Characterization of Al-stimulated efflux of malate from the apices of Al-tolerant wheat roots

Aluminium (Al) stimulates the efflux of malate from the apices of wheat (Triticum aestivum L.) roots (Delhaize et al. 1993, Plant Physiol. 103, 695-702). The response was five to tenfold higher in Al-tolerant seedlings than Al-sensitive seedlings and the capacity for Al-stimulated malate efflux was found to co-segregate with Al tolerance in a pair of near-isogenic wheat lines differing in Al-tolerance at a single dominant locus. We have investigated this response further using excised root apices. Half-maximal efflux of malate from apices of Al-tolerant seedlings was measured with 30 micromolar Al in 0.2 mM CaCl2, pH 4.2, while saturating rates of 2.0 nmol.apex(-1).h(-1) occurred with concentrations above 100 micrmolar Al. The stimulation of malate efflux by Al was accompanied by an increase in K+ efflux which appeared to account for electroneutrality. The greater stimulation of malate efflux from Al-tolerant apices compared to Al-sensitive apices could not be explained by differences in the activities of phosphoenolpyruvate carboxylase or NAD-malate dehydrogenase. Several other polyvalent cations, including gallium, indium and the tridecamer Al13, failed to elicit malate efflux. Aluminium-stimulated efflux of malate was correlated with the measured concentration of total monomeric Al present, and with the predicted concentrations of Al3 + and AlOH2 + ions in the solution. Several antagonists of anion channels inhibited Al-stimulated efflux of malate with the following order of effectiveness: niflumic acid approximately NPPB > IAA-94 approximately A-9-C > ethacrynic acid. Lanthanum, chlorate, perchlorate, zinc and alpha-cyano-4-hydroxycinnamic acid inhibited malate release by less than 30% at 100 micromolar while 4,4'-diisothiocyanatostilbene-2,2' disulphonate (DIDS) had no effect. These results suggest that the Al3+ cation stimulates malate efflux via anion channels in apical cells of Al-tolerant wheat roots.