Catalyzes the transport of dicarboxylates, such as oxoglutarate, oxaloacetate, malate, and succinate, and of tricarboxylates, such as citrate, isocitrate, cis-aconitate, and trans-aconitate by a counter-exchange mechanism across the inner mitochondrial membrane. Substrate preference in reconstituted proteoliposomes is oxaloacetate > malonate > malate > maleate > succinate > oxoglutarate > citrate > trans-aconitate > cis-aconitate > sulfate > isocitrate. May be important for plant metabolic functions requiring organic acid flux to or from the mitochondria, such as nitrogen assimilation, export of reducing equivalents from the mitochondria, and fatty acid elongation. Optimum pH is 5.5-6.0. Highly expressed in flower buds and at lower levels in roots, leaves and stems. Belongs to the mitochondrial carrier (TC 2.A.29) family.