description of cell to cell interactions and their functions

1. Cell-Cell interactions
D.INDRAJA

12. Plasmodesmata
• 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.
• Plasmodesmata are lined with plasma membrane that is continuous with the membranes of the
two cells. Each plasmodesma has a thread of cytoplasm extending through it

13. Formation and number
• 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.

14. 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

15. Functional states of plasmodesmata
Closed plasmodesmata
The plasmamembrane and desmotubule are appeared in a way that there is no
cytoplasmic sleeve. This may be temporary or permanent
Open basal plasmodesmata
The cytoplasmic channels are narrow. They have cell to cell transport of
micromolecules takes place
Intercellular exchange of ions, photoassimulators
Open dilated plasmodesmata
these are enlarged and more dilated for transport of macromolecules
Example: proteins of plant viruses
Dilation takes place due to physiological or developmental or specific target
proteins
Dilation takes place by specific targeted proteins is called Gatting

16. 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.