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Plant Physiol. 2018 Mar;176(3):2315-2329. doi: 10.1104/pp.17.01538. Epub 2017 Dec 28.

An Iron-Activated Citrate Transporter, MtMATE67, Is Required for Symbiotic Nitrogen Fixation.

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Noble Research Institute, Ardmore, Oklahoma 73401.
Department of Biochemistry and Cellular and Molecular Biology, University of Tennessee, Knoxville, Tennessee 37996.
Centro de Biotecnología y Genómica de Plantas (UPM-INIA), Universidad Politécnica de Madrid, Madrid 28223, Spain.
Argonne National Laboratory, Argonne, Illinois 60439.
Division of Plant and Soil Sciences, West Virginia University, Morgantown, West Virginia 26506.
Noble Research Institute, Ardmore, Oklahoma 73401


Iron (Fe) is an essential micronutrient for symbiotic nitrogen fixation in legume nodules, where it is required for the activity of bacterial nitrogenase, plant leghemoglobin, respiratory oxidases, and other Fe proteins in both organisms. Fe solubility and transport within and between plant tissues is facilitated by organic chelators, such as nicotianamine and citrate. We have characterized a nodule-specific citrate transporter of the multidrug and toxic compound extrusion family, MtMATE67 of Medicago truncatula The MtMATE67 gene was induced early during nodule development and expressed primarily in the invasion zone of mature nodules. The MtMATE67 protein was localized to the plasma membrane of nodule cells and also the symbiosome membrane surrounding bacteroids in infected cells. In oocytes, MtMATE67 transported citrate out of cells in an Fe-activated manner. Loss of MtMATE67 gene function resulted in accumulation of Fe in the apoplasm of nodule cells and a substantial decrease in symbiotic nitrogen fixation and plant growth. Taken together, the results point to a primary role of MtMATE67 in citrate efflux from nodule cells in response to an Fe signal. This efflux is necessary to ensure Fe(III) solubility and mobility in the apoplasm and uptake into nodule cells. Likewise, MtMATE67-mediated citrate transport into the symbiosome space would increase the solubility and availability of Fe(III) for rhizobial bacteroids.

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