The raffinose oligosaccharides (RFOs) is widely distributed in seeds and leaves of many plant species.RFOs is associated with plant abiotic stress tolerance.GALACTINOL SYNTHASE (GOLS) and RAFFINOSE SYNTHASE (RS) are two enzymes required for RFOs biosynthesis.GOLS has been extensively investigated while little is known about RS.We have retrieved one unique gene encoding maize RAFFINOSE SYNTHASE (ZmRS) from the maize B73 genome.The ZmRS bacterial expression product was able to synthesize raffinose or hydrolyze galactinol in vitro.The ZmRS gene is specifically expressed in leaves and embryos, and is induced by dehydration, heat shock,cold stress and salinity in maize seedlings.Mutator (Mu) interrupted zmrs plants failed to produce any raffinose in either leaves or seeds and seedlings deficient in raffinose are more sensitive to drought stress.The zmrs seed show significantly reduced seed vigor compared to its counterpart wild type control.Overexpression of ZmRS in Arabidopsis showed that ZmRS hydrolyzed galactinol in leaves and enhanced drought stress tolerance of the plant while it increased raffinose content, decreased stachyose or verbascose content, and reduced the artificial aging tolerance of the seeds.This work shows that the ZmRS plays important roles in plant drought stress tolerance and seed vigor.