GX abundance and xylan chain length are affected in all of these mutants, and xylan synthase (XylS) activity measured in microsome preparations is lower in the irx14 mutant and irx10 irx10L double mutant (Brown et al. 2009). However, XylT activity has only been demonstrated when coexpressing AtIRX9 and AtIRX14, or the respective putative orthologs from poplar (Populus trichocarpa) PtGT43B and PtGT43C, simultaneously in tobacco microsomes with exogenously fed xylooligomers (Lee et al. 2012c, d).