Minisymposium 24: Metabolic Engineering

Abs # 38001: Enhancing the nutritional value of soybean

Presenter:	Liang, Jihong , jihong.liang@renessen.com
Authors	Liang, Jihong  (A)   Rapp, Bill  (B)   Weaver, Lisa  (B)   Wang, Qi  (B)   Crow, Laura  (B)   Ream, Joel  (B)   Hill, Betsy  (B)   Brown, Wayne  (B)   Oulmassov, Tim  (B)   Gruys, Ken  (B)
Affiliations:	(A): Renessen LLC (B): Monsanto
Web Site:	http://www.renessen.com

Soybean is the major source of nitrogen and essential amino acids (EAAs) for animal feed. In corn and soybean based feed rations, lys and met are limiting. When synthetic lys and met are added to the ration, then a "second-tier" of EAAs becomes limiting. This group includes threonine, isoleucine, tryptophan, valine and arginine. The high cost of synthetic second-tier EAAs prohibits their use as a feed additive. An alternative approach is to increase the content of the desired EAAs in soybean seeds Creation of high tryptophan soybean: In the multiple-step biosynthetic pathway of tryptophan (trp), anthranilate synthase (AS) catalyzes the first reaction branching from the aromatic amino acid pathway to the biosynthesis of trp in plant, fungi and bacteria. Most common form of AS such as that from soybean or maize is a heterotetrameric enzyme consisting of two alpha- and two beta- subunits. The alpha-subunit is thought to contain the catalytic site for anthranilate synthesis and the regulatory site that binds to trp. The enzyme activity is inhibited when trp is bound on the regulatory site. Plants, fungi and bacteria use this mechanism to effectively regulate the levels of trp. To increase trp content in soybean seeds, a mutant variant of maize AS ƒÑ-subunit gene that is insensitive to trp inhibition is expressed in soybean using a seed specific promoter. Transgenic soybean seeds were grown in greenhouse and field for 5 generations. The free trp levels in the seeds are between 2,000-3,000 ppm, which is 20-30 times higher than that of the regular non-transgenic seeds. Protein and oil of the transgenic seeds are not significantly different from the non-transgenic controls. These plants are continually tested for agronomic performance, and the profiles of metabolites in the seeds Engineering of high isoleucine soybean storage protein: The amino acid sequences of ƒÒ-conglycinin (a major soybean storage protein) is modified to increase Ile content. Engineered proteins that contain between 10-20 substitutions of ile have been demonstrated to fold properly in an in vitro assay. Selected variants of the modified ƒÒ -conglycinin have been expressed at high levels in transgenic soybean seeds.