Beta-galactosyltransferase involved in elongation of beta-1,6-linked galactan side chains on arabinogalactan proteins. Required for the progression of embryogenesis beyond the globular stage (PubMed:23837821). Beta-galactosyltransferase involved in the biosynthesis of type II arabinogalactan. Transfers galactose from UDP-galactose to a mixture of various oligosaccharides derived from arabinogalactan proteins. Forms a complex with GALT29A that can work cooperatively to enhance the activities of adding galactose residues at O6 positions to beta-1,6-linked galactan and beta-1,3-linked galactan (PubMed:24693939). Protein modification; protein glycosylation. Interacts with GALT29A. Expressed in the suspensor of globular stage embryos. Embryonic lethality, due to the arrest of embryo development at the globular stage. Belongs to the glycosyltransferase 31 family.