Ion transport pathways across the root endodermis.

Casparian strips in endodermal cell walls prevent apoplastic transport across the root endodermis, limiting entry of harmful soil minerals, such as Al3+, and exit of useful minerals. Mineral nutrients that are transported into the cytosol of endodermal cells are able to pass through the endodermal barrier to xylem parenchyma cells via plasmodesmata. Once past the endodermis, nutrient ions, such as K+, are moved across plasma membranes to the apoplast of the vascular tissue and are thus able to enter xylem. The inset shows a transmission electron micrograph (TEM) of a Casparian strip in the wall of an endodermal cell.