SlSWEET15 exports sucrose from phloem and seed coat in tomato to supply carbon for fruit and seed development

Tomato, an important fruit crop worldwide, requires efficient sugar allocation for fruit development. However, molecular mechanisms for sugar import to fruits remain poorly understood. Expression of SWEET (Sugars Will Eventually be Exported Transporters) proteins is closely linked with hexose ratio in tomato fruits and may be involved in sugar allocation. Here, using quantitative PCR, we discovered that SlSWEET15 was highly expressed in developing fruits compared to vegetative organs. Based on in situ hybridization and GUS fusion analyses, SlSWEET15 proteins accumulated in vascular tissues and seed coats, major sites of sucrose unloading in fruits. Localizing SlSWEET15-GFP to the plasma membrane supported its putative role in apoplasmic sucrose unloading. The sucrose transport activity of SlSWEET15 was confirmed by complementary growth assays in a yeast mutant. Elimination of the SlSWEET15 function by CRISPR/cas9 gene editing significantly decreased average sizes and weights of fruits, with severe defects in seed filling and embryo development. Together, we confirmed the role of SlSWEET15 in mediating sucrose efflux from the releasing phloem to the fruit apoplasm and subsequent import into parenchyma cells during fruit development. Furthermore, SlSWEET15-mediated sucrose efflux was also required for sucrose unloading from the seed coat to the developing embryo. One-sentence SummarySlSWEET15, a specific sucrose uniporter in tomato, mediates apoplasmic sucrose unloading from releasing phloem cells and seed coat for carbon supply during fruit expansion and seed filling.