4.13
Plasmodesmata
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.
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.
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.
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4.13 : Plasmodesmata
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