4.13
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 tha…
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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Q1: What are plasmodesmata and why do plant cells need them?
Plasmodesmata are small channels connecting adjacent plant cells, allowing direct communication across rigid cell walls. These channels overcome the barrier created by cell walls, enabling the continuous cytoplasmic network called the symplast. Through plasmodesmata, plant cells exchange water, nutrients, and signaling molecules essential for coordinated growth and function.
Q2: What is the structure of a plasmodesma and how does it function?
Each plasmodesma consists of a plasma membrane continuation, a central desmotubule (an endoplasmic reticulum extension), and a surrounding cytoplasmic sleeve. The cytoplasmic sleeve forms the main pathway for molecular transport between cells. Water and small molecules like sugars and ions freely pass through, while larger molecules such as transcription factors and RNA are tightly regulated.
Q3: How do cells regulate the movement of large molecules through plasmodesmata?
Cells control plasmodesmal permeability by depositing callose, a polysaccharide, at the plasmodesma neck region. When callose accumulates, the opening narrows and transport is restricted. When callose breaks down, the opening widens, allowing larger molecules to pass. Complete callose accumulation can block channels entirely, preventing pathogen spread.
Q4: What is the difference between primary and secondary plasmodesmata?
Primary plasmodesmata form during cell division as new cell walls develop between daughter cells, appearing individually or in clustered pit fields. Secondary plasmodesmata emerge later in existing cell walls between neighboring cells, creating new connections after division. Both types contribute to the plant's continuous cytoplasmic network.
Q5: How many plasmodesmata does a typical plant cell have?
A single plant cell contains thousands of plasmodesmata perforating its cell wall, though the number and structure vary across different cell types. As cells grow, plasmodesmal density decreases unless cells produce secondary plasmodesmata. This extensive network of channels unifies most of a plant into a functional symplast.
Q6: What types of molecules can move through plasmodesmata?
Under normal conditions, water and small molecules like sugars and ions freely pass through plasmodesmata. Larger macromolecules including receptor-like protein kinases, signaling molecules, transcription factors, and RNA-protein complexes can also transport, though their movement is tightly regulated. This versatility allows plasmodesmata to coordinate complex cellular processes.
Q7: How do parasitic plants use plasmodesmata to extract nutrients from hosts?
Certain parasitic plants develop secondary plasmodesmata that connect them directly to host plant cells, creating a bridge for nutrient extraction. This specialized plasmodesmal connection allows parasites to tap into the host's nutrient transport system. The ability to form these connections demonstrates the dynamic nature of plasmodesmal development.