Minisymposium 17: Mineral Nutrition

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Abs # M1701: Functional characterization of FRD3, a novel organic acid effluxer involved in iron homeostasis

Presenter:	Rogers, Elizabeth E.       Contact Presenter
Authors	Durrett, Timothy P. (A)   Gassmann, Walter  (B)   Rogers, Elizabeth E. (A) (C)
Affiliations:	(A): Department of Biochemistry, University of Missouri (B): Division of Plant Sciences, University of Missouri (C): Department of Nutritional Sciences, University of Missouri

Iron, despite being an essential micronutrient, becomes toxic if present at high levels. Plants therefore possess carefully regulated mechanisms to acquire iron from the soil. The Strategy I response, found in all plants except the grasses, involves the release of protons into the rhizosphere, the induction of Fe(III) chelate reductase and increased Fe(II) transport activities. The frd3 mutant of Arabidopsis thaliana exhibits constitutive expression of these iron uptake responses and is chlorotic, both results of a reduction in the amount of iron present in frd3 leaf cells. frd3 xylem exudate also contains approximately half as much iron as does xylem exudate from wild type plants. The FRD3 protein belongs to the multidrug and toxin efflux (MATE) family; other MATE proteins are known to export low-molecular weight organic molecules. Therefore, we hypothesize that FRD3 loads into the xylem an iron chelator or other factor necessary for efficient iron translocation through the xylem or uptake from apoplastic space and into leaf cells. One such potential chelator is citrate. Xylem exudate from frd3 plants contains significantly less citrate than the exudate from wild type plants. Supplementation of growth media with citrate rescues the chlorotic appearance of frd3-1 plants. Additionally, the ectopic expression of FRD3-GFP also results in increased tolerance to aluminium in Arabidopsis roots, a hallmark of organic acid exudation. Finally, preliminary two-electrode voltage clamp data reveal that FRD3 expressed in Xenopus laevis oocytes mediates currents when exposed to citrate. These results all support the hypothesis that FRD3 transports an organic acid, most probably citrate. In this regard, FRD3 represents a novel effluxer of organic acids.