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.
[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.
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 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.
[11] This separation forces water and solutes to pass through the plasma membrane via a symplastic route in order to cross the endodermis layer.
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.
[15] In 1865, the German botanist Robert Caspary first described the endodermis of the root of plants, found that its cell wall was thickened, and named it Schuchtzscheide.
[note 1] In the past, some scholars believe that the formation of the endodermis of Casparian strip is the beginning of sublevel differentiation, but there is no direct relationship between the two.
[22] Some studies have shown that plants may form thicker Casparian strip in high-salt environments, and in areas closer to the tip of the roots, which may be an adaptation to the environment,[23][24] but compared with the endothelial sublevel differentiated wooden bolt walls, which are significantly thickened in high-salt adversity, the Casparian strip changes is smaller.
[27] For example, the conifers of Pinus bungeana and the stems of Pelargonium have the Casparian strip, which may be related to preventing water dispersion or pathogenic invasion.
