Summary Intracellular pH homeostasis is essential for all living cells. In plants, pH is usually maintained by three structurally distinct and differentially localized types of proton pump: P-type H+-ATPases in the plasma membrane, and multimeric vacuolar-type H+-ATPases (V-ATPases) and vacuolar H+-pyrophosphatases (H+-PPases) in endomembranes. Here, we show that reduced accumulation of proanthocyanidins (PAs) and hence the diminished brown seed coloration found in the Arabidopsis thaliana mutant transparent testa 13 (tt13) is caused by disruption of the gene encoding the P<inf>3A</inf>-ATPase AHA10. Identification of the gene encoded by the tt13 locus completes the molecular characterization of the classical set of transparent testa mutants. Cells of the tt13 seed coat endothelium do not contain PA-filled central vacuoles as observed in the wild-type. tt13 phenocopies tt12, a mutant that is defective in vacuolar import of the PA precursor epicatechin. Our data show that vacuolar loading with PA precursors depends on TT13. Consistent with the tt13 phenotype, but in contrast to other isoforms of P-type H+-ATPases, TT13 localizes to the tonoplast. PA accumulation in tt13 is partially restored by expression of the tonoplast localized H+-PPase VHP1. Our findings indicate that the P<inf>3A</inf>-ATPase TT13 functions as a proton pump in the tonoplast of seed coat endothelium cells, and generates the driving force for TT12-mediated transport of PA precursors to the vacuole. Significance StatementWe show that the Arabidopsis thaliana tt13 mutant is defective in the gene encoding the P<inf>3A</inf>-ATPase AHA10, and that this proton pump is localized in the tonoplast. TT13 is required for the formation of proanthocyanidin-filled vacuoles, by providing the proton gradient that energizes the vacuolar uptake of proanthocyanidin monomers.