Plasmodesma is a specialized bridge between two plant cells, allowing their contents to communicate with each other. This type of intercellular junction is a functional analogue of gap junctions in animals. With its help, ions, low molecular weight compounds and hormones are transported.
Plasmodesmata are communication structures characteristic only of plant cells, a feature of which is the presence of a dense cellulose membrane. The formation of intercellular contacts is inextricably linked with the features of its structure.
Cell wall and pores
The cell wall of plants consists of hemicellulose, pectin and a number of other polysaccharides present in smaller quantities. This is a very robust construction that performs protective, barrier and support functions, as well as giving the cell a fixed shape. However, in addition to the positive properties, the presence of such a dense shell imposes certain limitations, including the possibility of direct transport of substances through membrane contacts. To solve this problem, specialized channels are formed in the plant cell - pores and plasmodesmata.
The first are formed due to sections of the cell wall in which there is no secondary cellulose layer. The shell thickness in this region does not exceed 0.5 μm, and the cellulose microfibrils are more loose than in the primary wall of the rest of the cell surface. These sites are called pore channels. In neighboring cells, they are located opposite each other. In places of such contacts, the primary walls are tightly adjacent to each other, forming a closing film, separated by a very thin layer of intercellular substance.
The closing film is penetrated by many holes. Plasmodesmata pass through them, which, unlike pores, cannot be examined using light microscopy.
Features of communication between plant cells
Plasmodesma is the only class of intercellular contacts in plants. These structures directly bind the cytoplasm of neighboring cells, ensuring their communication. The thickness of the cell wall excludes the possibility of using for these purposes slotted contacts, which are characteristic of some animal tissues. Protoplasts connected through plasmodesmata form a single cell network - a symplast.
Usually, plant cells can transfer substances with a molecular mass of not more than 800 daltons through plasmodesmata. However, in some cases this limitation can be overcome. Thus, protein transcription factors, as well as mRNAs encoding them, can pass from one cell to another. Interacting with the structural elements of plasmodesm, these molecules cause the effective diameter of the hole to expand and pass freely through it. The same mechanism for the infection of new cells is used by some viruses that encode proteins that can affect the width of the lumen of the connecting channel.
The phenomenon of the movement of macromolecules through intercellular contact is a unique feature of plant tissues and plasmodesmata in particular. In animal cells, there are no mechanisms for overcoming the communicative barrier (the molecular threshold for gap contact is about 1000 daltons). The throughput of plasmodesmata can change under the control of cellular systems, the mechanism of action of which has not yet been studied. It is only known that in some cases, through morphologically normal contacts, the transfer of even low molecular weight substances is difficult.
The structure of plasmodesma
Plasmodesma is a narrow cytoplasmic channel from 30 to 60 nm in diameter. Its walls are lined with a plasma membrane passing from one cell to another. In the center there is a transport tube formed by eleven protein subunits - desmotubula. It connects the endoplasmic reticulum of neighboring cells. The gap between the desmotubole and the membrane is filled with hyaloplasm.
Holes through which plasmodesmata pass can be detected by electron microscopy.
Formation Features
The described type of compound can form both during division and in mature cells (de novo). The first method was studied using electron microscopy, which showed that during cytokenesis (separation of daughter cells) in certain areas of the primary pore fields, separation between cytoplasms does not occur. Then, structural elements of plasmodesm are formed at the site of the connecting tubes.
It has been proven that intercellular contacts in plants are present not only in sister cells. The formation of such plasmodesmata occurs due to the partial destruction of the cell wall.
Biological functions
Many of the functions of plasmodesma, as we have said, are similar to the gap junctions of animal cells. The main role of this structure is to ensure the active transport of metabolites (metabolic components). However, hormones or electrical signals that coordinate the work of plant cells within the same tissue can also be transmitted. Sometimes plasmodesmata transport regulatory proteins and messenger RNAs, thereby linking the biosynthetic processes of symplast cells.