FRD3 Controls Iron Localization in Arabidopsis

Abstract

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). These data imply there is an iron-requiring step between Mg-protoporphyrin IX monomethyl ester and protochlorophyllide (Spiller et al., 1982). Recently, CHL27...

Iron deficiency-mediated stress regulation of four subgroup Ib BHLH genes in Arabidopsis thaliana

“…Such functions could involve import or export of iron from the vascular system. In this respect it is noteworthy that a number of genes involved in aspects of iron transport were found expressed near the vasculature suggesting that these tissues are indeed involved in iron homeostasis (for example Bereczky et al 2003;Green and Rogers 2004;Jakoby et al 2004;Jean et al 2005).…”

Transition metal transport

“…AtFRD3 is expressed in the root pericycle and operates as an effluxer of citrate, a low molecular weight chelator molecule [61]. Thus, AtFRD3 is needed to maintain root-to-shoot Fe mobility [12,61]. It is possible that in A. halleri and T. caerulescens, FRD3 contributes to Fe homeostasis and, in particular, to Fe mobility in the presence of xylem Zn concentrations that are likely to be substantially higher than in non-accumulator species like A. thaliana [62].…”

“…Other membrane protein families that have been implicated in transition metal transport are the cation exchanger (CAX) family and three subfamilies of ATP-binding cassette (ABC) transporters, the multidrug resistance-associated proteins (MRP), the ABC transporters of the mitochondria (ATM) and the pleiotropic drug resistance (PDR) transporters [10]. Finally, putative transporters of metal ion ligands, the AtFRD3 (Ferric Reductase Defective 3) of the large multidrug and toxin efflux (MATE) family and AtZIF1 (Zinc Induced Facilitator 1) of the major facilitator superfamily (MFS), have key roles in iron and Zn homeostasis, respectively [11][12][13]. These plant metal transporter families and their biological roles have been reviewed by other authors recently [2,[14][15][16][17][18][19][20][21] (review Iron Transport, this issue).…”

The roles of organic anion permeases in aluminium resistance and mineral nutrition

“…The FRD3 gene isolated by map-based cloning encodes a MATE [30]. A role for the FRD3 protein in the long distance transport of Fe in the xylem was inferred from elegant experiments involving reciprocal grafts between roots and shoots of wild type and mutant plants [31]. It became apparent that instead of transporting Fe directly, the function of FRD3 Fig.…”