Biology Reference
In-Depth Information
A. 4-COUMARATE: COENZYME A LIGASE (4CL)
4CL catalyses the formation of activated thioesters of hydroxycinnamic acids.
Although 10 genes were identified in Brachypodium, only five of them are
similar to 4CL genes from other plants whereas two of these genes were found
in maize. Two homologous 4CL genes, Pv4CL1 and Pv4CL2,wereidentified
in switchgrass (P. virgatum) through phylogenetic analysis (
Xu et al., 2011
).
RNA interference of Pv4CL1 in switchgrass reduced extractable 4CL activity
by 80%. The transgenic T1 plants had similar S content but 47% less G content
and 45% more H content than non-transgenic control T1 plants. The dilute
acid-pretreated samples exhibited enhanced enzymatic digestibility for glucan
yield but not for xylan yield. The low lignin transgenic plant materials yielded
57.2% more fermentable sugar than the wild-type (WT) plant. Interestingly,
transgenics showed 'browning' on parts of the leaf mid-vein as observed
in brown midrib (bm) mutants. Roots were also reddish-brown at mature
stages. It is thus possible that some bm mutants with unknown function are
affected in 4CL genes.
B. CINNAMOYL-C
O
A REDUCTASE (CCR)
CCR catalyses the conversion of cinnamoyl-CoAs into their corresponding
cinnamaldehydes. In general, it is accepted that CCRs involved in lignifica-
tion use feruloyl-CoA as substrates, mainly in grasses (
Escamilla-Trevino
et al., 2010; Ma, 2007
). This enzyme is considered a potential control point
in phenylpropanoid pathway and since it is the first committed step of lignin-
specific pathway, it may regulate the overall carbon flux towards lignin pro-
duction and balance the abundance of phenolic compounds. However, until
recently, this role was poorly investigated in grasses. There are nine CCR genes
in Brachypodium (only one shows high similarities with known CCR genes
involved in lignification) and two genes are detectable in maize.
By screening a Mu insertional mutant collection of 27,500 maize lines,
Tamasloukht and colleagues isolated a mutant in the CCR1 gene named
Zmccr1 (
Tamasloukht et al., 2011
). This mutant carries a transposon in the
first intron, and CCR1 gene expression is reduced to 31% of the residual
WT level. Zmccr1 exhibited 10% Klason lignin (KL) reduction and
decreased H unit content when compared to the WT. The authors also
observed a delay in lignification of fibres in addition to cell shape modifica-
tions as sclerenchyma cells appeared more oblong in Zmccr1 mutants.
These modifications are associated with a modest but significant improve-
ment of polysaccharide cell wall degradability (4% increase of digestibility
using neutral detergent fibre (dNDF) method). Interestingly, field-grown
