Transcriptional Regulation of the Lignin Biosynthetic Pathway Revisited: New Players and Insights - Botanical Research - page 190

VND7. Therefore, the authors concluded that LBD18/ASL20 and LBD30/
ASL19 genes appear to be involved in a positive feedback loop for VND6
and VND7 expression ( Soyano et al., 2008 ). Another example of positive
regulation is the fibre-specific SND1 TF that can bind directly to a conserved
motif in its own promoter and stimulate its own expression as recently shown
by Wang et al. (2011) .
The SW NACs are also under negative feedback regulation. AtMYB32,
a negative regulator closely related to AtMYB4 is activated by AtMYB46
( Ko et al., 2009 ) and the expression of AtMYB32 is downregulated in the
nst1xnst3/snd1 double mutant of Arabidopsis ( Mitsuda et al., 2007 ). Using
transactivation assays and in vivo transgenic studies, Wang et al. (2011)
identified AtMYB32 as a direct target of SND1 and moreover they showed
that expression of SND1 is negatively regulated by AtMYB32.
F. OTHER NAC DOMAIN PROTEINS INVOLVED
IN SECONDARY WALL FORMATION
XYLEM NAC DOMAIN 1 (XND1) was initially isolated as a gene highly
expressed in xylem ( Zhao et al., 2005 ). Overexpression of XND1 inhibited
secondary wall deposition and autolysis in xylem vessels. Dominant repres-
sion of XND1 phenocopied gain-of-function plants, suggesting that XND1 is
a transcriptional repressor regulating the expression of genes involved in
both PCD and secondary wall formation ( Zhao et al., 2008 ). XND1 has
several potential orthologs in P. trichocarpa: PopNAC118, 122, 128, 129
( Grant et al., 2010 ). XND1 overexpression in poplar resulted in severe stunt-
ing and overexpressors were shown to lack phloem fibres and characterized
by reductions in cell size and number, vessel number, and frequency of rays in
the xylem. Overexpression of PopNAC122,anXND1 ortholog, yielded an
analogous phenotype in Arabidopsis ( Grant et al., 2010 ).
Zhong et al. (2008) reported that two genes encoding NAC domain pro-
teins, SND2 and SND3, are highly expressed in xylem and interfascicular
fibre cells and are expressed downstream of SND1. Overexpression of SND2
and SND3 increases secondary wall thickness of interfascicular fibres and
xylary fibres, and induces the expression of CesA8 gene ( Zhong et al., 2008 ),
suggesting that other NAC domain proteins also participate in SW forma-
tion. Overexpression of PopNAC154 (an SND2/SND3 ortholog) in poplar
reduced height growth and increased the relative proportion of bark versus
xylem ( Grant et al., 2010 ).
Recently, Hussey et al. (2011) overexpressed SND2 in Arabidopsis and they
found evidence for upregulation of biosynthetic genes associated with cellu-
lose, xylan, mannan and lignin polymerization but with only minor alterations