Biology Reference
In-Depth Information
Different MYB TFs (AtMYB4, EgMYB1, and ZmMYB31/42) have also
been shown to function as repressors of lignin biosynthesis (
Fornale et al.,
2006, 2010; Jin et al., 2000; Legay et al., 2010
). For example, ChIP assays
indicated that the ZmMYB31 protein interacts directly with the ZmCOMT
and ZmF5H genes in vivo (
Fornale et al., 2010
). Over-expression of the
ZmMYB31 gene in arabidopsis induced a dwarf phenotype, reduced lignifi-
cation, and downregulated several lignin biosynthesis genes. Similarly, over-
expression of EgMYB1 in poplar and arabidopsis is associated with reduced
lignin and downregulates several lignin biosynthesis genes (
Legay et al.,
2010
). However, in these studies laccases gene expression was not deter-
mined. It would obviously be of interest to see whether MYB proteins
capable of repressing lignin biosynthesis genes are also capable of directly
inhibiting laccases gene expression. The observation that potential binding
sites for the MYB4 repressor are statistically over-represented in arabidopsis
laccases promoters could support such a hypothesis. Differential expression
of laccases genes, as well as monolignols biosynthesis genes has also been
implicated in the natural hypolignification of flax bast fibres (
Fenart et al.,
2010
;
Huis et al., 2012
). Flax bast fibres, present in outer stem tissues of this
species possess thick cellulose-rich secondary cell walls and are naturally
hypolignified in contrast to inner stem (xylem) tissues that are normally
lignified (
Day et al., 2005
). Recent comparative transcriptomics of flax stem
inner- and outer-tissues show that transcripts of lignin biosynthesis and
laccases genes are significantly more abundant in lignified inner tissues than
in outer-tissues suggesting that bast fibre secondary cell wall hypolignifica-
tion is regulated at both the monolignols production step and at the poly-
merization step. Interestingly, a MYB TF (LuMYB4) belonging to the
subgroup 4 that includes the AtMYB4, EgMYB1 and ZmMYB31/42 repres-
sors was significantly upregulated in stem outer-tissues. Although functional
analyses are obviously necessary, it is possible that this MYB factor might
play a role in flax bast fibre hypolignification.
The expression of MYB genes is regulated by other TFs. For example,
AtMYB52 and AtMYB7 are directly activated by AtMYB46 (
Ko et al.,
2009
). MYB genes can also be regulated by other TFs and the AtMYB46
gene has been shown to be a direct target of the NAC (NAM, ATAF1,
and CUC2)factorSND1 (SECONDARYWALL-ASSOCIATEDNAC DO-
MAIN PROTEIN1) (
Zhong et al.,2007
). NAC proteins contain a conserved
domain, the NAC domain, and are encoded by a large multigene family
consisting of approximately 100 genes in arabidopsis (
Olsen et al.,2005
).
RNAi inhibition of AtSND1 and its homolog AtNST1 in arabidopsis down-
regulates the expression of the lignin-specific AtMYB58 and AtMYB63 while
overexpression of AtSND1 leads to up-regulation of AtMYB58 (but not
