symplast

Symplast is the living continuum of plant cell cytoplasm that is connected through cytoplasmic channels called plasmodesmata. Through these connections, most constituents of the cytoplasm can move from cell to cell, forming a coordinated network that contrasts with the apoplast, which includes the cell walls and extracellular spaces. The symplast thus represents the cell-to-cell living pathway for water, ions, metabolites, and signaling molecules.

Plasmodesmata are microscopic channels that traverse cell walls, linking adjacent cells’ cytoplasm and membranes. They often contain a central desmotubule derived from the endoplasmic reticulum. The permeability of plasmodesmata is not fixed; it is regulated by factors such as callose deposition around the neck region, developmental stage, and environmental cues. The size exclusion limit of the channels can be modulated, allowing or restricting the passage of molecules of various sizes. Transport within the symplast occurs by diffusion and cytoplasmic streaming, and is important for the rapid distribution of signaling molecules, RNAs, proteins, and certain metabolites that coordinate growth, development, and responses to stimuli.

In plant physiology, the symplast participates in processes such as phloem loading and unloading, where plasmodesmata connect companion cells and sieve elements. While phloem transport is often described as apoplastic, symplastic pathways exist in many species and developmental contexts, enabling the movement of sugars and signaling compounds through living tissue. The relative contribution of symplastic versus apoplastic routes can vary with tissue type, species, and physiological state.

Overall, the symplast provides a dynamic, living network essential for coordinated plant function, development, and response to the environment.