The class II PAL gene has the highest expression of all
three classes in all tissues, followed sequentially by class I and III genes.
Polymerization of monolignols occurs through the action of peroxidases and laccases. Thirteen
maize peroxidase sequences have been reported in the Maize Genetics and Genomics database
(Guillet-Claude et al. 2004), but only three have been characterized. Of the three, ZmPox1 occurs
in the epidermal cells of root tips where lignification does not occur, whereas ZmPox2 and ZmPox3
are observed in vascular tissues of the elongation zone of young roots, with the expression of
ZmPox2 being much more abundant than that of ZmPox3 . Analysis of a retrotransposon insertion
in exon 2 of the ZmPox3 gene provided further evidence for the role of this gene in monolignol
polymerization (Guillet-Claude et al. 2004). A deficiency in ZmPox3 activity has a negative effect
on cell wall digestibility. Genetic diversity analysis of this peroxidase indicates that it could be a
relevant target for improving digestibility through the use of specific allele introgressions (Guillet-
Claude et al. 2004).
16.5 cell Wall cross-lInkInG and mechanIcal strenGth
Lignin has long been a target for reduction because of its known adverse effect on rumen digestibility
as well as on ethanol production from stover biomass (Grabber 2005). The cross-links that lignin
forms with other wall polymers are believed to increase the recalcitrance of vegetative tissues to
hydrolytic enzymes (Jung 2003; Jung and Casler 2006).
Activated forms of the hydroxycinnamic acids ferulate and p -coumarate are, respectively,
feruloyl-CoA and p -coumaroyl-CoA, which, aside from being intermediates in the monolignol
biosynthetic pathway, act as substrates for feruloylation or coumaroylation of GAX (Campbell
and Sederoff 1996; Anterola and Lewis 2002). Ferulate and coumarate are linked to the
arabinosyl residues of GAX through an ester linkage in the Golgi compartment by a transferase
that remains to be identified (Figure 16.9; Iiyama et al. 1994). These feruloyl moieties can form
linkages with other reactive groups in the cell wall through a peroxidase-mediated reaction that
results in GAX-GAX and GAX-lignin linkages (Iiyama et al. 1994). Some cross-linking may
also occur in the Golgi itself (Fry et al. 2000). Another type of cross-linking is hypothesized to
occur among wall polysaccharides through a class of enzymes that have the ability to cut and
paste glycans (Fry 2004). A transglycosylase/hydrolase has recently been reported to mediate