Differential accumulation of soybean seed storage protein subunits in response to sulfur and nitrogen nutritional sources

Soybean (Glycine max [L.] Merr.) seed storage proteins consist of subunits that differ in amino acid profile, the β-subunit of 7S protein being essentially devoid of the S-containing amino acids, methionine and cysteine. Our objective was to examine the interaction of N and S nutrition on the relative abundance of these storage protein subunits in soybean seed. 'Kenwood' soybean was grown in hydroponic culture, and during vegetative growth (V2-R4.5) N was provided as 5 mM KNO₃ to plants grown under sulfur-deficient (0.004 mM Na₂SO₄) or sulfur-sufficient (0.4 mM Na₂SO₄) conditions. During seed fill (R4.5-R7) N was supplied as 5 mM KNO₃ or 2.5 mM urea. Each N group was given S treatments of 1) no sulfur, 2) 0.4mM Na₂SO₄, 3) 0.2 mM L-cystine, or 4) 0.4 mM L-methionine. Effects on seed protein quality of S deficiency during vegetative growth were essentially overcome by supplying sulfate as late as R4.5. Total protein and seed storage protein were increased with urea as N source, but urea also increased the β-subunit. Provision of reduced S as methionine essentially suppressed β-subunit production, but cystine did not, suggesting that cystine did not influence methionine level in the seed. We also report the accumulation of two as yet unreported proteins which occur at extremes of S nutrition: (1) a putative β-subunit of 7S protein occurring in the embryonic axis under S-deficiency; and (2) a ca. 14kD protein in cotyledon tissue under provision of L-methionine. Though S and N did interact to a limited extent to influence seed protein composition, major effects were from S or N acting individually.