Overexpression of BiP has inhibitory effects on the accumulation of seed storage proteins in endosperm cells of rice.

Seed storage proteins are specifically and highly synthesized during seed maturation and are deposited into protein bodies (PBs) via the endoplasmic reticulum (ER) lumen. The accumulation process is mediated by ER chaperones such as luminal binding protein (BiP) and protein disulfide isomerase (PDI). To examine the role of ER chaperones and the relationship between ER chaperones and levels of accumulation of seed storage proteins, we generated transgenic rice plants in which the rice BiP and PDI genes were overexpressed in an endosperm-specific manner under the control of the rice seed storage protein glutelin promoter. The seed phenotype of the PDI-overexpressing transformant was almost identical to that of the wild type, whereas overexpression of BiP resulted in transgenic rice seed that displayed an opaque phenotype with floury and shrunken features. In the BiP-overexpressing line, the levels of accumulation of seed storage proteins and starch contents were significantly lower compared with the wild type. Interestingly, overproduction of BiP in the endosperm of the transformant not only altered the morphological structure of ER-derived PB-I, but also generated unusual new PB-like structures composed of a high electron density matrix containing glutelin and BiP and a low electron density matrix containing prolamins. Notably, polysomes were attached around the aberrant PB-like structures, indicating that this aberrant structure is an ER-derived PB-I derivative. These results suggested that the PB-like structure may be formed in the ER lumen, resulting in inhibition of translation, folding and transport of seed proteins.