A patch-clamp study on the physiology of aluminum toxicity and aluminum tolerance in maize. Identification and characterization of Al3+-induced anion channels

The presence of Al3+ in the rhizosphere induces citrate efflux from the root apex of the Al-tolerant maize (Zea mays) hybrid South American 3, consequently chelating and reducing the activity of toxic Al3+ at the root surface. Because citrate is released from root epical cells as the deprotonated anion, we used the patch-clamp technique in protoplasts isolated from the terminal 5 mm of the root to study the plasma membrane ion transporters that could be involved in Al-tolerance and Al-toxicity responses. Acidification of the extracellular environment stimulated inward K+ currents while inhibiting outward K+ currents. Addition of extracellular Al3+ inhibited the remaining KC outward currents, blocked the Ki inward current, and caused the activation of an inward Cl- current (anion efflux). Studies with excised membrane patches revealed the existence of Al-dependent anion channels, which were highly selective for anions over cations. Our success in activating this channel with extracellular Al3+ in membrane patches excised prior to any Al3+ exposure indicates that the machinery required for Al3+ activation of this channel, and consequently the whole root Al3+ response, is localized to the root-cell plasma membrane. This Al3+-activated anion channel may also be permeable to organic acids, thus mediating the Al-tolerance response (i.e. Al-induced organic acid exudation) observed in intact maize root apices.