Plasmodesmata are microscopic channels that traverse the thick cell walls separating plant cells, creating a direct physical connection between the internal contents of neighboring cells. This connection is fundamental to the life of a multicellular plant, as the rigid cell wall would otherwise isolate each cell from its neighbors. By providing a continuous pathway, plasmodesmata effectively merge the separate cellular units into a single, interconnected system called the symplast. This structural arrangement serves as the plant’s primary means of communication and material exchange.
The Physical Structure of Plasmodesmata
It is a roughly cylindrical channel, approximately 50 to 60 nanometers in diameter at its midpoint, that tunnels through the cell wall and the intervening middle lamella. The outer boundary of this channel is lined by a continuous extension of the plasma membrane of both adjacent cells. This uninterrupted membrane lining means that the membranes of two separate cells become one.
Running through the center of the channel is a structure known as the desmotubule, which is a tightly packed, modified strand of the endoplasmic reticulum (ER). This strand of ER is continuous between the two connected cells, linking their internal membrane systems. The space between the outer plasma membrane and the central desmotubule is called the cytoplasmic sleeve or annulus. This fluid-filled sleeve is an extension of the cytosol, the watery substance inside the cells, making the cytoplasm of adjacent cells continuous. This physical union of the plasma membranes, the ER, and the cytoplasm represents the ultimate form of direct cell-to-cell contact in plants.
Mechanisms of Regulated Transport
Plasmodesmata are not merely passive open pores, but are instead highly regulated conduits that control the flow of substances between cells. This control is primarily achieved through the size exclusion limit (SEL), which is the maximum size of a molecule that can freely diffuse through the cytoplasmic sleeve. In a typical resting state, the SEL restricts the passage of molecules to those smaller than about one kilodalton, such as water, ions, and basic nutrients like sugars and amino acids. This restriction prevents large molecules from moving freely throughout the plant.
The plant can dynamically alter the channel’s permeability, a process known as gating, to allow for the passage of larger molecules. Gating involves modifying the aperture of the cytoplasmic sleeve to increase the SEL, sometimes permitting the transport of macromolecules like proteins and RNA. One of the main mechanisms for controlling this aperture is the reversible deposition of a carbohydrate called callose at the neck region of the plasmodesma. Enzymes like callose synthase deposit this substance to constrict the channel, while beta-1,3-glucanase removes it to widen the pore.
The active, regulated transport of large signaling molecules is mediated by specific proteins that can bind to the cargo and facilitate its movement through the pore. These proteins are capable of temporarily overriding the normal size limitations, allowing the passage of developmental and regulatory signals. This sophisticated regulation ensures that the direct contact provided by plasmodesmata is used selectively for coordinated communication.
The Role of Direct Contact in Plant Physiology
The direct cytoplasmic connectivity afforded by plasmodesmata is fundamental to the plant’s survival and its ability to function as a unified organism. This continuous pathway allows for efficient resource partitioning, particularly the rapid and widespread movement of photoassimilates, or sugars produced during photosynthesis, from source leaves to sink tissues like roots, fruits, and growing tips.
This pathway is also indispensable for developmental signaling, coordinating the growth and differentiation of cells across the plant body. For example, specific transcription factors, which are regulatory proteins, move through plasmodesmata to neighboring cells to instruct them on their future fate, ensuring proper patterning of tissues. By connecting cells into a functional symplast, the plasmodesmata enable the plant to orchestrate complex developmental programs, such as the formation of new organs or specialized tissue layers.
The direct connection also plays a role in the plant’s defense against threats, facilitating a localized stress response. A rapid change in plasmodesmatal permeability can quickly communicate an infection or a wound to adjacent cells, allowing for a coordinated defense reaction. However, this direct pathway is sometimes exploited by plant viruses, which encode specialized movement proteins that manipulate plasmodesmata structure to dramatically increase the SEL. This manipulation allows the viral genome or entire viral particles to pass through the channels, spreading infection from cell to cell throughout the plant.