The translocation of Fe and other minerals inside the plant body involves a sequence of processes that require various metal chelators and transporters (Clemens et al., 2002; Hell and Stephan, 2003). The rice genome putatively contains 18 OsYSL genes, 13 OsZIP genes, and eight OsNRAMP genes (Gross et al., 2003; Koike et al., 2004). The natural resistance-associated macrophage protein (NRAMP) family transporters are found in bacteria, fungi, plants, and animals; some of them have been shown to function as transporters of various cations including Fe(II), Mn(II), Cd(II), and Zn(II) (Gunshin et al., 1997; Chen et al., 1999).
Among the 18 OsYSL genes, OsYSL2 is expressed predominantly in Fe-deficient leaves (Koike et al., 2004). The OsYSL2-GFP fusion protein was localized in the plasma membrane of onion epidermal cells (Figure 20-7A). Electrophysiological analysis in Xenopus oocytes showed that OsYSL2 transported Fe(II)-NA and Mn(II)-NA, but did not transport Fe(III)-MAs. An analysis using transgenic rice containing the OsYSL2 promoter-GLS construct revealed that OsYSL2 was expressed in root companion cells (Figure 20-7C, D) and phloem cells of the leaves and leaf sheaths (Figure 20-7E-H). Fe deficiency induced the expression of OsYSL2 in all leaf tissues (Figure 20-7H). Strong expression of OsYSL2 was also observed in the vascular bundles of flowers and in developing seeds (Figure 20-7I-N). These results suggest that OsYSL2 functions as an Fe(II)-NA transporter responsible for the phloem transport of Fe, including the translocation of Fe into the grain (Koike et al., 2004). The characterization of the other OsYSL members is currently in progress.
The involvement of graminaceous ZIP and NRAMP genes in Fe translocation has not been well characterized. Ramesh et al. (2003) demonstrated that two of the OsZIP members, OsZIP1 and OsZIP3, possess a Zn(II)-transporting capacity. Graminaceous ZIP genes are expected to have diverse functions in the translocation of metals, including Fe(II) and Zn(II). Among the Arabidopsis and rice homologs of the NRAMP family genes, AtNRAMPl, AtNRAMP3, AtNRAMP4, and OsNRAMPl were reported to be able to complement yeast mutants defective in Fe uptake (Curie et al., 2000; Thomine et al., 2000). AtNRAMP3 was shown to be localized in the vacuolar membrane and was proposed to mobilize vacuolar metal pools to the cytosol (Thomine et al., 2003). Similarly, graminaceous NRAMP genes are expected to be involved in subcellular compartmentalization of metals, including Fe and Mn.
Additionally, a function of homologs of the vertebrate IREG1 Fe exporter has been suggested in plant Fe homeostasis (Honsbein et al., 2004). IDI7, which is an Fe-deficiency-responsive gene in barley roots that encodes a tonoplast-located ABC-type transporter, may also be involved in Fe translocation (Yamaguchi et al., 2002).
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