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-dimen sional network of continuous tubules and sheets that underlies the pla sma membrane, courses through the cytoplasm, and links up with the nuc lear envelope. Aside from discussing the dynamic properties of this ve rsatile and adaptable organelle, the review highlights the structure a nd 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 ul trarapid 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 doma ins; the vacuole-forming ER; the actin-binding, the plasma membrane-an choring and the vacuole and mitochondrion-attachment domains; the lipi d recycling ER cisternae and the plasmodesmata. Preliminary evidence s uggests that this list will have to be expanded in the near future. Un derstanding the assembly, the functional roles, and the developmental regulation of these domains has implications both for understanding ce ll structure and function, and for exploiting plants for agricultural and biotechnological purposes.