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 HPAT2 to arabinosylation of EXT3, but main contributor to arabinosylation of CLE peptides (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, but reduced level of arabinosylation of EXT3 (PubMed:24036508). Short-root-hair phenotype (PubMed:25944827). Hpat1 hpat3 double mutants have an impaired growth of pollen tubes, thereby causing a transmisson defect through the male gametophyte (PubMed:24036508, PubMed:26577059). Hpat1 hpat2 hpat3 triple mutants fail to produce detectable levels of Hyp-arabinosides, have low fertility and shorter pollen tubes (PubMed:26577059).