PLASMODESMETA

PLASMODESMETA

INTERNAL STRUCTURE

In vascular plants the basic plasmodesmal structure is a tube of plasma membrane surrounding a strand of modified ER, with particulate material between them. Fig I interprets the ultra structure of vascular plants in a cross and longitudinal sections. Essental features of plasmodesmeta 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 lies between the desmotubule and plasma membrane is considered as 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 plasma membrane. In cross section, 7-to-9 gaps occurs 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.

FUNCTIONS

It plays important role in cell to cell communication. To investigate this function microelectrodes have been employed to monitored the flow of electric current from cell to cell in plant tissue (as in case of animals gap junctions.). Such studies reveals that electric current passes between plant cells linked by plasmodesmeta more readily than it does between the cells that are not linked by plasmodesmeta. The magnitude of the current flow is directly related to the number of plasmodesmeta present.  This suggest that it plays role in cell-to-cell communication, comparable to that played by gap junctions in animals.

COMPARISON WITH GAP JUNCTION

§   In plants junctions are termed as plasmodesmeta, whereas in animals it is termed as cell junctions.

§   In plants plasmodesmeta's are found in pit-feilds where primary cell wall is thin and secondary cell wall is absent. In animals 3-types of junctions are found-

(i) Tight junction- which creates the permeability barriers across the layers of cell.

(ii) Plaque bearing junctions- which stablizes the cells against the mechanial stress.

(iii) Gap junctions- which prevents the movement across the cells.

§   Gap junctions in electron micrograph appears as regions in which the plasma membrane of the two adjacent cells are aligned in parallel and separated by a tiny gap of the 3nm.Plasmodesmeta electron micrograph reveals that these are narrow channels in the cell wall that are linend by plasma membrane and transverse by the tubules of ER.

§   Connexin proteins are observed on the membrane surface o the gap junctions, which are clusters of 6 connexin molecules surounded by aquoues channels. No such structures are seen in plasmodesmeta.

§   Permeability of the gap junctions can be altered (closed and opened) by the changes in the membrane potential, intracellular pH and cyclic AMP levels. Plasmodesmeta permeability is subjected to regulation by the internal conc. of Ca ions, but its mechanism is not well understood.

§   In addition to their role in electrical coupling, gap junctions may have developmental and metabolic functions. No such functions are performed by plasmodesmeta.