Genome-Wide Identification and Expression Analysis of Mitochondrial Dicarboxylate Carriers (DICs) in Medicago Under Aluminum Stress

Aluminum (Al) is solubilized as phytotoxic Al³⁺ in acidic soils, rapidly inhibiting root elongation. To detoxify Al, plant roots secrete organic acids that chelate the ion. The transcription factor SENSITIVE-TO-PROTON-RHIZOTOXICITY1 (STOP1) regulates the export, distribution and metabolism of organic acids, which is crucial for Al resistance. Plant DICARBOXYLATE-CARRIERs (DICs) located in the inner mitochondrial membrane are presumed to exchange the dicarboxylates. However, whether Al or STOP1 modulates <i>DIC</i> expression to coordinate the organic acid shuttle remains unclear. Here, in the model legume <i>Medicago truncatula</i>, we identified three <i>DIC</i> genes and twelve in tetraploid <i>Medicago sativa</i>. Phylogenetic analysis places all Medicago DICs in a clade with Arabidopsis AtDIC1 and AtDIC2, whereas <i>AtDIC3</i> lacks an ortholog in <i>M. truncatula</i>. Mining RNA-seq datasets followed by qRT-PCR validation showed that <i>MtDIC2</i> is upregulated by Al in roots in a MtSTOP1-dependent manner. Consistently, STOP1-binding motifs exist in the <i>MtDIC2</i> promoter, and MtSTOP1 binds to the <i>MtDIC2</i> promoter in yeast. Furthermore, <i>MsDIC2.4</i> shows an increase under Al treatment. Our study provides a genome-wide characterization of Medicago DICs and identifies <i>MtDIC2</i> as a candidate target of MtSTOP1, whose Al-responsive induction may enhance organic acid flux across the mitochondrial membrane.