Summary
Molybdenum, as a component of the iron-molybdenum cofactor of nitrogenase, is essential for symbiotic nitrogen fixation. This nutrient has to be provided by the host plant through molybdate transporters.
Members of the molybdate transporters family MOT1 were identified in the model legume Medicago truncatula and their expression in nodules determined. Yeast toxicity assays, confocal microscopy, and phenotypical characterization of a Tnt1 insertional mutant line were carried out in the one M. truncatula MOT1 family member expressed specifically in nodules.
Among the five MOT1 members present in M. truncatula genome, MtMOT1.3 is the only one uniquely expressed in nodules. MtMOT1.3 shows molybdate transport capabilities when expressed in yeast. Immunolocalization studies revealed that MtMOT1.3 is located in the plasma membrane of nodule cells. A mot1.3-1 knockout mutant showed an impaired growth concomitant with a reduction in nitrogenase activity. This phenotype was rescued by increasing molybdate concentrations in the nutritive solution, or upon addition of an assimilable nitrogen source. Furthermore, mot1.3-1 plants transformed with a functional copy of MtMOT1.3 showed a wild type-like phenotype.
These data are consistent with a model in which MtMOT1.3 would be responsible for introducing molybdate into nodule cells, which will be later used to synthesize functional nitrogenase.
