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in cell wall chemistry. In addition to SND1, AtMYB103 was also upregulated
in SND2-overexpressing plants. Strikingly, the authors found that some lines
with pronounced SND2 overexpression levels showed reduced fibre wall depo-
sition, which they attribute to excessive overexpression rather than co-suppres-
sion. This result clearly emphasizes the importance of taking into account
expression thresholds in TF studies. SND2 overexpression was also associated
with different phenotypes when expressed in woody and herbaceous stems
where it was observed that overexpression of SND2 in Eucalyptus stems led to
increased fibre cross-sectional cell area (
Hussey et al.,2011
).
V. OTHER TF FAMILIES REGULATING
LIGNIN BIOSYNTHESIS
The KNOTTED1-LIKE HOMEODOMAIN PROTEIN7 from A. thaliana
(KNAT7) was shown to be a direct target of SND1 (
Zhong et al.,2008
).
KNAT7 was first identified as an activator of secondary wall biosynthesis
because of the irregular xylem (irx) phenotype of a knat7 loss-of-function
mutant (irx11) and the thinner interfascicular cell walls in transgenic Arabi-
dopsis expressing a dominant repression of KNAT7. However, in addition to
the irx phenotype, knat7 loss-of-function mutants show increased interfascicu-
lar fibre cell wall thickness, while KNAT7 overexpression mutants have
the opposite phenotype, that is, thinner interfascicular fibre cell walls (
Li
et al.,2011
). One possible explanation for these apparently contradictory
results is that KNAT7 acts, in fact, as a transcriptional repressor and therefore
the dominant transcriptional repression variant of KNAT7 would enhance its
repressor function.
Li et al. (2012)
confirmed that KNAT7 is a negative regula-
tor of secondary wall biosynthesis both in Arabidopsis and poplar, and that it
functions in a negative feedback loop repressing metabolically inappropriate
commitment to SW formation, thereby maintaining metabolic homeostasis.
Recently, AtWRKY12, a member of the WRKY TF family was shown to
act as a negative regulator of SW in pith, a tissue with thin primary walls
adjacent to tissues with SW such as vascular bundles and interfascicular fibres
(
Wang et al., 2010
). The authors characterized mutants of M. truncatula and
A. thaliana with SW thickening in pith cells associated with ectopic deposition
of lignin, xylan and cellulose, leading to a 50% increase in biomass density in
stem tissue of the Arabidopsis mutants. The mutations are caused by disrup-
tion of a stem-expressed WRKY gene, AtWRKY12, which upregulates down-
stream genes encoding NST2 and a Cys3His zinc finger TFs that activate SW
synthesis. Direct binding of AtWRKY12 to the NAC gene promoter and
repression of three downstream TFs were confirmed by in vitro assays and in
planta transgenic experiments (
Wang et al.,2010
). Another member of the
WRKY family first shown to be involved in grapevine (Vitis vinifera)response
