Nicotianamine synthase 2 is required for symbiotic nitrogen fixation inMedicago truncatulanodules

Abstract

SUMMARYSymbiotic nitrogen fixation carried out by the interaction between legumes and diazotrophic bacteria known as rhizobia requires of relatively large levels of transition metals. These elements act as cofactors of many key enzymes involved in this process. Metallic micronutrients are obtained from soil by the roots and directed to sink organs by the vasculature, in a process participated by a number of metal transporters and small organic molecules that mediate metal delivery in the plant fluids. Among the later, nicotianamine is one of the most important. Synthesized by nicotianamine synthases (NAS), this non-proteinogenic amino acid forms metal complexes participating in intracellular metal homeostasis and long-distance metal trafficking. Here we characterized theNAS2gene from model legumeMedicago truncatula. MtNAS2 is located in the root vasculature and in all nodule tissues in the infection and fixation zones. Symbiotic nitrogen fixation requires ofMtNAS2function, as indicated by the loss of nitrogenase activity in the insertional mutantnas2-1, a phenotype reverted by reintroduction of a wild-type copy ofMtNAS2. This would be the result of the altered iron distribution innas2-1nodules, as indicated by X-ray fluorescence studies. Moreover, iron speciation is also affected in these nodules. These data suggest a role of nicotianamine in iron delivery for symbiotic nitrogen fixation.Significance StatementNicotianamine synthesis mediated by MtNAS2 is important for iron allocation for symbiotic nitrogen fixation by rhizobia inMedicago truncatularoot nodules.