Sugar transporters involved in flowering and grain development of rice

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 transporterOsSUT1 and the monosaccharide transportersOsMST1-3 have been previously characterized. To investigate sugar transport processes during flowering and in developing grains of rice, we newly isolated two genomic clonesOsSUT2(Oryzasativasucrosetransporter 2) andOsMST5(Oryzasativamonosaccharidetransporter 5) and their corresponding cDNAs.OsSUT2 andOsMST5are encoded by open reading frames of 1485 and 1557 bp encoding 495 and 519amino acids, respectively. The putative amino acid sequence ofOsSUT2showed 63.0 and 62.4 percnt; identity to that ofOsSUT1and barley transporterHvSUT1, respectively. Northern blot analysis revealed thatOsSUT2andOsMST5mRNA accumulates in panicles before pollination.In situ hybridization analysis showed thatOsSUT2transcript is specific to the developing pollen. These data presented suggest that bothOsSUT2andOsMST5 play a role during the development at the early stage of the seed development. On the other handOsSUT1 was expressed at the early stage of the grain development, suggesting a different physiological role compared toOsSUT2