Biology Reference
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phenylpropanoid biosynthetic genes in response to environmental or devel-
opmental cues (
Bonawitz et al., 2012
).
Zimmermann et al. (2004)
used a conserved motif in the R3 domain of
MYB proteins known to interact with bHLH to predict possible interactions
between MYB and bHLH proteins. Then, they validated the putative inter-
actions using the Y2H approach. Their work revealed interactions between
TT8 and another bHLH protein (bHLH012) and AtMYB4, enlarging the
number of candidate proteins involved in the WD40-bHLH-MYB com-
plexes. As shown by coimmunoprecipitation, AtMYB4 also interacts with
SAD2 (sensitive to abscisic acid (ABA) and drought) an importin
-like
protein (
Zhao et al., 2007
). This interaction is necessary to translocate
AtMYB4 from the cytoplasm to the nucleus to repress phenylpropanoid
gene expression.
Recent analyses demonstrated that VND7 interacts with the NAC domain
proteins, VND-INTERACTING1 (VNI1) and VNI2 (
Yamaguchi et al.,
2010b
). Transient reporter assays showed that VNI2 is a transcriptional
repressor and can repress the expression of vessel-specific genes regulated
by VND7. The expression of VNI2 under the control of the VND7 promoter
resulted in the inhibition of the normal development of xylem vessels in roots
and aerial organs. These data suggest that VNI2 regulates xylem cell specifi-
cation by functioning as a transcriptional repressor that interacts with VND7
(
Yamaguchi et al., 2010b
). In P. trichocarpa leaves, homologous genes to
AtVNI2 were upregulated by AtVND7 overexpression, whereas in Arabidop-
sis AtVND7 overexpression does not significantly induce AtVNI2 expression,
suggesting a difference in negative regulation among species (
Ohtani et al.,
2011
). Very recent work suggests that VNI2 is indeed a bifunctional TF
acting both as a transcriptional repressor in xylem vessel development and
as a transcriptional activator in stress-mediated regulation of leaf longevity
(
Yang et al., 2011
). These authors showed that VNI2 is responsive to ABA
and integrates ABA-mediated abiotic stress signals (high salinity) into leaf
ageing by regulating directly a subset of COLD-REGULATED (COR) and
RESPONSIVE TO DEHYDRATION (RD) genes. Leaf ageing was delayed
in transgenic plants overexpressing the VNI2 gene but significantly accelerated
in a VNI2-deficient mutant (
Yang et al.,2011
).
b
VIII. ENVIRONMENTAL REGULATION OF THE
LIGNIN BIOSYNTHETIC PATHWAY
Since the appearance of the first land plants, the environment where they
grow has dramatically influenced their evolution. Faced with a large range of
biotic and abiotic stresses, the responses to these stresses needed to be tightly
