Probable arabinosyl transferase responsible for the polymerization of arabinose into the arabinan of arabinogalactan. May function as inverting enzyme using UDP-beta-L-arabinopyranoside. May be important for arabinan side chains of rhamnogalacturonan I (RG-I), a major component of pectins. Cell wall pectic arabinans are involved in thigmomorphogenesis response of inflorescence stems to mechanical stress. Homodimer and heterodimer with ARAD2. Expressed in root vasculature, cotyledons, leaves, stems, vascular tissue of sepals, petals and stamens, pollen grains, mature siliques and abscission region of seeds. No visible phenotype under normal growth conditions, but mutant plants have strong reduction of arabinose content in leaves and stems due to a decreased content of pectic arabinan. Belongs to the glycosyltransferase 47 family.