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.