High level accumulation of soybean glycinin in vacuole-derived protein bodies in the endosperm tissue of transgenic tobacco seed

Abstract

Soybean glycinin genes are expressed specifically in the cotyledon and embryo of maturing soybean seed, from which the endosperm tissue is degenerated. To examine whether glycinin could be stably accumulated in endosperm tissue, the glycinin cDNA was transcriptionally fused to an endosperm-specific promoter of the rice storage protein glutelin gene and then introduced into tobacco genome via Agrobacterium-mediated transformation. Consequently the glycinin gene was expressed in a seed- and developmentally-specific manner in transgenic tobacco seeds. Glycinins were targeted to vacuole-derived protein bodies in the endosperm tissue and highly accumulated in the matrix region of many transgenic plants (1–4% of total seed proteins). Synthesized glycinin was processed into mature form, and assembled into a hexamer in a similar manner as the glycinin in soybean seed. However, it was noteworthy that about half of the synthesized glycinin was susceptible to limited degradation and that assembly into a hexamer was insufficient. Modified glycinins, in which 4 contiguous methionine residues were inserted at the variable regions corresponding to the C-terminal regions of the acidic and basic polypeptides, were also found to be accumulated similarly as in the normal glycinin. There was no apparent difference in the expression level, processing and targeting to protein bodies, or accumulation level between normal and modified glycinins.