Involved in the biosynthesis of the cyanogenic glycoside (R)-prunasin, a precursor of (R)-amygdalin, which at high concentrations is associated with intense bitterness in kernels of almond (PubMed:32688778). Stereo-selectively glucosylates (R)-mandelonitrile to produce (R)-prunasin (PubMed:32688778). (R)-mandelonitrile + UDP-alpha-D-glucose = (R)-prunasin + H(+) + UDP Belongs to the UDP-glycosyltransferase family.