Biology Reference
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lignin biosynthesis (
Zhou et al., 2009
). The hypothesis that AtMYB58 and
AtMYB63 are transcriptional activators of lignin biosynthesis was further
supported by the findings that they bind to the AC elements and directly
activate the expression of most lignin biosynthetic genes except F5H (
Zhou
et al., 2009
). AtMYB58 is also able to directly activate the expression of a
secondary wall-associated laccase (LAC4) gene that was recently shown to be
an important factor in monolignol polymerization in Arabidopsis (
Berthet
et al., 2011
). The expression of AtMYB58 and MYB63 is regulated by SND1
and its close homologs NST1, NST2, VND6 and VND7 as well as by their
downstream target AtMYB46. Orthologs of AtMYB58 and AtMYB63 are
present in tree species such as poplar. One of these orthologs, the P. tricho-
carpa PtrMYB28, is predominantly expressed in developing wood under-
going secondary wall thickening and lignification (
Zhong and Ye, 2009
).
Expression of PtrMYB58 in Arabidopsis protoplasts effectively induced the
expression of the lignin biosynthetic gene 4CL1 and this induction is depen-
dent on the presence of an AC element.
In addition to AtMYB58 and AtMYB63, another MYB TF, AtMYB85,
was found to activate lignin biosynthetic genes and cause ectopic lignin (but
not cellulose and xylan) deposition in epidermal and cortical cells in stems
when overexpressed. Its dominant repression significantly reduces secondary
wall thickening in fibre cells and causes deformation of vessels (
Zhong et al.,
2008
). Interestingly, AtMYB85 is regulated by SND1 (
Zhong et al., 2008
) but
is not a direct target of AtMYB46 (
Ko et al., 2009; Zhong and Ye, 2012
). The
P. taeda PtMYB1 (
Bomal et al., 2008; G´mez-Maldonado et al., 2004
)is
likely an ortholog of AtMYB85.
3. Other MYBs
Other MYB repressors not belonging to subgroup 4 have also been char-
acterized. This is the case of PttMYB21a from hybrid aspen which is highly
expressed in xylem and phloem fibres (
Karpinska et al., 2004
). Transgenic
aspen lines expressing antisense PttMYB21a were characterized by a
reduced growth and fewer internodes. No changes were observed in the
transcripts levels of PAL, CAD, C4H, 4CL and COMT in xylem and phloem
except for the CCoAOMT transcript which accumulated to higher amounts
in phloem. In addition, more acido-soluble lignin was found in the bark and
the total concentration of carbohydrates was decreased. The authors sug-
gested that CCoAOMT is downregulated by PttMYB21a, which could act
as a repressor.
Interestingly, PttMYB21a belongs to the same phylogenetic group as
AtMYB52 which is a direct target of AtMYB46 (see below) and has been
proposed to function as a transcriptional activator (
Zhong et al., 2008
). In
