Biochemical characterization of xylan xylosyltransferases involved in wood formation in poplar

The major polysaccharides in dicot wood biomass are cellulose and xylan. Although wood-associated cellulose synthase genes responsible for cellulose biosynthesis have been characterized, wood-associated xylan synthase genes have not been biochemically identified. A recent report by Lee et al. (2012) provides the first biochemical evidence that two functionally non-redundant Arabidopsis GT43 members are xylosyltransferases (XylTs) that function cooperatively in the elongation of the xylan backbone. We further extend this finding in the current report demonstrating that two poplar (Populus trichocarpa) GT43 glycosyltransferases, PtrGT43B and PtrGT43C, are xylan XylTs involved in wood formation. We show that microsomes from transgenic tobacco BY2 cells coexpressing PtrGT43B and PtrGT43C exhibited a high XylT activity capable of generating beta-(1,4)-linked xylooligosaccharides, whereas little XylT activity was detected in microsomes with expression of PtrGT43B or PtrGT43C alone. These findings indicate that poplar GT43 members are XylTs that act cooperatively in catalyzing the successive transfer of xylosyl residues during xylan backbone biosynthesis, which provides further support of the hypothesis that the biochemical functions of GT43 members in vascular plants are evolutionarily conserved.