Cosuppression of the alpha subunits of beta-conglycinin in transgenic soybean seeds induces the formation of endoplasmic reticulum-derived protein bodies

The expression of the alpha and alpha ' subunits of beta -conglycinin was s uppressed by sequence-mediated gene silencing in transgenic soybean seed. T he resulting seeds had similar total oil and protein content and ratio comp ared with the parent line. The decrease in beta -conglycinin protein was ap parently compensated by an increased accumulation of glycinin. In addition, proglycinin, the precursor of glycinin, was detected as a prominent polype ptide band in the protein profile of the transgenic seed extract. Electron microscopic analysis and immunocytochemistry of maturing transgenic soybean seeds indicated that the process of storage protein accumulation was alter ed in the transgenic line. In normal soybeans, the storage proteins are dep osited in pre-existing vacuoles by Golgi-derived vesicles. In contrast, in transgenic seed with reduced beta -conglycinin levels, endoplasmic reticulu m (ER)-derived vesicles were observed that resembled precursor accumulating -vesicles of pumpkin seeds and the protein bodies accumulated by cereal see ds. Their ER-derived membrane of the novel vesicles did not contain the pro tein storage vacuole tonoplast-specific protein alpha -TIP, and the sequest ered polypeptides did not contain complex glycans, indicating a preGolgi an d nonvacuolar nature. Glycinin was identified as a major component of these novel protein bodies and its diversion from normal storage protein traffic king appears to be related to the proglycinin buildup in the transgenic see d. The stable accumulation of proteins in a protein body compartment instea d of vacuolar accumulation of proteins may provide an alternative intracell ular site to sequester proteins when soybeans are used as protein factories .