Symplasti

Symplasti, also spelled symplast in English and symplasti in some languages, refers to the contiguous cytoplasm of plant cells that is connected through plasmodesmata. This cytoplasmic network forms a living, intracellular pathway that enables direct movement of water, ions, metabolites, RNA, and signaling molecules between neighboring cells without crossing a plasma membrane at every cell wall junction.

Plasmodesmata are narrow channels that traverse cell walls, creating cytoplasmic continuity among adjacent cells. The symplast is bounded by the plasma membrane; molecules crossing between external spaces and the symplast must pass through membranes or move through plasmodesmata depending on size and regulatory state. This symplastic transport works in concert with the apoplast, which comprises the cell walls and intercellular spaces outside the plasma membrane.

A key feature of symplastic transport is the size-exclusion limit (SEL) of plasmodesmata, which governs which molecules can pass passively. SEL is dynamically regulated by plants, notably through the deposition or removal of callose around the plasmodesmatal neck, allowing rapid changes in permeability in response to developmental signals or stress and infection.

Symplast connectivity supports coordinated tissue growth, development, and metabolism. It participates in phloem loading in some species (symplastic loading), assists in the distribution of nutrients and signaling molecules, and influences defense responses. Pathogens and some viruses can exploit plasmodesmata to move from cell to cell, prompting plant defenses that modify plasmodesmatal permeability.

In summary, the symplast denotes the interconnected cytoplasm of plant cells linked by plasmodesmata, forming a central route for intracellular communication and transport within the plant.