Probable glycosyltransferase involved in pectin and/or xylans biosynthesis in cell walls (By similarity). Together with IRX14, required for xylan and pectin synthesis in seed coat epidermal (SCE) cells (PubMed:30228108). Collaboratively with GAUT11, essential for the accumulation of seed mucilage, a gelatinous wall rich in unbranched rhamnogalacturonan I (RG I), and for shaping the surface morphology of seeds (PubMed:30228108). Glycan metabolism; pectin biosynthesis. Expressed in siliques and seeds. Accumulates in developing seeds in siliques, mainly in the seed coat and, to a lesser extent, in the embryo. Strong reduction of seed mucilage accumulation and major decrease in rhamnogalacturonan I (RG I), associated with reduced absolute levels of rhamnose (Rha), arabinose (Ara) and galacturonic acid (GalA) but increased absolute abundance of minor sugars (e.g. galactose (Gal), xylose (Xyl), glucose (Glc) and mannose (Man)) (PubMed:30228108). The double mutant muci70-1 gaut11-3 is completely defective in seed mucilage production and exhibits a strong release of minor sugars in total mucilage extracts (PubMed:30228108). 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 8 family.