Sulfur Availability, Cotyledon Nitrogen:Sulfur Ratio, and Relative Abundance of Seed Storage Proteins of Soybean

Abstract

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.