CHARACTERIZATION OF AL-STIMULATED EFFLUX OF MALATE FROM THE APICES OFAL-TOLERANT WHEAT ROOTS

Aluminium (Al) stimulates the efflux of malate from the apices of whea t (Triticum aestivum L.) roots (Delhaize et al. 1993, Plant Physiol. 1 03, 695-702). The response was five to tenfold higher in Al-tolerant s eedlings than Al-sensitive seedlings and the capacity for Al-stimulate d 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 dom inant 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 mu M Al in 0.2 mM CaCl2, pH 4.2, while saturating rates of 2.0 nmol . apex(-1). h(-1) occurred with concentra tions above 100 mu M Al, The stimulation of malate efflux by Al was ac companied by an increase in K+ efflux which appeared to account for el ectroneutrality. The greater stimulation of malate efflux from Al-tole rant apices compared to Al-sensitive apices could not be explained by differences in the activities of phosphoenolpyruvate carboxylase or NA D-malate dehydrogenase. Several other polyvalent cations, including ga llium, indium and the tridecamer Al-13, failed to elicit malate efflux . Aluminium-stimulated efflux of malate was correlated with the measur ed concentration of total monomeric Al present, and with the predicted concentrations of Al3+ and AlOH2+ ions in the solution. Several antag onists of anion channels inhibited Al-stimulated efflux of malate with the following order of effectiveness: niflumic acid approximate to NP PB > IAA-94 approximate to A-9-C > ethacrynic acid. Lanthanum, chlorat e, perchlorate, zinc and alpha-cyano-4-hydroxycinnamic acid inhibited malate release by less than 30% at 100 mu M while 4,4'-diisothiocyanat ostilbene-2,2'-disulphonate (DIDS) had no effect. These results sugges t that the Al3+ cation stimulates malate efflux via anion channels in apical cells of Al-tolerant wheat roots.