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differentiation were induced by the overexpression of VND7 (
Kubo et al.,
2005; Yamaguchi, 2010; Yamaguchi et al.,2010a
)orVND6, but not by the
NST genes (
Ohashi-Ito et al.,2010
).
Ohashi-Ito et al. (2010)
also showed that
VND6 and SND1 regulate a number of genes in common, specifically those
related to SW formation, but only VND6 directly regulates genes that harbour
a TE-specific cis-element, TERE. This element, with the CTTNAAAGCNA
conserved sequence, which contains a core motif AAAG for DOF (DNA with
one finger) TF binding, is able to confer TE-specific expression to both genes
related to SW formation or modification and PCD (
Pyo et al.,2007
).
By mapping the SND1-binding sequence using EMSA and transactivation
analysis, it has recently been demonstrated that SND1, together with other
secondary wall regulatory NACs, including VND6, VND7, NST1 and NST2
bind to imperfect palindromic 19-bp consensus sequence (T/A)NN(C/T) (T/C/
G)TNNNNNNNA(A/C) GN(A/C/T) (A/T) designated as the SNBE (
Zhong
et al.,2010c
). Genome-wide analysis of direct targets of SND1 and VND7
revealed that they directly activate the expression of not only downstreamTFs,
but also a battery of genes involved in secondary wall biosynthesis, cell wall
modification and PCD, the promoters of which all containmultiple SNBE sites.
SND1 and VND7 directly regulate the expression of a set of common targets
but each of themalso preferentially induces a distinct set of direct targets, which
is likely related to their differential activation strength toward SNBE sites
(
Zhong et al.,2010c
). However, since this element is quite degenerate, it is
very common in plant genomes and different NAC genes may bind to different
versions or alternatively, other factors may be involved in determining binding
specificity for this site (
Wang et al.,2011
). Among the TF downstreamof VND/
NST/SND1, the genes AtMYB46, AtMYB83, AtMYB103 and fromA. thaliana
are direct targets of SND1 and possibly of NST1, VND6 and VND7 (
Ko et al.,
2009; McCarthy et al., 2009; Yamaguchi, 2010; Yamaguchi et al., 2011; Zhong
and Ye, 2012; Zhong et al., 2007, 2008; Zhou et al.,2009
).
E. COMPLEX FEEDBACK REGULATION OF SW NACS
Increasing evidence suggests that SW NACs may be under both positive and
negative regulation. As an example of positive feedback regulation, there are
the LOB [LATERAL ORGAN BOUNDARIES DOMAIN (LBD)/ASYM-
METRIC LEAVES2 (AS2)] domain proteins LBD18/ASL20 and LBD30/
ASL19, whose genes are expressed in immature vessels and are upregulated
by VND6 and VND7 (
Soyano et al., 2008
). Recently, LBD18 and LBD30
were shown to be direct targets of VND7 (
Yamaguchi et al., 2011
). Over-
expression of LBD18/LBD30 induced transdifferentiation of cells from non-
vascular tissues into xylem vessels and ectopic expression of VND6 and
