Glycosyltransferase involved in the O-arabinosylation of several proteins including extensins and small signaling peptides (PubMed:24036508, PubMed:26577059). Catalyzes the transfer of the initial L-arabinose to the hydroxyl group of Hyp residues (PubMed:24036508). Contributes redundantly with HPAT1 and HPAT3 to arabinosylation of EXT3 (PubMed:24036508). trans-4-hydroxy-L-prolyl-[protein] + UDP-beta-L-arabinofuranose = H(+) + O-(beta-L-arabinofuranosyl)-trans-4-hydroxy-L-prolyl-[protein] + UDP Ubiquitous. No visible phenotype (PubMed:24036508). Short-root-hair phenotype (PubMed:25944827). Hpat1 hpat2 double mutants have longer hypocotyls, are early flowering and show early senescence in leaves associated with a decrease in chlorophyll content (PubMed:24036508). Hpat1 hpat2 hpat3 triple mutants fail to produce detectable levels of Hyp-arabinosides, have low fertility and shorter pollen tubes (PubMed:26577059).