Transcriptome Analysis Revealed a Key Sugar Transporter CitSWEET15 and Its Regulatory Network for Sugar Accumulation in Citrus Fruit

Sugar accumulation is a major determinant of citrus fruit quality, yet the underlying molecular mechanisms remain unclear. We investigated the dynamic changes in sugars and organic acids during the development of 'Ehime 38' citrus fruit and explored the regulatory mechanisms of sugar accumulation through integrated physiological, transcriptomic, and functional analyses. Fruit maturation was accompanied by increased sugar accumulation and reduced organic acid levels, reshaping the sugar-acid balance for flavor formation. Weighted gene coexpression network analysis identified CitSWEET15, encoding a plasma-membrane-localized sucrose transporter, as a key candidate. Transient overexpression of CitSWEET15 promoted the accumulation of sucrose, glucose, and fructose in fruit. Furthermore, CitLBD18 directly bound to the CitSWEET15 promoter and activated its transcription, establishing a CitLBD18–CitSWEET15 regulatory module associated with sugar accumulation. These findings provide new insights into the transcriptional regulation of sugar accumulation in citrus fruit and identify potential molecular targets for improving fruit quality.