Transcriptional Regulation of Sugar Accumulation and Composition in Jujube Fruit: The Roles of ZjSWEET15 and ZjSUS2 Genes

• The sweetness difference between cultivated and sour jujubes is associated with sucrose levels, rather than fructose or glucose contents. • The Sugar Will Eventually be Exported Transporter Gene ZjSWEET15 contributed to high sucrose content in cultivated jujube fruit. • Sucrose synthase gene ZjSUS2 played a crucial role in sucrose degradation, potentially hindering sucrose accumulation in sour jujube fruit. The composition and concentration of sugars are pivotal in determining fruit quality. Cultivated jujube ( Ziziphus jujuba Mill.) fruit exhibits significantly higher sweetness compared to its wild-type counterpart, the sour jujube, due to increased sugar content. Despite this, our understanding of the key genes regulating sugar content and composition in jujube fruit is limited. This study reveals that the sweetness difference between cultivated and sour jujubes is associated with sucrose levels, with the sucrose content of cultivated jujubes (10.5 g/100 g FW) approximately four times greater than that of sour jujubes (2.6 g/100 g FW). Through transcriptomic analysis, spatiotemporal expression analysis, overexpression experiments, and subcellular localization, we identified ZjSWEET15 (sugar will eventually be exported transporter gene) contributing to high sucrose content in cultivated jujube fruit. Enzyme activity assay revealed that the sucrose decomposition activity was significantly higher in sour jujube compared to cultivated jujube. Overexpression and VIGS experiments identified the sucrose synthase gene ZjSUS2 playing a crucial role in sucrose cleavage, potentially hindering sucrose accumulation in sour jujube fruit. Collectively, these findings shed light on the transcriptional mechanisms of ZjSWEET15 and ZjSUS2 in sugar accumulation and composition in jujube fruits.