Transcriptional Regulation of the Lignin Biosynthetic Pathway Revisited: New Players and Insights - Botanical Research - page 180

activities suggesting that it could act as a negative regulator of lignin biosyn-
thesis gene expression in planta ( Legay et al.,2007 ). The role of EgMYB1 was
further investigated through overexpression in Arabidopsis and poplar by
Legay et al. (2010) . Histochemical analyses indicated that in stems, SW of
vascular tissue were significantly thinner in both Arabidopsis and poplar
EgMYB1-overexpressing plants. In agreement with these observations,
qRT-PCR analysis in both species indicated that EgMYB1 overexpression
downregulated three different SW cellulose synthase genes (CESA4/IRX5,
CESA7/IRX3 and CESA8/IRX1) and two glycosyltransferases (IRX8 and
IRX7/FRA8) associated with xylan biosynthesis, in addition to a number of
lignin biosynthetic genes. The negative impact on overall SW thickening could
explain the apparent discrepancy between the moderate decrease in Klason
lignin (11% relative to cell wall weight) and the marked effect on lignin staining
visualized by histochemistry (particularly dramatic in Arabidopsis interfasci-
cular zones). Together, these results strongly suggest that EgMYB1 functions
as a negative regulator of the whole SW developmental programme and is not
just as a negative regulator of lignification ( Legay et al.,2010 ). This was the
first example of a master repressor of SW formation.
b. Monocotyledon subgroup 4. The MYBs described below belong to a sub-
clade of subgroup 4 which seems to be specific to monocots ( Fig. 2 ). In maize,
two closely related MYB proteins, ZmMYB31 and ZmMYB42 were shown to
play non-redundant repressor roles in the regulation of the phenylpropanoid
pathway affecting a partially overlapping distinct set of genes ( Fornal´ et al.,
2006, 2010; Sonbol et al.,2009 ).When expressed inA. thaliana, they function as
repressors of both the maize and the A. thaliana COMT gene. In addition, both
factors repress the A. thaliana 4CL1 gene and provoke a strong reduction in
lignin content (60% for ZmMYB31 overexpression). ZmMYB42 but not
ZmMYB31 reduces the S/G ratio of the polymer. In both cases, plants are
dwarfed and present alterations in their leaf morphology ( Fornal ´ et al. 2006,
2010; Sonbol et al. 2009 ). Both MYBs negatively regulate the accumulation of
sinapoyl malatemaking plantsmore sensitive toUV radiation. ZmMYB42 also
represses flavonol biosynthesis ( Sonbol et al.,2009 )whereasZmMYB31
increases the accumulation of anthocyanins suggesting that the reduction of
lignin redirects the carbon flux towards the biosynthesis of anthocyanins
( Fornal ´ et al.,2010 ). Chromatin immunoprecipitation (ChIP) assays demon-
strated that ZmMYB31 interacts with the AC-II element of the maize COMT
gene promoter in vitro but not with the Arabidopsis COMT promoter, which
only contains a MYB type-I cis-element ( Fornal´ et al.,2010 ). In addition,
ZmMYB31 interacts directly with the maize F5H gene, whereas Arabidopsis
F5H does not contain MYB-binding sites ( Raes et al.,2003 ). This suggests an