PLASMODESMATA

Plasmodesmata are microscopic channels which transverse the cell wall of plant cell and some algal cell and enabling transport and communication between them
Neighboring plant cells are therefore separated by a pair of cell wall and the intervening middle lamella, forming an extra cellular domain known as the apoplast.
Cell wall are permeable to small soluble protein and other solute.
Plasmodesmata enable direct regulated symplastic intracellular transport of substance between cells.
There are two forms of plasmodesmata .

The plasmodesmata are narrow channels that act as intercellular cytoplasmic bridge to facilitate communication and transport of material between plant cells.
The plasmodesmata serve to connect the symplastic space in the plant and are extremely specialize channel that allow for intracellular movement of water various nutrient and other molecules.
These are located in narrow areas of cell walls called primary pit fields and they are so dense in these area.
They make up one percent of the entire area of cell wall

Structure of Plasmodesmata

plasma membrane is continuous between cell
with in the plasmodesmal pore a tightly wound cylinder of membrane termed the desmotubule runs the length of the plasmodesmata
This structure is studied by Tilney by using plasmolysis, Trion X detergent extraction and protease digestion.
Thus he suggests that desmotubule provide a rigid stability to plasmodesmata and confer a fixed diameter and pore size to plamodesmal canal.
Desmotuble is linked to E.R. in each of the adjacent cells forming a dynamic endomembrane continuum in the symplastic space
A typical plant cell may have between 103 and 105 plasmodesmata connecting it with adjacent cell to between 1-10µm²
Plasmodesmata are approximately 50-60 nm in diameter, at the midpoint are constructed of three main layers: the plasma membrane, cytoplasmic sleeve, and desmotubules.

It is fluid filled space enclosed by plasma membrane
The larger molecules including protein and RNA pass through the cytoplasmic sleeve diffusively.
The mechanism of regulation of plasmodesmatal transport is the accumulation of the polysaccharide callose that accumulates around the neck region of plasmodesmata to form a collar reducing their diameter and thereby controlling permeability to substance in the cytoplasm.

These are tube of appressed endoplasmic reticulum that run between two adjacent cell.
some molecule are known to be transported through this channel
Around the desmotubule and P.M. area of and electron dense material have been seen often joined together by spoke -like structures that seem to split the plasmodesmata in to smaller channel.
These may be composed of myosin and actin which are part of the cyto-skeleton.

In vascular plants the basic plasmodesmal structure is a tube of plasma membrane surrounding a strand of modified ER, with particulate material between them.
interprets the ultrastructure of vascular plants in a cross and longitudinal sections.
Essential features of plasmodesmata is a cell to cell tubule of the plasma membrane that surrounds a cylindrical strand of tightly furled ER, the desmotubule.
A thin darkly stained central rod occupies the center of the desmotubule.
A cytoplasmic sleeve or the cytoplasmic annulus lying between the desmotubule and plasma membrane is considered as a possible pathway for the cell-to-cell water and solute movement.

The desmotubule is essentially a solid strand of lipid, the central rod is composed of the lipid polar groups and a few proteins that can physically occupy the inner core of the tightly furled lipid bilayer.
Much of the cytoplasmic annulus is occupied by proteinaceous material.
These particles are associated with both the outer surface of the desmotubules and the inner surface of the plasma membrane.
In the cross section, 7-to-9 gaps occur between the particles.
The distance across the gaps is about 2-3nm as comparable with the 4nm channel diameter.
These gaps are the physical basis of cell-to-cell transport.
A wide range of plasmodesmal morphologies have been observed, within the same plants.
These include differences in length, branching and size of the central cavity.
The functional significance of the variations is largely unknown.

It plays an important role in cell to cell communication.
To investigate this function microelectrodes have been employed to monitor the flow of electric current from cell to cell in plant tissue (as in case of animal gap junctions.).
Such studies reveal that electric current passes between plant cells linked by plasmodesmata more readily than it does between the cells that are not linked by plasmodesmata.
The magnitude of the current flow is directly related to the number of plasmodesmata present.
This suggests that it plays a role in cell-to-cell communication, comparable to that played by gap junctions in animals.
They serve to connect the symplastic space in the plant and are extremely specialized channels that allow for intercellular movement of water, various nutrients, and other molecules.
They allow the passage of molecules with molecular weights of less than 800 daltons.
Plasmodesmata have been shown to transport proteins (including transcription factors), short interfering RNA, messenger RNA, viroids, and viral genomes from cell to cell.
Plasmodesmata are also used by cells in the phloem, and symplastic transport is used to regulate the sieve-tube cells by the companion cells.