Probable arabinosyl transferase responsible for the polymerization of arabinose into the arabinan of arabinogalactan. May function as inverting enzyme using UDP-beta-L-arabinopyranoside. Cell wall pectic arabinans are involved in thigmomorphogenesis response of inflorescence stems to mechanical stress. Homodimer and heterodimer with ARAD1. Belongs to the glycosyltransferase 47 family.