Biology Reference
In-Depth Information
to pathogens, VvWRKY2, is specifically expressed in cells undergoing lignifica-
tion in stems and was proposed to play a role in regulating lignification in
grapevine possibly in response to biotic or abiotic stresses (
Guillaumie et al.,
2010
). Transgenic tobacco overexpressing VvWRKY2 showed a decrease of the
S/G ratio and altered expression of genes involved in the lignin biosynthesis
pathway and cell wall formation. The ability of VvWRKY2 to activate the
promoter of the VvC4H gene was confirmed by transient transcriptional acti-
vation assays in tobacco protoplasts (
Guillaumie et al.,2010
).
Interestingly,
Ambavaram et al. (2011)
discovered a novel function for the
SHINE/WAX INDUCER (SHN/WIN) member of the APETALA2/
ETHYLENE RESPONSE FACTOR (AP2/ERF) TF family, previously
shown to be involved in wax/cutin lipid regulation and drought tolerance
in Arabidopsis. They showed that expression of an AtSHN in rice (Oryza
sativa) causes a 34% increase in cellulose and a 45% reduction in lignin
content. Using a detailed systems-level analysis of global gene expression in
rice, the authors proposed that SHN/WIN represses the SND1/NST1/NST2
and VND6 NAC TFs, and also directly represses AtMYB58 and AtMYB63
and directly activates AtMYB20 and AtMYB43. Due to these three activities,
SHN/WIN can bypass the main switches (NACs) and selectively up- and
downregulates downstream TF (MYBs) specific to lignin biosynthetic genes
and other SW components like cellulose (
Ambavaram et al., 2011
).
VI. THE SPECIFIC REGULATION
OF THE SYRINGYL PATHWAY
Among higher plants, S lignin is generally considered to be restricted to
angiosperms, which contain the S lignin-specific cytochrome P450-dependent
monooxygenase, F5H. No MYB sites were found in the Arabidopsis F5H
promoter (
Raes et al.,2003
), and the lignin-specific AtMYB58 and AtMYB63
directly regulate the expression of monolignol pathway genes through AC-
binding sites except for F5H (
Zhou et al.,2009
). In a transient expression
system,
Zhao et al. (2010b)
showed that the F5H promoter is activated by
SND1, but the other promoters of the lignin pathway genes are not activated.
In fact, SND1 can bind the promoter region of F5H and activate F5H expres-
sion without de novo protein synthesis, indicating that the regulation of F5H by
SND1 is through direct binding to the F5H promoter rather than through
downstreamTFs such as AtMYB46 and/or AtMYB58/AtMYB58. In addition,
inhibition of NST1 expression in M. truncatula leads to a loss of S lignin
associated with a more than 25-fold reduction of F5H expression, but only
around a twofold reduction in the expression of other lignin pathway genes
(
Zhao et al.,2010b
). Moreover, a detailed phylogenetic analysis showed that
