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are able to regulate all the genes involved in the lignin biosynthetic pathway
except F5H, a key enzyme in S lignin biosynthesis (
Zhou et al., 2009
). All
these transcriptional factors bind to AC elements placed in the promoters of
the genes involved in lignin biosynthesis (
Raes et al., 2003
), except C4H,
COMT and F5H. However, C4H and COMT are regulated by a lignin-
specific MYB transcription factor, so their regulatory regions are supposed
to contain AC elements (
Zhao and Dixon, 2011; Zhou et al., 2009
). Never-
theless, the gene encoding F5H is not reported to contain AC elements.
Instead, F5H, and therefore syringyl lignin biosynthesis, is directly regulated
by NST1/SND1 (
Zhao et al., 2010
), which has previously been shown to
regulate MYB46, as well as other transcription factors involved in secondary
cell wall regulation (
Zhong et al., 2006
). It was suggested that MYB46 was
already present by the appearance of F5H, and NST1/SND1 evolved to
coordinately activate secondary cell wall machinery and F5H (
Zhao et al.,
2010
). There are no homologs of NST1/SND1 in gymnosperms, ferns,
mosses or algae, a fact that supports the evolution of F5H after
gymnosperm-angiosperm divergence (
Zhao et al., 2010
). However, the ge-
nome has been fully sequenced only in a few non-angiosperm species, so the
presence of both F5H and NST1/SND1 cannot be ruled out. On the other
hand, S. moellendorffii has been shown to possess an enzyme able to produce
syringyl lignin that is not homologous to angiosperm F5H (
Weng et al.,
2008
). Hence, this species (and perhaps other non-angiosperm species with
syringyl lignin in their cell walls) may have a different transcriptional net-
work regulating syringyl lignin biosynthesis.
IV. CONCLUSION
As discussed above, in the past decades the knowledge about lignins has
considerably increased. Lignin occurrence has been described in non-land
and non-vascular plants. Moreover, the presence of lignins in non-vascular
tissues of bryophytes and ferns, an observation with no counterpart in
gymnosperms or angiosperms (
G
ยด
mez Ros et al., 2007a,b
), supports the
view (
Boyce et al., 2003; Friedman and Cook, 2000
) that lignification might
have originated in the peripheral tissues of protracheophytes and was only
later co-opted to strengthen the tracheids in eutracheophytes. The presence
of lignins in peripheral cells of the red alga Calliarthron also seems to confirm
this hypothesis (
Martone et al., 2009
). As a whole, this scenario is reminiscent
of Gould's exaptation concept (
Gould, 2002
), and probably reflects what
Donoghue (2005)
termed 'developmental enablers', that is, early changes in
sequences that open up new design options. This working hypothesis is
