Plasmodesmata

In 1879, Eduard Tangl discovered plasmodesmata’s presence in the symplasm, the inner side of the cell membrane where water low-molecular-weight solutes diffuse freely. Eduard Strasburger named them in 1901.

Shape and Size

Plasmodesmata are mostly cylindrical-shaped, membrane-lined channels having a diameter of 20 to 40 nm. Their morphology is found to vary significantly during cell growth and differentiation or depending upon the transport requirement of the cell.

Composition

Plasmodesmata consist of three main layers: the cell membrane, the axial center called the desmotubule, and the region between the desmotubule and the cell membrane called the cytoplasmic sleeve. The cylindrical-shaped desmotubule remains continuous with the outer membrane of the smooth endoplasmic reticulum of each connected cell. At the same time, the cytoplasmic sleeve acts as the principal conduit for cellular passage. The cell membrane portion of the plasmodesmata consists of the phospholipid bilayer.

Between the outside face of the desmotubule and the cell membrane’s inner face is an annulus of cytoplasm that appears constricted at each end of plasmodesmata. The presence of actin and myosin protein along the length of plasmodesmata provides flexibility to regulate its aperture.

Plasmodesmata are formed during cell division when parts of the parent cell endoplasmic reticulum get trapped in the new cell wall formed to create new daughter cells.

Plasmodesmata act as cytoplasmic bridges to facilitate the transport and communication of different materials in and out of the cell. Plasmodesmata allow the passage of molecules less than 800 Daltons, functioning similarly to gap junctions in animal cells. The different roles of plasmodesmata are given below:

• Connecting symplastic space in plant cells and specialized channels allowing the movement of water, ions, and small signaling molecules such as sugars and amino acids between them.
• Transporting small proteins such as transcription factors, short interfering RNA, messenger RNA, viroids, and viral genomes.
• Helping to regulate the function of sieve-tube cells by the companion cells in the phloem through symplastic transport.
• Linking tissue cells to one another thus helping in tissue growth and development.