Casparian strips

In plant anatomy, the Casparian strip is a band of cell wall material deposited in the radial and transverse walls of the endodermis, and is chemically different from the rest of the cell wall - the cell wall being made of lignin and without suberin - whereas the Casparian strip is made of suberin and sometimes lignin. The Casparian strip might be viewed rather like mortar in a wall that blocks the passive flow of materials such as water and solutes. Water and solutes are pumped into the stele through special endodermal cells called passage cells. The Casparian strip prevents those from leaking back out to the cortex. The band was first recognized as a wall structure by Robert Caspary (1818–1887).

The strip forms during the early development of the cell and is a part of the primary wall of the cell of the endodermis. It varies in thickness and is often much thinner than the wall in which it occurs. It is typically located closer to the inner tangential wall than to the outer wall.

The chemistry of the Casparian strip has been described as composed of suberin. According to some studies, the Casparian strip begins as a localized deposition of phenolic and unsaturated fatty substances in the middle lamella between the radial walls, as partly oxidized films. The primary wall becomes encrusted with and later thickened by deposits of similar substances on the inside of that wall. The encrustation of the cell wall by the material constituting the Casparian strip presumably plugs the pores that would have otherwise allowed the movement of water and nutrients via capillary action along that path. The cytoplasm of the endodermal cell is firmly attached to the Casparian strip so that it does not readily separate from the strip when the cells are subjected to contraction of the protoplasts. At the root, the casparian strip is embedded within the cell wall of endodermal cells in the non-growing region of the root behind the root tip. Here, the casparian strip serves as a boundary layer separating the apoplast of cortex from the apoplast of the vascular tissue thereby blocking diffusion of material between the two. This separation forces water and solutes to pass through the plasma membrane via a symplastic route in order to cross the endodermis layer.

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