Maize - Bioenergy Crop Plants - page 421

The genes responsible for the bm2 and bm4 mutations have not been identified yet. In both the
cases, the amount of lignin and ferulate ether is reduced, whereas the amounts of p -coumarate and
ferulate esters are slightly increased.
Several studies on QTL analyses for cell wall digestibility and lignification in maize have been
reported (Mechin et al. 2001; Roussel et al. 2002; Cardinal et al. 2003; Barrieire et al. 2004). Five major
QTL clusters can be found in bins 6.06, 3.05/06, 1.03, 8.05, and 9.02, whereas additional locations and
clusters involved in these traits have been located in nine other bins. The genes underlying these QTL
have not been identified yet, although some of the lignin biosynthesis genes colocalize with these
QTL. The QTL for cell wall digestibility colocalize with lignification and cross-linking traits, and in
several cases they also colocalize with QTL for tolerance to European corn borer. Thus, breeding for
improved digestibility through modification of lignin biosynthesis should consider the implications
to the plant's defense against insect pests. Yet, two important QTL for cell wall digestibility (i.e., bins
1.03 and 6.06) that do not colocalize with any published lignin-biosynthetic pathway sequence do not
show co-localization with European corn borer tolerance either.
16.6.2 B ioEnErgy and a gronomic t raitS
The goal of higher ethanol productivity, if only looked at from the viewpoint of reducing lignin,
is at odds with that of stalk standability, an important agronomic trait. Vascular bundles and
sclerenchymatous cells have lignified, cellulose-rich walls. Lignin is generally believed to contribute
to mechanical strength; however, it plays a role in compression rather than tensile strength (Ching et al.
2006). To maximize mechanical resistance to breakage, for example, cellulose microfibrils in the stem
tissue subjected to mechanical bending are oriented along the axis of tension (Green 1962). Similarly,
cellulose microfibrils are oriented perpendicular to the axis of growth in an expanding cell, again
to maximize radial strength so as to allow longitudinal expansion (Green 1962). Lignin synthesis is
downregulated in the cells subjected to tension but is upregulated in those subjected to compression,
suggesting that it is not lignin but cellulose that is primarily responsible for tensile strength (Donaldson
et al. 1999; Joseleau et al. 2004; Andersson-Gunneras et al. 2006; Schmitt et al. 2006).
The relationship between planting density and biomass increase is nonlinear, particularly at
higher densities, i.e., biomass increase is less than would be expected from the number of plants per
unit land area. Eventually, the reduction in individual plant biomass that accompanies increasing
planting density will make the plants mechanically unstable to the extent that any gain in grain yield
will be negated by an increase in crop lodging (Appenzeller et al. 2004). At that point, about the
only option left to the breeders would be to sacrifice HI to maintain standability, which again will
offset the gain in grain yield from increased planting density (Dhugga 2007).
16.7 systems aPProach In ImProvInG maIze BIoProductIvIty
Plant breeders, by selecting for grain yield, essentially use a systems approach because they
indirectly select simultaneously for many gene combinations that underlie the desired phenotype.
However, it is challenging to model plant performance using theoretical knowledge of the
metabolic pathways because of the enormous plasticity of central metabolism (Carrari et al. 2003).
Grain yield and biomass are complex traits that result from an integration of many developmental
steps during the ontogeny of the plant. Throughout its development, a plant is exposed to variable
environmental factors, which vary across geographic locations. Additional unpredictability results
from the year-to-year variation of these environmental factors. For example, moisture content of the
soil and soil temperature can affect germination, which eventually manifests in the final crop stand
in the field. Available nutrients in the soil can vary depending upon the soil type and unpredictable
moisture content, which affects plant growth at different stages of development. Mineralization,
which means release of organically bound nutrients for utilization by the plant, also varies as a