Plasmodesmata

Plasmodesmata (singular, plasmodesma) are microscopic channels of plants traversing the cell walls and middle lamella between pairs of plant cells and facilitating transport and communication between them. Unlike animal cells, plant cells are protected by a cell wall that is impermeable to large molecules such as proteins and in these cells plasmodesmata enable regulated intercellular transport. Plant cells can use both passive and active transport to move molecules and ions through the passage. A plasmodesma is constructed of three main layers, the plasma membrane, the cytoplasmic sleeve, and the desmotubule.

Plasmodesmal plasma membrane
The plasma membrane portion of the plasmodesma is a continuous extension of the cellular plasmalemma. It is similar in structure to most cellular phospholipid bilayers.

Cytoplasmic sleeve
The cytoplasmic sleeve is enclosed by the plasma membrane and is an extension of the cytosol. Trafficking through plasmodesmata is assumed to occur through this passage. Smaller molecules (e.g. monosaccharides) and ions can easily pass through plasmodesmata by diffusion without the need for additional chemical energy. It is unknown how the selective transport of larger molecules, such as proteins, occurs. One hypothesis is that the polysaccharide callose accumulates around the neck region of plasmodesmata reducing their diameter and thereby controlling permeability to substances in the cytoplasm.

Desmotubule
The desmotubule is a tube of appressed endoplasmic reticulum that runs between two adjacent cells. Some molecules are known to be transported through this channel, but it is not thought to be the main route for plasmodesmatal transport.

Around the desmotubule and the plasmamembrane areas of an electron dense material have been seen, often joined together by spoke-like structures that seem to split the plasmodesma into smaller channels. These structures may be composed of myosin and actin, which are part of the cell's cystoskeleton. If this is the case these proteins could be used in the selective transport of large molecules between the two cells.

Transport
Plasmodesmata have been shown to transport proteins, messenger RNA and viral genomes from cell to cell. The best studied of these are viral movement proteins such as those of the tobacco mosaic virus MP-30. MP-30 is thought to bind to the virus's own genome and shuttle it from infected cells to uninfected cells through plasmodesmata. Flowering Locus T protein moves from leaves to the shoot apical meristem through plasmodesmata to initiate flowering.

Links
Video showing vesicular transport through the plasmodesmata of a Water Lily petal cell. Kinesin and Dynein actively move the vesicles along microtubule highways, into and out of the cell.