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First published online August 13, 2004; 10.1104/pp.104.045633 Plant Physiology 136:2523-2531 (2004) © 2004 American Society of Plant Biologists FRD3 Controls Iron Localization in Arabidopsis1Departments of Biochemistry and Nutritional Sciences, University of Missouri, Columbia, Missouri 65211
The frd3 mutant of Arabidopsis exhibits constitutive expression of its iron uptake responses and is chlorotic. These phenotypes are consistent with defects either in iron deficiency signaling or in iron translocation and localization. Here we present several experiments demonstrating that a functional FRD3 gene is necessary for correct iron localization in both the root and shoot of Arabidopsis plants. Reciprocal grafting experiments with frd3 and wild-type Arabidopsis plants reveal that the phenotype of a grafted plant is determined by the genotype of the root, not by the genotype of the shoot. This indicates that FRD3 function is root-specific and points to a role for FRD3 in delivering iron to the shoot in a usable form. When grown under certain conditions, frd3 mutant plants overaccumulate iron in their shoot tissues. However, we demonstrate by direct measurement of iron levels in shoot protoplasts that intracellular iron levels in frd3 are only about one-half the levels in wild type. Histochemical staining for iron reveals that frd3 mutants accumulate high levels of ferric iron in their root vascular cylinder, the same tissues in which the FRD3 gene is expressed. Taken together, these results clearly indicate a role for FRD3 in iron localization in Arabidopsis. Specifically, FRD3 is likely to function in root xylem loading of an iron chelator or other factor necessary for efficient iron uptake out of the xylem or apoplastic space and into leaf cells.
Iron is both necessary for plant growth and toxic in excess. It participates as a redox cofactor in a number of metalloenzymes involved in respiration and photosynthesis. These same redox properties allow iron to catalyze the formation of damaging oxygen radicals (Halliwell and Gutteridge, 1992  ). Although iron is plentiful in the earth's crust, it exists primarily in the insoluble ferric, Fe(III), form. Therefore, plants need specific mechanisms to obtain sufficient amounts of this important nutrient. Dicots rely on acidification of the rhizosphere to solubilize ferric iron, reduction of ferric iron to the more soluble ferrous form, and transport of the ferrous iron into the root epidermal cells. These activities are collectively termed iron uptake responses and are maximally expressed under conditions of iron deficiency. The genes responsible for the root iron-deficiency inducible ferric chelate reductase activity and the major ferrous uptake transporter have been identified as FRO2 and IRT1, respectively, in the model plant Arabidopsis (Eide et al., 1996; Robinson et al., 1999; Vert et al., 2002).
It is well known that iron deficiency causes chlorosis in plants. On a molecular level, this chlorosis is caused by a reduction in the amount of chlorophyll synthesized and an accumulation of both Mg-protoporphyrin IX and Mg-protoporphyrin IX monomethyl ester that are chlorophyll precursors (Spiller et al., 1982
Relatively little is known about either the mechanisms that control the expression of iron uptake responses or those involved in iron translocation throughout the plant. Previously, we reported that the frd3 mutant of Arabidopsis constitutively exhibits symptoms of iron deficiency (Rogers and Guerinot, 2002a Here we report the results of several experiments designed to distinguish between these two models for FRD3 action. First, we show that detached roots of the frd3 mutant are capable of repressing iron uptake responses when cultured under iron-sufficient conditions. Furthermore, grafting experiments show that when frd3 mutant shoots are appropriately supplied with iron, they regreen and the plants are capable of correctly regulating root iron uptake responses. We also show that protoplasts isolated from frd3 mutants have lower iron levels than those from wild-type plants and that frd3 mutant plants accumulate abnormal levels of ferric iron in their root vasculature. FRD3 is expressed in the pericycle and other vascular cylinder cells in the mature portion of the root. All of these data are consistent with a role for FRD3 in the delivery of iron to the shoot in a useable form.
If the wild-type FRD3 protein were involved in transmission or perception of a systemic iron deficiency signal, the loss-of-function frd3 mutants presumably would not be able to down-regulate their iron uptake responses even though there is sufficient iron in the leaf tissues. In this signaling model, shoot chlorosis would be caused by excess iron or manganese. A signaling defect could be shoot-specific, in which case the frd3 mutant shoot would be defective in signaling the shoot iron status to the root. Alternatively, a signaling defect could be root-specific. In this case the frd3 mutant roots would be defective in perceiving a shoot-derived iron status signal or could not regulate their iron uptake responses as directed by the shoot-derived signal. In the other model, the FRD3 protein would be involved in iron localization. In this case, iron would not get to the leaf cells in a usable form in the frd3 mutant, causing the shoot to be functionally iron deficient. This model is consistent with the lack of accumulation of the ferritin protein in frd3 leaf tissue. With this model, the constitutive iron uptake responses exhibited by frd3 mutant roots would be an appropriate response to a shoot iron deficiency signal and shoot chlorosis could be caused by iron deficiency.
