Overexpression of OsSTP1 increases grain yield via enhancing carbohydrate metabolism and transport in rice.

Overexpression of OsSTP1 enhances the non-structural carbohydrate remobilization in the source, starch accumulation in grains, and the transportation of carbohydrates from source to sink during the filling stage. The sugar transporter protein (STP) is the best-characterized subfamily of the monosaccharide transporter (MST) family and plays critical roles in regulating plant stress tolerance, growth, and development. However, the role of STPs in regulating rice yield is poorly understood. In this study, we report that compared with Taipei 309, overexpression of OsSTP1 can achieve higher rice yield. We demonstrate that OsSTP1 mRNA levels are higher than those of the other seven STPs in mixed samples of leaf sheaths, stems, and nodes at 12 days after pollination (DAP). OsSTP1 is prominently expressed in the leaf sheaths, stems, and nodes at the grain filling stage. Subcellular localization analysis revealed that OsSTP1 is localized in the plasma membrane. Overexpression of OsSTP1 increased the activities of amylase (AMY) and sucrose phosphate synthase (SPS) in mixed samples of leaf sheaths, stems, and nodes at 12 DAP, the sucrose content of the phloem exudate, and accumulation of soluble sugars and starch in panicles, ultimately increasing seed-setting rates and grain yields in the Taipei 309 cultivar. These findings indicate that overexpression of OsSTP1 can improve grain yield by synergistically promoting non-structural carbohydrate (NSC) remobilization and transportation.