Glycosyltransferase involved in the formation of rhamnogalacturonan I (RG-I) oligosaccharides in the seed coat mucilage, which is a specialized cell wall with abundant RG-I (PubMed:30082766). Transfers the rhamnose residue from UDP-beta-L-rhamnose to RG-I oligosaccharides (PubMed:30082766). Prefers RG-I oligosaccharides with a degree of polymerization of 5 or larger than 5 (PubMed:30082766). Does not act on oligosaccharides with a degree of polymerization of 4 or smaller than 4 (PubMed:30082766). Does not require metal ions for its activity (PubMed:30082766). alpha-D-galacturonosyl-[(1->2)-alpha-L-rhamnosyl-(1->4)-alpha-D-galacturonosyl](n) + UDP-beta-L-rhamnose = [(1->2)-alpha-L-rhamnosyl-(1->4)-alpha-D-galacturonosyl](n+1) + H(+) + UDP Optimum pH is 7.0. Glycan metabolism; pectin biosynthesis. Highly expressed in siliques (PubMed:30082766). Expressed in stems and flowers (PubMed:30082766). Expressed at low levels in roots and rosette leaves (PubMed:30082766). During seed coat development, expressed from linear cotyledon to mature green stages, with a peak at the bent cotyledon stage. Reduced volume of the seed coat mucilage due to reduced levels of rhamnogalacturonan I (RG-I) in the seed coat mucilage. Belongs to the glycosyltransferase GT106 family. Truncated N-terminus.