Plasmodesmata Are Membrane Lined Cytoplasmic Channels

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PD are specialized intercellular channels that establish both cytoplasmic and endomembrane continuity in plants (Robards and Lucas 1990; Lucas and Wolf 1993; Roberts and Oparka 2003). Unlike animal cells, plant protoplasts are encased by rigid cell walls, preventing the PMs of neighboring cells from making direct contact. This physical limitation imposed upon plant cells may have driven the creation of membrane-lined cytoplasmic pores as means of a symplasmic communication, which is essential to function as a complex multicellular organism. Consequently, this event is considered a major factor in the evolution of higher plants (Lucas et al. 1993).

At the ultrastructural level, individual primary plasmodesma in higher plants is composed of tubular cytoplasmic channels that are lined externally with the PM and internally with the ER membrane (Ding et al. 1992a; Overall and Blackman 1996; Lee et al. 2000) (Fig. 1). The cytoplasmic space between the PM and ER membrane is divided into microchannels (~3-4 nm in diameter) by crosslinking of presumed globular proteins that are embedded within the PM inner leaflet and ER outer leaflet (Robards and Lucas 1990; Overall and Blackman 1996). The basal SEL of these microchannels is estimated to be ~800 Da, which is sufficient for the diffusion of small molecules such as nutrients and ions. The central or core region of a plasmodesma is occupied by an appressed form of the ER, which is connected to the cytoplasmic ER cisternae. This unique structural organization enables the establishment of an endomem-brane continuum between adjoining cells throughout plant tissues or symplasmic domains (Overall et al. 1982; Robards and Lucas 1990; Roberts and Oparka 2003). The internal structure of PD corresponding to the appressed ER was originally termed desmotubule (Robards 1968) because the appearance of the structure was interpreted as something similar to cytoplasmic microtubules. Later, Lucas et al. (1993) proposed renaming it to "appressed ER" to reflect the fact that the core region constitutes a modified form of ER strand continuous with

Bordered Pits Plants

Fig. 1 Ultrastructure of PD. Transmission (a) and scanning (b) electron micrographs showing side and top views, respectively, of numerous PD, clustered in a pit field. Corresponding schematic diagrams (right panels) illustrate basic architecture of a plasmodesma embedded in the cell wall and delimited by PM inner and ER outer leaflets (modified from Lee et al. 2000). These membranes appear to contain electron-dense globular proteins (Ding et al. 1992a) that may constitute putative plasmodesmal channel proteins (PCPs)

Fig. 1 Ultrastructure of PD. Transmission (a) and scanning (b) electron micrographs showing side and top views, respectively, of numerous PD, clustered in a pit field. Corresponding schematic diagrams (right panels) illustrate basic architecture of a plasmodesma embedded in the cell wall and delimited by PM inner and ER outer leaflets (modified from Lee et al. 2000). These membranes appear to contain electron-dense globular proteins (Ding et al. 1992a) that may constitute putative plasmodesmal channel proteins (PCPs)

the cortical ER. Appressed ER is thought to provide PD channels with structural stability within the rigid cell wall environment. In addition, the presence of proteinaceous molecules, apparently embedded in the appressed ER membrane, raises the possibility that the appressed ER may enable PD to control SEL and facilitate macromolecular trafficking (Lucas et al. 1993).

Another important role provided by the appressed ER is a functional membrane coupling between neighboring cells. Fluorescently tagged lipid molecules that label the ER membrane in a target cell were shown to spread into and label the ER membrane in neighboring cells, supporting the idea that the ER membrane is functionally coupled through PD (Grabski et al. 1993; Martens et al. 2006). By contrast, no occurrence of a membrane coupling through the PM has been reported. Interestingly, the extent of ER membrane coupling is not constant across the boundaries of different cell types. A higher degree of the coupling was revealed at the cell junctions between the companion cell (CC) and the sieve element (SE) than other junctions (Martens et al. 2006). Consistent with previous structural observations that the appressed ER structurally lacks the lumen space in general (Overall et al. 1982; Ding et al. 1992a), the ER coupling through the lumen was not detected even at the CC-SE junction when a ER targeted green fluorescent protein (GFP) reporter was employed (Martens et al. 2006).

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