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Plant cells have rigid cell walls that help regulate cell shape and osmotic pressure. These walls create a challenge for communication between neighboring cells.
To overcome this challenge, plant cells connect through small channels called plasmodesmata that allow direct communication from one cell to another.
Each plasmodesma pore is a continuation of the plasma membrane of adjacent cells. Running through the center of the channel is the desmotubule, a narrow extension of the endoplasmic reticulum, or ER, that connects the ER of the neighboring cell.
Between the desmotubule and the surrounding plasma membrane is a space called the cytoplasmic sleeve.
This sleeve forms the main pathway for molecules that move between cells.
The cytosol is connected between the two cells, making a continuous network called the symplast.
Under normal conditions, water and small molecules, such as sugars and ions, can freely pass between cells. The desmotubule itself is tightly compressed, leaving very little, if any, open space in its center for molecules to pass through.
The exchange of larger molecules, such as small RNAs, transcription factors, and other cytosolic proteins, is tightly regulated.
Cells regulate this movement by depositing a polysaccharide called callose at the neck region of the plasmodesma.
When callose accumulates at the neck, the opening narrows, and transport is restricted. When callose breaks down, the opening widens, and larger molecules can pass through the plasmodesma.
In some cases, callose accumulates enough to completely block the channel. This is, for example, beneficial to restrict the movement of plant viruses that use these channels to spread to neighboring cells.
Plasmodesmata can originate in two ways. Primary plasmodesmata form during cell division as the new cell wall develops between daughter cells. These channels may appear individually or in groups, sometimes forming clustered regions called pit fields. Secondary plasmodesmata emerge later in existing cell walls between neighboring cells, adding new connections after cell division.
