Occurrence of the primary cell wall polysaccharide rhamnogalacturonan II in bryophytes:
Implications for the evolution of vascular plants.
The first plants that colonised the land approximately 480 million years ago are believed to be related to extant bryophytes. These early land plants subsequently gave rise to the trachaeophytes that now dominate the terrestrial environment. One of the most important biochemical changes that allowed plants to adapt to life on land is borate ester cross-linking of the cell wall, pectic polysaccharide rhamnogalacturonan Il (RG II). Recent studies have shown that boron has a major role in maintaining wall structure. Boron requirements of flowering plants are correlated with the amounts of pectin in their primary walls. Boron exits in primary walls predominantly as 1:2 borate diol ester that covalently cross-links two chains of the pectic polysaccharides RG Il. Plants carrying mutations, result in abnormally low amounts of this cross-link. Such plants are dwarfed or exhibit reduced intercellular attachment. The gametophytic generation of members of the avascular bryophytes (Bryopsida, Hepaticopsida and Anthocerotopsida) have primary cell walls that conta
small amount of an RG Il like polysaccharide. The amount of RG Il incorporated into the walls of plants increased during evloution of vascular plants from their bryophyte like ancestors. Thus, it can be concluded that during evolution bryophytes gave rise to higher plants
