- Every living cell in a higher plant is connected to its living neighbors by fine cytoplasmic channels, each of which is called a plasmodesma which passes through the intervening cell walls.
- They act like tunnels running through the cell wall which allow communication with the other cells in a tissue.
- The plasma membrane of one cell is continuous with that of its neighbor at each plasmodesma.
- They are formed around the elements of the smooth endoplasmic reticulum that become trapped during cytokinesis (of mitotic cell division) within the new cell wall that will bisect the parental cell. Here the wall is not thickened further, and depressions or thin areas known as pits are formed in the walls. Pits normally pair up between adjacent cells.
- They can also be inserted into existing cell walls between non-dividing cells (secondary plasmodesmata).
- They occur in varying numbers.
- For example, plasmodesmata are especially common and abundant in the walls of columns of cells that lead toward sites of intense secretion, such as in nectar-secreting glands (trichomes of Abutilon nectaries).
- In such cells, there may be 15 or more plasmodesmata per square micrometer of wall surface, whereas there is often less than 1 per square micrometer in other cell walls.
- A typical plant cell may have between 103 and 105 plasmodesmata connecting it with adjacent cells equating to between 1 and 10 per µm2.
Figure: Diagram of Plasmodesmata. Image Source: Public domain/Wikimedia Commons
Structure of Plasmodesmata
- They are roughly cylindrical, membrane-lined channels with a diameter of 20 to 40 nm.
- They are constructed of three main layers, the plasma membrane, the cytoplasmic sleeve, and the desmotubule.
- Running from cell to cell through the center of most plasmodesmata is a narrower cylindrical structure, the desmotubule, which remains, continuous with elements of the SER membranes of each of the connected cells.
- Between the outside of the desmotubule and the inner face of the cylindrical plasma membrane is an annulus of cytosol, which often appears to be constricted at each end of the plasmodesmata.
- These constrictions may regulate the flux of molecules through the annulus that joins the two cytosols.
- The plasma membrane portion of the plasmodesma is a continuous extension of the cell membrane or plasmalemma and has a similar phospholipid bilayer structure.
Functions of Plasmodesmata
They are narrow channels that act as intercellular cytoplasmic bridges to facilitate communication and transport of materials between plant cells. 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.
- Plasmodesmata function in intercellular communication, i.e., they allow molecules to pass directly from cell to cell.
- It has been suggested that plasmodesmata mediate transport between adjacent plant cells, much as gap junctions of animal cells. 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.
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