To assess the signaling capabilities of the frd3 mutant, reciprocal grafting experiments were performed with frd3 and wild-type seedlings. Approximately 2 weeks after grafting, plants were transferred to iron-sufficient media for 3 d, after which root ferric chelate reductase activity, and shoot chlorophyll and ferritin levels were measured (Fig. 1). Self-grafted plants behave similarly to the ungrafted controls, indicating that the grafting process itself does not affect iron homeostasis. Examining the reciprocal grafts, it is clear the phenotype of the grafted plant follows the genotype of the root, not the genotype of the shoot. For example, when a frd3 shoot is grafted onto a wild-type root, the frd3 shoot is not chlorotic and accumulates the ferritin protein, and the wild-type root does not express its ferric chelate reductase activity. This indicates that the frd3 shoot is capable of transmitting its iron sufficient status to the root and that when appropriately supplied with iron by the wild-type root, a frd3 shoot does accumulate chlorophyll and ferritin protein. However, when a wild-type shoot is grafted onto a frd3 mutant root, this wild-type shoot becomes chlorotic and no longer accumulates ferritin protein, phenocopying shoots of intact frd3 plants. This result is consistent with a model in which iron signaling is intact in the frd3 mutant, but frd3 roots are not capable of supplying iron to the shoots in a usable form, causing the shoots to become functionally iron deficient.
The expression of root iron uptake responses appears to be regulated by a combination of two signals, a systemic one originating in the shoot and a local one endogenous to the root (Grusak and Pezeshgi, 1996 ; Schikora and Schmidt, 2001). To examine local, root-endogenous signaling, we determined whether frd3 mutant roots are capable of repressing their iron uptake responses in the absence of a shoot signal. Hypocotyls were cut and the shoot portion removed from 2-week-old seedlings that had been growing on standard media. The root portion was then transferred to either iron-sufficient or deficient media for 5 d before the roots were assayed for ferric chelate reductase activity. Measuring ferric chelate reductase activity is straightforward and was used here as a marker of all three iron deficiency responses. The absolute levels of ferric chelate reductase activity measured in the detached roots from either wild type or frd3 are significantly lower than the reductase activity exhibited by roots of intact plants. This drop in ferric chelate reductase activity occurs immediately after shoot removal and probably reflects the nonphysiological nature of detached roots. Nevertheless, the detached roots continued to grow throughout the experiment and were able to respond to iron levels in the media (see below). As is shown in Figure 2, wild-type detached roots behaved similarly to roots of intact wild-type plants, with ferric chelate reductase activity repressed under iron-sufficient conditions and elevated under iron deficiency. Interestingly, frd3 mutant roots, in the absence of a shoot signal, were capable of appropriately regulating their ferric chelate reductase activity. By 5 d after transfer, the frd3 mutant roots grown under iron-deficient conditions expressed high levels of reductase activity. More importantly, the detached frd3 mutant roots appropriately repressed their ferric chelate reductase activity under iron-sufficient conditions, a response never seen in intact frd3 mutant seedlings. This result is also consistent with the idea that the root mechanisms that control the iron uptake responses are functional in the frd3 mutant, implying that the defect in the frd3 mutant is not in its ability to down-regulate its root iron uptake responses under conditions of iron sufficiency.
Causes of Chlorosis in the frd3 Mutant
There are two potential causes of the chlorosis observed in frd3 mutant shoots. One is iron deficiency; that in spite of the high iron levels present in whole leaves of the frd3 mutant, the chloroplastic or intracellular iron levels could be low. The other potential cause of chlorosis is an excess of iron or other redox active metals such as manganese (Gonzalez et al., 1998
Iron Localization in the Root
The roots of the frd3 mutant also accumulate excess iron (Delhaize, 1996
Figure 4, section E shows an immunoblot of total root proteins probed with anti-ferritin antibodies. Root tissue from frd3 mutants grown under iron-sufficient conditions shows significantly higher levels of the ferritin protein than does wild-type root tissue. This result is consistent with the higher iron levels demonstrated by elemental analysis and by Perls' stain. It also indicates that much of the iron found in frd3 roots is intracellular, since it triggers the accumulation of the ferritin protein.
We had previously reported that, by RNA blot and reverse transcription-PCR analysis, the wild-type FRD3 gene was only expressed to detectable levels in Arabidopsis root tissue. To more precisely localize the expression of the FRD3 gene, an FRD3-green fluorescent protein (GFP) fusion construct was used. This fusion protein is capable of complementing the frd3-1 mutant phenotypes. Expression of the fusion protein is driven by the FRD3 promoter to accurately reflect endogenous FRD3 gene expression. Figure 5 shows that the FRD3 gene is expressed in the pericycle and cells internal to the pericycle and surrounding the vascular tissue. The green fluorescence observed in the epidermis is autofluorescence; it is also present in untransformed controls and so is not due to GFP (Fig. 5, section E). The location of FRD3 gene expression is very similar to the site of ferric iron accumulation in the frd3 mutant.
