Biology Reference
In-Depth Information
poplar. However, transgenic plants expressing the dominant negative forms
of VND7 driven by its own promoter exhibit a dwarf phenotype with
inhibition of protoxylem and metaxylem vessel formation in roots associated
with continuous vessel formation in the aerial parts (
Yamaguchi et al., 2008
).
Thus, VND7 plays a critical role in the formation of all types of vessels, and
together with other VND proteins appears to function synergistically and/or
redundantly during xylem vessel differentiation (
Yamaguchi, 2010
).
C. POPLAR NAC
S
In contrast to the Arabidopsis SW NACs that exhibit either fibre- or vessel-
specific expression, the PtrWNDs are expressed in both vessels and fibres and
also in ray parenchyma cells of P. trichocarpa (
Zhong et al.,2010b
).
Ohtani
et al. (2011)
identified 12 PtVNS/PtrWND genes that redundantly control the
differentiation of both vessels and fibre cells during xylem tissue formation
by modulating their activity depending on the situation, probably through a
change in expression levels and/or dimer formation of the corresponding
proteins. They act not only on genes involved in secondary wall formation
and PCD but also on other TFs. Similar conclusions were drawn by
Zhong
et al. (2010b)
who analysed more particularly PtrWND2N and PtrWND6B.
D. UPSTREAM REGULATORS AND DOWNSTREAM TARGET GENES
1. Upstream regulators
AtMYB26 was shown to regulate the expression of NST1 and NST2. Loss-
of-function mutation in the AtMYB26 gene induces a defect in the secondary
wall thickening of anther walls with resultant indehiscent anthers (
Steiner-
Lange et al., 2003; Yang et al., 2007
).
In addition, AtWRKY12, a member of the WRKY family of TF, was
shown to act as a negative regulator of SW in pith. It binds directly to the
NAC gene (NST2) promoter likely through a W box TTGACT/C and
represses NST2
Wang et al. (2010)
.
2. Downstream target genes
As described above, overexpression of VND6, VND7 and NST genes can
induce the ectopic secondary wall formation of various types of cells.
Expression analyses demonstrated that VND7 and NSTs upregulate a num-
ber of genes previously shown to be associated with pathways required for
SW formation such as cellulose, hemicellulose and lignin biosynthesis (
Kubo
et al., 2005; Mitsuda et al., 2005; Yamaguchi et al. 2010a; 2010b; Zhong et al.,
2006, 2007
). However, genes
involved in PCD during xylem vessel
