Aluminum activates a citrate-permeable anion channel in the aluminum-sensitive zone of the maize root apex. A comparison between an aluminum-sensitive and an aluminum-resistant cultivar

In search for the cellular and molecular basis for differences in aluminum (Al) resistance between maize (Zea mays) cultivars we applied the patch-cla mp technique to protoplasts isolated from the apical root cortex of two mai ze cultivars differing in Al resistance. Measurements were performed on pro toplasts from two apical root zones: The 1- to 2-mm zone (DTZ), described a s most Al-sensitive, and the main Elongation zone (3-5 mm), the site of Al- induced inhibition of cell elongation. Al stimulated citrate and malate eff lux from intact root apices, revealing cultivar differences. In the elongat ion zone, anion channels were not observed in the absence and presence of A l. Preincubation of intact roots with 90 muM Al for 1 h induced a citrate- and malate-permeable, large conductance anion channel in 80% of the DTZ pro toplasts from the resistant cultivar, but only 30% from the sensitive culti var. When Al was applied to the protoplasts in the whole-cell configuration , anion currents were elicited within 10 min in the resistant cultivar only . La3+ was not able to replace or counteract with Al3+ in the activation of this channel. Ln the presence of the anion-channel blockers, niflumic acid and 4, 4 ' -dinitrostilbene-2, 2 ' disulfonic acid, anion currents as well as Exudation rates were strongly inhibited. Application of cycloheximide d id not affect the Al response, suggesting that the channel is activated thr ough post-translational modifications. We propose that the Al-activated lar ge anion channel described here contributes to enhanced genotypical Al resi stance by facilitating the exudation of organic acid anions from the DTZ of the maize root apex.