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Enhanced Protein Content and Quality in Sweetpotato Engineered with a Synthetic Storage Protein Gene. (Annual Meeting of the American Society of Plant Physiology, Baltimore, MD)
| Egnin M., Shireen, K., Walker, M., Lewis, J., Pace, R., Prakash, C. S., Jaynes, J (1999).
| We sought to improve the nutritive quality of plant proteins by enhancing their essential amino acids (EAA) content. A novel storage protein (asp-1) with many EAA and with improved protein stability was rationally developed. The synthetic asp-1 gene under the control of CaMV 35S promoter was introduced into sweetpotato (Ipomoea batatas L) using the Agrobacterium gene transfer system. Transgenic sweetpotato plants grew normally in a field trial, and in an unanticipated twist, showed 300 - 500% increase in the total protein content in their storage roots (tuber). Levels of many EAA such as methionine, threonine, isoleucine, and lysine also increased proportionally, while tryptophan increased by several orders of magnitude. Although transgenic plants expressed the asp-1 protein detectable by immunoblot analysis, the increased protein content in the roots was primarily due to enhanced levels of several native proteins, especially sporamin and beta-amylase. Animals (golden Syrian hamsters) fed with transgenic sweetpotato had 56% more live body weight over the control-fed animals and exhibited lowered total cholesterol, triglycerides, and LDL-cholesterol levels in their plasma and liver. The animal feeding also showed the superiority of asp-1 sweetpotatoes in terms of the true protein digestibility, net protein utilization and biological value of the protein. The corrected protein efficiency ratio of transgenic sweetpotato (3.71) was comparable to that of soy protein (3.72) and higher than control sweetpotato (2.57) or casein (2.49). The animal histopathological studies with brain, liver, kidney, intestine, and bone showed that transgenic sweetpotato lines did not have any detectable toxic effects. Our research now is testing various hypotheses for the causes of dramatic increase in the total protein content in sweetpotato transformed with the asp-1 gene. Research supported by NASA and USDA.
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