PLASMODESMATA: CHARACTERISTICS, STRUCTURE AND FUNCTIONS - BIOLOGY - 2025

The plasmodesmata are cytosolic connections that exist between adjacent plant cells, ie, communicating protoplasts (cytosol and plasma membrane) through the cell wall, forming a continuous symplastic.

These structures are functionally analogous or equivalent to the gap junctions observed between the cells of an animal tissue and their main function is to communicate the cells with each other and serve as a channel for the transport of different types of ions and molecules.

The simplistic and apoplastic pathways and the involvement of plasmodesmata (Source: Jackacon, vectorised by Smartse via Wikimedia Commons)

Plasmodesmata were described more than 100 years ago by Tangl and, since then, hundreds of studies have been published in which their functioning mechanism, their structure and other related aspects have been detailed in detail.

At present it is known that these cytosolic "channels" or "connections" between cells are structures under strict control mechanisms and it has also been determined that they are composed mainly of integral membrane proteins, chaperone proteins and other proteins specialized in the transport of substances.

Characteristics of plasmodesmata

Plasmodesmata connect cells belonging to the same "simplistic domain" in a plant tissue, which means that not all cells of a plant are connected to each other, but there are different specific "regions" in a tissue in which the cells present there exchange information permanently.

These are highly dynamic structures; their number, structure and operation can be modified in response to a specific functional demand on a fabric.

In addition, these channels can be degraded or "sealed" in some cellular interfaces (the space between two cells), which implies the formation of a simplistic "barrier" between the cells of some plant tissues and promoting the isolation of defined regions in a tissue.

Some bibliographic citations suggest that plasmodesmata are structures as complex as the so-called nuclear pore complexes, which perform similar functions but in the translocation of molecular information from the cytosolic environment to the interior of the nucleus.

Structure

A quick glance at a plant tissue is enough to verify that there are multiple types of plasmodesmata.

According to some authors, these can be classified as primary and secondary, according to the moment in which they are formed during the life of a cell; or as simple and branched, depending on the morphology of the channels that are formed between cell and cell.

Regardless of the type of plasmodesmus in question, its "structural architecture" is more or less equivalent, since it is almost always a question of conduits with a diameter that varies between 20 and 50 nm, whose entrances or orifices are a little more narrow, constituting what is known as a “bottleneck constriction”.

Some scientists have proposed that such constriction in the orifices of the plasmodesmata participates in the regulation of the flow of substances through these, that is, that their dilation (expansion) or constriction (reduction in diameter) determines the amount and speed of flow.

These "bottlenecks" are composed of a substance known as callose (β-1,3-glucan) and, as can be inferred, they are found in the areas closest to the wall of plant cells connected by these channels.

Graphic representation of plasmodesmata (Source: User: Zlir'a via Wikimedia Commons)

Primary plasmodesmata

Primary plasmodesmata form in the "cell plate" during cytokinesis, which is the time of mitosis where the two daughter cells separate. However, these can undergo structural modifications and change their distribution and operation during the development of the plant to which they belong.

These plasmodesmata are actually membranous environments that consist of pores in the plasma membrane that form a kind of bridge between the cell wall and an axial element of the "trapped" endoplasmic reticulum known as the desmotubule.

A demotubule is a cylindrical structure of about 15 nm in diameter, composed of the endoplasmic reticulum of one cell that is continuous with the cisternae of the endoplasmic reticulum of the neighboring cell that is connected through the plasmodesm.

Between the "strand" represented by the desmotubule and the plasma membrane that makes up the cylindrical cavity that is the plasmodesmus, there is a space known as the "cytoplasmic sleeve" (from the English Cytoplasmic sleeve), which is through which it is thought to occur the flow of substances from one cell to another.

Secondary plasmodesmata

These are the ones that can form de novo between two cell walls independently of cytokinesis, that is, without the need for a cell division event to occur. Secondary plasmodesmata are considered to have special functional and structural properties.

Secondary plasmodesmata are formed thanks to the fusion of opposite ends of pre-existing "halves" of plasmodesmata, which are usually established in regions of the cell wall that have been thinned. Each fused half creates the central cavities of a plasmodesmus.

The central strands in this type of plasmodesm are subsequently added by passive "enclosure" of endoplasmic reticulum tubules and the resulting morphology is very similar to that of primary plasmodesmata.

Experts in the field suggest that secondary plasmodesmata are formed in cells that undergo extensive growth processes (elongation), that is, between longitudinal cell walls, in order to compensate for the progressive “dilution” of the number of plasmodesmata that can occur thanks to to growth.

Features

Plasmodesmata represent one of the main cell-cell communication pathways in plant tissue. These structures also offer a channel for electrical signaling, for the diffusion of lipids and small soluble molecules and even for the exchange of transcription factors and macromolecules such as proteins and nucleic acids.

These communication pathways provided by the plasmodesmata appear to have an essential function in programming plant development and also in coordinating the physiological functioning of a mature plant.

They participate in the regulation of the release of important molecules from the physiological and developmental point of view towards the phloem (which carries the sap); they intervene in the physical isolation of some cells and tissues during development, which is why they are said to coordinate growth, development and defense against pathogens.

After invasion by a pathogenic fungus, plasmodesmata are also involved, since they correspond to the main intracellular or simplistic invasion pathways in plant tissues.

References

1. Ehlers, K., & Kollmann, R. (2001). Primary and secondary plasmodesmata: structure, origin, and functioning. Protoplasm, 216 (1-2), 1.
2. Lucas, WJ, & Lee, JY (2004). Plasmodesmata as a supracellular control network in plants. Nature Reviews Molecular Cell Biology, 5 (9), 712.
3. Maule, AJ (2008). Plasmodesmata: structure, function and biogenesis. Current opinion in plant biology, 11 (6), 680-686.
4. Robards, AW, & Lucas, WJ (1990). Plasmodesmata. Annual review of plant biology, 41 (1), 369-419.
5. Roberts, A., & Oparka, KJ (2003). Plasmodesmata and the control of symplastic transport. Plant, Cell & Environment, 26 (1), 103-124.
6. Turgeon, R. (1996). Phloem loading and plasmodesmata. Trends in Plant Science, 1 (12), 418-423.