Malate-Permeable Channels and Cation Channels Activated by Aluminum in the Apical Cells of Wheat Roots

Abstract

Aluminum (Al 3ϩ )-dependent efflux of malate from root apices is a mechanism for Al 3ϩ tolerance in wheat (Triticum aestivum). The malate anions protect the sensitive root tips by chelating the toxic Al 3ϩ cations in the rhizosphere to form non-toxic complexes. Activation of malate-permeable channels in the plasma membrane could be critical in regulating this malate efflux. We examined this by investigating Al 3ϩ -activated channels in protoplasts from root apices of near-isogenic wheat differing in Al 3ϩ tolerance at a single locus. Using whole-cell patch clamp we found that Al 3ϩ stimulated an electrical current carried by anion efflux across the plasma membrane in the Al 3ϩ -tolerant (ET8) and Al 3ϩ -sensitive (ES8) genotypes. This current occurred more frequently, had a greater current density, and remained active for longer in ET8 protoplasts than for ES8 protoplasts. The Al 3ϩ -activated current exhibited higher permeability to malate 2Ϫ than to Cl Ϫ (P mal /P Cl Ն 2.6) and was inhibited by anion channel antagonists, niflumate and diphenylamine-2-carboxylic acid. In ET8, but not ES8, protoplasts an outward-rectifying K ϩ current was activated in the presence of Al 3ϩ when cAMP was included in the pipette solution. These findings provide evidence that the difference in Al 3ϩ -induced malate efflux between Al 3ϩ -tolerant and Al 3ϩ -sensitive genotypes lies in the differing capacity for Al 3ϩ to activate malate permeable channels and cation channels for sustained malate release.

The roles of organic anion permeases in aluminium resistance and mineral nutrition

“…These Al 3+ -activated inward currents behave similarly to the malate efflux measured from intact roots as well as excised root apices [11,22]. Both are activated by Al 3+ but not La 3+ , both occur in cells from the root apex but not from mature root cells and both are sensitive to the same range of channel antagonists.…”

“…The conductance of these channels is 66 pS at À50 mV ($100 mM [Cl À ] pipette , $20 mM [Cl À ] external ). Subsequent whole-cell studies described large inward currents that are carried by malate [22]. Assuming the whole-cell currents are mediated by channels then the permeability for malate relative to Cl À (P mal /P Cl-) is 7.8 in wheat protoplasts (corrected from Zhang et al [22]).…”

“…Subsequent whole-cell studies described large inward currents that are carried by malate [22]. Assuming the whole-cell currents are mediated by channels then the permeability for malate relative to Cl À (P mal /P Cl-) is 7.8 in wheat protoplasts (corrected from Zhang et al [22]). Measurments of tobacco-suspension cells expressing TaALMT indicates that P mal /P ClÀ is 38 (W.-H. Zhang, personal communication) which make this the most malate-permeable plant channel described to date.…”

“…Measurments of tobacco-suspension cells expressing TaALMT indicates that P mal /P ClÀ is 38 (W.-H. Zhang, personal communication) which make this the most malate-permeable plant channel described to date. Surprisingly, single-channel events have not been detected when malate is the major permeable anion present in wheat protoplasts [22] or in tobacco-suspension cells expressing TaALMT1 (W. (Fig. 3) and it is tempting to speculate that Al 3+ interacts with amino acid residues in that region.…”

The Physiology and Biophysics of an Aluminum Tolerance Mechanism Based on Root Citrate Exudation in Maize

“…exudation) down the outwardly directed electrochemical gradient. Anion channels that are specifically activated by extracellular Al 3ϩ have recently been reported in protoplasts isolated from root tips of Al-tolerant wheat (Ryan et al, 1997;Zhang et al, 2001) and maize (Kollmeier et al, 2001;Piñ eros and Kochian, 2001). Given that these channels are specifically activated by Al and are permeable to organic acids such as citrate and malate, these transport systems are good candidates for mediating the Al-activated root organic acid release in Al-tolerant wheat and maize.…”