ER Patterns During Cell Differentiation and Organ Development

Attempts to follow in vivo the dynamics of ER organization during development and differentiation of plant organs are still rather scarce. McCauley and Hepler (1990, 1992) studied the ER organization during bud formation in the moss Funaria by visualizing the ER with the fluorochrome DiOC6(3), and Ridge et al. (1999) followed the ER transition in developing Arabidopsis root and hypocotyl cells by employing GFP technology.

In tip-growing moss protonema cells, the dense ER meshwork in the cell cortex changes to a more open reticulate tubular network with occasional small lamellar sheets when growth slows down (see Fig. 2b) (McCauley and Hepler 1990, 1992). In bud initiative cells this wide tER network regains compactness after the asymmetric division and becomes even tighter during the succeeding developmental phase of the bud initial. In caulonema cells and developing buds, ribosomes are found attached to the reticular tubular network as well as to the lamellar ER. The ER network of buds is covered more densely, whereas the lamallae apparently show no distinct difference in caulonema, branches, and buds (McCauley and Hepler 1992).

Root development involves the differentiation of meristematic cells into highly expanded parenchyma and epidermal cells, or in the root cap to elongated statocytes or secretory cells. Root but also hypocotyl cells of Arabidospis are distinguished in the cell cortex by highly perforated lamellar ER sheets during the early phase of expansion (Ridge et al. 1999). The overall size and the diameter of the holes of these perforated sheets are larger than those reported for the fenestrated lamellae observed in barley mitotic cells (Hepler 1981). The perforated ER sheets were only detectable in fast-growing cells and they began to diminish when growth came to an end. The perforation holes dilated, became segregated, and finally the cortical ER pattern resembled that of highly vacuolated non-growing cells. In root epidermal cells which had ceased expansion growth but entered polar root hair differentiation, the polygonal reticulum again condensed in the trichoblast starting at the site of root hair outgrowth (Ridge et al. 1999). The ER pattern in growing root hairs resembled that of tip-growing Funaria protonema cells (McCauley and Hepler 1992). The compact polygonal reticulum in the tip and in the non-growing basal region became gradually wider after growth ceased.

The observations by Ridge et al. (1999) are in agreement with previous ultrastructural studies of root cap differentiation (Stephenson and Hawes 1986) and with cotyledons during seedling growth (Harris and Chrispeels 1980), although the extent of the tubular and lamellar ER is not easily deduced by electron microscopy even using impregnation procedures that allow the study of thicker sections. During sieve cell maturation the transition from lamellar to tER is followed by tER relocation (Schulz 1988). The opposite, i.e. the formation of lamellar ER stacks, has been shown to occur during mustard seedling maturation (Bergfeld and Schopfer 1984). ER stack formation correlates with the disappearance of ER tubules and with seed dehydration. These ER aggregates probably represent a form of ER membrane storage during periods of dehydrated protoplasm, which is a situation also characteristic of mature pollen grains.

In general, a relatively wide polygonal network of tER in the cell cortex seems to be characteristic for very slow or non-growing plant cells, but in swiftly differentiating cells the amount, the distribution, and the proportion of ER tubules to lamellae undergo distinct changes.

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