Malate transporter critical for aluminum tolerance. The STOP1 transcription factor is required for ALMT1 expression. Activated by external aluminum. Expressed in roots, but not in shoots. Detected in the root apex in absence of aluminum stress and in root apices, the stele and endodermis of the elongating zone of primary and lateral roots after aluminum stress. Not expressed in cortical and epidermal cells. Up-regulated by aluminum. Small induction by erbium (Er) and low pH stress, but no induction by cadmium, copper, lanthanum or sodium. Phosphorylated. A reversible phosphorylation is required for activation (By similarity). No visible phenotype when grown under normal conditions. Lack of aluminum-activated malate release and shorter roots when submitted to aluminum stress. Essential factor for aluminum (Al) tolerance but does not represent the major Al tolerance QTL also found on chromosome 1. Acts in parallel but independently of MATE, an aluminum-activated root citrate transporter. A reversible phosphorylation acting both at the transcriptional and post-transcriptional levels is required to activate malate release in response to aluminum. Belongs to the aromatic acid exporter (TC 2.A.85) family.