Biology Reference
In-Depth Information
not only in angiosperms but also in Gnetales, which bear G,S lignins; in
Coniferales, such as Pinus sylvestris, a species that lacks S lignins; in basal
gymnosperms (Cycadales and Ginkgoales); in basal vascular plants, such as
the terrestrial lycopod S. moellendorffii and the aquatic fern Ceratopteris
richardii, both containing G,S lignins, in non-vascular plants such as
P. patens and M. polymorpha (
G´mez Ros et al., 2007a; Logan and
Thomas, 1985; Ros Barcel ´ et al., 2007
). The presence of S peroxidases in
M. polymorpha is in concordance with the occurrence of S-type lignin and
lignin-like substances in the cell wall of liverworts (
Espi
˜
eira et al., 2011;
Logan and Thomas, 1985
).
It has been suggested that S peroxidases are not as exceptional as previ-
ously assumed, and their presence predates not only the gymnosperm-
angiosperm divergence but also the radiation of tracheophytes itself. This
observation suggests that the genes coding these enzymes predate the appear-
ance of vascular plants (
Duroux and Welinder, 2003; G
´
mez Ros et al.,
2007a
), and might even be contemporaneous with the acquisition of the
most primitive short-distance water and nutrient transport systems that
coevolved with mosses and liverworts (
Ligrone et al., 2000
). Because the
main characteristic of the S peroxidases is the absence of steric restrictions
at the substrate-binding site, these observations support the existence of an
ancestral basic model of cell wall architecture and function, that would have
evolved before the evolutionary divergence of bryophytes, ferns and seed
plants (
G´mez Ros et al., 2007a; Ligrone et al., 2002
), and that have been
finely tuned in response to specific evolutionary pressures. In this model, G
peroxidases constitute the most evolved state, as would be expected from the
constraints that arise to increased enzyme specificity, during plant evolution.
B. TRANSCRIPTIONAL REGULATION
Lignified cells are an important carbon sink and unable to expand due to
lignin deposition, so lignification must occur after cell division. Given the
metabolic cost of building lignin polymer, along with the persistence and
properties of lignified cells, the timing and localization of lignification must
be strongly regulated (
Rogers and Campbell, 2004
). In recent years, many
advances have been made in the knowledge of the transcriptional regulation
of lignification. Some transcription factors that regulate the formation of
secondary cell wall formation have been identified, as well as regulators of
lignin biosynthesis. MYB46 regulates secondary cell wall formation in Ara-
bidopsis (
Zhong et al., 2007
) and upregulates two transcriptional factors,
MYB58 and MYB63, that are specifically involved in lignin biosynthetic
regulation in Arabidopsis (
Zhou et al., 2009
). Both MYB58 and MYB63
