Trifunctional enzyme involved in UDP-beta-L-rhamnose biosynthesis, a precursor of the primary cell wall components rhamnogalacturonan I (RG-I) and rhamnogalacturonan II (RG-II). Catalyzes the dehydration of UDP-glucose to form UDP-4-dehydro-6-deoxy-D-glucose followed by the epimerization of the C3' and C5' positions of UDP-4-dehydro-6-deoxy-D-glucose to form UDP-4-keto-beta-L-rhamnose and the reduction of UDP-4-keto-beta-L-rhamnose to yield UDP-beta-L-rhamnose (PubMed:17190829). Required for the normal seed coat epidermal development (PubMed:14671019). UDP-alpha-D-glucose = H2O + UDP-4-dehydro-6-deoxy-alpha-D-glucose kcat is 2860 sec(-1) with UDP-glucose as substrate. Optimum pH is 7.5. Optimum temperature is 35 degrees Celsius. Carbohydrate biosynthesis. Expressed in roots, stems, leaves, seedlings, inflorescence tips, and siliques. Up-regulated at the time of mucilage production during seed coat differentiation. The dehydratase activity is contained in the N-terminal region while the epimerase and reductase activities are in the C-terminal region. In bacteria, TDP-L-rhamnose is formed by the successive action of three different enzymes on TDP-D-glucose. In plants, on the other hand, a single polypeptide probably catalyzes all three reactions that lead to the conversion of UDP-D-glucose to UDP-L-rhamnose. RHM2 appears to be more highly expressed in cv. Landsberg erecta than in cv. Col-2. Transcriptions factors AP2, TTG1, and GL2 are required for maximum levels of RHM2 expression at the time of mucilage production. In the N-terminal section; belongs to the NAD(P)-dependent epimerase/dehydratase family. dTDP-glucose dehydratase subfamily. In the C-terminal section; belongs to the dTDP-4-dehydrorhamnose reductase family.