Biology Reference
In-Depth Information
underwater meadows in the Mediterranean Sea. Posidonia belongs to a group
of land angiosperms that returned to the sea in the Cretacic period, 100 mya
(
Klap et al., 2000; Larkum and Den Hartog, 1989
). The P. oceanica lignin
composition mirrors the angiosperm pattern, with an H:G:S ratio of 8:30:62
(
Espi˜ eira et al., 2011
), despite its aquatic habitat (
Fig. 9
).
The higher complexity of monocot cell walls is not only restricted to lignin
composition but also polysaccharide compounds. In their primary cell wall,
poales possess more xylan and less polysaccharides derived from galactose and
fucose than other angiosperms. They also contain MLG, which was thought
to be a unique component of poales cell walls (
Sørensen and Willats, 2008
).
The concentration of expansin is much higher in cell walls from poales
than in dicots and is absent in bryophytes. This distribution links the pres-
ence of expansin to xylem development, and suggests a divergence of the
genes before the appearance of angiosperms (
Carey and Cosgrove, 2007
).
Fig. 9. Lignins in an aquatic monocotyledon, Posidonia oceanica. (A) View of
several Posidonia plants. (B) Mass spectrum and gas chromatography profiles (inner
box) of the thioethylated monomers (erythro and threo isomers) arising from aryl-
glycerol-
b
-aryl ether (
b
-O-4) structures derived from sinapyl alcohol from P. oceanica
lignins.
