Plasmodesma (plural: plasmodesmata) is thin irregular cylinder of cytoplasm lined by plasmalemma, passing through fine pores in the cell walls, thus forming a connection between the cytoplasm of adjacent cells.
They are found in higher plants and fluctuate widely in abundance and distribution. They are commonly present in primary pit fields and pit membranes of young and mature living cells respectively. They may be scattered over the entire wall or occur in groups when they are concentrated on primary fields (e.g. cambium of Pinus strobus, ray cells of Sequoia sempervirens etc.).
In a meristematic cell, the number of plasmodesma ranges from 1000 to 10,000 and their distribution may not be uniform. The frequency of distribution may vary even in different walls of a single cell. Plasmodesmata, at the intercellular canal between the common walls of living cells, are encircled by plasmalemma, which is continuous with that of the adjacent cells.
At the centre of plasmodesma, there occurs a tube of membrane, termed desmotubule. The desmotubule is composed of protein sub-units and contains an axial central rod. Diameter of the lumen of plasmodesma is very narrow, 30 nm to 60 nm in diameter through which the cell organelles cannot move to the adjacent cells. The diameter of desmotubule ranges from 16 nm to 20 nm.
A space is present in between the plasmalemma and desmotubule termed cytoplasmic annulus. Sometimes plasmodesmata are branched; it is observed in the pit membranes of living fibres of Tamarix and between sieve tube and companion cell where it is branched towards the latter side. The above structure of plasmodesmata is revealed by electron microscopic study (Fig. 3.7).
1. Diagrammatic representation of a plasmodesma between adjoining cell wall. 2. Cross sectional view of the plasmodesma at different levels. 3. Diagram showing plasmodesmata.
Plasmodesma originates during cytokinesis when cell plate is formed. It is formed at those regions of the cell plate where the endoplasmic reticulum (ER) is present and prevents the fusion of vesicles.
At this region, the cellulose microfibrils and pectic substances are not accumulated. As a result intercellular canal is formed. It is observed that the desmotubules are continuous with the ER of adjoining cells through the intercellular canals.
Therefore, it is regarded that the desmotubules are derived from ER. There are reports of the formation of plasmodesmata between mature cells, between the host cell and haustorium of parasite, between tyloses, in grafts between the cells of stalk and scion etc.
In these cases there are wall degrading enzymes and signaling devices, which perforate the common wall to form intercellular canal and align the plasmodesma situated at the adjoining cells.
Plasmodesmata exist in thick cell wall also, e.g. endosperm of the seeds of Phoenix dactylifera, Coffea arabica etc. They can be easily observed in the endosperm of seeds of Aesculns, Diospyros etc. It is best studied in plasmolysed cells where the protoplast shrinks from all the regions of cell wall, except the places where plasmodesmata occur.
Function:
(1) It helps in the short distance transport of materials;
(2) The relay of stimulus occurs through it;
(3) Viruses can pass through plasmodesmata;
(4) Plant hormones move through plasmodesma;
(5) The movement through plasmodesma is bi-directional. It is suggested that the desmotubules act as a valve and regulate the direction of flow; and
(6) Small molecules and ions pass readily through plasmodesma.