The FRD3 protein is predicted to contain 14 transmembrane domains and to be a member of the MATE family of membrane proteins. Therefore, the FRD3 protein is predicted to localize to a membrane. Close inspection of Figure 5, section A reveals green fluorescence both internal to the pericycle cells and outlining a number of pericycle cells. While this could indicate the presence of FRD3 in the cytoplasm or on intracellular vesicles, we believe this is more likely the result of degradation of some of the FRD3-GFP fusion protein. By immunoblot, there are four protein species that cross-react with anti-GFP antibodies (data not shown). The largest of these is approximately the size of a full-length FRD3 protein fused to GFP; the other three cross-reactive bands are significantly smaller and probably represent degradation products from which some or all of the FRD3 portion of the fusion protein has been removed. For this reason, it is not possible to definitively localize the FRD3 protein on a subcellular level using this FRD3-GFP fusion protein. However, the presence of FRD3-GFP degradation products does not affect the result that the FRD3 gene is expressed in the pericycle and vascular cylinder. A construct containing FRD3 protein fused to the FLAG epitope tag was also constructed and transformed into wild type and the frd3-1 mutant. By immunoblotting, there is a single anti-FLAG antibody cross-reactive species of the appropriate Mr to be the full-length FRD3-FLAG protein (data not shown). Like the FRD3-GFP fusion, the FRD3-FLAG is able to complement frd3-1. Immunofluorescence experiments were performed on roots expressing the FRD3-FLAG protein and showed that the FRD3-FLAG protein is present in the central vascular cylinder, the pericycle cells and smaller cells surrounding the vascular tissue (Fig. 5, section F), confirming the results obtained for the FRD3-GFP fusion.
Previous work suggested two possible models of FRD3 action. The first was that FRD3 is part of an iron signaling pathway. It is likely that plant root iron uptake responses are controlled at least in part by a shoot-derived signal of shoot iron status (Grusak and Pezeshgi, 1996 ). This signal must both be produced by the shoot and detected in the root. If the frd3 mutant were defective in iron signaling, the defect would likely be specific to either the shoot or the root. The reciprocal grafting experiments presented in Figure 1 indicate that the phenotype of a grafted plant is determined by the genotype of the root, not the genotype of the shoot. The fact that a wild-type root appropriately regulates its iron uptake responses when grafted to a frd3 mutant shoot demonstrates that frd3 mutant shoots are capable of generating a functional signal of iron status. The competency of frd3 mutant roots to perceive a shoot-generated signal and appropriately regulate iron uptake responses is demonstrated in the detached root experiments presented in Figure 2. Figure 2 clearly demonstrates that in the absence of a shoot and therefore lacking a shoot-derived signal of iron status, frd3 mutant roots appropriately regulate their ferric chelate reductase activity. This includes repressing their reductase activity when grown under iron-sufficient conditions, which is never seen in intact frd3 mutants. These results are inconsistent with the hypothesis that the frd3 mutant is defective in any aspect of iron deficiency signaling.
Our second hypothesis concerning FRD3 action is that FRD3 is involved in shoot iron localization. In this case, the chlorosis observed in the frd3 would be caused by low iron levels in the shoot cells or organelles where it is needed. Here we have presented several lines of evidence that demonstrate that frd3 mutants mislocalize iron in their tissues. Even though whole leaves of frd3 have elevated levels of iron and manganese, frd3 leaf protoplasts have only approximately one-half as much iron as do those from wild type (Fig. 3), and manganese levels in the two are similar. Since frd3 shoots as a whole overaccumulate both iron and manganese, two redox active metals, it was possible that the chlorosis in frd3 shoots was caused by oxidative damage (Gonzalez et al., 1998
It is formally possible that the phenotypes of the frd3 mutant are not caused by changes in iron uptake or localization but rather by the overaccumulation of manganese observed in frd3 (Rogers and Guerinot, 2002a
The FRD3 gene is expressed in the central cylinder of the root, in the pericycle cells, and cells surrounding the vascular tissues (Fig. 5). Roots of the frd3 mutant also overaccumulate iron in this central vascular cylinder. The FRD3 gene encodes a protein containing 14 predicted transmembrane domains. Therefore, the FRD3 protein is likely to be associated with a cell membrane. Both the FRD3-GFP fusion fluorescence and the FRD3-FLAG immunofluorescence are brightest at the plasma membranes. However, some of the FRD3-GFP fusion protein is degraded and therefore cannot be used to determine subcellular localization. Additionally, cells in the central vascular cylinder are too small to be able to clearly observe plasma membrane localization. Unfortunately, the FRD3-FLAG protein is expressed at low levels and was not detectable by immunoelectron microscopy (data not shown). Nevertheless, the localization results presented in Figure 5 are consistent with at least some of the FRD3 protein localizing to the plasma membrane. FRD3 belongs to the MATE family of transmembrane proteins (Rogers and Guerinot, 2002a
