Genes encoding the ironIIIMAs transporter

Early studies proposed the presence of a specific transport system for the Fe(III)-MAs complex in the plasma membrane of root cells in graminaceous plants (Romheld and Marschner, 1986; Mihashi and Mori, 1989). Efforts to isolate the corresponding transporter gene by functional complementation of yeast mutants were not successful. The maize yellow stripe 1 (ys1) mutant is defective in Fe(III)-MAs uptake (von Wiren et al., 1994); therefore, YS1 has been suggested to be the Fe(III)-MAs transporter. Molecular cloning of the

Root cell

YSl gene was performed by Curie et al. (2001) using a transposon-tagged population. A yeast strain defective in Fe uptake was complemented by the expression of YSl when supplied with Fe(III)-DMA. YSl expression in maize is increased in both roots and shoots under conditions of Fe deficiency, but is not strongly affected by Zn or Cu deficiency (Curie et al., 2001; Roberts et al., 2004). Schaff et al. (2004) investigated the precise transporting properties of YS1 in Xenopus oocytes by electrophysiological analysis. YS1 functions as a proton-coupled symporter for various DMA-bound metals including Fe(III), Zn(II) Cu(II), and Ni(II). YS1 also transports NA-chelated Ni(II), Fe(II), and Fe(III) complexes.

OsYSL3 OsYSL4

Figure 20-6. Unrooted phylogenic tree for ZmYSl and 18 OsYSL amino acid sequences (original figure: Koike et al., 2004).

OsYSL3 OsYSL4

Figure 20-6. Unrooted phylogenic tree for ZmYSl and 18 OsYSL amino acid sequences (original figure: Koike et al., 2004).

Our search for YSl homologs in the rice genome database identified 18 putative OsYSL (Oryza sativa YSl-like) genes (Figure 20-6; Koike et al., 2004). Northern analysis detected the transcripts of OsYSL2, 6, l3, l4, l5, and l6 in roots or leaves of Fe-deficient or Fe-sufficient rice plants. Among these, the expression of OsYSLl5 and l6 was induced in Fe-deficient roots, suggesting the possibility that these genes play a role in Fe-MAs uptake from the rhizosphere. We are currently analyzing the transporting properties of these OsYSL genes.

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