Evolutionary History of Lignins - Botanical Research - page 331

question this long held point of view. These data suggest syringyl lignins have
emerged at least five times during plant history by convergent evolution (in
algae, bryophytes, lycopods, gymnosperms and angiosperms) or that syringyl
lignins are an ancient feature that emerged in algae, even before land coloni-
zation and was lost a posteriori, except in some specific groups and angios-
perms. Recent data appear to point to convergent evolution, which suggests
that syringyl lignins play a key role in plant physiology, even though the
precise functions of S lignin in the plant have not been unravelled. In angios-
perms, most syringyl lignins are located in the vessel fibres, whereas xylem is
mainly composed of guaiacyl lignin, which suggests strong selective pressure
towards the presence of G lignin in conducting tissues ( Peter andNeale, 2004 ).
In a similar fashion, the Selaginella xylem contains guaiacyl lignin, while
syringyl lignin is restricted to epidermal and subepidermial/cortical tissues
( Fig. 6 ). An analogous pattern has also been shown in some ferns, such as
Ceratopteris ( Fig. 7 ). Some reports have associated the induction of S lignin
biosynthesis as a response against pathogens ( Wuyts et al., 2006 ), whereas
other authors describe an increase in lignin amount after pathogen challeng-
ing, but due to an increase in H and G units, while S units remain unaltered
( Gayoso et al., 2010; Pomar et al., 2004 ). The structural differences between
G and S lignin are caused by the presence of the methoxyl group at the
5-position, which results in S lignin being more linear and less condensed.
Bonawitz and Chapple (2010) have suggested S lignin confers flexibility to
plants. The flexible polymer may be important for herbaceous plants, which
every year grow their aerial biomass and therefore must grow quickly, a
requirement woody angiosperms and gymnosperms do not have.
Some lignins are known to be acetylated at the g -carbon of the side chain.
Recently, it has been reported this acetylation occurred naturally in lignins,
to a greater extent than previously thought ( del R´o et al., 2007; Ralph, 1996 ).
Acetylated lignin does not derive from acetylation of the growing polymer of
lignin, but from acetylated monolignols themselves, which are incorporated
into the lignin by the typical radical coupling polymerization process ( Lu and
Ralph, 2002, 2008 ). del R ´ o et al. (2007) reported the occurrence of naturally
acetylated lignins in all studied angiosperms, both woody and herbaceous,
but did not detect it in either of two gymnosperms (pine and spruce). The
presence of acetylation was restricted to the g -carbon and occurred predom-
inantly on S units, whereas acetylated G units were barely detected, though
bamboo and eucalyptus were some exceptions ( del R´o et al., 2007 ). Thus,
lignin acetylation was constrained to angiosperms and to species with a high
S/G ratio, as shown by the occurrence in abaca, kenaf and sisal ( del R´o et al.,
2004 ). Lu and Ralph (2002, 2008) established g -acetylated monolignols alter
lignin structure because the g -OH group participates in some post-coupling