Transporter SISWEET15 unloads sucrose from phloem and seed coat for fruit and seed development in tomato

Tomato (Solanum lycopersium), 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 sugars will eventually be exported transporters (SWEETs) proteins is closely linked to high fructose/glucose ratios in tomato fruits and may be involved in sugar allocation. Here, we discovered that SISWEET15 is highly expressed in developing fruits compared to vegetative organs. In situ hybridization and beta-glucuronidase fusion analyses revealed SISWEET15 proteins accumulate in vascular tissues and seed coats, major sites of sucrose unloading in fruits. Localizing SISWEET15-green fluorescent protein to the plasma membrane supported its putative role in apoplasmic sucrose unloading. The sucrose transport activity of SISWEET15 was confirmed by complementary growth assays in a yeast (Saccharomyces cerevisiae) mutant. Elimination of SISWEET15 function by clustered regularly interspaced short palindromic repeats (CRISPRs)/CRISPR-associated protein gene editing significantly decreased average sizes and weights of fruits, with severe defects in seed filling and embryo development. Altogether, our studies suggest a role of SISWEET15 in mediating sucrose efflux from the releasing phloem cells to the fruit apoplasm and subsequent import into storage parenchyma cells during fruit development. Furthermore, SISWEET15-mediated sucrose efflux is likely required for sucrose unloading from the seed coat to the developing embryo.