| Summary: | ABSTRACT 27 Symbiotic nitrogen fixation in legume root nodules requires a steady supply of 28 molybdenum for synthesis of the iron-molybdenum cofactor of nitrogenase. This nutrient 29 has to be provided by the host plant from the soil, crossing several symplastically 30 disconnected compartments through molybdate transporters, including members of the 31 MOT1 family. MtMOT1.2 is a Medicago truncatula MOT1 family member located in 32 the endodermal cells in roots and nodules. Immunolocalization of a tagged MtMOT1.2 33 indicates that it is associated to the plasma membrane and to intracellular membrane 34 systems, where it would be transporting molybdate towards the cytosol, as indicated in 35 yeast transport assays. Loss-of-function mot1.2-1 mutant showed reduced growth 36 compared to wild-type plants when nitrogen fixation was required, but not when nitrogen 37 was provided as nitrate. While no effect on molybdenum-dependent nitrate reductase 38 activity was observed, nitrogenase activity was severely affected, explaining the observed 39 difference of growth depending on nitrogen source. This phenotype was the result of 40 molybdate not reaching the nitrogen-fixing nodules, since genetic complementation with 41 a wild-type MtMOT1.2 gene or molybdate-fortification of the nutrient solution, both 42 restored wild-type levels of growth and nitrogenase activity. These results support a 43 model in which MtMOT1.2 would mediate molybdate delivery by the vasculature into 44 the nodules. 45 46
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