Factors Affecting The Accumulation Of Sulfur-Rich Proteins In Soybean Seeds.

While some progress has been achieved through the use of a biotechnology-based strategy for increasing expression of sulfur-rich proteins in legumes, these studies have shown that seed-specific expression of sulfur-rich proteins alone is not sufficient to improve nutritive value.  Moreover, there is a decrease in accumulation of endogenous sulfur-rich proteins that offsets any increase in the sulfur -amino acid content of these transgenic soybeans.  Several sulfur-rich polypeptides have been identified in soybean seeds. Prominent among these are the bioactive peptides Bowman-Birk protease inhibitor (BBI), leginsulin and lunasin. The BBI comprises 110 amino acid residues, 15 of which are Cys; leginsulin is synthesized as a 146 amino acid precursor which is subsequently processed to a 37-residue peptide that includes 6 Cys residues; and lunasin is a 43-residue peptide that includes 2 Cys-residues. Despite their potential importance, there is limited information available on factors that influence the accumulation of these Cys-rich proteins in soybean seeds. Studies conducted in our laboratory suggest that modification in the expression of these proteins should facilitate accumulation of sulfur-rich proteins to a level sufficient to meet the nutritional requirements of livestock and poultry.