Mitochondrial pyruvate carrier 1 (MPC1) regulates the sugar-acid metabolism in citrus

The sugar-acid balance in citrus fruits is widely acknowledged as a critical determinant of overall flavor quality, with the pronounced accumulation of citric acid representing a defining metabolic feature. The mitochondrial pyruvate carrier (MPC), which functions as the primary protein for pyruvate transport from the cytoplasm into the mitochondria, plays a key role in connecting glycolysis to the tricarboxylic acid (TCA) cycle. However, its role in prototypical citric acid-accumulating citrus fruit remains unexplored. This study identified three genes encoding MPC proteins in the sweet orange (Citrus sinensis) genome. Through comprehensive phylogenetic analysis and functional characterization, we determined that CsMPC1 is the core protein localized to mitochondria with pyruvate transport activity. Experimental analyses conducted in citrus callus and fruits showed that both inhibitor treatment with UK5099 and RNAi-mediated silencing of CsMPC1 reduced soluble sugar content but elevated citric acid level. Through integrated transcriptomic and metabolomic profiling, we uncovered that the inhibition of CsMPC1 triggered metabolic reprogramming, redirecting carbon flux toward citrate synthesis via pyruvate kinase (PK) downregulation and phosphoenolpyruvate (PEP) carboxylase (PEPC) genes upregulation. Collectively, these findings identify pyruvate transport facilitated by CsMPC1 as a central regulator of sugar-acid homeostasis in citrus fruit. This work provides a fundamental advance in the theoretical understanding of quality metabolism in citrus.