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of MYB factors in the regulation of the synthesis of phenylpropanoids and
derived products has been demonstrated since the 1990s ( Grotewold et al.
1994; Martin and Paz-Ares, 1997; Sablowski et al. 1994; Weisshaar and
Jenkins, 1998 ). Taken together, these results suggested that MYB TFs play a
central role in regulating lignin biosynthetic genes. Here, we will focus on the
MYB TFs regulating SW and lignin biosynthesis, which were discovered later.
Figure 1 illustrates the phylogenetic relationships between the different MYBs
described in this review.
A. MYB PROTEINS REPRESSING THE
PHENYLPROPANOID AND LIGNIN PATHWAY
1. MYB repressors belonging to subgroup 4
a. Dicotyledon subgroup 4. The first line of genetic evidence on the possible
involvement of MYBs in the regulation of lignin biosynthesis came from
the pioneer study of two MYB proteins, AmMYB308 and AmMYB330,
from Antirrhinum majus ( Tamagnone et al., 1998 ). Overexpression of the
AmMYB308 and AmMYB330 genes caused a decrease in hydroxycinnamic
acid and monolignol accumulation in tobacco (17% reduction of xylem lignin
content), by reducing expression of phenylpropanoid and lignin biosynthetic
genes (C4H, 4CL and CAD). It was hypothesized that AmMYB308 and
AmMYB330 acted as very weak transcriptional activators, competing with
endogenous strong activators and thereby inhibiting gene activation. Two
years later, a knockout (KO) mutant for AtMYB4 (the proposed Arabidopsis
thaliana ortholog of AmMYB308) exhibited increased amounts of sinapate
esters, resulting in better tolerance to UV-B ( Jin et al., 2000 ). The increase in
sinapate ester accumulation in the mutant was associated with an enhanced
expression of the gene encoding C4H, which appears to be the principal
target of AtMYB4. AtMYB4 was shown to be an active repressor and
includes an Ethylene-responsive element binding factor-associated amphi-
philic repression-(EAR)-like repression motif in its C terminus ( Kazan, 2006;
Ohta et al., 2001 ). AtMYB4 belongs to the subgroup 4 ( Kranz et al., 1998 )
and all the proteins belonging to this subgroup contain this active repression
motif (EAR-like). Since this early work, several subgroup 4 genes were
shown to be involved in the transcriptional silencing of phenylpropanoid
and lignin genes ( Figs. 1 and 2 ). AtMYB32, a TF closely related to AtMYB4
appears to negatively control the expression of genes involved in the phenyl-
propanoid and lignin pathways through different targets, affecting the com-
position of the pollen wall ( Preston et al., 2004 ). The mutation of AtMYB32
leads to aberrant pollen and partial male sterility. The Atmyb32 mutant
shows increased expression of COMT and decreased expression of dihydro-
flavonol 4-reductase (DFR) and anthocyanin synthase (AS). In addition,
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