Investigating the relationship between aluminium toxicity, root growth and root-generated ion currents

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One of the primary initial symptoms of Al toxicity is inhibition of root growth. Also, there is a growing body of evidence indicating that natural electric currents play a major role in the differentiation and growth of cells, tissues, and organs, including apical growth of roots. In this study, we are using a highly sensitive extracellular vibrating microelectrode system to map the ion current patterns surrounding growing root apices of wheat cultivars (Triticum aestivum L. ‘Atlas 66’ and ‘Scout 66’) which differ in Al tolerance. When the seedlings were grown in a solution consisting of 0.6 mol m−3 CaCl2 (pH 4.50), a positive current (1-2 μA cm−2) was measured entering the root cap, apical meristem, and region just behind the apical meristem in both ‘Scout’ and ‘Atlas’. Exposure of the roots to 10 mmol m−3 AlCl3 elicited a significant inhibition of this inward current in ‘Scout’ after approx. 3 hours exposure to Al. This same level of Al inhibited root growth in ‘Scout’; the time course for Al-induced inhibition of root growth and ion currents were nearly identical. In ‘Atlas’, 10 mmol m−3 Al had no effect on root-generated currents or root growth. A higher level of Al (50 mmol m−3 AlCl3), that had some inhibitory effect on ‘Atlas’ root growth, was also used. This level of Al more dramatically inhibited ‘Scout’ root growth and the ion currents associated with the root apex. However, an exposure period of about 3 hrs was still necessary to observe an inhibition of growth and root currents. This same Al level caused a moderate inhibition of both ‘Atlas’ root growth and inward current with similar time courses. The striking similarities between the time course of Al-induced inhibition of root growth and root-associated currents strongly suggest that the ion transport processes resulting in inward current at the root apex are involved in root growth, and Al-induced alteration of these transport processes may be an important aspect of the early stages of Al toxicity.