Abstract
Iron is critical for symbiotic nitrogen fixation (SNF) as a key component of multiple ferroproteins involved in this biological process. In the model legume Medicago truncatula, iron is delivered by the vasculature to the infection/maturation zone (zone II) of the nodule, where it is released to the apoplast. From there, plasma membrane iron transporters move it into rhizobia-containing cells, where iron is used as the cofactor of multiple plant and rhizobial proteins (e.g. plant leghemoglobin and bacterial nitrogenase). MtNramp1 (Medtr3g088460) is the M. truncatula Natural Resistance-Associated Macrophage Protein family member, with the highest expression levels in roots and nodules. Immunolocalization studies indicate that MtNramp1 is mainly targeted to the plasma membrane. A loss-of-function nramp1 mutant exhibited reduced growth compared with the wild type under symbiotic conditions, but not when fertilized with mineral nitrogen. Nitrogenase activity was low in the mutant, whereas exogenous iron and expression of wild-type MtNramp1 in mutant nodules increased nitrogen fixation to normal levels. These data are consistent with a model in which MtNramp1 is the main transporter responsible for apoplastic iron uptake by rhizobia-infected cells in zone II.
Highlights
symbiotic nitrogen fixation (SNF) is carried out by the endosymbiosis between legumes and diazotrophic bacteria called rhizobia
Sequence comparison of the seven MtNramp sequences with known plant Natural Resistance-Associated Macrophage Protein (Nramp) transporters showed that they cluster in two different groups (Fig. 1A)
Like in other nongraminaceous plants, in legumes, iron uptake from soil follows what is known as Strategy I, i.e. Fe3+ solubility is increased by acidification of the surrounding soil and Fe3+ is reduced by a ferroreductase to Fe2+, which is imported into plants via specific transporters in the root epidermis (Andaluz et al, 2009)
Summary
SNF is carried out by the endosymbiosis between legumes and diazotrophic bacteria called rhizobia (van Rhijn and Vanderleyden, 1995). MtNramp Provides Iron for Symbiotic Nitrogen Fixation supply of nutrients from the host: photosynthates, macronutrients such as phosphate and sulfate, amino acids, at least prior to nitrogen fixation, and metal micronutrients (Udvardi and Poole, 2013). Among the latter, iron is one of the most critical (Brear et al, 2013; González-Guerrero et al, 2014). Iron deficiency is prevalent in plants and hampers crop production and human health (Grotz and Guerinot, 2006; Mayer et al, 2008) This is even more so when legumes are nodulated (Terry et al, 1991; Tang et al, 1992). Knowing how iron homeostasis is maintained in nodulated legumes, including how this micronutrient is delivered to the nodule, is important for understanding and improving SNF
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