The plant ER: A dynamic organelle composed of a large number of discrete functional domains

The endoplasmic reticulum (ER) of plants is comprised of a three-dimensional network of continuous tubules and sheets that underlies the plasma membrane, courses through the cytoplasm, and links up with the nuclear envelope. Aside from discussing the dynamic properties of this versatile and adaptable organelle, the review highlights the structure and the functional properties of 16 types of morphologically defined ER membrane domains. Owing to their labile or transient nature, several of these domains can only be visualized reliably through the use of ultrarapid freezing techniques. The ER domains discussed are: the lamin receptor domain; the nuclear pores; the nuclear envelope-ER gates, the microtubule nucleation domains; the protein and oil body-forming domains; the vacuole-forming ER; the actin-binding, the plasma membrane-anchoring and the vacuole and mitochondrion-attachment domains; the lipid recycling ER cisternae and the plasmodesmata. Preliminary evidence suggests that this list will have to be expanded in the near future. Understanding the assembly, the functional roles, and the developmental regulation of these domains has implications both for understanding cell structure and function, and for exploiting plants for agricultural and biotechnological purposes.