Galactinol synthase involved in the biosynthesis of raffinose family oligosaccharides (RFOs) that function as osmoprotectants. Promotes plant stress tolerance such as heat, chilling, salinity and methylviologen (MV), a superoxide radical generating drug, by mediating raffinose accumulation, an osmoprotective substance. myo-inositol + UDP-alpha-D-galactose = alpha-D-galactosyl-(1->3)-1D-myo-inositol + H(+) + UDP Accumulates in mature seeds. Expressed in seedlings (axes and cotyledons), meristems, vascular tissues and emerging lateral roots. Present in abscission zones. Induced by abscisic acid (ABA), heat, drought and high-salinity stresses. Promoted by methylviologen (MV), a superoxide radical generating drug. Impaired raffinose accumulation in response to heat stress. Belongs to the glycosyltransferase 8 family. Galactosyltransferase subfamily.