Submitted to: Plant Biotechnology Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/18/2003
Publication Date: 5/1/2004
Citation: Kim, W-S, Krishnan, H. B. 2004. Expression of an 11kd methionine-rich delta zein in transgenic soybean results in the formation of two types of novel protein bodies in transitional cells situated between the vascular tissue and storage parenchyma cells. Plant Biotechnology. 2(3):199-210. Interpretive Summary: Soybean is a rich source of protein. However, soybean proteins contain low amounts of two important amino acids, methionine and cysteine, that are vital for optimal growth of humans and animals. Therefore, attempts are being made to increase the amount of these two amino acids in soybean proteins. This study reports the expression of a methionine-rich protein from corn in transgenic soybeans. Accumulation of the corn protein marginally increased soybean methionine content. Our results demonstrate that it should be possible to increase the overall methionine content of soybean by this approach. Availability of methionine-enriched soybean will enhance the profits of soybean farmers and also benefit animal industry by eliminating the use of synthetic methionine, which costs about 100 million dollars annually.
Technical Abstract: Soybean (Glycine max (L.) Merr.) is an important protein source in human diets and animal feeds. The sulfur content of soybean seed proteins, however, is not optimal for ration formulations. Thus, increasing the methionine and cysteine content of soybean seed proteins would enhance the nutritional quality of this widely-utilized legume. We have earlier reported the isolation of an 11 kD delta-zein protein rich in methionine from the endosperm of the maize (Zea mays L.) inbred line W23a1. Using Agrobacterium-mediated transformation, a construct consisting of the coding region of the cloned delta-zein gene under regulation of beta-conglycinin alpha'-promoter was introduced into the soybean genome. The 11 kD delta-zein gene was expressed predominantly in the seeds of transgenic soybeans though low-level expression was also detected in the leaves. In situ hybridization indicated that the 11 kD delta-zein mRNA was expressed predominantly in transitional cells located between the vascular tissue and storage parenchyma cells. Immunohistochemistry of developing transgenic soybeans revealed that the accumulation of the 11 kD delta-zein occurred primarily in these transitional cells. Expression of the 11 kD delta-zein gene in transgenic soybean resulted in the formation of two endoplasmic reticulum-derived protein bodies that were designated as either spherical or complex. Immunocytochemical localization demonstrated that both the spherical and complex protein bodies accumulated the 11 kD delta-zein. Although expression of the 11 kD delta-zein gene elevated the methionine content of the alcohol-soluble protein fraction 1.5 to 1.7 fold above non-transgenic control line, the overall methionine content of seed flour was not increased. Our results suggest that the confined expression of the 11 kD delta-zein gene in transitional cells could be limiting the increase of methionine content in transgenic soybean seeds.