Apoplast

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The apoplast is the extracellular space outside of plant cell membranes, especially the fluid-filled cell walls of adjacent cells where water and dissolved material can flow and diffuse freely. Fluid and material flows occurring in any extracellular space are called apoplastic flow or apoplastic transport. The apoplastic pathway is one route by which water and solutes are transported and distributed to different places through tissues and organs, contrasting with the symplastic pathway.

To prevent uncontrolled leakage to unwanted places, in certain areas there are barriers to the apoplastic flow: in roots the Casparian strip has this function^{[clarification needed]} Outside the plant epidermis of aerial plant parts is a protective waxy film called plant cuticle that protects against drying out, but also waterproofs the plant against external water.

The apoplast is important for all the plant's interaction with its environment: The main carbon source (carbon dioxide) needs to be solubilized, which happens in the apoplast, before it diffuses through the cell wall and across the plasma membrane, into the cell's inner content, the cytoplasm, where it diffuses in the symplast to the chloroplasts for photosynthesis. In the roots, ions diffuse into the apoplast of the epidermis before diffusing into the symplast, or in some cases being taken up by specific ion channels, and being pulled by the plant's transpiration stream, which also occurs completely within the boundaries of the apoplast.^{[clarification needed]} Similarly, all gaseous molecules emitted and received by plants such as oxygen must pass through the apoplast.

In nitrate poor soils, acidification of the apoplast increases cell wall extensibility and root growth rate. This is believed to be caused by a decrease in nitrate uptake (due to deficit in the soil medium) and supplanted with an increase in chloride uptake. H+ATPase increases the efflux of H+, thus acidifying the apoplast.^{[clarification needed] [1]}

The apoplast is a site for cell-to-cell communication. During local oxidative stress, hydrogen peroxide and superoxide anions can diffuse through the apoplast and transport a warning signal to neighbouring cells. In addition, a local alkalinization of the apoplast due to such stress can travel within minutes to the rest of the plant body via the xylem and trigger systemic acquired resistance.^[2]

The apoplast also plays an important role in resistance to aluminium toxicity.

In addition to resistance to chemicals, the apoplast provides the rich environment for microorganisms endophytes which arises[??] the abiotic resistance of plants.^{[clarification needed] [3]} Exclusion of aluminium ions in^{[clarification needed]} the apoplast prevent toxic levels which inhibit shoot growth, reducing[?] crop yields.^[4]