Impact of co-expression of maize 11 and 18 kDa delta-zeins and 27 kDa gamma-zein in transgenic soybeans on protein body structure and sulfur amino acid content

Authors: KIM, WON-SEOK - University Of Missouri; Krishnan, Hari

Submitted to: Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/13/2018
Publication Date: 12/18/2018
Publication URL: https://handle.nal.usda.gov/10113/6264997
Citation: Kim, W., Krishnan, H.B. 2018. Impact of co-expression of maize 11 and 18 kDa delta-zeins and 27 kDa gamma-zein in transgenic soybeans on protein body structure and sulfur amino acid content. Plant Science. 280:340-347. https://doi.org/10.1016/j.plantsci.2018.12.016.
DOI: https://doi.org/10.1016/j.plantsci.2018.12.016

Interpretive Summary: Soybean is a major source of protein for animals and humans. However, it contains 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. Previous attempts to increase the sulfur amino acid content of soybeans through expression of methionine-rich proteins have been met with limited success. Co-expression of different class of methionine-rich proteins could result in their stable accumulation in transgenic plants. To test this hypothesis, we have made conventional crosses between transgenic plants to obtain plants that simultaneously express different classes of methionine-rich proteins. Our study demonstrates that co-expression of certain class of methionine-rich proteins can result in significant increase in sulfur amino acid content of soybean seeds. The information obtained from this study will benefit the US soybean farmers by providing tools that will aid in the development of soybeans with higher nutritive value.

Technical Abstract: The methionine-rich seed storage proteins of maize have been expressed in transgenic plants as a means to improve the overall sulfur amino acid content of seed, but previous attempts to increase the sulfur amino acid content of soybean seeds using this approach have met with limited success. In this study, conventional crosses between transgenic plants individually expressing 11, 18 kDa delta-zeins and 27 kDa gamma-zein were made to obtain plants that simultaneously express both the delta-zein and gamma-zein transgenes. Transmission electron microscopic observation of thin-sections of transgenic soybean seeds revealed that the zeins accumulated in ER-derived protein bodies (PBs) which were found sparsely scattered in cytoplasm. The size of these PBs varied from 0.2 to 0.6 µm in soybean plants individually expressing 11, 18 kDa delta-zeins and 27 kDa gamma-zein. In contrast, soybeans co-expressing the 18 kDa delta-zein and 27 kDa gamma-zein the PBs were 3-4 times larger. Amino acid analysis of transgenic soybean individually expressing 11, 18 kDa delta-zeins and 27 kDa gamma-zein revealed only a minimal increase in the overall methionine content compared to the wild-type. In contrast, plants co-expressing 18 kDa delta-zein and 27 kDa gamma-zein showed a significant increase (27%) in the methionine content compared to the control seeds.