Maize - Bioenergy Crop Plants - page 424

“nose-backing,” which is indicative of sink not being limiting. This apparently ensures capturing of the
maximal amount of available photosynthate for grain production. Biomass of space-planted individual
plants or trees is not thus an indicator of the potential biomass production per unit land area.
Complexity of the cellulose synthase system poses a challenge to increasing its activity through
transgenic manipulation. For an overexpressed component to have an effect, a parallel increase
in the expression of all other components of the complex must be assumed (Sere 1987; Barreiro
and Dhugga 2007). Alternatively, the component being overexpressed must indeed be limiting in
the assembly of the cellulose synthase complex. Regardless of these apprehensions, preliminary
experiments suggest that it may be possible to affect cellulose synthesis with the overexpression of
individual CesA genes (Dhugga et al. 2005).
Association of the rate of cellulose synthesis or tissue cellulose concentration with available
haplotypes and specifically with allelic variants of different genes known to be involved
in cellulose formation offers perhaps more promise in accomplishing the objective of increasing
cellulose in plant tissues. The associated haplotypes could then be used to select for increased
cellulose production in breeding populations. Crystalline cellulose can be determined by a
simple chemical method that is amenable to scaling-up for high throughput analysis (Updegraff
1969). Direct measurement of cellulose would allow for the mapping of this trait using recombinant
lines derived from divergent parents, which might also reveal as yet unknown effectors of
cellulose formation. Mutational genetics has revealed a number of non- CesA genes to be involved
in cellulose formation.
Upregulation of cellulose synthesis may eventually lead to altered composition of stover by
competing with GAX and lignin in secondary walls, which account for most of the biomass.
For this approach to affect biomass production, vegetative sink, which may be approximated
as an aggregate of nonphotosynthetic, nongrain plant parts, must be assumed to be limiting in
the current varieties. HI in maize is held nearly constant even when grain yield varies widely
under varying environmental conditions, suggesting that the source-to-grain sink ratio is already
optimized (Sinclair et al. 1990).
Overexpression of a cellulose-binding domain (CBD) from microbial cellulose hydrolases in
poplar has been claimed to enhance growth (Levy et al. 2002). Microbial and fungal cellulases
possess a CBD and a hydrolytic domain. CBD enhances hydrolysis of cellulose by acting as an
anchor for the hydrolytic domain and also perhaps by depolymerizing the cellulose crystal into
glucan chains. Separated from the hydrolytic domain, CBD still retains the ability to bind crystalline
cellulose. The explanation for enhanced growth rate in poplar overexpressing CBD was that by
binding to the nascent glucan chains coming out of the cellulose synthase complex, CBD slowed the
rate of crystallization and thus enhanced the rate of polymerization (Levy et al. 2002). The same
group claimed that overexpression of an endo-β-1,4-glucanase accelerated growth in tobacco and
Arabidopsis . Implicit in these studies is the assumption that carbon supply (source) is nonlimiting
and only cellulose polymerization or wall properties limit growth. Follow-up long-term studies are
not available yet, so it is not possible to determine if the reported increase in cellulose synthesis or
accelerated growth indeed resulted in increased biomass per unit land area per unit time.
Disruption of the enzymes that potentially add side chains onto the xylan backbone of GAX causes
a severe disruption in secondary wall synthesis (Zhong et al. 2005). A number of fragile fiber ( fra )
and irregular xylem ( irx ) mutants are caused by defects in genes encoding the enzymes potentially
involved in the addition of a side group onto the xylan backbone (Burk and Ye 2002; Zhong et al.
2004, 2005; Persson et al. 2007). However, complete knockouts caused severe phenotypes, and it
remains to be seen whether partial downregulation of these genes can quantitatively reduce GAX
content without adversely affecting plant growth (Burk and Ye 2002; Zhong et al. 2004, 2005;
Persson et al. 2007).
A candidate gene for feruloylation of GAX has been proposed based on comparative genomics
(Mitchell et al. 2007). It can be downregulated by transgenic means to determine whether it reduces
the degree of cross-linking in the wall and, if so, whether and to what extent it is biologically