Abstract
Sweet orange (Citrus sinensis) is recognized as one of the most significant citrus fruits globally. The sugar content of fruits is the most critical internal quality associated with taste in sweet oranges, serving as a vital determinant of fruit quality and commercial value. Therefore, a comprehensive exploration of the regulatory mechanisms governing sugar accumulation during fruit ripening holds substantial value for high-quality fruit breeding. In this study, we investigated citrus sugar accumulation using the flesh of the Newhall navel orange and its high-sugar-content mutant cultivar, Ganmi, as experimental materials. RNA sequencing of the flesh from both Ganmi and Newhall oranges at 180 and 200 days after flowering identified 642 and 493 differentially expressed genes (DEGs), respectively. Functional enrichment analysis indicated that DEGs were mainly enriched in the sugar metabolism pathways, sugar transporters, and plant hormone signal transduction. Important DEGs associated with fruit sugar accumulation in Ganmi included Cs_ont_2g004470 (CsNAC73) and Cs_ont_9g005250 (CsSTP13) involved in sugar accumulation. Weighted gene co-expression network analysis showed that 20 co-expression modules were obtained, and the brown1 module had the strongest correlations with sugar content. Based on gene functionality and gene expression analyses of 1189 genes in this module, three genes (Cs_ont_2g004470 (CsNAC73), Cs_ont_5g050360 (CsMYC2) and Cs_ont_3g002820 (CsBBX21)) were identified as key genes potentially related to sugar accumulation during the ripening. These findings may contribute to elucidating the mechanisms underlying sugar accumulation during ripening and provide insights for the molecular breeding of citrus varieties.