Biology Reference
In-Depth Information
the developmentally regulated pattern of F5H transcript accumulation is
distinct from that of the other phenylpropanoid genes and that the promoter
region alone is not sufficient to drive F5H expression (except in the case of
embryos) and requires a positive regulatory element in the downstream 3
0
region (
Ruegger et al., 1999
).
The discovery that the AC elements help to regulate the expression of
lignin biosynthetic genes and serve as a common means for the coordinated
regulation of genes in the lignin biosynthetic pathway was a breakthrough in
understanding how lignin biosynthesis is coordinately regulated. As a conse-
quence, a considerable effort has been invested in the characterization of TFs
that bind to these AC-rich elements. Separate Southwestern experiments
identified two tobacco AC-rich motif-binding proteins, ACBF (
S
ยด
guin
et al., 1997
) and NtLIM1 (
Kawaoka et al., 2000
). The NtLIM1 protein
binds in a sequence-specific manner an AC element called the PAL-box
motif in the horseradish wound-inducible peroxidase promoter (prxC2)and
activate its transcription (
Kaothien et al., 2002
). The PAL-box motif is an
elicitor-responsive element found in the promoter of genes involved in the
phenylpropanoid biosynthesis pathway (
Logemann et al., 1995; Lois et al.,
1989
). The role of NtLIM1 in the regulation of the lignin pathway was
further supported by experiments on transgenic tobacco and Eucalyptus
camaldulensis plants carrying an antisense NtLIM1. The modified plants
showed decreased accumulation levels of transcripts for phenylpropanoid
genes such as PAL and 4CL and a reduction of 27-29% of their lignin content
(
Kawaoka and Ebinuma, 2001; Kawaoka et al., 2000, 2007
).
III. MYB TF
S
AND THE REGULATION
OF THE LIGNIN PATHWAY
AC-rich sequences are also known to be putative targets for TFs belonging to
the MYB family. The family of MYB TFs is one of the most abundant classes
of TFs in plants, and the subfamily containing the two-repeat R2R3 DNA-
binding domain is the largest (
Dubos et al., 2010; Stracke et al., 2001
). It is
divided into subgroups according to the conserved C-terminal motifs puta-
tively involved in their function (
Kranz et al., 1998
). Compilation of studies
on sequence specificity led to the identification of three classes of MYB-
binding sites called MBSI (C/TAACNG), MBSII (CTAACA/TAA/CC), and
MBSIIG (C/TACCA/TAA/CC) (
Romero et al., 1998
). AC-rich sequences
found in promoters of phenylpropanoid and lignin genes such as PAL2, 4CL1,
EgCCR and EgCAD2 fit perfectly with the MBSIIG consensus sequence
(
Hatton et al.,1995;Hauffeet al.,1993
). Consistent with this, the involvement
