Abstract
Arabinogalactan-proteins (AGPs) are highly glycosylated hydroxyproline-rich glycoproteins present in plant cell walls. AGPs are characterized by arabinose-/galactose-rich side chains, which define their interactive molecular surface. Fucose residues are found in some dicotyledon AGPs, and AGP fucosylation is developmentally regulated. We previously identified Arabidopsis thaliana FUT4 and FUT6 genes as AGP-specific fucosyltransferases (FUTs) based on their enzymatic activities when heterologously expressed in tobacco (Nicotiana tabacum) BY2 suspension-cultured cells. Here, the functions of FUT4 and FUT6 and the physiological roles of fucosylated AGPs were further investigated using Arabidopsis fut4, fut6, and fut4/fut6 mutant plants. All mutant plants showed no phenotypic differences compared to wild-type plants under physiological conditions, but showed reduced root growth in the presence of elevated NaCl. However, roots of wild-type and fut4 mutant plants contained terminal fucose epitopes, which were absent in fut6 and fut4/fut6 mutant plants as indicated by eel lectin staining. Monosaccharide analysis showed fucose was present in wild-type leaf and root AGPs, but absent in fut4 leaf AGPs and in fut4/fut6 double mutant leaf and root AGPs, indicating that FUT4 was required for fucosylation of leaf AGPs while both FUT4 and FUT6 contributed to fucosylation of root AGPs. Glycome profiling of cell wall fractions from mutant roots and leaves showed distinct glycome profiles compared to wild-type plants, indicating that fucosyl residues on AGPs may regulate intermolecular interactions between AGPs and other wall components. The current work exemplifies the possibilities of refinement of cell wall structures by manipulation of a single or a few cell wall biosynthetic genes.
Highlights
Primary plant cell walls are a composite of complex carbo- microfibres cross-linked by xyloglucan hemicellulose are thought hydrates and protein components
Cellulose to constitute the load-bearing framework, which is believed to Abbreviations: AG, arabinogalactan; AGP, arabinogalactan-protein; ANOVA, analysis of variance; Ara, arabinose; CCRC, Complex Carbohydrate Research Center; DAG, days after germination; ELISA, enzyme-linked immunosorbent assay; Fuc, fucose; FUC XG, fucosylated xyloglucans; FUT, fucosyltransferase; Gal, galactose; gas chromatographymass spectrometry (GC-MS), gas chromatography-mass spectrometry; GT, glycosyltransferase; HG, homogalacturonan; HRGP, hydroxyproline-rich glycoprotein; MS, Murashige and Skoog; NON-FUC XG, non-fucosylated xyloglucans; qPCR, quantitative PCR; RG, rhamnogalacturonan
When whole roots of 14-day-old Arabidopsis plants were subjected to staining with eel lectin conjugated to Texas Red, the wild-type and fut4 mutant plants showed a bright, patchy staining pattern on the root surface, which was absent in the fut6 and fut4/fut6 mutant roots (Fig. 6)
Summary
Primary plant cell walls are a composite of complex carbo- microfibres cross-linked by xyloglucan hemicellulose are thought hydrates and protein components. AGPs, having the greatest number of family members and the highest level of glycosylation of all the HRGPs, are implicated in various aspects of plant growth and development (Ellis et al, 2010; Nothnagel, 1997; Seifert and Roberts, 2007; Showalter, 2001) As their name implies, AGPs are extensively glycosylated with type II arabinogalactan (AG) polysaccharides, which are mainly composed of galactose (Gal) and arabinose (Ara) residues, but may contain other sugars, including rhamnose, glucuronic acid, galacturonic acid, and fucose (Fuc) (Ellis et al, 2010; Nothnagel, 1997; Tan et al, 2004). Given that the sugar side chains typically account for more than 90% of the molecular mass of AGPs, they are likely to define the interactive surface of the molecule and its function
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