Biology Reference
In-Depth Information
proportional to lignin levels and growth. In addition, the reduction of SA
levels restores growth in lignin-downregulated plants (
Gallego-Giraldo et al.,
2011
). Despite numerous studies linking plant hormones and lignin biosyn-
thesis, the molecular link between the hormone-signalling pathway and
lignin biosynthesis regulation is still far from being understood.
X. CONCLUDING REMARKS
Tremendous progress has been made in the last decade with regard to our
understanding of the regulation of the lignin biosynthetic pathway and more
globally of SW formation. The initial picture of a hierarchical network in
which the Arabidopsis secondary wall-related NAC TFs, SND1/NST3,
NST1, VND6 and VND7, operate as the first layer of master switches, and
the MYB TFs AtMYB46/AtMYB83 act as a second layer of master switches
which regulate downstream genes in secondary wall formation, needs to be
revisited in order to take into account more recent findings. For instance,
genome-wide analysis of the direct targets of SW-related NAC and MYB46,
respectively, shows that they not only control a number of downstream TF
but can also directly regulate a battery of genes involved in SW biosynthesis,
modification and PCD (
Zhong and Ye, 2012, Zhong et al., 2010c
). Some
target genes are common to SW-related NAC and MYB46 whereas some are
controlled by only one category of master switches. SW-related NAC can be
controlled both by positive (
Yang et al., 2007
) and negative upstream reg-
ulators (
Wang et al., 2010
) and in addition, are submitted to both negative
(
Wang et al., 2011
) and positive (
Soyano et al., 2008
) feedback regulations
including self-feedback regulation (
Wang et al., 2011
). These complex and
subtle regulations including multilevelled feed-forward loop regulatory
structure may give plants more flexibility in controlling secondary wall
formation to better cope with their ever-changing environments. Another
good example, although less studied than Arabidopsis, includes two impor-
tant Eucalyptus MYB master regulators, EgMYB1 and EgMYB2, both
preferentially expressed in xylem (
Goicoechea et al., 2005; Legay et al.,
2007, 2010
) and playing antagonistic roles in SW biogenesis and lignin
biosynthesis. A dynamic competition between EgMYB1 and EgMYB2 for
the same promoters can allow the formation of either a repressing or an
activating regulatory complex, thereby providing a sophisticated mechanism
for the spatial and temporal control of lignified SW formation. Consistent
with this idea, the EgCAD2 promoter has a complex organization with its cis-
elements arranged in two similar modules (containing MYB sites) suggesting
that redundancy and mechanisms of cooperation or competition between cis-
