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ment by forming extensive surface areas. There are clear similarities and diVerences in functional properties and Based on the characterization of the chemical com- structural demands of these above-ground and below- position of endodermal and hypodermal cell walls ground interfaces across which interactions occur. Both isolated from seven monocotyledonous and three dico- interfaces have in common that they must protect the tyledonous plant species, a model of the composition living tissue from infection by pathogens ( Kerstiens, 1996; of apoplastic barriers in roots is proposed. Depending Agrios, 1997). The leaf epidermis carries stomata allowing on the species, endodermal and hypodermal cell walls the regulation of gas exchange and transpiration (Larcher, of roots contained varying amounts of the biopolymers 1994). In addition, more than 90% of the leaf epidermis suberin, lignin, cell wall proteins, and carbohydrates. is covered by a lipophilic cuticle which prevents uncon- Although analysis of the chemical composition of these trolled water loss and leaching of nutrients from the apoplastic barriers of roots is now possible, it is pointed apoplast when stomata are closed (Schonherr, 1982). out that conclusions from these data concerning the In contrast to the leaf, the root is designed for the functional properties of these cell walls can not easily uptake of water and nutrients across the apoplast into the be drawn. However, in analogy to suberized periderms symplast (Marschner, 1995). Simultaneously, the root must it is argued that the suberin should play a role in estab- be able to exclude potentially harmful substances and it lishing an apoplastic transport barrier in roots, albeit must prevent infection by pathogens. This necessary select- not a perfect barrier. Furthermore, due to the combined ivity in root function is accompanied by a complex root occurrence of suberin, lignin and cell wall proteins it is anatomy regarding the soil/root interface (Esau, 1977). argued that endodermal and hypodermal cell walls also Going from the root surface to the root centre, the have an important function as barriers towards patho- outermost cell layer of mature, primary roots and thus the gens. Finally, it is pointed out that additional experi- soil/root interface is formed by the rhizodermis (root mental approaches combining the investigation of epidermis) carrying the root hairs. Under natural condi- transport properties and of the chemical composition tions the rhizodermis, which is the root zone with the of apoplastic transport barriers in roots are necessary highest absorption activity, stays alive only for several before the function of endodermal and hypodermal cell days before it is shed, leaving the hypodermis as the walls in roots can be fully understood. outermost cell layer of the root surface in most species. In addition to the rhizo- and hypodermis there is the endo-

Apoplastic barriers in roots: chemical composition of endodermal and hypodermal cell walls