Involved in the biosynthesis of galactolipids found in the photosynthetic membranes. Catalyzes the isomerization of monoglucosyldiacylglycerol (GlcDG) to yield monogalactosyldiacylglycerol (MGDG). a 1,2-diacyl-3-O-(beta-D-glucopyranosyl)-sn-glycerol = a 1,2-diacyl-3-O-(beta-D-galactosyl)-sn-glycerol The mutant lacks both monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), and accumulates GlcDG. It possesses intact thylakoid membranes and shows normal maximal photosynthetic activity, albeit with reduced utilization of light energy. Belongs to the short-chain dehydrogenases/reductases (SDR) family.