Identification of an arabinopyranosyltransferase from Physcomitrella patens involved in the synthesis of the hemicellulose xyloglucan

The hemicellulose xyloglucan consists of a backbone of a {beta}-1,4 glucan substituted with xylosyl moieties and many other, diverse sidechains 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 Physcomitrella patens, where it acts as a Xyloglucan \"D\"-side-chain Transferase (XDT). Heterologous expression of XDT in the Arabidopsis thaliana double mutant mur3.1 xlt2, 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 hortholog of XLT2 leads to the production of the galactose-containing \"L\" side chain. The presence of the \"D\" and \"L\" xyloglucan side chains in PpXDT mur3.1 xlt2 and PpXLT2 mur3.1 xlt2 transgenic plants, respectively, rescue the dwarfed phenotype of untransformed mur3.1 xlt2 mutants to nearly wild-type height. Expression of PpXDT and PpXLT2 in the Arabidopsis mur3.1 xlt2 mutant also enhanced root growth.