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supported by recent advances in knowledge of transcriptional regulation of
the biosynthesis of lignins. The AC elements are conserved cytosine- and
adenosine-rich motifs that are present in the promoters of genes encoding
enzymes of the phenylpropanoid pathway. AC elements contained in the
promoter region of lignin biosynthetic genes are thought to enhance their
expression in the xylem and, at the same time, to prevent their expression in
peripheral tissues (
Peter and Neale, 2004
). Since the deletion of the AC
element results in de-repression of these genes in tissues foreign to xylem
tissues (
Raes et al., 2003
), it has been suggested that a tissue-specific repressor
(transcription factor) is normally bound to the AC element, preventing
expression in cells foreign to the xylem tissues (
Leyva et al., 1992
).
The presence of such transcription factors in tissues other than the xylem
might reflect the evolution of AC-rich element-containing promoters from a
primitive non-specific to a specific vascular promoter through the introduc-
tion of a transcription factor that suppresses foreign tissue expression.
On the other hand, syringyl lignins are distributed with an unknown pattern
among plants, from algae to angiosperms. As far as is known, angiosperms and
Selaginella have evolved two enzymes (F5H and COMT) that catalyse the
production of syringyl lignins and, given the presence of syringyl lignins in
liverworts, lycopods, ferns and basal living gymnosperms, it is possible that the
pathway for sinapyl alcohol has evolved (independently or not) several times
during the evolution of land plants. Alternatively, sinapyl alcohol may have
been incorporated into the earliest land plants, and was subsequently lost or
repressed (
Novo-Uzal et al., 2009
) in some but not all of the extant diverging
basal liverworts, lycopods, equisetopsids, ferns and even algae. The recruitment
of sinapyl alcohol for lignin biosynthesis would thus be the 'primitive state'.
All the technical research advances described in this review led to a better
understanding about why, when and how lignins appeared in plants. Hope-
fully in the following years, the analysis regarding the presence and compo-
sition of lignins in new species, the sequencing of genes encoding lignin
biosynthesis enzymes and their gene regulation in several new and distantly
related species will let us know and understand, step by step, the evolutionary
history of lignins.
ACKNOWLEDGEMENTS
The authors thank Dr. Ignacio B´rbara for providing Posidonia images and
Teresa Mart´nez for assistance in the histochemical analysis. This work was
supported in part by a grant from Xunta de Galicia (INCITE08P-
xIB103182PR) and Ministerio de Ciencia e Innovaci ´n (BFU2009-08151).
