The cell walls and arabinogalactan-proteins of streptophyte algae, bryophytes and ferns in the context of evolution

While much is known about the cell walls of seed plants, there is a significant gap in the knowledge on the cell walls of streptophyte algae and spore-producing land plants. To expand this knowledge, the cell walls of several streptophyte algae, bryophytes and ferns were isolated and characterized in this work. Analytical investigations revealed that the main polysaccharide classes of the cell walls of seed plants were also present in the cell walls of the species investigated, although there were differences in quantity and structure within the subclasses.
In addition to polysaccharides, cell wall glycoproteins, especially arabinogalactan-proteins (AGPs), are also part of the cell wall. The search for these AGPs in the cell walls of the Charales members demonstrated that only the galactan core structure of pyranosidic 1,3-, 1,6- and 1,3,6-galactose (Galp) known from seed plant AGPs was found, but not the connection of the carbohydrate and the protein part via hydroxyproline. Furthermore, the branched galactans were characterized by a unique feature: high amounts of the unusual monosaccharide 3-O-methyl Gal.
In all investigated land plant species, AGPs were present. Within the bryophytes, a hornwort AGP was isolated for the first time and revealed an unusual 1,3,4-Galp branching point. Furthermore, terminal furanosidic arabinose (t-Araf) residues dominated and were accompanied by low amounts of pyranosidic t-Ara and 1,3-Araf. The AGP of the eusporangiate fern Psilotum exhibited mainly 1,3-linked Araf and both t-Ara residues, but these in an equal ratio and thus resembled the AGP isolated from a lycophyte. In contrast, all leptosporangiate ferns were characterized by 1,2-linked Araf and t-Araf. All AGPs of the analysed species had in common that they contained end groups with rhamnose (Rha) and/or the unusual 3-O-methylated Rha, which is not part of angiosperm cell walls.