Submitted to: Plant Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/20/2002
Publication Date: 7/20/2002
Citation: PINEROS, M.A., MAGALHAES, J.V., ALVES, V., KOCHIAN, L.V. A PHYSIOLOGICAL AND BIOPHYSICAL EXAMINATION OF ALUMINUM TOLERANCE IN MAIZE SUGGESTS THE EXISTENCE OF MULTIPLE TOLERANCE MECHANISMS. PLANT PHYSIOLOGY. 2002. 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 physiological methods to duct a detailed characterization of a proposed Al tolerance mechanism in maize based on Al activated citric acid release from the root tip (the site of Al toxicity) Citrate very effectively binds Al in the soil and keeps it out of the root. These studies were integrated with electrophysiological approaches using the patch clamp technique to study the functioning of single ion transport proteins in the outer membrane of plant cells that are activated by Al, and mediate the citric acid release. We found that unlike previous work in wheat, the Al-activated citrate release is not localized to the root tip. We also found that it is not localized to cells of the root periphery, as previously believed, but occurs in all of the cells of the root. We also found evidence for a second Al tolerance mechanism based on Al-induced increases in internal citrate levels, which could detoxify Al that enters root cells. These findings are providing important new information for our overall goal of isolating the genes that confer the Al tolerance trait.
Technical Abstract: Aluminum (Al)-induced release of Al-chelating organic acids into the rhizosphere from the root apex has been identified as a major Al tolerance mechanism in a number of plant species. In the present study with the very tolerant maize cultivar, Cateto-Columbia, physiological investigations were conducted to study the spatial and temporal characteristics of Al-activated droot organic acid exudation, as well as changes in root organic acid content & Al accumulation. These were integrated with biophysical studies using the patch clamp technique to examine Al-activated anion channel activity in protoplasts isolated from different regions of the maize root. It was found that Al nearly instantaneously activated a large citrate exudation in Cateto; this exudation is not confined to the root apex, but could be found as far as 5 cm back from the root cap. We also found that the root cap is not involved in either the citrate exudation or perception of the Al signal, and that both root cortical and stelar cells exhibit thi Al-activated citrate release. The use of the patch clamp technique confirmed the presence of an Al-inducible plasma membrane anion channel, which appears to be the transporter mediating this response, in protoplasts isolated from either stelar or cortical tissues. In addition to the rapid Al activation of citrate release, a slower, Al-inducible increase in root citrate content was observed. These findings led us to speculate that in addition to the Al exclusion mechanism based on root citrate exudation, a second internal Al tolerance mechanism may be operating based on Al-inducible changes in organic acid synthesis and compartmentation. This would fit with findings from recent genetic studies of Al tolerance in maize, that suggest Al tolerance in maize is a complex trait.