Submitted to: Planta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/7/2011
Publication Date: 7/30/2011
Publication URL: http://handle.nal.usda.gov/10113/54520
Citation: Kim, W., Chronis, D., Juergens, M., Schroeder, A.C., Jez, J.M., Krishnan, H.B. 2011. Transgenic soybean plants overexpressing O-acetylserine sulfhydrylase accumulate enhanced levels of cysteine and bowman-birk protease inhibitor in seeds. Planta. 235(13-23). 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. In this study we have taken an alternative approach to generate transgenic soybeans overexpressing a key enzyme involved in the synthesis of cysteine cytosolic isoform of soybean OASS. Our study demonstrates soybean plants overexpressing this enzyme (OASS) contain elevated amounts of Bowman-Birk protease inhibitor, a cysteine-rich protein, when compared to non-transgenic plants. Additionally, amino acid analysis reveals our OASS overproducing transgenic soybean seed contains a 74% increase in protein-bound cysteine content. 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: Soybeans provide an excellent source of protein in animal feed. Soybean protein quality can be enhanced by increasing the concentration of sulfur containing amino acids. Previous attempts to increase the concentration of sulfur-containing amino acids through the expression of heterologous proteins have been met with limited success. Here, we report a successful strategy to increase the cysteine content of soybean seed through the overexpression of a key sulfur assimilatory enzyme. We have generated several transgenic soybean plants that overexpress a cytosolic isoform of O-acetylserine sulfhydrylase (OASS). These transgenic soybean plants exhibit a 4- to 10-fold increase in OASS activity when compared to non-transformed wild-type. The OASS activity in the transgenic soybeans was significantly higher at all the stages of seed development. Unlike the non-transformed soybean plants, there was no marked decrease in the OASS activity even at later stages of seed development. Overexpression of cytosolic OASS resulted in a 74% increase in protein-bound cysteine levels (2.77 ± 0.01% total amino acid content) compared to non-transformed wild-type soybean seeds (1.59 ± 0.03% total amino acid content). A 32% increase in the free cysteine levels was also observed in transgenic soybeans overexpressing OASS. Furthermore, these transgenic soybean plants showed a marked increase in the accumulation of Bowman-Birk protease inhibitor, a cysteine-rich protein. The overall increase in soybean total cysteine content (both free and protein-bound) satisfies the recommended levels required for the optimal growth of monogastric animals.