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.