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The apoplastic and symplastic pathways

Within a plant, the apoplast is the free diffusional space outside the plasma membrane. It is interrupted by the Casparian strip in roots, by air spaces between plant cells and by the plant cuticle.

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.^[1] 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.^[2]

Notes

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.

References

^ 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.

Footnotes

Salibury F; Ross C (1991). Plant Physiology. Brooks Cole. p. 682. ISBN 0-534-15162-0. .

English Journal

Roles of apoplastic peroxidases in plant response to wounding.

Minibayeva F1, Beckett RP2, Kranner I3.
Phytochemistry.Phytochemistry.2014 Jul 12. pii: S0031-9422(14)00259-3. doi: 10.1016/j.phytochem.2014.06.008. [Epub ahead of print]
Apoplastic class III peroxidases (EC 1.11.1.7) play key roles in the response of plants to pathogen infection and abiotic stresses, including wounding. Wounding is a common stress for plants that can be caused by insect or animal grazing or trampling, or result from agricultural practices. Typically
PMID 25027646

Can silicon partially alleviate micronutrient deficiency in plants? a review.

Hernandez-Apaolaza L.
Planta.Planta.2014 Jul 11. [Epub ahead of print]
Silicon protects plants against various biotic and abiotic stresses, including metal toxicity. Under a high metal concentration, Si can externally decrease metal availability to the plant by its precipitation in the growth media, and Si also affects the metal distribution inside the plant, diminishi
PMID 25011646

Light regulates motility, attachment and virulence in the plant pathogen Pseudomonas syringae pv tomato DC3000.

Río-Álvarez I1, Rodríguez-Herva JJ, Martínez PM, González-Melendi P, García-Casado G, Rodríguez-Palenzuela P, López-Solanilla E.
Environmental microbiology.Environ Microbiol.2014 Jul;16(7):2072-85. doi: 10.1111/1462-2920.12240. Epub 2013 Aug 27.
Pseudomonas syringae pv tomato DC3000 (Pto) is the causal agent of the bacterial speck of tomato, which leads to significant economic losses in this crop. Pto inhabits the tomato phyllosphere, where the pathogen is highly exposed to light, among other environmental factors. Light represents a stress
PMID 24033935