Involved in the synthesis of the hemicellulose glucuronoxylan, a major component of secondary cell walls (PubMed:17944810, PubMed:20595206, PubMed:20335400, PubMed:20424005). Involved in the elongation of glucuronoxylan xylosyl backbone (PubMed:17944810, PubMed:20595206, PubMed:20335400, PubMed:20424005). Xylan xylosyltransferase that acts cooperatively with IRX9 to achieve the successive addition of xylosyl residues during xylan backbone elongation (PubMed:22080591, PubMed:25118690). Required for the proper composition and structural properties of released seed coat mucilage (PubMed:26482889, PubMed:26834178). Required for the production of highly branched xylan polymers in seed coat mucilage (PubMed:26482889, PubMed:26834178). Xylan with xylose side chains seems to be necessary for pectin attachment to the seed surface (PubMed:26482889, PubMed:26834178). Together with MUCI70, required for xylan and pectin synthesis in seed coat epidermal (SCE) cells (PubMed:26482889, PubMed:26834178, PubMed:30228108). [(1->4)-beta-D-xylan](n) + UDP-alpha-D-xylose = [(1->4)-beta-D-xylan](n+1) + H(+) + UDP Expressed in developing interfascicular fibers and xylem cells in stems and developing secondary xylem in roots. Expressed in the seed coat at the linear cotyledon and mature green stages, when mucilage synthesis occurs. Reduced xylan content in cell wall (PubMed:17944810). Reduced amount and altered composition of seed coat mucilage (PubMed:26482889, PubMed:26834178). Plants missing both MUCI70 and IRX14 exhibit a severe reduction in both xylan- and pectin-related sugars in total seed mucilage extracts (PubMed:30228108). Belongs to the glycosyltransferase 43 family.