Together with the data presented, this leads to a new question: how can a putative effluxer of low Mr compounds that is expressed around the root vasculature affect iron localization in the shoot? Iron reaches the shoot tissue in sufficient amounts in the frd3 mutant; even in soil-grown frd3 shoots, shoots grown under conditions of low iron availability, iron levels are only about 10% lower than in wild type (Lahner et al., 2003
Given FRD3's expression in cells surrounding the root vasculature, we hypothesize that FRD3 effluxes into the xylem a low Mr compound that is necessary for correct iron unloading from the xylem in the shoot. This compound could be an iron chelator. Certainly, given the low solubility of ferric iron at the pH of the xylem, approximately pH 6, ferric iron would need to be chelated to move efficiently. It is widely thought that iron moves in the xylem as ferric citrate (Pich et al., 1994
The FRD3 protein also could transport one of a variety of compounds necessary for iron uptake in the shoot. Once the ferric iron reaches the shoot apoplast, it is thought to be reduced, probably by a member of the FRO (ferric reductase oxidase) family. FRD3's substrate could be involved in presenting iron to the reductase. After reduction, the ferrous iron could be transported inside the leaf cells either on its own, perhaps by a member of the ZIP (Zrt- Irt- like protein) family (Rogers and Guerinot, 2002b
Arabidopsis Lines and Growth Conditions
The Arabidopsis mutants frd3-1 and frd3-3 and the corresponding wild type Columbia gl-1 have been described previously (Rogers and Guerinot, 2002a
Micrografting of Arabidopsis seedlings was performed as described (Turnbull et al., 2002
Arabidopsis seedlings were grown on Gamborg's B5 medium (Caisson Labs, Sugar City, ID) for approximately 2 weeks. The hypocotyl was cut with a sharp razor blade and shoot was removed. The root was immediately transferred to iron sufficient or iron deficient media prepared as described except for the addition of 20 g/L Suc. After 5 d, ferric chelate reductase activity was assayed as described (Yi and Guerinot, 1996
Chlorophyll was extracted in methanol and absorbance measured at 652, 665, and 750 nm. Total chlorophyll concentration was calculated as described (Porra et al., 1989
Immunoblots were performed as previously described (Connolly et al., 2002
Total protein (10 µg) was separated by SDS-PAGE (Laemmli, 1970
Arabidopsis protoplasts were isolated as previously described (Fitzpatrick and Keegstra, 2001
Arabidopsis roots were treated with Perls' stain according to established histological methods for mammalian tissues. Briefly, equal amounts of solutions of 4% (v/v) HCl and 4% (w/v) potassium ferrocyanide were mixed immediately prior to use. The stain solution was vacuum infiltrated into 6- or 7-d-old Arabidopsis seedlings for approximately 15 min. Seedlings were rinsed in water and Perls' staining was observed immediately in whole roots. For cross-sections, after staining seedlings were fixed in 4% (w/v) paraformaldehyde and imbedded in 4% (w/v) low-melt agarose. Approximately 70-µm sections were cut with a Lancer series 1000 Vibratome (Vibratome, St. Louis).
The FRD3 genomic sequence cloned into pCAMBIA2300 has been previously described (Rogers and Guerinot, 2002a
GFP fluorescence was visualized in living whole-mount roots using a Bio-Rad (Hercules, CA) Radiance 2000 confocal system coupled to an Olympus IX70 inverted microscope at the University of Missouri Molecular Cytology Core facility. Roots from FRD3-FLAG transgenics were fixed, embeded in methacrylate, sectioned, and stained for immunofluorescence as previously described (Baskin and Wilson, 1997 Upon request, all novel materials described in this publication will be made available in a timely manner for noncommercial research purposes.
The authors thank Tobias Baskin and Jan Judy-March for help with microscopy and immunofluorescence, Amanda Crawford and Dave Robertson for the ICP-MS analysis, Fritz Bienfait for helpful suggestions, and David Eide, Dirk Charlson, and Mary Lou Guerinot for critical reading of the manuscript. Received April 30, 2004; returned for revision May 25, 2004; accepted May 30, 2004.
1 This work was supported by an MU Research Board grant and a USDA CSREES grant (2002–35100–12331 to E.E.R.). 
2 Present address: Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst, MA 01003. Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.045633. * Corresponding author; e-mail rogersee{at}missouri.edu; fax 573–882–0185.
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