Involvement of the sucrose transporter, OsSUT1, in the long-distance pathway for assimilate transport in rice

The roles of the rice sucrose transporter, OsSUT1, have previously been examined in filling grain, germination, and early seedling growth. In the current work, the role that OsSUT1 plays in the transport of assimilate along the entire long-distance pathway, from the flag leaf blade to the base of the filling grain, was investigated. OsSUT1 promoter::GUS (beta-glucuronidase) reporter gene analysis and immunolocalization revealed that both OsSUT1 promoter::GUS activity and OsSUT protein were present in the mature phloem of all the vegetative tissues involved in the long-distance assimilate transport pathway during grain filling. In addition, expression was observed in the flag leaf blade and sheath prior to heading. The OsSUT1 promoter::GUS activity appeared to be largely confined to the companion cells within the phloem, whereas the protein localized to both the sieve tubes and the companion cells. RT-PCR analysis confirmed that the OsSUT1 transcript is expressed in the uppermost internode of the rice plant (internode-1). These OsSUT localization data were related to measurements of starch and soluble sugar content of these tissues, and localization of the carbohydrate reserves stored in the stem. Results from dye feeding experiments, to examine cellular connections, revealed a symplastic continuity between the phloem and surrounding parenchyma in the flag leaf blade, sheath, and internode-1 tissues. It is proposed that OsSUT1 may primarily play a role in phloem loading of sucrose retrieved from the apoplasm along the transport pathway.