Primary Plasmodesmata

Based on where and how they are formed, plasmodesmata are categorised as either primary or secondary. Primary plasmodesmata form during cytokinesis at sites where ER tubules cross the phragmoplast of a dividing cell (Fig. 2e) (Hepler 1982; Staehelin and Hepler 1996). ER tubules, surrounded by cytoplasm, are trapped among the fusing Golgi vesicles that deliver wall material to the developing cell plate (Fig. 3a). As the cytoplasmic bridges become increasingly constricted they develop into plasmodesmal strands that are lined with plasma membrane derived from the Golgi membrane (Fig. 3b). Although transformed into the desmotubule, the ER tubules remain connected with the cortical ER network of the adjacent cells. When formed these plasmodesmata are simple in structure, but during the course of growth and differentiation may be modified into branched forms (for reviews, see Ding and Lucas 1996; Kollmann and Glockmann 1999; Ehlers and Kollmann 2001).

One mechanism of plasmodesmal branching, observed in the division walls of protoplast-derived calluses (Ehlers and Kollmann 1996), is similar to the mode of formation of primary plasmodesmata. As the cell walls thicken the simple primary plasmodesmata have to elongate to maintain intercellular communication. Golgi-derived vesicles carrying wall material fuse with the plasma membrane and trap cytoplasmic strands, enclosing straight or branched cytoplasmic ER cisternae which are continuous with the desmo-tubule (Fig. 3c). The shape of the entrapped ER determines the shape of the resulting branched plasmodesmata (Ehlers and Kollmann 1996, 2001).

Biogenesis Plasmodesmata

Fig. 3 Diagrammatic representation of plasmodesma biogenesis. Primary plasmodesmata (left) are formed during cytokinesis at sites where the ER tubules cross the phragmo-plast (a). ER and cytoplasm are trapped among the fusing Golgi vesicles forming the desmotubule and cytoplasmic sleeve (b). These simple plasmodesmata may be modified into branched forms by the entrapment of additional ER strands during wall thickening (c). Secondary plasmodesmata (right) form in existing cell walls by the fusion of ER across thinned wall areas (d), establishing usually branched connections (e) (Modified from Kragler et al. 1998)

Fig. 3 Diagrammatic representation of plasmodesma biogenesis. Primary plasmodesmata (left) are formed during cytokinesis at sites where the ER tubules cross the phragmo-plast (a). ER and cytoplasm are trapped among the fusing Golgi vesicles forming the desmotubule and cytoplasmic sleeve (b). These simple plasmodesmata may be modified into branched forms by the entrapment of additional ER strands during wall thickening (c). Secondary plasmodesmata (right) form in existing cell walls by the fusion of ER across thinned wall areas (d), establishing usually branched connections (e) (Modified from Kragler et al. 1998)

Similar entrapment of ER cisternae during cell wall deposition also results in the conversion of simple, primary plasmodesmata into complex plasmo-desmatal morphotypes with median branching planes and central cavities. Primary plasmodesmata may become considerably extended in width during cell expansion growth and thinning of the cell wall. In this case, the dilated primary plasmodesmata form the median plasmodesmal parts in the middle lamella which later become the central cavities (for details, see Kollmann and Glockmann 1999; Ehlers and Kollmann 2001), with uneven wall expansion of the adjacent cells resulting in the formation of dilated central cavities (van der Schoot and Rinne 1999).

The lateral fusion of neighbouring simple primary plasmodesmata to form H-shaped plasmodesmata, which may possess dilated central cavities (Ding et al. 1992a, 1993; Glockmann and Kollmann 1996; Volk et al. 1996; Itaya et al. 1998; Kollmann and Glockmann 1999; Oparka et al. 1999), may involve local enzymatic digestion of existing cell wall to allow penetration of additional plasmodesmatal strands (Jones 1976; Ding et al. 1992a, 1993,1999; Lucas et al. 1993; Lucas and Gilbertson 1994; Ding and Lucas 1996; Itaya et al. 1998), or the formation of further branches by ER entrapment (Ehlers and Kollmann 1996, 2001; Glockmann and Kollmann 1996; Kollmann and Glockmann 1999).

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  • merigo
    What is desmotubules or er tubules?
    3 years ago
  • FRANK
    How plasmodesmata connect with er?
    2 years ago

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