Sucrose and hexoses derived from sucrose breakdown are importance for early grain development in cereals. These assimilates were used to promote division and elongation, differentiation of the cells and deposition in storage organs. OsMST5 (Oryza sativa monosaccharide transporter 5), OsSUT1 (Oryza sativa sucrosetransporter 1) and OsSUT2 (Oryza saliva sucrose transporter 2) have been cloned from rice. All mRNAs of these functional sugar transporters were detected during flowering and grain filling stage. The OsMST5 and OsSUT2 mRNAs were exclusively detectable before the pollinating stage by Northern analysis and the OsSUT2 mRNA was localized in archespores and pollen mother cells by in situ mRNA detection, indicating that these gene products are involved in import of sucrose to promote pollen development at the early stage. Moreover, the OsSUT1 whiclh is known to be phloem companion cell-specific expression in source organs, was also expressed in the sink organs as well as modified aleurone cells of the developing endosperm. These results indicatethat the OsSUT1 plays a role in re-uptake of sucrose from endosperm cavity to accumulate starch in grain filling stage.