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Title: The Ferroportin Metal Efflux Proteins Function in Iron and Cobalt Homeostasis in Arabidopsis

Author
item MORRISSEY, JOE - Dartmouth University
item Baxter, Ivan
item LEE, JOOHYUN - Dartmouth University
item LI, LIANGTAO - University Of Utah
item LAHNER, BRETT - Purdue University
item GROTZ, NATASHA - Dartmouth University
item KAPLAN, JERRY - University Of Utah
item SALT, DAVID - Purdue University
item GUERINOT, MARY LOU - Dartmouth University

Submitted to: The Plant Cell
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/29/2009
Publication Date: 10/27/2009
Citation: Morrissey, J., Baxter, I.R., Lee, J., Li, L., Lahner, B., Grotz, N., Kaplan, J., Salt, D.E., Guerinot, M. 2009. The Ferroportin Metal Efflux Proteins Function in Iron and Cobalt Homeostasis in Arabidopsis. The Plant Cell. 21:3326-3338.

Interpretive Summary: Iron (Fe) is an essential element for both plants and animals, with insufficient Fe causing reduced plant growth and severe human health effects including anemia. While the basic mechanisms that plants use to take up Fe from the soil are known, relatively little is known about how the Fe is moved through the root to the vasculature, which takes it up to the shoot. Here we characterize two genes related to the mammalian Fe transporter, Ferroportin, FPN1 and FPN2, in the model plant Arabidopsis thaliana. The two proteins are expressed in different cell layers and go to two different cellular locations. FPN1 is localized to the plasma membrane (the cells outer layer) and is expressed around the vasculature, suggesting that it is involved in loading Fe into the vasculature. FPN2 is localized to the vacuole, an internal storage compartment that performs a variety of functions, and is expressed in the outer root layers. This suggests that FPN2 is working to buffer the levels of Fe in these cells by sequestering Fe in the vacuole. Consistent with these roles, we show that lines where these genes are disrupted have altered responses to Fe deficient conditions. We also show that these genes are involved in the homeostasis of Co, which is chemically similar to Fe but can be toxic to plants. These results will allow us to develop strategies to improve Fe uptake in plants which in turn will result in increased yields and improved nutritional qualities of seeds for human consumption.

Technical Abstract: Relatively little is known about how metals such as iron are effluxed from cells, a necessary step for transport from the root to the shoot. Ferroportin is the sole iron efflux transporter in animals, and there are two closely related orthologs in Arabidopsis, FPN1 and FPN2. FPN1 localizes to the plasma membrane and is expressed in the stele, suggesting a role in vascular loading; FPN2 localizes to the vacuole and is expressed in the two outermost layers of the root in response to iron deficiency, suggesting a role in buffering metal influx. Consistent with these roles, fpn2 has a reduced iron deficiency response whereas fpn1 fpn2 has an elevated iron deficiency response. Ferroportins also play a role in cobalt homeostasis; a survey of Arabidopsis accessions for ionomic phenotypes showed that truncation of FPN2 results in elevated shoot cobalt levels and leads to increased sensitivity to the metal. Conversely, loss of FPN1 abolishes shoot cobalt accumulation, even in the cobalt accumulating mutant frd3. Consequently, in the fpn1 fpn2 double mutant, cobalt cannot move to the shoot via FPN1 and is not sequestered in the root vacuoles via FPN2; instead, cobalt likely accumulates in the root cytoplasm causing fpn1 fpn2 to be even more sensitive to cobalt than fpn2 mutants.