Diversity and Formation of Endoplasmic Reticulum-Derived Compartments in Plants. Are These Compartments Specific to Plant Cells?

Unlike animals, plants are not able to escape from adverse circumstances. To cope with external stresses, plants modulate the endomembrane systems, espe- cially the endoplasmic reticulum (ER), which is the most flexible and adaptable organelle (Staehelin, 1997). Alternatively, plants generate a specialized compart- ment from the ER to accumulate an enormous amount of proteins that are actively synthesized on the ER (Chrispeels and Herman, 2000). Active protein synthe- sis can be regarded as an internal stress. In response to the internal and external stresses, the ER differentiates into various types of compartments, each of which has its own responsibility. The diversity of ER-derived compartments has been getting increased attention in plant cell biology. The ER networks of membranous tubules and cisternae are extended throughout the cytosol area surrounding the vacuoles. The ER membrane, which occupies nearly one-half the total area of membrane in cells, can be