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

Won-Seok Kim, Joseph M. Jez, Hari B. Krishnan
Frontiers in plant science 2014 v.5 no.Article 633 pp. 1-12
RNA interference, Western blotting, amino acid composition, beta-conglycinin, bioaccumulation, delta-zein, endoplasmic reticulum, glycinin, growing media, introgression, lipid content, methionine, promoter regions, protein bodies, protein synthesis, proteome, seeds, soy protein, soybeans, sulfur, transgenes, transgenic plants, transmission electron microscopy, wild relatives
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.