Résumé
Legumes have a key role to play in both agroecological and food transitions due to their ability to accumulate large amountsof seed proteins without nitrogen fertilization thanks to symbiotic N2 fixation in the root nodules. However, in agroecologicalsystems, legumes are more exposed to nutrient deficiencies, including sulfur deficiency, than in conventional systems, makingit important to optimize nutrient use efficiency for maintain seed protein quality, in particular the level of (semi) essentialamino acids like methionine and cysteine. These sulfur-containing amino acids are synthetized through the sulfur metabolicpathway 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 remainsscarce. Here, we investigated the contribution of vacuolar sulfate to seed yield and quality by using two mutants of the onlySULTR4 gene that exists in pea (Pisum sativum). Seed yield of the two mutants was significantly reduced in response to sulfurdeficiency, 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 ahigher sulfate concentration compared to the wild-type seeds. These data and the unchanged sulfur content of the mature sultr4seeds suggest that vacuolar sulfate remobilization within developing seeds contributes to storage protein synthesis. A geneexpression analysis, by qRT-PCR, revealed an up-regulation of genes involved in sulfate reduction and a down-regulation ofgenes encoding sulfur-rich storage proteins in both mutant seeds. Based on these results, we present a hypothetical model ofthe 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.