Biology Reference
In-Depth Information
evolutionary divergence between the regulation of themaize and the A. thaliana
COMT and F5H genes by R2R3-MYB factors. In wheat (Triticum aestivum),
the gene TaMYB4 which is phylogenetically closer to ZmMYB42 than to
ZmMYB31 (
Fig. 2
), is highly expressed in stems and roots (
Ma et al.,2011
).
Its overexpression in transgenic tobacco plants led to transcriptional reduction
of both CAD and CCR genes and substantially decreased the levels of total
lignin but increased the S/G ratio. In addition, the total flavonoid content was
increased in transgenic tobacco leaves, suggesting that the overexpression of
TaMYB4 probably led to a redirection of the metabolic flux from the lignin
pathway to the flavonoid pathway (
Ma et al.,2011
).
The lignin repressor PvMYB4 from switchgrass (Panicum virgatum) has also
recently been characterized (
Shen et al.,2012
). Overexpression of PvMYB4 in
transgenic tobacco and switchgrass suggests that the gene is functionally
orthologous to AtMYB4 and ZmMYB31. PvMYB4 binds directly to AC-I
(preferred), -II and -III elements, both in vitro and in a yeast transcription
system. Lignin biosynthetic genes are significantly downregulated in overex-
pressing plants, associated with reductions in lignin content and in the ester-
linked p-coumaric acid: ferulic acid ratio (
Shen et al.,2012
).
B. MYB TRANSCRIPTIONAL ACTIVATORS
1. Master switches
The first MYBs acting as transcriptional activators of the lignin pathway were
found in the differentiating xylem of trees. For example,
Patzlaff et al. (2003)
identified PtMYB4 from Pinus taeda expressed in lignifying tissues. In tobacco
plants overexpressing PtMYB4, the expression of the genes encoding coumaroyl
shikimate 3-hydroxylase (C3H), caffeoyl CoA 3-o-methyltransferase
(CCoAOMT), COMT, CCR and CAD was increased while PAL gene expres-
sion was decreased and 4CL and C4H gene expression was unchanged. The
increased transcription of the monolignol-specific genes was associated with
an increase of lignin deposition extended to cell types that do not normally
lignify such as pith and phloem (
Patzlaff et al., 2003
).
In E. gunnii, EgMYB2, a member of the R2R3-MYB family of TFs not
classified in any of the subgroups defined by
Kranz et al. (1998)
, was cloned
from a xylem cDNA library (
Goicoechea et al., 2005
). This gene is present in
a single copy and maps to a quantitative trait locus (QTL) for lignin content.
The EgMYB2 protein binds specifically the regulatory regions of the EgCCR
and EgCAD2 promoters in vitro, and activates their transcription both in
transient and stable expression assays. Transgenic tobacco plants overex-
pressing EgMYB2 displayed a dramatic increase of the SW thickness and an
alteration in lignin profiles (increase in the S/G ratio). Transcript analysis of
