There is no standardized endoplasmic reticulum (ER) in plants. Descending from a single cell, in higher plants the zygote, plant cells may undergo rigorous morphological and metabolic changes before reaching their destination within an organism. Therefore, the course of development with periods of division and differentiation leads to various cell types, which will all have distinct ER organization according to their function. In the past few years our understanding of ER organization during the cell cycle, and in fully differentiated or differentiating cells during plant development, has improved considerably.
The ER extends as a highly anastomosing membranous network throughout the cytoplasm and represents in most plant cells the largest membrane system. It is the major compartment of membrane biogenesis and, ever since the membrane flow hypothesis was proposed (Mollenhauer and Morre 1980), also acts as the portal to the secretory pathway. It also contributes to several principal anabolic and catabolic cellular pathways, including the fine-tuning of the cytosolic Ca2+ concentration.
Morphologically the ER can be divided into three sub-domains: the smooth ER (sER), the rough ER (rER) (Fig. 1a), and the nuclear envelope (NE). In plants, desmotubule ER crossing plasmodesmata may be added as a fourth ER sub-domain linking the ER network of neighbouring cells. Plasmodesmata are structures which mediate intercellular communication (Staehelin 1996; see also Oparka and Wright, this volume). The amount of the NE remains almost constant during interphase, whereas the amount of the other ER sub-domains may vary according to metabolic demands.
Conventional ultrastructural studies showed longitudinally sectioned ER membrane fragments either without any associated ribosomes (the sER) or studded with ribosomes in a linear, spiral, or no clear arrangement (the rER). Ultrastructural studies using high-voltage electron microscopy have extended our view of ER organization in both animal (Walz 1982) and plant cells (Harris 1979). Such studies employed new post-fixation and post-staining techniques like osmium tetroxide/potassium ferricyanide (Hepler 1981) or zinc iodide/osmium tetroxide (Hawes et al. 1981), or high-pressure freezing (Craig and Staehelin 1988). These techniques not only provided evidence in favour of ER continuity, but also indicated that rER and sER occur in the form of flat, sheet-like sacs and tubular elements, referred to as cisternal ER (cER) and tubular ER (tER), respectively.
An even more complex view of ER morphology is obtained after visualizing the ER by means of light microscopy with ultraviolet microscopy, video-enhanced differential interference phase contrast microscopy (VeDIC) or fluorescence microscopy (Lichtscheidl and Hepler 1996). Actually, these procedures provided a reasonable insight into ER morphology some 40 years ago, but did not enjoy much recognition by the scientific community at the time (Drawert and Rüffer-Bock 1964; Url 1964). Light microscopy techniques, however, became the procedures of choice with the general availability of
VeDIC and confocal laser scanning microscopy (CLSM). ER visualization by CLSM profited enormously from the introduction of selective fluorescent stains about 20 years ago (Terasaki et al. 1984; Quader and Schnepf 1986; Hep-ler and Gunning 1998), and from indirect immunofluorescence techniques using antibodies that recognize the ER retention signal of ER luminal proteins (Napier et al. 1992) or ER resident proteins like the Ca2+-binding protein cal-reticulin (Denecke et al. 1995). About a decade ago, green fluorescent protein (GFP) technology (Boevink et al. 1996; Haseloff et al. 1997; Hawes et al. 2001) provided an even greater input into ER morphology studies. A significant benefit of CLSM is the possibility of carrying out three-dimensional studies of ER organization in living cells (Hepler and Gunning 1998; Ridge et al. 1999; Cutler and Erhard 2002).
In general, ER morphology and its dynamics depend on the particular functions the cells of different tissues will have to perform for the sake of the organ, and thus of the whole organism. In the past, particular aspects of the organization and function of plant cell ER have been reviewed by several authors (Hepler et al. 1990; Lichtscheidl and Hepler 1996; Staehelin 1997; Hawes et al. 2001). In this chapter, we discuss the changes of ER morphology in meristem and differentiated cells, as related to cell cycle stages or physiological conditions.
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