A patch-clamp study on the physiology of aluminum toxicity and aluminum tolerance in maize. Identification and characterization of Al(3+)-induced anion channels.
The presence of Al(3+) 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 Al(3+) at the root surface. Because citrate is released from root apical 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 Al(3+) inhibited the remaining K(+) outward currents, blocked the K(+) 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 Al(3+) in membrane patches excised prior to any Al(3+) exposure indicates that the machinery required for Al(3+) activation of this channel, and consequently the whole root Al(3+) response, is localized to the root-cell plasma membrane. This Al(3+)-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.
Piñeros
M A
MA
United States Plant, Soil, and Nutrition Laboratory, United States Department of Agriculture, Agricultural Research Service, Cornell University, Ithaca, New York 14853, USA.
Kochian
L V
LV
D000535:Aluminum / Q000494:pharmacology / Q000633:toxicity*; D004361:Drug Tolerance; D007473:Ion Channels / Q000187:drug effects / Q000502:physiology*; D007700:Kinetics; D018408:Patch-Clamp Techniques; D018517:Plant Roots / Q000187:drug effects; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology; D011523:Protoplasts / Q000187:drug effects; D003313:Zea mays / Q000187:drug effects / Q000502:physiology*
D007473:Ion Channels; D015221:Potassium Channels; D000535:Aluminum