Biology Reference
In-Depth Information
for several bryophytes, which, according to the authors, indicates the absence
of cinnamaldehydes, the main target of phloroglucinol (
Pomar et al., 2002
),
but a categorical absence of lignin cannot be ruled out (
Ligrone et al., 2008
).
It has been reported (
Xu et al., 2009
) that the first appearance of the entire
lignin biosynthetic pathway enzymes (excluding the pathway that leads to
syringyl lignin formation) from the catalysis of phenylalanine to coniferyl
alcohol formation, took place in mosses (Physcomitrella patens). However,
the presence of lignin in more basal plants cannot be ruled out, as the analysis
these authors performed was based on searching for sequences with Arabi-
dopsis gene homologs as templates for lignin biosynthetic gene identification
(
Xu et al., 2009
). Though this kind of study is highly useful, enzyme-coding
genes that could have arisen from convergent evolution are not considered.
Moreover, the number of fully sequenced species is very limited, most of
them are angiosperms, and only a few basal species have a fully sequenced
genome. As mentioned above, the presence of lignin has been reported in a
red alga and a marchantiopsid, neither of them fully sequenced species.
2. Lycophytes
Arising at least 400 mya, this lineage appeared in the fossil record very soon
after the first appearance of vascular plants. During the Carboniferous
period (345-290 mya), lycophytes were especially diverse and abundant,
dominating coastal swamps of tropical lowlands (
Bateman et al., 1998
).
The remains of these plants account for our major coal deposits.
Currently, there are some 1280 species of lycophytes belonging to three
major lines: Lycopodiaceae, which is homosporous, Selaginellaceae and
Isoetaceae, both of them heterosporous. Lycopodiaceae are terrestrial or
epiphytic, cosmopolitan and most diverse in tropical and alpine habitats.
Selaginella (
Fig. 6
) comprises 750 species, mostly terrestrial, herbaceous and
perennial, and mostly occurring in tropical forests, although many species
are able to survive long periods of drought. Isoetes is the only living remnant
of the clade that included the giant lycopods of the Carboniferous period.
Isoetes has retained the cambium and some secondary growth and it has
rootlets that resemble those of the extinct trees. This family has both terres-
trial and aquatic members, with a nearly cosmopolitan distribution (
Judd
et al., 2008
).
The distribution of syringyl lignins among extant lycophytes is not
uniform or entirely understood. Syringyl lignin derivatives cannot usually
be detected in extant Lycopodiaceae (i.e. Lycopodium) by chemical depoly-
merizing methods such as derivatization followed by reductive cleavage
(DFRC;
Weng et al., 2008
) or cupric oxide alkaline oxidation (
Logan and
Thomas, 1987
). It is perhaps evolutionarily significant that syringyl lignin
