Plays a role in the arabinosylation of cell wall components. Involved in the arabinosylation of extensin proteins in root hair cells (PubMed:21680836). Extensins are structural glycoproteins present in cell walls and its arabinosylation is important for root hair cell development and root hair tip growth (PubMed:21680836, PubMed:25944827). The conserved DXD motif is involved in enzyme activity. Reduced root hair length and content of arabinosylated extensins in root cell walls. Belongs to the glycosyltransferase 77 family.