Major galactinol synthase mainly involved in the biosynthesis of storage raffinose family oligosaccharides (RFOs) that function as osmoprotectants. May promote plant stress tolerance. myo-inositol + UDP-alpha-D-galactose = alpha-D-galactosyl-(1->3)-1D-myo-inositol + H(+) + UDP Expressed in source leaves, specifically in the mesophyll. By cold. Follows a circadian rhythm; accumulates mostly at midday during the light phase. Belongs to the glycosyltransferase 8 family. Galactosyltransferase subfamily.