Acts as a UDP-D-xylose 4-epimerase but lacks both UDP-D-glucose and UDP-D-glucuronic acid 4-epimerase activities in vitro. UDP-beta-L-arabinopyranose = UDP-alpha-D-xylose Nucleotide-sugar biosynthesis; UDP-L-arabinose biosynthesis; UDP-L-arabinose from UDP-alpha-D-xylose: step 1/1. Cell wall biogenesis; cell wall polysaccharide biosynthesis. A number of isoforms are produced. According to EST sequences. High expression in roots. Also found in leaves, stems, flowers, and siliques. Belongs to the NAD(P)-dependent epimerase/dehydratase family.