Malate-permeable channels and cation channels activated by aluminum in the apical cells of wheat roots

Aluminum (Al3+)-dependent efflux of malate from root apices is a mechanism for Al3+ tolerance in wheat (Triticum aestivum). The malate anions protect the sensitive root tips by chelating the toxic Al3+ cations in the rhizosphere to form non-toxic complexes. Activation of malate-permeable channels in the plasma membrane could be critical in regulating this malate efflux, We examined this by investigating Al3+-activated channels in protoplasts from root apices of near-isogenic wheat differing in Al3+ tolerance at a single locus. Using whole-cell patch clamp we found that Al3+ stimulated an electrical current carried by anion efflux across the plasma membrane in the Al3+-tolerant (ET8) and Al3+-sensitive (ES8) genotypes. This current occurred more frequently, had a greater current density, and remained active for longer in ET8 protoplasts than for ES8 protoplasts. The Al3+-activated current exhibited higher permeability to malate(2-) than to Cl- (P-mal/P-C1 greater than or equal to 2.6) and was inhibited by anion channel antagonists, niflumate and diphenylamine-2-carboxylic acid. In ET8, but not ESS, protoplasts an outward-rectifying K+ current was activated in the presence of Al3+ when cAMP was included in the pipette solution. These findings provide evidence that the difference in Al3+-induced malate efflux between Al3+-tolerant and Al3+-sensitive genotypes lies in the differing capacity for Al3+ to activate malate permeable channels and cation channels for sustained malate release.