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Title: Effects of proteome rebalancing and sulfur nutrition on the accumulation of methionine rich d-zein in transgenic soybeans

Author
item KIM, WON-SEOK - University Of Missouri
item JEZ, JOSEPH - Washington University
item Krishnan, Hari

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/24/2014
Publication Date: 11/11/2014
Publication URL: https://handle.nal.usda.gov/10113/59989
Citation: Kim, W., Jez, J., Krishnan, H.B. 2014. Effects of proteome rebalancing and sulfur nutrition on the accumulation of methionine rich d-zein in transgenic soybeans. Frontiers in Plant Science. 5:633.

Interpretive Summary: Soybean is a rich source of protein. Unfortunately, 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. Previous attempts to increase the sulfur amino acid content of soybeans through expression of heterologous methionine-rich proteins have been met with limited success. It has been suggested that by suppressing the production of endogenous seed storage proteins, one could redirect the available protein synthesis capacity to the synthesis of introduced foreign proteins. To test this hypothesis, we have expressed a methionine-rich protein in ß-conglycinin suppressed transgenic soybeans. Our study demonstrates that the availability of sulfur, not proteome rebalancing, is more critical for high-level accumulation of methionine-rich proteins in soybean seeds. The information obtained from this study will help biotechnologists to genetically manipulate the sulfur-assimilatory enzyme expression levels so that we can improve the overall quality of soybean seed proteins. Superior quality soy proteins can be utilized to meet the nutritional requirements of the multitude of malnourished people around the world.

Technical Abstract: Expression of heterologous methionine-rich proteins to increase the overall sulfur amino acid content of soybean seeds has been only marginally successful, presumably due to low accumulation of transgenes in soybeans. Proteome rebalancing of seed proteins has been shown to promote the accumulation of foreign proteins. In this study, we have utilized RNAi technology to suppress the expression of the ß-conglycinin, the abundant 7S seed storage proteins of soybean. Western blot and 2D-DIGE analysis revealed that ß-conglycinin knockdown line (SAM) failed to accumulate the a', a, and ß-subunits of ß-conglycinin. The proteome rebalanced SAM retained the normal overall protein and oil content similar to that of wild-type soybean. We also generated transgenic soybean lines expressing methionine-rich 11 kDa d-zein under the control of either the glycinin or ß-conglycinin promoter. The introgression of the 11 kDa d-zein into ß-conglycinin knockdown line did not enhance the accumulation of the 11 kDa d-zein. However, when the same plants were grown in sulfur-rich medium, we observed 3- to 16-fold increased accumulation of the 11 kDa d-zein. Transmission electron microscopy observation revealed that seeds grown in sulfur-rich medium contained numerous endoplasmic reticulum derived protein bodies. Our findings suggest that sulfur availability, not proteome rebalancing, is needed for high-level accumulation of methionine-rich proteins in soybean seeds.