Sulfur availability, cotyledon nitrogen:sulfur ratio, and relative abundance of seed storage proteins of soybean

The nutritional value of soybean [Glycine max (L.) Merr.] seed protein could be enhanced by increasing its concentration of the S-containing amino acids, methionine and cysteine. Two greenhouse pot studies and one field study were conducted with soybean grown under varying levels of S availability to observe the relationship between S availability, seed S content, and relative abundance of poor and high quality storage proteins. Abundances of the β-subunit of β-conglycinin (poor quality) and of glycinin (high quality) seed storage proteins were determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Cotyledon-S concentration more than doubled, and the N:S ratio of the seed decreased sharply (from about 40-20 g N g S^-1), as S availability increased from 12 to 62 mg available S per plant in the first greenhouse trial. The amount of the poor-quality β-subunit of β-conglycinin was linearly related to the N:S ratio of cotyledon tissue and varied from less than 15 up to 40% of storage proteins. On the other hand, the high-quality glycinin fraction of storage protein showed a linear, negative relation to N:S ratio of cotyledon tissue and decreased from 60 to less than 30% of storage proteins as the N:S ratio increased under S stress. Even in high S environments the β-subunit of β-conglycinin comprised 10% or more of total storage proteins. Since poor quality storage protein was synthesized even in high S environments, we hypothesize that the plant's ability to reduce sulfate and synthesize S-containing amino acids during seed filling may be a factor limiting soybean protein quality.