Arabinogalactan-proteins of the eusporangiate fern Psilotum nudum show atypical structural features compared to other ferns

During the colonization of land, abiotic and biotic challenges induced strong adaptations in plants, including changes of their cell walls. Up to now, cell walls of spore-bearing plants have been only sparsely investigated. As ferns are the sister group to seed plants, studying their cell walls is key to understanding the evolutionary development of the cell wall in the tracheophyte lineage. Beside polysaccharides, especially arabinogalactan-proteins (AGPs) are important cell wall components involved in structural but also signalling functions. We chose a member of the eusporangiate ferns, <i>Psilotum nudum</i>, to characterize and immunolocalize AGPs. Biochemical analyses identified special structural features of <i>P. nudum</i> AGPs in comparison to <i>A. thaliana</i> AGPs, including lower amounts of 1,6-linkage in the galactan core. With regard to the arabinose (Ara) residues, <i>A. thaliana</i> AGP was dominated by terminal furanosidic Ara, followed by 1,5-Ara, whereas in <i>P. nudum</i> AGPs, 1,3-Ara, terminal furanosidic Ara and terminal pyranosidic Ara occurred in comparable amounts. Terminal pyranosidic rhamnose was only found in the fern AGP. Immunofluorescence analysis using JIM13 antibody detected AGPs in all tissues of the <i>P. nudum</i> stem with the exception of the mature protoxylem. Localization was observed along the plasma membrane as well as tonoplast.