Malate matters: disrupting bacterial-type phosphoenolpyruvate carboxylase (BTPC) rewires tomato fruit development

Plant phosphoenolpyruvate carboxylases (PEPCs) are ubiquitously expressed as cytosolic Class-1 PEPC homotetramers composed of 107 kDa plant-type PEPC (PTPC) subunits that are highly sensitive to allosteric inhibition by malate. Class-2 PEPC heterooctameric complexes that are desensitized to malate inhibition also exist in certain sink tissues due to the interaction of a Class-1 PEPC with unrelated 118 kDa bacterial-type PEPC (BTPC) polypeptides. Class-2 PEPCs dynamically associate with the mitochondrial outer envelope and have been hypothesized to support sustained anaplerotic flux and respiratory CO₂ refixation in malate-rich sink tissues, including immature tomato fruit. The current study generated CRISPR-Cas9-edited tomato lines with targeted disruption of the BTPC gene and investigated the impact on fruit development, metabolism, and transcriptional regulation. Immunoblotting and co-immunoprecipitation confirmed the absence of BTPC polypeptides and Class-2 PEPC complexes in the edited lines. Fruits from the edited plants were 25% smaller and 40% lighter and required up to 10 additional days to complete ripening compared to the WT. Metabolomic analysis across ripening stages revealed substantial reductions in malate and citrate, with elevated sugars and amino acids, indicating reprogrammed carbon flux. RNA-seq data showed downregulation of genes for cell wall remodeling, sugar transport, and ethylene-responsive transcription factors. These results provide direct evidence that BTPC is essential for organic acid balance, sugar metabolism, and ripening regulation in tomato. Its absence perturbs metabolic homeostasis and developmental progression, positioning BTPC as a strategic target for enhancing fruit quality traits through genetic engineering.