Interrelation between N and S Nutrition on Accumulation of Storage Protein in Soybean Seed

Soybean is an important crop because its seed has very high protein relative to others. The quality of soy protein is limited by the concentration of the sulfur-containing amino acids in the amino acid profile. Among the supply of various forms of 0.4mM sulfur as S nutrition during seed fill, only 0.4mM L-methionine can inhibit B-subunit synthesis completely and produce the highest glycinin-containing seeds. Compared to 0.4mM sulfate control, seeds supplied by 0.4mM L-methionine have lower a-, no B-subunit, and highly increased glycinin without altering total protein concentration. Supply of 0.2mM cystine (0.4mM S) did not affect the accumulative pattern of seed storage protein (SSP) subunits. In the supply of L-methionine, 0.2mM treatment showed higher glycinin in seeds but 0.05mm resulted in lower glycinin than the sulfate control. The relative abundance of a -subunit was not altered by any N or S nutrition. Under 5mM nitrogen, protein concentration was increased about 3-5% by substituting ammonia for nitrate during seed fill independent of S nutrition. This increase resulted in the only increase of 7S protein, mainly B-subunit. Our data suggest that the regulatory system of SSP genes responds to the balance between N and S assimilates supplied from mother plant, and controls the differential synthesis of their subunits for the maximum protein accumulation in developing soybean seed.