MATE transporter GFD1 cooperates with sugar transporters, mediates carbohydrate partitioning, and controls grain filling duration, grain size and number in rice.

More than half of the world's food is provided by cereals, as humans obtain >60% of daily calories from grains. Producing more carbohydrates is always the final target of crop cultivation. The carbohydrate partitioning pathway directly affects grain yield, but the molecular mechanisms and biological functions are poorly understood, including rice (Oryza sativa L.), one of the most important food sources. Here, we reported a prolonged grain filling duration mutant 1 (gfd1), exhibiting a long grain filling duration, less grain number per panicle, and bigger grain size without changing grain weight. Map-based cloning and molecular biological analyses revealed that GFD1 encoded a MATE transporter and expressed high in vascular tissues of the stem, spikelet hulls, and rachilla, but low in the leaf, controlling carbohydrate partitioning in the stem and grain but not in the leaf. GFD1 protein was partially localized on the plasma membrane and in the Golgi apparatus, and was finally verified to interact with two sugar transporters, OsSWEET4 and OsSUT2. Genetics analyses showed that GFD1 might control grain filling duration through OsSWEET4, adjust grain size with OsSUT2, and synergistically modulate grain number per panicle with both OsSUT2 and OsSWEET4. Together, our work proved that, the three transporters, which are all initially classified in the MFS family, could control starch storage in both the primary sink (grain) and temporary sink (stem), and affect carbohydrate partitioning in the whole plant through physical interaction, giving a new vision of sugar transporter interactome and providing a tool for rice yield improvement.