Maize - Bioenergy Crop Plants - page 420

Theoretical conversions, which help define upper limits for productivity of biomass of a particular
composition, do not always lead to expected results and may require substantial breeding effort in
selecting desirable recombinants (Dhugga and Waines 1989). For example, a reduction in lignin is
expected to be accompanied by an increased bioproductivity but an opposite outcome is observed
because of the associated pleiotropic effects (Figure 16.11) (Pedersen et al. 2005).
Increasing starch in the grain is yet another option, but the starch content of the grain is already
high at more than 70% (Earle et al. 1946; Belyea et al. 2004). Additional starch can be accumulated
at the expense of embryo size, grain protein in the endosperm, and/or grain fiber. Reductions in
endosperm protein as well as embryo size below a certain threshold may cause viability problems
and a reduction in fiber content may affect grain strength, causing increased breakage. Most of
the fiber in the grain is in the pericarp, which is rich in GAX (Hazen et al. 2003). Incrementally
replacing GAX with cellulose, which is mechanically stronger but also more crystalline, may
allow for maintenance of grain strength yet allocate more carbon to additional starch formation
(Dhugga 2007).
16.6.1 m utantS r ElEvant to B ioEnErgy
Improved cell wall degradability through the alteration of lignin content and/or composition has
been observed in the naturally occurring brown midrib mutants of maize, the understanding of
which has led to the exploration of transgenic manipulation to improve digestibility. Several QTL
have been identified for cell wall lignin content, the amount of p -coumarate and ferulate esters, and
for digestibility. Some of these overlap with QTL for insect tolerance and with the localization of
lignin biosynthetic genes. Brown midrib mutants have been reported in maize, sorghum, and millet,
in which tissues associated with lignification, such as leaf midrib, exhibit a reddish brown coloration
(Barrieire et al. 2004). This coloration is intrinsic to the lignin polymer itself because it remains in
the tissue after soluble metabolites and cell wall polysaccharides are removed. The actual reason for
the coloration and the interesting fact that, to date, bmr mutants have only been reported in diploid
Panicoideae C4 grass species, are aspects that remain unanswered. The bmr mutants reported in
maize are naturally occurring, whereas those in sorghum and millet were obtained through chemical
mutagenesis. Four bmr mutants are known in maize and 19 in sorghum.
The most well characterized of the bmr mutants of maize is the bm3 mutant, where the
mutation is known to be caused by a partial reduction in the activity of the enzyme, cinnamoyl
o -methyltransferase (COMT) (Morrow et al. 1997). The bm3 mutant exhibits a reduction in lignin
content on the order of 25-40%, a reduction in the S unit composition, and also the occurrence of
unusual lignin units, specifically, 5-hydroxyguaiacyl units (Barrieire et al. 2004). The S/G ratio is
reduced in the bm3 mutants by more than 3-fold. These mutants have fewer p -coumarate esters,
which is consistent with the preferential acylation of S units by p -coumaric acid, and noticeably
elevated levels of ferulate in younger tissues (Marita et al. 2003). Disruption of the COMT gene
in the bm3 mutant has been shown to result in an overexpression of other monolignol biosynthetic
genes in the basal and ear internodes, with the exception of PAL (Guillaumie et al. 2008). These
mutants also showed overexpression of several transcription factors, cell signaling genes, and
transport and detoxification genes in these tissues. Of the four bm mutants reported in maize, bm3
causes most improvement in plant stover digestibility.
The maize bm1 mutation reduces the expression of the cinnamoyl alcohol dehydrogenase
(CAD) gene, the activity of which is reduced 60-70% in aerial tissues and 90-97% in roots
of these mutants (Halpin et al. 1998). The amounts of lignin, p -coumarate esters, and ferulate
ethers are significantly reduced in the stem of bm1 mutants, whereas the amount of ferulate
esters is only slightly reduced. There is no alteration in the lignin composition of bm1 mutants,
and the amount of S, G, and H units remain similar to that of the wild type. The lignin of bm1
mutant incorporates coniferaldehyde and sinapaldehyde units as a result of CAD deficiency
(Barrieire et al. 2004).