Submitted to: Plant Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/15/2001
Publication Date: N/A
Citation: Interpretive Summary: Large areas of land within the U.S. and over 40% of the world's arable lands are acidic. In these acid soils, aluminum (Al) toxicity is the primary factor limiting crop production via Al-induced inhibition of root growth. There is considerable genetic variation in tolerance to Al between different plant species and genotypes, yet the physiological and molecular basis for Al tolerance is still poorly understood. Thus, we need a more complete understanding of the mechanisms underlying Al tolerance if we are going to be able to develop more Al tolerant crop plants for improved cultivation on acid soils. In this paper, we used an electrophysiological approach, the patch clamp technique, that allows us to study the functioning of single ion transport proteins in the outer membrane of plant cells. Using this technique, we studied a mechanism of Al tolerance in maize that involves Al-induced release of the organic acid citrate from the eroot tip of Al tolerant maize; citrate very effectively binds Al in the soil and keeps it out of the root. Using the patch clamp technique, we identified a very specialized ion transporter, an Al-activated anion channel, that is likely the transporter that mediates the citrate release. We found that Al directly binds to the channel protein during activation, opening the channel and allowing citrate to flow out of the root into the soil. These findings are significant, for they suggest that the gene encoding this anion channel may be a key Al tolerance gene. We are currently attempting to isolate this gene, for use in facilitating crop improvement via biotechnological approaches.
Technical Abstract: The presence of Al3+ in the rhizosphere induces citrate efflux from the root apex of the Al tolerant maize hybrid SA3, consequently chelating and reducing the activity of toxic Al3+ 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 eroot 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 K+ outward currents, blocked the K+ inward current, and caused the activation of an inward Cl- current (anion efflux). 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).