Involved in the synthesis of the hemicellulose glucuronoxylan, a major component of secondary cell walls (PubMed:15980264, PubMed:16844780, PubMed:17322407, PubMed:17938130, PubMed:17944810, PubMed:20335400, PubMed:20424005). Xylan xylosyltransferase that acts cooperatively with IRX14 to achieve the successive addition of xylosyl residues during xylan backbone elongation (PubMed:22080591, PubMed:25118690). [(1->4)-beta-D-xylan](n) + UDP-alpha-D-xylose = [(1->4)-beta-D-xylan](n+1) + H(+) + UDP Expressed in developing interfascicular fibers, primary and secondary xylem in stems and developing secondary xylem in roots. Dwarf phenotype. Collapsed xylem vessels and reduced xylan content in cell wall. Belongs to the glycosyltransferase 43 family.