Biology Reference
In-Depth Information
In vitro COMT is able to O-methylate caffeic acid at the C3 position to produce
FA despite the lack of convincing data. In addition, it is unclear whether free
caffeic acid is available to be methoxylated by COMT in grasses. Interestingly,
the same enzyme is able to accept different substrates and is involved in other
metabolic processes such as sinapate esters and isorhamnetin biosynthesis in
Arabidopsis (
Goujon et al., 2003; Muzac et al., 2000
). However, the capacity of
grass COMT to accept a substrate other than coniferaldehyde has not been
shown. While there are four COMT genes clearly identified in Brachypodium,
the ZmCOMT is a unique gene in maize.
The most striking effects on lignin composition in plants deficient in
COMT were shown in maize bm3 mutant lines (
Barri
`
re et al., 2004a; Kuc
and Nelson, 1964
). A quasi-lack of COMT activity was thus established in
maize bm3 plants (
Grand et al., 1985
), corresponding to an modified exon
2 in the COMT sequence (
Morrow et al., 1997; Vignols et al., 1995
). When
harvested at the silage maturity stage, bm3 plants have reduced lignin content
by 25-40%, a lower frequency of S lignin units, and incorporated in their
lignin 5-hydroxyguaiacyl (5-OH G) units in benzodioxane structures however,
the reduced lignin content did not affect the yield of G thioacidolysis monomers
(
Barri`re et al., 2004a; Lapierre et al., 1988
). Noteworthy, although COMT is
completely deficient in bm3 maize mutants, S lignin units are heavily depleted
but not absent suggesting that other, still unknown enzyme(s), may be involved
in the methylation of 5-hydroxyconiferaldehyde.
An involvement of a CCoAOMT might be hypothesized, but 'there is as
yet no report of the activity of this enzyme towards the aldehydes or alcohols'
(
Anterola and Lewis, 2002
). Therefore, ZRP4-like OMT may be a potential
candidate in regulating this step. At least 11 ZRP4-like OMTs are present in
the maize genome whose roles are likely not limited to methylation of suberin
subunit precursors in plant roots as initially described by
Held et al. (1993)
.
Bm3 plants also have fewer p-coumarate esters, a reduction consistent with
the preferential p-coumaroylation of S units. Conversely, bm3 plants were
shown unaltered in their content of alkali-releasable FA. This result strength-
ens the hypothesis that the COMT gene is not involved in FA biosynthesis,
with a methylation occurring in grass on caffeoyl-CoA and feruloylation of
arabinoxylans from feruloyl-CoA. Finally, transgenic maize lines in which
COMT was downregulated (
He et al., 2003; Pichon et al., 2006
), or bm12 and
bm18 sorghum mutant with a disrupted COMT (
Bout and Vermerris, 2003;
Oliver et al., 2004; Palmer et al., 2008
), showed lignin levels and composition
profiles that were comparable with maize bm3 plants.
A significant improvement in cell wall digestibility was shown in bm3
plants (
Ballard et al., 2001; Barnes et al., 1971; Barri`re and Argillier, 1993;
Barri`re et al., 2004b
); however, relative to their lignin content, bm3 plants
