Casparian strip

The Casparian strip is a band-like thickening in the center of the root endodermis (radial and tangential walls of endodermal cells) of vascular plants (Pteridophytes ^[1] and Spermatophytes). The composition of the region is mainly suberin, lignin and some structural proteins, which are capable of reducing the diffusive apoplastic flow of water and solutes into the stele and its width varies between species.^[2][3] The Casparian strip is impervious to water so can control the transportation of water and inorganic salts between the cortex and the vascular bundle, preventing water and inorganic salts from being transported to the stele through the apoplast, so that it must enter the cell membrane and move to the stele through the symplastic pathway, blocking the internal and external objects of the cell.^{[clarification needed]} The function of mass transportation are similar to that of animal tissues.^{[clarification needed]}.^[4][5] The development of the Casparian strip is regulated by transcription factors such as SHORT-ROOT (SHR), SCARECROW (SCR) and MYB36, as well as polypeptide hormone synthesised by midcolumn cells.^[6][7]

The chemistry of the Casparian strip has been described as composed of suberin. According to some studies,^[8] 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.^[9] 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.^[10] Here, the Casparian strip serves as a boundary layer separating the apoplast of the cortex from the apoplast of the vascular tissue thereby blocking diffusion of material between the two.^[11] This separation forces water and solutes to pass through the plasma membrane via a symplastic route in order to cross the endodermis layer.^[10]

Casparian strips differentiate after an outward growth of the cortex is completed. At this level of the root development, the primary xylem of its vascular cylinder is only partly advanced. In gymnosperms and angiosperms displaying secondary growth, the roots commonly develop only endodermis with Casparian strips. In many of those, the endodermis is later discarded, together with the cortex, when the periderm develops from the pericycle. If the pericycle is superficial and the cortex is retained, either the endodermis is stretched or crushed or it keeps pace with the expansion of the vascular cylinder by radial anticlinal divisions, and the new walls develop Casparian strips in continuity with the old ones.^[12]

In the absence of secondary growth (most monocotyledons and a few eudicots), the endodermis commonly undergoes wall modifications. There are two developmental stages beyond the development of the Casparian strip. In the second stage suberin (or endoderm^[9]) coats the entire wall on the inside of the cell. As a result, the Casparian strip is separated from the cytoplasm and the connection between the two ceases to be evident. In the third stage, a thick cellulose layer is deposited over the suberin, sometimes mainly on the inner tangential walls. The thickened wall, as well as the original wall in which the Casparian strip is located, may become lignified, creating a secondary cell wall. The Casparian strip may be identifiable after the thickening of the endodermal wall has occurred. The thickened endodermal wall may have pits. The successive development of endodermal walls is clearly expressed in monocotyledons.^[13][14]