Glycosyltransferase involved in pectin and/or xylans biosynthesis in cell walls (By similarity). Required for the biosynthesis of pectin in seed coat epidermal (SCE) cells (PubMed:30228108). Collaboratively with MUCI70, 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). Catalyzes homogalacturonan (HG) elongation by acting as an HG alpha-1,4 galacturonic acid transferase (PubMed:30228108). [(1->4)-alpha-D-galacturonosyl](n) + UDP-alpha-D-galacturonate = [(1->4)-alpha-D-galacturonosyl](n+1) + H(+) + UDP Glycan metabolism; pectin biosynthesis. Monomer. Expressed in roots, inflorescences, siliques, seeds, leaves and stems. 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, associated with reduced absolute levels of rhamnose (Rha), galacturonic acid (GalA) and xylose (Xyl) but increased absolute abundance of minor sugars (e.g. galactose (Gal), 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). Belongs to the glycosyltransferase 8 family.