Required for xylan biosynthesis, but not directly involved in catalyzing the addition of sugars to the growing polymer. Also located in an unknown intracellular compartment that may have some role in the transport of cell wall polysaccharides into the cell wall. Expressed in rosette leaves, stems and siliques. Expressed in the xylem. Up-regulated during secondary cell wall deposition. No visible phenotype; due to the redundancy with IRX15-L. Irx15 and irx15-l double mutants have a mild collapsed xylem phenotype, irregular secondary cell wall margins in fiber cells, uneven distribution of xylan in the cell wall and a lower degree of xylan polymerization, but no visible growth phenotype. Extended C-terminus.