Possesses hydroxyproline O-galactosyltransferase activity. Transfers galactose from UDP-galactose to hydroxyproline residues in the arabinogalactan proteins (AGPs). Is specific for AGPs containing non-contiguous peptidyl hydroxyproline residues. Utilizes UDP-galactose solely as sugar donor. The addition of galactose onto the peptidyl hydroxyproline residues in AGP core proteins represents the first committed step in arabinogalactan polysaccharide addition. AGP glycans play essential roles in both vegetative and reproductive plant growth. Optimum pH is 6.5. Protein modification; protein glycosylation. A number of isoforms are produced. According to EST sequences. Expressed in stems and at lower levels in cauline leaves and siliques. By salt stress. Reduced levels of arabinogalactan proteins (PubMed:26690932, PubMed:25974423). Defects in root hair growth, root elongation, pollen tube growth, flowering time, leaf development, silique length and inflorescence growth. Increased sensitivity to salt stress (PubMed:25974423). Belongs to the glycosyltransferase 31 family.