Rice protein-body formation: all types are initiated by dilation of the endoplasmic reticulum.

The ultrastructure of protein deposition in the starchy endosperm of developing rice (Oryza sativa L.) grains was examined in conventionally fixed (glutaraldehyde and osmium tetroxide) tissues and also in thick sections (0.3 μm) of zinc iodide-osmium tetroxide post-fixed tissue. Three types of previously characterised protein body were observed and it was shown that each type was initiated by dilations of the endoplasmic reticulum. Crystalline type protein bodies were initiated by a ribosome-free dilation from rough cisternal endoplasmic reticulum and developed by inclusion of protein from dictyosome-derived vesicles. The large spherical and small spherical protein bodies developed within the cisternae of the rough endoplasmic reticulum.