Plasmodesmata

Plasmodesma is a singular form of plasmodesmata. The plasmodesmata definition states that it is a microscopic cytoplasmic canal, which can pass through the plant cell walls and allows the molecules to directly communicate with the adjacent plant cells. Plasmodesmata usually occur during the cell division process. Here, the traces of the endoplasmic reticulum caught by the new cell walls that can develop into daughter cells. 

The plasmodesmata function is to regulate the passage of molecules between the plant cells, the two progeny cells get connected by thousands of rings of membrane plasmodesmata. In order to overcome the plasma membrane barrier, plasmodesmata link the plant cells into syncytial tissues. 

The plasmodesmata are of two types. They are primary plasmodesmata and secondary plasmodesmata. If the plasmodesma occurs during the cell division. It is termed primary plasmodesmata. If the plasmodesmata developed between mature cells, it is termed secondary plasmodesmata.  

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Plasmodesmata Structure

Each typical plant cell contains 103 -105 plasmodesmata, which are connecting space between one cell to adjacent cell of about 1 to 10 µm2. The diameter of plasmodesma is approximately ranged from 50-60 nm from its midpoint. Each plasmodesma is made up of three main layers namely, plasma membrane, desmotubles, and cytoplasmic sleeve. They can transverse the cell wall thickness up to 90nm. The plasmodesma’s plasma membrane portion continuous to extend the cell membrane or plasmalemma, which has a similar bilayer structure as phospholipid. 

The fluid-filled space enclosed by the plasmalemma is known as the cytoplasmic sleeve, which can continuous extension of the cytosol. It also helps to traffic the molecules and ions through plasmodesmata that occur in this space. Smaller ions and molecules like sugar and amino acids can easily pass through the plasmodesmata by diffusion. This does not require additional chemical energy. Likewise, the larger molecules like proteins and RNA can also pass through the cytoplasmic sleeve by diffusing through it. The transport of some larger molecules into plasmodesmata in plant cells is facilitated by special mechanisms. 

One mechanism behind regulating the permeability is the polysaccharide callose accumulated around the neck region in a collar form around the plasmodesmata allows permeability. This may result in a reduction in the diameter of the pores, which are transporting substances to cells. Likewise, the permeability of plasmodesmata in plant cells is increased while going through dilation, active gating, or structural remodelling. The increase of plasmodesmata pore permeability allows the macromolecules like signalling molecules, transcription factors, and RNA-protein complexes to transport to various cells. 

A tube of appressed endoplasmic reticulum, which turns between two cells adjacent to each other is known as desmotubule. This is not the main route for plasmodesmata transport. But some molecules using this channel to transport the nutritions. The electron-dense material found across the desmotubule and the plasma membrane areas is often joined together by the spoke-like structures present between the plasmodesma into smaller channels. These structures also composed of myosin and actin a part of the cell’s cytoskeleton. These can be used in the selective transport of large molecules between the two cells.

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Plasmodesmata Function

Plasmodesmata play an important in both molecular translocation and cellular communication. Each cell in the plants plays an important role to support plant growth. Therefore, the communication between the cells is predominant for plant survival. The plant cells usually have tough, rigid cell walls. So, it is much difficult for the larger molecules to penetrate into the cell wall of plants. The plasmodesmata in the plant cell help these molecules to enter the cell wall. 

Plasmodesmata meaning is the linking of cells of one tissue to another. So, this has functional importance for tissue growth and development. According to a research study in 2009, researchers found that the design of major organs depends on the transport of transcription factors like protein help to convert RNA and DNA through the plasmodesmata in plant cell.  

Plasmodesmata contain both passive and active pores. Through the passive pores of plasmodesmata in plant cells, it transmits water and nutrients.  

Actin structures found in plasmodesmata help to move transcription factors such as short interfering RNA, messenger RNA, viroids, and plant viruses. Some mechanism of plasmodesmata still remains unknown. But, some larger molecules can cause the plasmodesmata channels to open wider.  

To find the average width of the plasmodesmal space, fluorescent probes can be used. The approximate width of the space ranges from 3-4 nanometers. The plasmodesmal space will get vary depends on the species types and cell types. However, the plasmodesmata can alter their dimensions depending on molecules transported through them. The plant viruses which are travelling through plasmodesmata can create a problem for the plant cells. Since the viruses can infect the entire cells of the plant. Some viruses can also manipulate the plasmodesma size to allow the larger viral particles. This affects the entire plant’s growth.

According to the research study, the sugar molecules that can control the mechanism for closing the plasmodesmal pore are termed callose. It supports a trigger like a pathogen invader. Callose usually deposits in the cell wall around the plasmodesmal pore and the pore closes. Generally, genes will give the command for callose to be synthesis and deposit, it is known as CalS3. So, the plasmodesmata density will affect the induced resistance response to pathogen attack in plants. 

To avoid pathogenic bacterial attacks in plants, researchers discovered the protein namely plasmodesmata-located protein 5 PDLP5, which can produce salicylic acid to enhance defence against the plant pathogenic attack.