Sugar uptake and transport in rice embryo. Expression of companion cell-specific sucrose transporter (OsSUT1) induced by sugar and light

We investigated sugar uptake and transport in rice (Oryza sativa) embryo du ring grain germination. Endogenous sugar levels, accumulation of starch gra nules, and gene expression of a rice sucrose transporter (OsSUT1) were exam ined using rice embryos germinated with or without exogenous sugar supply. Starch granules remarkably accumulated in the cells around vascular bundles as a consequence of the sugar taken up by the embryos, indicating that the taken-up sugars are transiently converted into starch. In situ detection f or OsSUT1 mRNA indicated its localization in the phloem companion cells. Fu rthermore, northern-blot and in situ hybridization analyses showed that OsS UT1 expression is not detectable in embryos subjected to sugar starvation c onditions, whereas its expression is enhanced by an increased endogenous su gar level. Overall results indicate that the expression of companion cell-s pecific sucrose transporter, OsSUT1 is regulated by the endogenous sugar st atus as well as light exposure.