Involved in pectin biosynthesis. Catalyzes the transfer of galacturonic acid from uridine 5'-diphosphogalacturonic acid onto the pectic polysaccharide homogalacturonan. [(1->4)-alpha-D-galacturonosyl](n) + UDP-alpha-D-galacturonate = [(1->4)-alpha-D-galacturonosyl](n+1) + H(+) + UDP Glycan metabolism; pectin biosynthesis. Expressed in seedlings, inflorescences, flowers, siliques, pollen, roots, stems and leaves. Belongs to the glycosyltransferase 8 family.