MtESN2 is a subgroup II sulphate transporter required for symbiotic nitrogen fixation and prevention of nodule early senescence in Medicago truncatula.

Adequate distribution of mineral sulphur (S) nutrition to nodules mediated by sulphate transporters is crucial for nitrogen fixation in symbiosis establishment process. However, the molecular mechanisms underlying this process remain largely unknown. In this study, we characterized the function of Early Senescent Nodule 2 (MtESN2), a gene crucial to nitrogen fixation in Medicago truncatula. Mutations in MtESN2 resulted in severe developmental and functional defects including dwarf shoots, early senescent nodules, and lower nitrogenase activity under symbiotic conditions compared to wild‐type plants. MtESN2 encodes an M. truncatula sulphate transporter that is expressed only in roots and nodules, with the highest expression levels in the transition zone and nitrogen‐fixing zone of nodules. MtESN2 exhibited sulphate transport activity when expressed in yeast. Immunolocalization analysis showed that MtESN2‐yellow fluorescent protein fusion protein was localized to the plasma membranes of both uninfected and infected cells of nodules, where it might transport sulphate into both rhizobia‐infected and uninfected cells within the nodules. Our results reveal an unreported sulphate transporter that contributes to effective symbiosis and prevents nodule early senescence in M. truncatula. Summary statement: In the present study, we performed detailed phenotypic studies of loss‐of‐function mutants and functional characterization of MtESN2 and found that MtESN2 is a sulphate transporter localized on the plasma membrane of infected and uninfected nodule cells with an important role in nitrogen fixation. Our findings have expanded knowledge of an indispensable role of the subgroup II sulphate transporter MtESN2 in legume‐rhizobial symbiosis and nodule senescence. [ABSTRACT FROM AUTHOR]