Endodermis and Exodermis in Roots

Abstract Roots of terrestrial plants are designed to take up water and nutrients. At the same time, uptake of unwanted compounds, for example toxic, and infection by soil borne pathogens must be avoided. Specific unicellular tissues, the endodermis and the exodermis, allow roots to establish and maintain this selectivity. The endodermis represents an unicellular cell layer separating the central cylinder of the root from the cortex. The exodermis represents an unicellular cell layer located at the outer surface of the root directly below the root epidermis. Both tissues are characterised by specific cell wall modifications. In early developmental stages the anticlinal radial walls exhibit Casparian bands, composed of the polymers suberin and lignin. In a subsequent developmental state a suberin lamella is deposited on the inner surface of endo‐ and exodermal cell walls. These apoplastic barriers, mainly composed of suberin, significantly affect radial uptake of water and dissolved nutrients and radial loss of oxygen. Key Concepts: Casparian bands, composed of lignin and suberin, form characteristic cell wall modifications in radial walls of endodermal and exodermal cells. Suberin lamellae are deposited onto the inner surface of endodermal and exodermal cell walls. Apoplastic barriers in roots are established by the deposition of suberin into and onto the cell wall. Suberised apoplastic barriers in roots help to establish root selectivity in nutrient uptake. The endodermis, forming an ‘inner’ apoplastic barrier in the root, is important in preventing nutrients actively concentrated in the xylem from passively diffusing back to the soil. The exodermis, forming an ‘outer’ apoplastic barrier at the root surface, mainly establishes the root/soil interface between the root and the soil environment surrounding the root. As an adaptation to various environmental stress factors (e.g. drought, salt, heavy metal stress, oxygen deficiency, etc.) suberisation of apoplastic barriers is significantly modified. Fatty acid elongases and cytochrome P450 hydroxylases represent important key enzymes in suberin biosynthesis.