Fig. 6.

Schematic diagrams of the coordinated functions of ALMT1 and NIP1;2 in Al tolerance in Arabidopsis. (A) Toxic Al³⁺ ions that enter the root cell wall activate ALMT1-mediated malate release from the root cells into the root apoplast and rhizosphere, where nontoxic Al-Mal complexes form. (B) Al-Mal in the rhizosphere is unable to enter the root apoplast, whereas the Al-Mal formed within the root cell wall is subjected to NIP1;2-mediated uptake into the root cytosol. Once inside the root cells, Al-Mal moves across the Casparian strip of endodermal cells through symplastic flow into xylem parenchyma cells, where NIP1;2 facilitates xylem loading of Al-Mal into the xylem vessels, followed by translocation of Al-Mal to the shoot. The vertical gray bar on the endodermis represents the Casparian strip.