Sugar content serves as a crucial determinant of fruit flavor quality and nutritional value. Calcium plays extensive regulatory roles in fruit development and quality formation, yet the molecular mechanisms underlying calcium-mediated sugar accumulation remain poorly understood. In this study, we demonstrate that calcium treatment enhances sugar accumulation in both citrus fruits and calli, concomitant with upregulated expression of the sucrose transporter gene CsSWEET17. Functional characterization revealed that the membrane-localized CsSWEET17 protein exhibits sucrose transport activity. Transgenic overexpression of CsSWEET17 in citrus juice sacs, calli and heterologous tomato systems consistently elevated sucrose levels. Conversely, suppression of CsSWEET17 expression through either virus-induced gene silencing or RNA interference significantly reduced sucrose content in citrus. Further investigation identified CsMYB36 as a calcium-responsive transcription factor that directly activates CsSWEET17 expression. Transgenic validation demonstrated that both calcium signaling and CsMYB36-mediated sucrose accumulation strictly depend on CsSWEET17 transcriptional regulation. Our findings elucidate a novel calcium-MYB36-SWEET17 regulatory module controlling sucrose accumulation, providing molecular insights into calcium-based strategies in citrus quality improvement and informing fundamental mechanisms of sugar transporter regulation in fruit crops.

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