SlSPL10 mediates nitric oxide-induced suppression of the sugar-acid ratio in postharvest tomato fruit

The sugar-acid ratio is a key determinant of tomato flavor quality during postharvest ripening. Although nitric oxide (NO) has been shown to influence sugar and organic acid metabolism in various fruits, the molecular mechanisms by which NO regulates sugar-acid balance in tomato fruit, particularly the downstream transcriptional regulators and target metabolic genes involved, remain incompletely understood. Here, we investigated the role of SlSPL10 in NO-mediated regulation of sugar and organic acid metabolism in postharvest tomato fruit. Treatment with the NO donor sodium nitroprusside (SNP) suppressed the increase in the sugar-acid ratio by reducing fructose, glucose, sucrose, and total soluble sugars accumulation while maintaining higher citric acid, malic acid, and titratable acidity. Moreover, slspl10 mutants accumulated more soluble sugars and exhibited an elevated sugar-acid ratio, whereas SlSPL10 -overexpression lines showed reduced sugar accumulation, enhanced organic acid retention, and a lower sugar-acid ratio. Importantly, the NO-induced suppression of the sugar-acid ratio was attenuated in slspl10 mutants but strengthened in SlSPL10 -overexpression fruit, indicating that SlSPL10 is required for NO-mediated regulation. Transcriptome and RT-qPCR analyses identified SlSUS6 , SlPPC4 and SlCSY2 as candidate genes responsive to SNP and SlSPL10. Yeast one-hybrid and ChIP-qPCR assays demonstrated that SlSPL10 directly binds the promoters of SlSUS6 and SlPPC4 , but not SlCSY2 . Dual-luciferase assays further revealed that SlSPL10 represses SlSUS6 and activates SlPPC4 , and that SNP treatment further strengthened these SlSPL10-associated transcriptional effects. Functional validation showed that transient overexpression of SlSUS6 or silencing of SlPPC4 partially compromised the SlSPL10-associated reduction of the sugar-acid ratio under SNP treatment. Collectively, these findings establish that NO suppresses the sugar-acid ratio in postharvest tomato fruit through an SlSPL10-dependent transcriptional regulatory pathway, providing new insights into the molecular regulation of tomato flavor quality during storage.