Role of vacuolar sulfate in nutritional quality of pea seeds

International audienceGrain legumes have a key role to play in both agroecological and food transitions. Indeed, these plants are able to accumulate large amounts of proteins in their seeds even in the absence of nitrogen fertilization thanks to symbiotic N2 fixation in the root nodules. However, legumes are exposed to abiotic stresses, including nutrient deficiencies, making it important to optimize nutrient use efficiency for maintaining seed protein content and quality. Seed protein quality refers to the ability of the seed proteins to meet the body’s requirements for essential amino acids. It strongly depends on the amino acid balance, which determines protein digestibility. In pea (Pisum sativum) seeds, methionine and cysteine are, with tryptophan, the most limiting essential amino acids. Although the literature suggests that sulfate assimilation during seed development may be a source of sulfur amino acids for the synthesis of sulfur-rich storage proteins, work is still needed to understand how sulfate transport and metabolism regulate seed protein composition.Sulfate ions are transported in plant tissues by sulfate transporters (SULTR) encoded by the SULTR gene family. These ions are taken up from the rhizosphere by high-affinity transporters of group 1 (SULTR1) and their root-to-shoot transport is ensured by SULTR2 and SULTR3 transporters, which have also been shown to play a role in the translocation of sulfate within developing siliques or seeds in Arabidopsis. In the sink organs, sulfate can be stored in the vacuoles and remobilized via SULTR4 transporters, which allow sulfate efflux for further assimilation. Sulfate assimilation starts by a reaction catalyzed by ATP sulfurylase (ATPS), which produces Adenosine 5’-PhosphoSulfate (APS). APS can be reduced into sulfite ion (SO32-) and sulfide (S2-) by APS reductase (APR) and sulfite reductase (SiR), respectively. Sulfide can be incorporated into O-acetylserine (OAS) to provide cysteine, which is the precursor for the synthesis of methionine and glutathione (GSH). Here, we focused on the only gene encoding a SULTR4 transporter in pea to investigate its role in providing sulfate for the synthesis of sulfur-rich storage proteins (2S albumins, 11S globulins) in seeds. Two mutants of this gene were identified through a screening of the TILLING (Targeting Induced Local Lesions IN Genomes) population developed using the pea ‘Caméor’ cultivar. The first mutant, called W/-, has a nonsense mutation at position W78. The second mutant, called E/K, has a missense mutation leading to a substitution of a glutamate to a lysine at position 586 (E586K) in the STAS domain (for Sulfate Transporter and Anti-Sigma antagonist), which is essential for the transport function [1]. These mutants and their corresponding wild-type lines (same genetic background) were phenotyped under sulfur-sufficient and sulfur-deficient conditions. Seed yield of the sultr4 mutants was unchanged compared to the wild-type when the plants were cultivated under sulfur sufficiency, suggesting that sultr4 mutants use sulfate absorbed by roots and/or sulfur metabolites instead of vacuolar sulfate to maintain seed production. Nevertheless, seed yield of sultr4 mutants was significantly reduced under sulfur deficiency, suggesting a pivotal role for the remobilization of vacuolar sulfate to maintain protein biosynthesis in seeds when external sulfate supply is low. The results also revealed significant changes in seed protein composition of the sultr4 mutants, even under sulfur sufficiency. In this condition, the relative abundance of the sulfur-rich PA1 albumins was lower in mutant seeds. Moreover, sulfate content of these mature mutant seeds was two times higher than that of wild-type seeds, but seed sulfur content (and sulfur quantity per seed) were unchanged. Altogether, the data uncover a defect in the utilization of vacuolar sulfate within the developing mutant seeds. Based on these findings, we suggest that vacuolar sulfate remobilization during seed development is a vital sulfur source for the synthesis of sulfur-rich 2S albumins in seeds even under ample external sulfur supply. We studied the kinetics of seed development in the mutant and wild-type lines by measuring the fresh weight, dry weight and water content of seeds collected under sulfur-sufficient conditions. There was no change in these parameters kinetics compared to the wild-type until around 400 degree-days after pollination (that is to say that 400 is cumulative sum of daily mean temperature in °C between pollination day and seed sampling day). This result indicates that both sultr4 mutants, E/K and W/-, seem to have a normal early seed development. After this stage, water content of the W/- mutant seeds decreased faster than that of wild-type seeds, indicating that a complete loss of function of the SULTR4 transporter accelerates seed maturation. We further investigated the contribution of vacuolar sulfate to seed protein accumulation by focusing on the stages where no change in the seed development kinetics was observed. Wild-type and mutant seeds were harvested at 257 degree-days (embryogenesis) and 312 degree-days (early seed filling). We studied, by qRT-PCR, the expression of genes encoding enzymes of sulfur metabolism and seed storage proteins. For both mutants, the expression of genes involved in sulfate reduction (ATPS1 and APR3) increased during embryogenesis compared to the wild type, whereas the expression of several genes encoding sulfur-rich storage proteins (PA1 and PA2 albumins, and legumin) decreased at the early filling stage. This suggests that vacuolar sulfate availability could regulate storage protein production and consequently influence nutritional seed quality. Based on these data, 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