Plants have evolved unique microscopic channels called plasmodesmata to overcome the rigid cell walls that impede direct communication between cells. These channels connect the cytoplasm of adjacent cells, forming a continuous network throughout the plant body. Plasmodesmata are essential for coordinating physiological processes, enabling efficient intercellular communication and the movement of substances vital for plant life.
Anatomy of a Plant Connection
Plasmodesmata are intricate structures that pierce through the plant cell wall, forming a direct bridge between neighboring cells. Each plasmodesma is a membrane-lined channel, where its plasma membrane extends continuously from the connected cells. This continuity creates a unified cellular environment, allowing for the direct passage of molecules.
Running through the center of most plasmodesmata is a modified tubule of the endoplasmic reticulum, known as the desmotubule. This desmotubule is tightly appressed to the plasma membrane lining the channel. The space between the desmotubule and the surrounding plasma membrane is called the cytoplasmic sleeve. This sleeve, filled with cytosol, is where most molecular transfer occurs.
Plasmodesmata vary in complexity, appearing as simple, unbranched channels in younger tissues, or as branched structures in mature tissues. Branched plasmodesmata may consist of multiple interconnecting channels. A typical plant cell can possess thousands to hundreds of thousands of plasmodesmata, with their diameter ranging from 50 to 60 nanometers at their midpoint.
The Plant’s Internal Highway
Plasmodesmata function as the plant’s internal highway, facilitating the direct and regulated movement of molecules between cells. This intercellular transport is essential for processes like nutrient distribution, developmental signaling, and defense responses. They allow for the passage of water, small solutes, amino acids, and sugars.
Beyond small molecules, plasmodesmata also enable the transport of larger substances, including hormones, proteins, and RNA molecules such as messenger RNA (mRNA) and small interfering RNA (siRNA). This directed movement of macromolecules plays a role in coordinating growth and development. For instance, signaling molecules and transcription factors can move through plasmodesmata to influence cell differentiation and patterning.
The continuous network established by plasmodesmata, often referred to as the symplast, allows for efficient cell-to-cell communication, important given that plant cells are largely immobile. This direct contact facilitates rapid signal transduction, which is important for processes like flowering induction or root architecture establishment. Plant viruses, however, exploit this internal highway by producing “movement proteins” that help them traverse plasmodesmata, spreading infection throughout the plant.
Dynamic Gatekeepers
Plasmodesmata are not passive open channels but dynamic structures whose permeability is tightly regulated by the plant. This regulation involves controlling the “size exclusion limit” (SEL), which determines the maximum size of molecules that can pass through them. The plant can modulate this limit, allowing for selective transport of specific molecules at different times or in response to stimuli.
A primary mechanism for regulating plasmodesmata permeability involves the deposition and removal of a polysaccharide called callose. Callose accumulates around the neck region of the plasmodesmata, constricting the channel and reducing its aperture. Conversely, the degradation of callose by specific enzymes, such as β-1,3-glucanases, promotes the opening of these channels, increasing their conductivity. This balance of callose synthesis and degradation acts as a gatekeeping mechanism.
Plants actively regulate plasmodesmata in response to internal and external cues. For example, during stress or pathogen attack, plants rapidly increase callose deposition to close off plasmodesmata, isolating infected cells and preventing disease spread. This response serves as an important defense mechanism. Plasmodesmata also exhibit regulated permeability during different developmental stages, with changes in their aperture influencing processes like growth in meristems or the coordination of cell differentiation.