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sclerenchyma. However, lignin composition varies among species, phylogenetic groups,
cell types, developmental stages and even seasonal growth. In recent decades, the
number of species where lignins have been detected has notably increased, such as
the red alga Calliarthron, some Coleochaetale species and some bryophytes. Moreover,
the presence of syringyl lignins, which has been restricted solely to angiosperms, has been
confirmed also in most studied phylogenetic groups, from rhodophytes to gymnos-
perms, including non-vascular plants. The discovery in Selaginella of a novel enzyme
analogous to angiospermferulate-5-hydroxylase supports the existence of evolutionarily
convergent pathways that lead to syringyl lignin biosynthesis. In this review, the role of
lignins in the development of vascular system and the presence and composition of
lignins in different phylogenetic groups will be discussed, paying special attention to new
contributions and the evolutionary development of syringyl lignins.
I. INTRODUCTION
Lignins are abundant organic polymers, accounting for
25% of plant
biomass ( Higuchi, 1990 ). They are important compounds of cell walls,
where they are imbibed in a polysaccharide matrix ( Boudet et al., 1995 ).
These polymers impart water impermeability, including resistance against
tensile forces of the water columns, and confer structural support and flexural
stiffness to the aerial organs. Lignins can also provide protection against the
microbial degradation of cell walls ( Ros BarcelĀ“,1997 ).
Lignins are three-dimensional heteropolymers resulting from the oxidative
coupling of primarily the three p-hydroxycinnamyl alcohols (monolignols):
p-coumaryl, coniferyl and sinapyl alcohols. The cross-coupling reaction of
monolignol radicals produces a hydrophobic heteropolymer composed of
p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) units ( Vanholme et al.,
2010 ; Fig. 1 ).
Fig. 1. Structures of p-coumaryl (A), coniferyl (B), and sinapyl (C) alcohols,
precursors of p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) lignins units,
respectively. O atoms are shaded in red, H atoms in pale grey and C atoms in dark
grey.
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