Identification of an arabinopyranosyltransferase from involved in the synthesis of the hemicellulose xyloglucan.

The hemicellulose xyloglucan consists of a backbone of a β-1,4 glucan substituted with xylosyl moieties and many other, diverse side chains that are important for its proper function. Many, but not all glycosyltransferases involved in the biosynthesis of xyloglucan have been identified. Here, we report the identification of an hitherto elusive xyloglucan:arabinopyranosyltransferase. This glycosyltransferase was isolated from the moss , where it acts as a xyloglucan "D"-side chain transferase (XDT). Heterologous expression of in the double mutant , where xyloglucan consists of a xylosylated glucan without further glycosyl substituents, results in the production of the arabinopyranose-containing "D" side chain as characterized by oligosaccharide mass profiling, glycosidic linkage analysis, and NMR analysis. In addition, expression of a related Physcomitrella glycosyltransferase ortholog of leads to the production of the galactose-containing "L" side chain. The presence of the "D" and "L" xyloglucan side chains in the Arabidopsis double mutant expressing and , respectively, rescues the dwarfed phenotype of untransformed mutants to nearly wild-type height. Expression of and in the mutant also enhanced root growth.