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Structurally, the apoplast is formed by the continuum of cell walls of adjacent cells as well as the extracellular spaces, forming a tissue level compartment comparable to the symplast. The apoplastic route facilitates the transport of water and solutes across a tissue or organ. This process is known as apoplastic transport.
The apoplast is important for all the plant's interaction with its environment. The main carbon source (carbon dioxide) needs to be solubilized in the apoplast before it diffuses through the plasma membrane into the cell's cytoplasm (symplast) and is used by the chloroplasts during 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. Similarly, all gaseous molecules emitted and received by plants such as plant hormones and other pheromones must pass the apoplast. The apoplast is also a site for cell-to-cell communication. During local oxidative stress, hydrogen peroxide and superoxide anion can diffuse through the apoplast and transport a warning signal to neighbouring cells. In addition, a local alkalinization of the apoplast due to such a stress can travel within minutes to the rest of the plant body via the xylem and trigger systemic acquired resistance.
- Apoplast was previously defined as "everything but the symplast, consisting of cell walls and spaces between cells in which water and solutes can move freely". However, since solutes can neither freely move through the air spaces between plant cells nor through the cuticle, this definition has been changed. When referring to "everything outside the plasma membrane", the term "extracellular space" is in use.
- The word apoplasm is also in use with similar meaning as apoplast, although less common.
- Campbell, N.A.; Reece, J.B. (2002). Biology. California: Benjamin Cummings. pp. 753–754. ISBN 0-8053-6624-5.
- H. H. Felle; A. Herrmann; R. Hückelhoven; K.-H. Kogel (December 2005). "Root-to-shoot signalling: apoplastic alkalinization, a general stress response and defence factor in barley (Hordeum vulgare)". Protoplasma (Springer Wien) 227 (1): 17–24. doi:10.1007/s00709-005-0131-5. PMID 16389490.