Molecular characterization of sugar transporters involved in flowering and grain development of rice

Summary Developing seeds offer a fine experimental system for study on sugar transport mechanism(s) in sink tissue. The sugar transporters identified as being specific to the seed development may play a crucial role for flowering and grain development. The rice sucrose transporter OsSUT1 and the monosaccharide transporters OsMST1–3 have been previously characterized. To investigate sugar transport processes during flowering and in developing grains of rice, we newly isolated two genomic clones OsSUT2 (Oryza sativa sucrose transporter 2) and OsMST5 (Oryza sativa monosaccharide transporter 5) and their corresponding cDNAs. OsSUT2 and OsMST5 are encoded by open reading frames of 1485 and 1557bp encoding 495 and 519 amino acids, respectively. The putative amino acid sequence of OsSUT2 showed 63.0 and 62.4 % identity to that of OsSUT1 and barley transporter HvSUT1, respectively. Northern blot analysis revealed that OsSUT2 and OsMST5 mRNA accumulates in panicles before pollination. In situ hybridization analysis showed that OsSUT2 transcript is specific to the developing pollen. These data presented suggest that both OsSUT2 and OsMST5 play a role during the development at the early stage of the seed development. On the other hand OsSUT1 was expressed at the early stage of the grain development, suggesting a different physiological role compared to OsSUT2.

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