A comprehensive analysis of the carbohydrate-containing macromolecules from the coencocytic green seaweed Codium fragile and their arrangement in the cell wall was carried out. Cell walls in this seaweed are highly complex structures composed of 31% (w/w) of linear (1→4)-β-D-mannans, 9% (w/w) of pyruvylated arabinogalactan sulfates (pAGS), and low amounts of hydroxyproline rich-glycoprotein epitopes (HRGP). In situ chemical imaging by synchrotron radiation Fourier transform infrared (SR-FTIR) microspectroscopy and by immunolabeling using antibodies against specific cell wall carbohydrate epitopes revealed that β-D-mannans and pAGS are placed in the middle part of the cell wall, whereas HRGP epitopes (arabinogalactan proteins (AGPs) and extensins) are located on the wall boundaries, especially in the utricle apical zone. pAGS are sulfated at C-2 and/or C-4 of the 3-linked β-L-arabinopyranose units and at C-4 and/or C-6 of the 3-linked β-D-galactopyranose residues. In addition, high levels of ketals of pyruvic acid were found mainly at 3,4- of some terminal β-D-Gal p units forming a five-membered ring. Ramification was found at some C-6 of the 3-linked β-D-Gal p units. In agreement with the immunolabeled AGP epitopes, a nonsulfated branched furanosidic arabinan with 5-linked α-L-Ara f, 3,5-linked α-L-Ara f, and terminal α-L-Ara f units and a nonsulfated galactan structure composed of 3-(3,6)-linked β-D-Gal p residues, both typical of type-II AG glycans were found, suggesting that AGP structures are present at low levels in the cell walls of this seaweed. Based on this study, it is starting to emerge that Codium has developed unique cell wall architecture, when compared, not only with that of vascular plants, but also with other related green seaweeds and algae.
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