Potassium Promotes Citric Acid Accumulation by Regulating Its Synthesis and Vacuole Storage in Newhall Navel Orange (Citrus sinensis)

Potassium (K) is recognized as a crucial element affecting fruit flavor quality and influences the accumulation of citric acid (CA). To gain deeper insights into how K regulates CA accumulation, we investigated CA synthesis in mitochondria, decomposition pathways, and vacuolar storage under both pot and field culture in Newhall navel orange (Citrus sinensis). Our findings demonstrated that an appropriate level of K enhanced CA concentrations primarily by modulating CA synthesis, as evidenced by the increased activities and gene expressions of citrate synthase (CS) and phosphoenolpyruvate carboxylase (PEPC) in the early stages of fruit development. Integration of mitochondrial proteomic and targeted metabolomic revealed that K promoted CA synthesis by accelerating the TCA cycle, which were supported by the upregulation of TCA cycle-associated proteins, including malate dehydrogenase, 2-oxoglutarate dehydrogenase E2 component, pyruvate dehydrogenase E2 component, and dihydrolipoamide dehydrogenase, along with elevated levels of TCA cycle-related metabolites such as CA, isocitrate, 2-oxoglutarate, and succinate. Additionally, K enhanced CA concentration in vacuoles by promoting vacuolar acidification, as evidenced by the upregulation of P-type ATPase (CsPH8) expression. In summary, our findings provide novel insights into K facilitating CA accumulation by modulating its synthesis and vacuole storage in Citrus.