Evidence for covalent linkage between xyloglucan and acidic pectins in suspension-cultured rose cells

Abstract

Neutral xyloglucan was purified from the cell walls of suspension-cultured rose (Rosa sp. 'Paul's Scarlet') cells by alkali extraction, ethanol precipitation and anion-exchange chromatography on 'Q-Sepharose FastFlow'. The procedure recovered 70% of the total xyloglucan at about 95% purity in the neutral fraction. The remaining 30% of the xyloglucan was anionic, as demonstrated both by anion-exchange chromatography at pH 4.7 and by high-voltage electrophoresis at pH 6.5. Alkali did not cause neutral xyloglucan to become anionic, indicating that the anionic nature of the rose xyloglucan was not an artefact of the extraction procedure. Pre-incubation of neutral [3H]xyloglucan with any of ten non-radioactive acidic polysaccharides did not cause the radioactive material to become anionic as judged by electrophoresis, indicating that stable complexes between neutral xyloglucan and acidic polysaccharides were not readily formed in vitro. The anionic xyloglucan did not lose its charge in the presence of 8 M urea or after a second treatment with NaOH, indicating that its anionic nature was not due to hydrogen-bonding of xyloglucan to an acidic polymer. Proteinase did not affect the anionic xyloglucan, indicating that it was not associated with an acidic protein. Cellulase converted the anionic xyloglucan to the expected neutral nonasaccharide and heptasaccharide, indicating that the repeatunits of the xyloglucan did not contain acidic residues. Endo-polygalacturonase converted about 40% of the anionic xyloglucan to neutral material. Arabinanase and galactanase also converted appreciable proportions of the anionic xyloglucan to neutral material. These results show that about 30% of the xyloglucan in the cell walls of suspension-cultured rose cells exists in covalently-linked complexes with acidic pectins.

Wall Structure and Wall Loosening. A Look Backwards and Forwards

“…Recent work has yielded new evidence that a small amount of xyloglucan is indeed attached to pectic polysaccharides (23), thus reviving the model of Figure 1A. Whether this pectin-xyloglucan molecular functions to hold microfibrils together remains to be seen.…”

Revolutionary Times in Our Understanding of Cell Wall Biosynthesis and Remodeling in the Grasses

“…There have also been suggestions that pectic polysaccharides might be covalently linked with xyloglucans in plant cell walls (Keegstra et al, 1973;Thompson and Fry, 2000;Cumming et al, 2005;Popper and Fry, 2005). However, the HvXET5 enzyme purified by Hrmova et al (2007) did not link pectinrelated polysaccharides such as polygalacturonan or b-D-galactans to xyloglucan, nor did it link arabinoxylans to xyloglucans, despite suggestions based on molecular modeling (Strohmeier et al, 2004) that this was a possibility.…”

Cell wall biosynthesis and the molecular mechanism of plant enlargement

“…Xyloglucans are thought to cross-bridge between pectins and cellulose microfibrils with intertwining segments of the molecules or with hydrogen bonds (Passioura and Fry 1992; (Thompson and Fry 2000). A substantial fraction of xyloglucan is covalently linked to pectin in terrestrial species (Thompson and Fry 2000;Fry 2005, 2008).…”

“…A substantial fraction of xyloglucan is covalently linked to pectin in terrestrial species (Thompson and Fry 2000;Fry 2005, 2008). This suggests that, when present in the wall, xyloglucans contribute to load-bearing by pectins but are not essential.…”