Protein malnutrition is a serious problem in some developing countries, especially those in sub-Saharan Africa and South Asia. While the industrialized countries have a reliable and affordable source of dietary protein supply, primarily through animal products, the developing country's population depends mostly on plants as a source of protein. Although plants are an efficient, inexpensive, and environmentally friendly producer of proteins, many crops such as sweetpotato cassava, potato, and plantain that are staples in the sub-Saharan Africa have a low protein content. Further, when compared to animal protein sources such as beef, milk, or egg, the plant proteins are seriously deficient in essential amino acids that are critical to the growth and development of the body especially in growing children. Greater food and nutrition security in the developing world can be achieved by enhancing the quality and sustainable productivity of local food crops that are important to this region. Sweetpotato [Ipomoea batatas L. (Lam.)] is one such crop grown by poor farmers and is a major source of calories, protein, and micronutrients. The seventh most important crop in the world, sweetpotato is especially popular among low-resource farmers because of its high yield, adaptability, and drought tolerance, and requires minimal or no chemical inputs. The goal of our research is to improve the nutritional protein content and quality in food crops, using sweetpotato as a model system.

To improve the nutritive quality of plant proteins, a novel storage protein (asp-1) with many essential amino acids and with enhanced stability features was synthesized. The asp-1 gene under the transcriptional control of the constitutive CaMV 35S promoter was introduced into sweetpotato using the Agrobacterium vector. Genetically modified sweetpotato plants with the asp-1 gene were normal in growth and phenotypic appearance under laboratory, greenhouse, hydroponic and field conditions of testing. Interestingly, the storage roots (‘tuber’) of asp-1 plants exhibited a three to five-fold increase in their total protein content. A proportional increase in the levels of many essential amino acids such as methionine, threonine, isoleucine, and lysine was also observed, while tryptophan increased by several orders of magnitude.

In a controlled animal feeding study, golden Syrian hamsters fed with high-protein sweetpotato showed 56% more live body weight over the control-fed animals. Animals fed with the engineered sweetpotato also had lower levels of total cholesterol, triglycerides, and LDL-cholesterol in their plasma and liver. Our animal feeding test also demonstrated 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). Histopathological studies with brain, liver, kidney, intestine, and bone showed that transgenic sweetpotato lines had no detectable toxic effects. We are now introducing the asp-1 gene into other food crops of importance to developing countries such as cassava, rice, and potato in collaboration with other research groups.