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Ko et al. (2009)
and
Zhong and Ye (2012)
, who identified the direct targets of
AtMYB46 using inducible direct activation systems in Arabidopsis coupled to
whole-transcriptome profiling analysis. Genome-wide analysis of direct targets
of AtMYB46 demonstrated that it directly regulates the expression of not only
a number of downstream TFs such as AtMYB43, AtMYB52, AtMYB54,
AtMYB58, AtMYB63, AtC3H14 and the Class-II KNOTTED1-like homeo-
box (KNOX)geneAtKNAT7, but also a suite of secondary wall biosynthetic
genes, cell wall modification and cell death genes (
Zhong and Ye, 2012
). Some
of them are also directly activated by secondary wall NAC (SWN) master
switches or by AtMYB46-direct TF targets. Interestingly, the repressors
AtMYB7, AtMYB4 and AtMYB32 were activated only by AtMYB46. In
particular, AtMYB7 was activated early and very strongly by AtMYB46 (
Ko
et al. 2009
). A 7-bp sequence ACC(A/T)A(A/C)(T/C) called secondary wall
MYB responsive element (SMRE) is sufficient to be responsive to AtMYB46
activation (
Zhong and Ye, 2012
). Furthermore, AtMYB83, a functional
homolog of AtMYB46 (
McCarthy et al.,2009
)wasfoundtobindtothe
same SMRE consensus sequence and activate the same set of direct targets
as AtMYB46 (
Zhong and Ye, 2012
). These new results illustrate the complexi-
ty of the transcriptional programme regulating secondary wall biosynthesis
that involves a multilevelled feed-forward loop regulatory structure.
Recently, PtrMYB3 and PtrMYB20 from Populus trichocarpa were shown to
be functional orthologs of Arabidopsis AtMYB46 and AtMYB83 and their
expression is directly activated by PtrWND2, a poplar ortholog of SND1
(
McCarthy et al., 2010
). Another ortholog of AtMYB46, PtxtMYB021 was
clonedinhybridaspen(Populus tremula
Populus tremuloides)(
Winzell et al.,
2010
) and shown to transactivate promoters of xylan-active CAZymes most
likely via CCACCAAC (or ACTYP), a motif similar to the AC elements
mediating transactivation of lignin biosynthetic genes. Orthologs of AtMYB46
were also found in rice (OsMYB46)andmaize(ZmMYB46) and their promoters
were shown to contain SWN-binding element (SNBE) sites (explained below),
which are known to be bound by NAC TFs (
Zhong et al.,2011
).
2. Lignin-specific MYBs: AtMYB58, AtMYB63 and AtMYB85
Unlike the other MYB activators outlined above which cause ectopic activa-
tion of the entire SW programme when overexpressed, the lignin-specific
AtMYB58, 63 and 85 only affect lignin biosynthesis. AtMYB58 and
AtMYB63 are specifically expressed in fibres and vessels undergoing lignifi-
cation. Their overexpression was found to induce ectopic deposition of lignin
but not cellulose and xylan, whereas their dominant repression resulted in a
reduction in secondary wall thickening and lignin deposition, indicating
that AtMYB58 and AtMYB63 are specifically involved in the regulation of
