Biology Reference
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are less digestible than expected. Lignins of bm3 mutants appear to inhibit
proportionally more cell wall digestibility than normal lignin (
Thorstensson
et al., 1992
). Plants deficient in OMT1 using an RNA interference-mediated
silencing strategy (
Tu et al., 2010
) caused reduced levels of S units without
significant changes in G unit levels that coincide with the incorporation of
5-hydroxyconiferyl alcohol into the lignin polymer. Acetyl bromide-soluble
lignin content was reduced by 6% in transgenic lines when compared to WT.
A similar phenotype, characterized by a reduced level of S lignin and a lower
S/G ratio, is seen in transgenic tall fescue plants in which the COMT1a gene
has been downregulated (
Chen et al., 2004
). Transgenic maize and tall fescue
lines show enhanced cell wall digestibility. The combined results strongly
support a key role for the OMT1 gene product in the biosynthesis of both
S and G lignin units in perennial ryegrass. Field-grown OMT1-deficient
perennial ryegrass plants showed enhanced digestibility without obvious
detrimental effects on either plant fitness or biomass production. This high-
lights the potential of metabolic engineering not only to enhance the forage
quality of grasses but also to produce optimal feedstock plants for biofuel
production.
Noteworthy, five Brachypodium mutations recently identified in the same
COMT gene (Bradi3g16530) exhibit a strong decrease in S/G ratio that also
accumulate 5-OH G units with a 20% decrease of Klason content. Notewor-
thy, mutants displayed no obvious bm phenotype as detected in bm3 or bm12
lines (R. Sibout, unpublished data). If confirmed, this result would highlight
some differences in metabolisms between C3 and C4 plants.
D. CINNAMYL ALCOHOL DEHYDROGENASE (CAD)
CAD catalyses the last step of monolignol biosynthesis by reducing cinnamal-
dehydes into cinnamyl alcohols prior to their transport through the membranes
and polymerization in the cell wall. There are seven CADs in Brachypodium
with a unique ortholog similar to other known CAD genes involved in lignifi-
cation of dicots. Only two similar CADs were identified in maize.
CAD activity was first characterized in the maize bm1 mutation. The loss
of bm1 resulted in the reddish-brown color in leaf midribs and stem scleren-
chyma and was linked to a reduced CAD activity and accumulation of
cinnamaldehydes (
Halpin et al., 1998
). Brown-midrib mutants were also
identified in sorghum, another C4 plant (
Bucholtz et al., 1980; Palmer
et al., 2008
). The bm mutants of sorghum are characterized by brown
vascular tissue associated with altered lignin content (with few exceptions).
Interestingly, three allelic bm mutants display mutations in SbCAD2 gene.
Thanks to the sorghum genome release,
Saballos et al. (2009)
showed that
