Journal of Experimental Botany, Vol. 52, No. 365, pp. 2245-2264,
December 1, 2001
© 2001 Oxford University Press
Review Article |
The exodermis: a variable apoplastic barrier
1 Julius-von-Sachs-Institut für Biowissenschaften der Universität, Lehrstuhl Botanik I, Julius-von-Sachs-Platz 2, D-97082 Würzburg, Germany
2 IACR-Long Ashton Research Station, Long Ashton, Bristol BS18 9AF, UK
3 Lehrstuhl für Pflanzenökologie der Universität Bayreuth, Universitätsstrasse 30, D-95440 Bayreuth, Germany
4 Botanisches Institut der Universität, Abteilung Ökophysiologie, Kirschallee 1, D-53115 Bonn, Germany
The exodermis (hypodermis with Casparian bands) of plant roots represents a barrier of variable resistance to the radial flow of both water and solutes and may contribute substantially to the overall resistance. The variability is a result largely of changes in structure and anatomy of developing roots. The extent and rate at which apoplastic exodermal barriers (Casparian bands and suberin lamellae) are laid down in radial transverse and tangential walls depends on the response to conditions in a given habitat such as drought, anoxia, salinity, heavy metal or nutrient stresses. As Casparian bands and suberin lamellae form in the exodermis, the permeability to water and solutes is differentially reduced. Apoplastic barriers do not function in an all-or-none fashion. Rather, they exhibit a selectivity pattern which is useful for the plant and provides an adaptive mechanism under given circumstances. This is demonstrated for the apoplastic passage of water which appears to have an unusually high mobility, ions, the apoplastic tracer PTS, and the stress hormone ABA. Results of permeation properties of apoplastic barriers are related to their chemical composition. Depending on the growth regime (e.g. stresses applied) barriers contain aliphatic and aromatic suberin and lignin in different amounts and proportion. It is concluded that, by regulating the extent of apoplastic barriers and their chemical composition, plants can effectively regulate the uptake or loss of water and solutes. Compared with the uptake by root membranes (symplastic and transcellular pathways), which is under metabolic control, this appears to be an additional or compensatory strategy of plants to acquire water and solutes.
Key words: Exodermis, plant roots, barrier, variable resistance, solutes, water.
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