Legumes have a key role to play in both agroecological and food transitions due to their ability to accumulate large amounts of seed proteins without nitrogen fertilization thanks to symbiotic N2 fixation in the root nodules. However, in agroecological systems, legumes are more exposed to nutrient deficiencies, including sulfur deficiency, than in conventional systems, making it important to optimize nutrient use efficiency for maintain seed protein quality, in particular the level of (semi) essential amino acids like methionine and cysteine. These sulfur-containing amino acids are synthetized through the sulfur metabolic pathway starting from sulfate reduction. Sulfate is taken up from the soil and transported in plant parts by sulfate transporters (SULTR) and can be stored in the vacuoles for further remobilization by SULTR4 transporters when sulfur availability remains scarce. Here, we investigated the contribution of vacuolar sulfate to seed yield and quality by using two mutants of the only SULTR4 gene that exists in pea (Pisum sativum). Seed yield of the two mutants was significantly reduced in response to sulfur deficiency, highlighting the prominent role of this transporter in stabilizing seed yield when sulfate is limiting. Interestingly, under sulfur-sufficient conditions, the mature sultr4 seeds accumulate lower amounts of sulfur-rich proteins but displayed a higher sulfate concentration compared to the wild-type seeds. These data and the unchanged sulfur content of the mature sultr4 seeds suggest that vacuolar sulfate remobilization within developing seeds contributes to storage protein synthesis. A gene expression analysis, by qRT-PCR, revealed an up-regulation of genes involved in sulfate reduction and a down-regulation of genes encoding sulfur-rich storage proteins in both mutant seeds. Based on these results, we present a hypothetical model of the impact of vacuolar sulfate in fine-tuning sulfur metabolism and storage protein synthesis during pea seed development. Further experiments are currently performed to confirm this model.