Plasmodesmata

Plasmodesmata (singular, plasmodesma) are microscopic channels of plants traversing the cell walls of plant cells^[1] and enabling transport and communication between them. Unlike animal cells, every plant cell is surrounded by a polysaccharide cell wall. Neighbouring plant cells are therefore separated by a pair of cell walls and the intervening middle lamella, forming an extracellular domain known as the apoplast. Although cell walls are permeable to small soluble proteins and other solutes, plasmodesmata enable direct, regulated, symplastic intercellular transport of substances between cells. Plant cells can use both passive and active transport to move molecules and ions through the passage^{[citation needed]}.

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Structure

Plasmodesmatal plasma membrane

A typical plant cell may have between 10³ and 10⁵ plasmodesmata connecting it with adjacent cells. Plasmodesmata are approximately 50-60nm in diameter at the mid-point and are constructed of three main layers, the plasma membrane, the cytoplasmic sleeve, and the desmotubule ^[2]

The plasma membrane portion of the plasmodesma is a continuous extension of the cell membrane or plasmalemma ^[3] It is similar in structure to the cellular phospholipid bilayers.

Cytoplasmic sleeve

The cytoplasmic sleeve is a fluid-filled space enclosed by the plasmalemma and a continuous extension of the cytosol. Trafficking of molecules and ions through plasmodesmata is assumed to occur through this passage. Smaller molecules (e.g. sugars and amino acids) and ions can easily pass through plasmodesmata by diffusion without the need for additional chemical energy. It is not yet known 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 to form a collar, reducing their diameter and thereby controlling permeability to substances in the cytoplasm^{[citation needed]}.

Desmotubule

The desmotubule is a tube of appressed endoplasmic reticulum that runs between two adjacent cells^{[citation needed]}. Some molecules are known to be transported through this channel^{[citation needed]}, 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^{[citation needed]}. These structures may be composed of myosin and actin^{[citation needed]}, which are part of the cell's cytoskeleton. 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^{[citation needed]}. The best studied of these are viral movement proteins such as those of the tobacco mosaic virus MP-30^{[citation needed]}. MP-30 is thought to bind to the virus's own genome and shuttle it from infected cells to uninfected cells through plasmodesmata^{[citation needed]}. Flowering Locus T protein moves from leaves to the shoot apical meristem through plasmodesmata to initiate flowering^{[citation needed]}.

References

1. ^ Plasmodesmata (www.dictionary.com)
2. ^ AW Robards (1975) Plasmodesmata. Annual Review of Plant Physiology 26, 13-29
3. ^ AW Robards (1976) Plasmodesmata in higher plants. In: Intercellular communications in plants: studies on plasmodesmata. Edited by BES Gunning and AW Robards Springer-Verlag Berlin pps 15-57.