Galactinol synthase involved in the biosynthesis of raffinose family oligosaccharides (RFOs) that function as osmoprotectants. Promotes stress tolerance of factors such as drought, chilling, salinity and methylviologen (MV), a superoxide radical generating drug, by mediating an increase in levels of the endogenous osmoprotective compounds, galactinol and raffinose. myo-inositol + UDP-alpha-D-galactose = alpha-D-galactosyl-(1->3)-1D-myo-inositol + H(+) + UDP Accumulates in mature seeds. Induced by abscisic acid (ABA), drought and high-salinity stresses. Promoted by methylviologen (MV), a superoxide radical generating drug, a superoxide radical generating drug. Belongs to the glycosyltransferase 8 family. Galactosyltransferase subfamily.