SlABCB13 Regulates Hexose and Sucrose Homeostasis during Tomato Fruit Development

Sugar accumulation in fleshy fruits is largely dependent on membrane-localized sugar transporters, among which SWEET proteins play important roles. Although ABCB transporters, a subclass of the ATP-binding cassette (ABC) superfamily, mediate the transport of diverse substrates and participate in various physiological processes in plants, their direct involvement in fruit sugar accumulation remains unclear. In this study, SlABCB13, an ABCB transporter in tomato, was characterized. Multiple in vitro and in vivo assays demonstrated that SlABCB13 physically interacts with SlSWEET14. SlABCB13 was predominantly expressed in fruits and localized to the plasma membrane. Heterologous expression in yeast showed that SlABCB13 transported fructose and glucose, but not sucrose. Genetic analysis revealed that CRISPR/Cas9-mediated knockout of SlABCB13 significantly increased fruit hexose, sucrose, and total sugar contents while reducing fruit weight. SlABCB13-overexpressing lines also demonstrated reduced fruit weight, accompanied by stage-specific alterations in sugar levels. Transcriptome analysis revealed distinct transcriptional reprogramming in SlABCB13 knockout and overexpression lines, characterized by downregulation of sugar transporter genes in the overexpression lines, consistent with feedback inhibition, and upregulation in the knockout lines, indicative of compensatory activation. These results identify SlABCB13 as a plant ABCB transporter that mediates hexose transport in yeast and as an important regulator of sugar homeostasis in tomato fruits through coordinated feedback and compensatory mechanisms. This study broadens the functional spectrum of ABCB transporters and highlights SlABCB13 as a potential target for molecular breeding to improve fruit quality.