Symbiosome membrane-localized cationic amino acid transporters support symbiotic nitrogen fixation in Medicago truncatula

Legumes engage in nitrogen-fixing symbiosis with rhizobia, wherein it is well established that host legumes supply dicarboxylates as a carbon source to rhizobia, while rhizobia reciprocate by providing ammonium to the host plants. Apart from the classical model, accumulating evidence suggests that amino acid exchange is also essential to legume-rhizobium symbiosis. However, it remains unclear whether amino acid transporters are present on the symbiosome membrane (SM) to mediate amino acid exchange in symbiotic nitrogen fixation (SNF). In this study, we identified three amino acid transporters in Medicago truncatula-MtCAT1a, MtCAT1b, and MtCAT1c-belonging to a clade of the plant Cationic Amino acid Transporter (CAT) family known for transporting a wide range of amino acids. Notably, MtCAT1b and MtCAT1c are predominantly expressed in infected cells of nodules and are localized to the SM. Genetic analyses further demonstrate that both MtCAT1b and MtCAT1c are required for amino acid exchange on the SM, with additional evidence indicating that metabolism of bacteroids is disturbed in the mutant. Transport assays reveal that both MtCAT1b and MtCAT1c exhibit broad substrate specificity. Collectively, our findings identify MtCAT1b and MtCAT1c as key mediators of cross-kingdom amino acid exchange, essential for maintaining efficient SNF in root nodules.