ER Morphology and the Cytoskeleton

Many metabolic pathways in plant cells are under strict spatial control, as is apparent from distinct organelle distributions which are guaranteed by the two major cytoskeletal elements: microtubules (MTs) and actin filaments (AFs). MTs and AFs function in organelle patterning and motility as relatively stiff tracks along which the organelles are transported via specific motor proteins like dynein or kinesin (for MTs), and myosin (for AFs).

The controlled arrangement of the ER membrane meshwork throughout the cytoplasm is not conceivable without a guiding skeleton. In animal cells, MTs form tracks along which the ER network is constituted and altered in connection with the appropriate motor protein (Lee and Chen 1988; Terasaki 2000). In plant cells, ER tubules and small ER sheets have also been observed by electron microscopy in close nearness to MTs in the cell cortex of developing guard cells (Hodge and Palevitz 1984) and in pollen tubes (Lancelle et al. 1987). Most distinct are the MT-ER associations found during the mitotic stages (Hepler 1980) and during cytokinesis (Segui-Simarro et al. 2004). However, all the examples of spatial nearness between MTs and the ER during interphase resemble static situations, and bear the blemish that MTs and the ER have not been detected to co-align in plant cells over a longer distance. It thus seems very doubtful that these scarce contact sites are sufficient for the observed controlled dynamic changes of the ER pattern. In vivo observations of ER organization and motility in onion epidermal cells show no perturbation in the presence of MT disassembling drugs, such as colchicine (Knebel et al. 1990), oryzalin, or trifluralin (Quader et al. 1989), when applied at concentrations specific for plant cells and for reasonable experimental periods. These results strongly support the view that MTs are not involved in ER organization in interphase and highly differentiated cells, although it has been claimed that MTs might be involved in the polar distribution of membranes including the ER (Mathur et al. 2003). A different situation may exist during mitosis and cytokinesis (see below).

Electron microscopy has revealed a spatial proximity between AF bundles and ER elements in fusiform cambium cells (Goosen de Roo et al. 1983), in developing guard cells (Palevitz and Hodge 1984), in differentiated root statocytes (Hensel 1987), in onion epidermis cells (Quader et al. 1987), in parenchyma cells of Drosera (Lichtscheidl et al. 1990), and in internodal cells of the giant alga Chara (Kachar and Reese 1988). Sliding of ER tubules along AFs has been convincingly demonstrated by in vitro video microscopy studies after gently extracting the cytoplasm of internodal cells of giant algae such as Nitella (Higashi-Fujime 1988) and Chara (Kachar and Reese 1988). We approached this question in onion bulb scale epidermal cells by following the redistribution of the ER after previously dislocating it by centrifugation (Quader et al. 1987). The ER starts to relocate in the form of bundles of long ER tubules and attains its former pattern after several hours depending on the centrifugal force applied. However, no recovery is observed in the presence of the AF disassembling drug cytochalasin D. This causes the transformation of parts of the tER into flat lamellar ER sheets (Quader et al. 1989). Lamellae form, in particular, at sites in the polygonal network where organelles accumulate (Quader et al. 1996). Depending on the physiological activity of the cells studied, the lamellar ER sheets may occupy large areas of the peripheral ER network. In mesophyll cells from Vallisneria, cytochalasin D also caused a change of the polygonal cortical ER tubules into lamellar sheets (Liebe and Menzel 1995).

AFs can only function as a framework for ER organization in correlation with a linking motor protein creating the force for ER motility. Using perfused internodal cells of C. corallina, Williamson (1979) showed that myosin-like filaments are associated in a stationary way with the ER at one end, while the other end is apparently in loose contact with the sub-cortical AF bundles. These filaments were later proved to be myosin (Grolig et al. 1988). The involvement of myosin in the dynamic distribution of cell organelles including the ER was confirmed in onion epidermal cells by localizing myosin through immunofluorescence, and by blocking its activity with the sulfhydryl reagent N-ethylmaleimide (NEM, Liebe and Quader 1994). Displacement of the ER by centrifugation leads to the dislocation of the ER and of myosin to the region of the centrifugal pole. After centrifugation, the previous ER and myosin pat tern was gradually restored. NEM, which leads to the complete inhibition of actomyosin-dependent organelle movement (Kohno and Shimmen 1988), not only arrested ER translocation, but also caused in onion epidermal cells the partial conversion of tER elements into lamellar sheets, whereas this agent apparently only froze the polygonal network in the alga Vallisneria (Liebe and Menzel 1995). Blocking the action of the myosin ATPase by 2,3-butanedione 2-monoxime caused the bulging and dilation of cortical ER tubules in the vicinity of plasmodesmata (Samaj et al. 2000).

In conclusion, while it is generally accepted for many algae and differentiated higher plant cells that ER organization depends on an intact actomyosin system, little is known regarding the situation in mosses and fern protonema cells.

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