All plants, except grasses, use Strategy I to acquire Fe by three reactions: (i) initial excretion of protons carried out by a plasma membrane P-type ATPase to acidify the surrounding rhizosphere therefore increasing Fe3+ solubility; (ii) reduction of Fe3+ to Fe2+ completed by a Fe3+-chelate reductase; and finally (iii) transport of Fe2+ by transporters across the plasmalemma of root epidermal cells (Hell and Stephan, 2003; Fox and Guerinot, 1998). It has been shown that both the Fe3+-chelate reductase and Fe2+-transport activities are enhanced under Fe deficiency (Connolly et al., 2002; Fox and Guerinot, 1998; Robinson et al., 1999; Vert et al., 2002).
In Fe acquisition, genes encoding for Fe3+-chelate reductase have been cloned from A. thaliana, FRO2 (Robinson et al., 1999) and pea, FRO1 (Waters et al., 2002). Both research groups reported the two genes are up-regulated in roots in response to Fe deficiency. Moreover, A. thaliana lines with an inactivated FRO2 gene show reduced Fe3+-chelate-reductase activity during both Fe-sufficient and Fe-deficient conditions with chlorotic leaves and plant growth impaired on Fe-deficient medium (Robinson et al., 1999).
Once the Fe has been reduced at the root, the next step is transport into the plant. The Fe-transporter genes, IRT1 and IRT2, have been cloned from Arabidopsis (Eide et al., 1996; Vert et al., 2001) as well as its orthologs from pea (RIT1) (Cohen et al., 1998) and tomato (IRT1 and IRT2) (Eckhardt et al., 2001). The Arabidopsis gene IRT1 has been established as the major Fe-uptake component for acquisition of Fe from soil (Vert et al., 2002). In addition, Arabidopsis IRT1 also transports zinc, manganese, and cadmium (Fox and Guerinot, 1998; Guerinot, 2000; Korshunova et al., 1999). Arabidopsis IRT2 is also induced in roots of Arabidopsis in response to Fe deficiency (Eide et al., 1996; Vert et al., 2001). Transporters involved in Fe, and other metals have been identified as belonging to the ZIP family of transporters (Eng et al., 1998; Fox and Guerinot, 1998; Guerinot, 2000). Other Fe transporter genes (Nramp) have also been cloned from A. thaliana (Curie et al., 2000; Lanquar et al., 2004; Thomine et al., 2000) that share important similarity with their mammalian counterparts. Although their role is not yet completely clear, at least NRAMP 1, 3, and 4 seem to be induced by Fe deficiency, and are expressed in roots and leaves of A. thaliana. It is thought they might contribute to Fe homoestasis in addition to IRT transporters (Curie et al., 2000; Thomine et al., 2000). An additional transporter gene, FRD3, has been recently identified in A. thaliana (Rogers and Guerinot, 2002). The authors hypothesize that FDR3 is involved in transporting a low-molecular weight iron chelator into the xylem. The reader is referred to Chapter 16 on FRD3's role in iron homeostasis.
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