The phytotoxic effects of aluminum (Al) on root systems of crop plants constitute a major agricultural problem in many areas of the world. Root exudation of Al-chelating molecules such as low-molecular-weight organic acids has been shown to be an important mechanism of plant Al tolerance/resistance. Differences observed in the physiology and electrophysiology of root function for two maize genotypes with contrasting Al tolerance revealed an association between rates of Al-activated root organic acid release and Al tolerance. Using these genotypes, we cloned ZmALMT1, a maize gene homologous to the wheat ALMT1 and Arabidopsis AtALMT1 genes that have recently been described as encoding functional, Al-activated transporters that play a role in tolerance by mediating Al-activated organic acid exudation in roots. The ZmALMT1 cDNA encodes a 451 amino acid protein containing six transmembrane helices. Transient expression of a ZmALMT1532352367