Pj. Sexton et al., SULFUR AVAILABILITY, COTYLEDON NITROGEN SULFUR RATIO, AND RELATIVE ABUNDANCE OF SEED STORAGE PROTEINS OF SOYBEAN/, Crop science, 38(4), 1998, pp. 983-986
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 stora
ge proteins. Abundances of the beta-subunit of beta-conglycinin (poor
quality) and of glycinin (high quality) seed storage proteins were det
ermined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (
SDS-PAGE). Cotyledon-S concentration more than doubled, and the N:S ra
tio of the seed decreased sharply (from about 40-20 g N g S-1), as S a
vailability increased from 12 to 62 mg available S per plant in the fi
rst greenhouse trial. The amount of the poor-quality beta-subunit of b
eta-conglycinin was linearly related to the N:S ratio of cotyledon tis
sue and varied from less than 15 up to 40% of storage proteins. On the
other hand, the high-quality glycinin fraction of storage protein sho
wed a linear, negative relation to N:S ratio of cotyledon tissue and d
ecreased from 60 to less than 30% of storage proteins as the N:S ratio
increased under S stress. Even in high S environments the beta-subuni
t of beta-conglycinin comprised 10% or more of total storage proteins.
Since poor quality storage protein was synthesized even in high S env
ironments, we hypothesize that the plant's ability to reduce sulfate a
nd synthesize S-containing amino acids during seed filling may be a fa
ctor limiting soybean protein quality.