Simplasto

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The symplast (from Greek sym "together" + plasma "formed or moulded substance") is the continuous, living network of cytoplasm that extends through most plant tissues. Its continuity is established by thousands of plasmodesmata — plasma-membrane-lined nanoscopic tunnels that pierce the cell walls and join the cytosol and endoplasmic reticulum of adjacent cells. Because those channels also open into the phloem sieve elements, the symplast provides both short-range and long-range conduits for water, nutrients, metabolites, proteins and RNAs to move between cells along concentration or pressure gradients. By contrast, the apoplast comprises the porous cell-wall matrix and extracellular spaces through which solutes diffuse outside the plasma membrane. Electron microscopy shows that a single plasmodesma consists of a narrow cylindrical sleeve of cytosol bounded by plasma membrane and traversed by a central rod of endoplasmic reticulum called the desmotubule. Cross-sections reveal protein "spokes”" connecting these membranes; the surrounding wall is enriched in pectins rather than cellulose, a composition thought to confer the flexibility needed for rapid aperture changes. Primary plasmodesmata form during cytokinesis when strands of endoplasmic reticulum become trapped in the cell plate, whereas secondary pores are inserted later into existing walls, often budding next to older channels to create pit fields. In highly specialised interfaces— for example between companion cells and sieve elements—plasmodesmata widen asymmetrically into pore plasmodesmata or sieve pores that sustain bulk phloem flow. Diffusion through plasmodesmata is gated by a reversible 'size-exclusion limit. Deposition of the β-1,3-glucan callose around the pore narrows or seals the cytoplasmic sleeve, while callose hydrolases reopen it.