The role of sulfur (S) in both drought tolerance and seed quality makes it important to understand the trade-off between these two traits with respect to S utilization (Bonnot et al. 2023). This is particularly important for legumes, whose seeds contribute to the human requirement for S-amino acids. To investigate the contribution of S stored as sulfate in the vacuole to seed protein quality in pea, we focused on the only vacuolar sulfate transporter (SULTR4) present in pea (PsSULTR4). Its simulated 3D structure appeared to be conserved with that experimentally determined for its Arabidopsis homolog AtSULTR4;1 (available on PDB, https://www.rcsb.org/structure/7LHV). This observation, together with its experimentally confirmed tonoplast localization, suggests that it is the functional homolog of AtSULTR4;1, which mediates sulfate efflux from the vacuole (Kataoka et al. 2004). Five mutant and wild-type lines were phenotyped under S-sufficient and S-deficient conditions. One mutation in the cytosolic domain (E568K) reduced seed yield under S deficiency to a similar extent as a nonsense mutation, highlighting this residue as essential for sulfate transport activity. Interestingly, mature seeds of these two mutants developed under S-sufficiency showed changes in seed protein composition characteristic of S-deficiency, with less S-rich storage proteins. This was associated with reduced sulfate utilization within the seed. Gene expression studies and sulfate measurements in developing seed tissues revealed the importance of vacuolar sulfate remobilization in the embryo-surrounding tissues for the synthesis of S-rich seed proteins. These data will be presented together with perspectives on the contribution of vacuolar sulfate to seed quality