Time-resolved elemental mapping and characterisation of storage proteins during soybean seeds development: a bottom-up strategy to increase protein content and quality

Soybean (Glycine max (L.) Merrill) is one of the most important crops worldwide and stands out as major a source of protein meal and vegetal oil for humans and animals. In this scenario, most of the proteins in soybean seeds are stowed to be used a nitrogen supply during germination and early seedling development processes. These storage proteins are mainly composed of legumin (glycinin) and vicilin (β-conglycinin), which mostly accumulate in the seed cotyledon’s cells. Nevertheless, the relative abundance of β-conglycinin over glycinin leads to a remarkable deficiency in sulphur-amino acids such as methionine and cysteine, essential and conditionally essential amino acids for many vertebrates, including humans. As the biosynthesis of storage proteins is dependent on mineral nutrients, we have investigated the proteomic and ionomic composition in 95 mature soybean varieties widely cultivated in Brazil, as well as their accumulation dynamics during the seed development of five genotypes cultivated under field conditions. This investigation revealed an intricate and conserved association between the content of total and storage proteins and sulphur (S), zinc (Zn), and manganese (Mn). In this regard, a foliar application of S, Zn, and Mn prior to the onset of storage protein biosynthesis, resulted in increased S and Zn concentrations in cotyledon cells, as well as enhanced total protein content and increased glycinin without affecting or even increasing the seed yield. These findings elucidate the complex association between mineral nutrients and protein composition in soybean seeds and demonstrate that a stage-specific foliar supplementation of S, Zn, and Mn can enhance the seed's sulphur and zinc availability and affects protein biosynthesis. These findings highlight the mineral requirements that might limit SSPs biosynthesis and suggest promising fertilisation strategies to improve both protein quantity and quality in soybean seeds.