The Bridge Between Plant Cells


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Have you ever wondered how plant cells talk to one another? It is rather a childlike thing to wonder about, although the answer is far from childlike and instead rather complicated. You may know that plant cells differ in many different ways from animal cells, both in terms of some of their internal organelles and the fact that plant cells have cell walls, whereas animal cells do not. The two cell types also differ in the way they communicate with one another and in how they translocate molecules.

What Are Plasmodesmata?

Plasmodesmata (singular form: plasmodesma) are intercellular organelles found only in plant and algal cells. (The animal cell "equivalent" is called the gap junction.) The plasmodesmata consist of pores, or channels, lying between individual plant cells, and connect the symplastic space in the plant. They can also be termed as "bridges" between two plant cells. The plasmodesmata separate the outer cell membranes of the plant cells. The actual air space separating the cells is called the desmotubule. The desmotubule possesses a rigid membrane that runs the length of the plasmodesma. Cytoplasm lies between the cell membrane and the desmotubule. The entire plasmodesma is covered with the smooth endoplasmic reticulum of the connected cells.

Plasmodesmata form during periods of cell division during plant development. They form when parts of the smooth endoplasmic reticulum from the parent cells become trapped in the newly formed plant cell wall. Primary plasmodesmata are formed while the cell wall and endoplasmic reticulum are formed, as well; secondary plasmodesmata are formed afterward. Secondary plasmodesmata are more complex and may have different functional properties in terms of the size and nature of the molecules able to pass through.

Activity and Function of Plasmodesmata

Plasmodesmata play roles in both cellular communication and in molecule translocation. Plant cells must work together as part of a multicellular organism (the plant); in other words, the individual cells must work to benefit the common good. Therefore, communication between cells is crucial for plant survival. However, the problem with the plant cells is the tough, rigid cell wall. It is difficult for larger molecules to penetrate the cell wall, which is why plasmodesmata are necessary.

The plasmodesmata link tissue cells to one another, so they have functional importance for tissue growth and development. It was clarified in 2009 that the development and design of major organs was dependent on the transport of transcription factors through the plasmodesmata.

Plasmodesmata were previously thought to be passive pores through which nutrients and water moved, but now it is known that there are active dynamics involved. Actin structures were found to help move transcription factors and even plant viruses through the plasmodesma. The exact mechanism of how the plasmodesmata regulate the transport of nutrients is not well understood, but it is known that some molecules can cause the plasmodesma channels to open more widely. It was determined by using fluorescent probes that the average width of the plasmodesmal space is approximately 3-4 nanometers; however, this can vary between plant species and even cell types. The plasmodesmata may even be able to alter their dimensions outward so that larger molecules can be transported. Plant viruses may be able to move through plasmodesmata, which can be problematic for the plant since the viruses can travel around and infect the entire plant. The viruses may even be able to manipulate the plasmodesma size so that larger viral particles can move through.

Researchers believe that the sugar molecule controlling the mechanism for closing the plasmodesmal pore is callose. In response to a trigger such as a pathogen invader, callose is deposited in the cell wall around the plasmodesmal pore and the pore closes. The gene that gives the command for callose to be synthesized and deposited is called CalS3. Therefore, it is likely that the plasmodesmata density may affect the induced resistance response to pathogen attack in plants. This idea was clarified when it was discovered that a protein, named PDLP5 (plasmodesmata-located protein 5), causes the production of salicylic acid, which enhances the defense response against plant pathogenic bacterial attack.

History of Plasmodesma Research

In 1897, Eduard Tangl noticed the presence of the plasmodesmata within the symplasm, but it wasn't until 1901 when Eduard Strasburger named them plasmodesmata. Naturally, the introduction of the electron microscope allowed the plasmodesmata to be studied more closely. In the 1980s, scientists could study the movement of molecules through the plasmodesmata using fluorescent probes. However, our knowledge of plasmodesmata structure and function remains rudimentary, and more research needs to be performed before all is fully understood.

What hinders further research? Put simply, it is because plasmodesmata are associated so closely with the cell wall. Scientists have attempted to remove the cell wall in order to characterize the chemical structure of the plasmodesmata. In 2011, this was accomplished, and many receptor proteins were found and characterized.