4.16:
Plasmodesmata
Plant cells have rigid cell walls that help regulate cell shape and tonicity. However, this barrier presents a special challenge for communication between cells.
To overcome this challenge, plant cells connect via plasmodesmata—small channels that allow for cell-to-cell communication.
Each plasmodesma pore is a continuation of the plasma membranes of adjacent cells. In the center is a structure known as the desmotubule—an extension of the endoplasmic reticulum, or ER—that runs from one cell into the neighboring cell. The cytosol is continuous between the two connected cells. In this way, the plasmodesmata create a continuous network of cytoplasm, called the symplast.
The desmotubule penetrates the channel and creates a cytoplasmic sleeve, which can be dilated or constricted to regulate the permeability of the plasmodesma. For example, under normal conditions, water and small molecules, such as sugars and ions, can freely pass between cells. The desmotubule is so tightly squeezed, however, that very little—if any—lumen exists to allow passage of molecules.
The exchange of larger molecules—small RNA, transcription factors, and other cytosolic proteins—is tightly regulated. An accumulation of the polysaccharide callose narrows the opening in the cell wall, preventing the flow of these molecules. When callose is broken down, the opening widens and macromolecules can pass through the plasmodesma.
Additionally, callose can accumulate and close off the movement of all molecules. This is, for example, beneficial to restrict the movement of plant viruses that use the channels to spread to neighboring cells.
Plasmodesmata can originate in two ways. Primary plasmodesmata form during cell division in early development and are often found in clusters, called pit fields. Secondary plasmodesmata emerge during later stages in existing cell walls of neighboring cells.
Finally, plasmodesmata can be degraded based on the needs of the cells, for instance when cells need to isolate from the symplast.
4.16:
Plasmodesmata
The organs in a multicellular organism’s body are made up of tissues formed by cells. To work together cohesively, cells must communicate. One way that cells communicate is through direct contact with other cells. The points of contact that connect adjacent cells are called intercellular junctions.
Intercellular junctions are a feature of fungal, plant, and animal cells alike. However, different types of junctions are found in different kinds of cells. Intercellular junctions found in animal cells include tight junctions, gap junctions, and desmosomes. The junctions connecting plant cells are called plasmodesmata. Of the junctions found in animal cells, gap junctions are the most similar to plasmodesmata.
Plasmodesmata are passageways that connect adjacent plant cells. Just as two rooms connected by a doorway share a wall, two plant cells connected by a plasmodesma share a cell wall.
The plasmodesma “doorway” creates a continuous network of cytoplasm—like air flowing between rooms. It is through this cytoplasmic network—called the symplast—that most nutrients and molecules are transferred among plant cells.
A single plant cell has thousands of plasmodesmata perforating its cell wall, although the number and structure of plasmodesmata can vary across cells and change in individual cells. The continuum of cytoplasm created by plasmodesmata unifies most of a plant.
Most of the water and nutrients that move through a plant are transported by vascular tissue—xylem and phloem. However, plasmodesmata also transport these materials among cells and ultimately throughout the plant.
Plasmodesmata are versatile, and continuously alter their permeability. In addition to water and small molecules, they can also transport certain macromolecules, such as receptor-like protein kinases, signaling molecules, transcription factors, and RNA-protein complexes.
As cells grow, their density of plasmodesmata decreases unless they produce secondary plasmodesmata. Certain parasitic plants develop secondary plasmodesmata that connect them to hosts, allowing them to extract nutrients.