Plant Physiology Preview
Published on March 9, 2007; 10.1104/pp.107.097162
OPEN ACCESS ARTICLE
Received February 1, 2007
Accepted February 26, 2007
The FRD3-mediated Efflux of Citrate into the Root Vasculature Is Necessary for Efficient Iron Translocation
Timothy P. Durrett , Walter Gassmann , and Elizabeth E. Rogers *
Department of Biochemistry; Division of Plant Sciences; Department of Nutritional Sciences; Life Sciences Center; University of Missouri, Columbia, MO 65211.
* Corresponding author; email: rogersee{at}missouri.edu.
Iron, despite being an essential micronutrient, becomes toxic if present at high levels. As a result, plants possess carefully regulated mechanisms to acquire iron from the soil. The frd3 mutant of Arabidopsis thaliana is chlorotic and exhibits constitutive expression of its iron uptake responses. Consequently, frd3 mutants over-accumulate iron; yet paradoxically, the frd3 phenotypes are due to a reduction in the amount of iron present inside frd3 leaf cells. The FRD3 protein belongs to the multidrug and toxin efflux (MATE) family, members of which are known to export low-molecular weight organic molecules. We therefore hypothesized that FRD3 loads an iron chelator necessary for the correct distribution of iron throughout the plant into the xylem. One such potential chelator is citrate. Xylem exudate from frd3 plants contains significantly less citrate and iron than the exudate from wild type plants. Additionally, supplementation of growth media with citrate rescues the frd3 phenotypes. The ectopic expression of FRD3GFP results in enhanced tolerance to aluminum in Arabidopsis roots, a hallmark of organic acid exudation. Consistent with this result, approximately three times more citrate was detected in root exudate from plants ectopically expressing FRD3GFP. Finally, heterologous studies in Xenopus laevis oocytes reveal that FRD3 mediates the transport of citrate. These results all strongly support the hypothesis that FRD3 effluxes citrate into the root vasculature, a process important for the translocation of iron to the leaves, as well as confirming previous reports suggesting that iron moves through the xylem as a ferric-citrate complex. Our results provide additional answers to long-standing questions about iron chelation in the vasculature and organic acid transport.
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