**5. Breeding efforts**

Soybean has an extensive cultivated history dating back thousands of years to its country of origin, China. *Glycine max*, the contemporary species of soybean, was domesticated from the wild species *Glycine soja* and has been continually been improved through selective and molecular breeding [48–50]. Once harvesting traits such as seed shatter and lodging were improved in the late 1930s to make soybean a competitive row crop, other cultivar improvements became a valuable research goal [49]. Current breeding programs tend to focus on traits such as yield, disease resistance, abiotic stress tolerance, and seed composition. Seed composition improvements include protein and oil content, fatty acid levels, anti-nutritional factors, isoflavones, and amino acids profiles. Before 1972, there had been zero reported research for improvement of soybean amino acid profiles, rather with emphasis on overall protein content [51]. Modern breeders are also inclined to concentrate efforts on protein content and consider amino acid levels an afterthought [52]. TN04-5321 is the only released germplasm in the United States that maintains yield and protein content while improving amino acid balance by increasing methionine and cysteine to levels recommended livestock needs [53].

The major soybean storage proteins are 11S (glycinin) and 7S (conglycinin) and provide the bulk of amino acids with limited non-proteinogenic amino acids in seed [54–56]. By increasing 11S and 7S quantity, more protein can ultimately be present in food and feed. Overemphasis on crude protein content can have negative ramifications on overall protein quality, specifically deficient amino acids. While an increase in protein content would theoretically entail an increase in amino acids including methionine and cysteine [57], the opposite effect has been more notable [58, 59]. Molecular breeding techniques have recently improved the understanding of amino acid genomic regulation. Multiple quantitative trait loci (QTL) studies have been performed to identify genomic regions that control amino acids levels in soybean seed [60–64]. A myriad of QTL's were found to create amino acid phenotypic variation. Individual amino acids had reoccurring or proximal QTL's discovered such as Satt 518 [60], ss107913002 [61], and BARC-048619 [62] for glycine and threonine. QTL's for methionine and cysteine were also discovered which could lead to valuable improvements for soybean livestock feed [60–64]. Other genomic studies such as genome-wide association studies (GWAS) and genetic diversity analyses would further improve genetic understanding.
