- 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 .
#Primary – It is formed during cell division
#Secondary – It is form between mature cell
- 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.
1 – Plasmodesmatal Plasma membrane
- This is continuous extension of the cell membrane or plasma lemma.
- 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.