Glycobiology Advance Access published online on August 14, 2008
Glycobiology, doi:10.1093/glycob/cwn078
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Moss and liverwort xyloglucans contain galacturonic acid and are structurally distinct from the xyloglucans synthesized by hornworts and vascular plants+
2 Complex Carbohydrate Research Center, University of Georgia, Athens, GA 30602, USA
3 Department of Biochemistry and Molecular Biology, University of Georgia, Athens, GA 30602, USA
1 To whom correspondence should be addressed: Tel: +1-706-542-4419; Fax: +1-706-542-4412; e-mail: mpena{at}ccrc.uga.edu
Received on July 7, 2008; accepted on August 7, 2008
Xyloglucan is a well-characterized hemicellulosic polysaccharide that is present in the cell walls of all seed-bearing plants. The cell walls of avascular and seedless vascular plants are also believed to contain xyloglucan. However, these xyloglucans have not been structurally characterized. This lack of information is an impediment to understanding changes in xyloglucan structure that occurred during land plant evolution. In this study, xyloglucans were isolated from the walls of avascular (liverworts, mosses, and hornworts) and seedless vascular plants (club and spike mosses, ferns and fern allies). Each xyloglucan was fragmented with a xyloglucan-specific endo-glucanase and the resulting oligosaccharides then structurally characterized using NMR spectroscopy, MALDI-TOF and electrospray mass spectrometry, and glycosyl-linkage and glycosyl residue composition analyses. Our data show that xyloglucan is present in the cell walls of all major divisions of land plants and that these xyloglucans have several common structural motifs. However, these polysaccharides are not identical because specific plant groups synthesize xyloglucans with unique structural motifs. For example, the moss Physcomitrella patens and the liverwort Marchantia polymorpha synthesize XXGGG- and XXGG-type xyloglucans, respectively, with side chains that contain a β-D-galactosyluronic acid and a branched xylosyl residue. By contrast, hornworts synthesize XXXG-type xyloglucans that are structurally homologous to the xyloglucans synthesized by many seed-bearing and seedless vascular plants. Our results increase our understanding of the evolution, diversity and function of structural motifs in land plant xyloglucans and provide support to the proposal that hornworts are sister to the vascular plants.
Key words: Xyloglucan / plant cell wall / land plants / evolution
+ Dedicated to the memory of Roger A. O’Neill, 1957 – 2008.