Grasses and important food plants such as rice (Oryza sativa) and maize (Zea mays) use Strategy II for Fe acquisition. Roots release (i) phytosidero-phores (PSs) that chelate Fe3+ in the rhizosphere, and (ii) specific plasmalemma transporter proteins import the Fe3+-PS complexes into the plant (Romheld and Marschner, 1986). Both processes are enhanced in response to Fe deficiency via up-regulation of the underlying genes (Hell and Stephan, 2003) indicating their importance in Fe acquisition.
Most of the underlying genes for Strategy II have been cloned from barley and rice, and Mori has scholarly reviewed the topic (1999). Fe chelation was recognised when plant PSs were first identified by Takagi in 1976 (Mori et al., 1991). PSs belong to the mugineic acid (MA) family of chelators, solubilizing inorganic Fe3+ compounds by chelation, and the Fe-PS complexes are taken into the root cells by a specific transport system of the plasma membrane (Mori, 1999). Biosynthesis of MA starts from three molecules of methionine from the Yang cycle that are integrated into one molecule of nicotianamine (NA) in an enzymatic step catalyzed by nicotianamine synthase (Hell and Stephan, 2003). NA synthase has been cloned from tomato, barley, and rice; and its expression in roots is strongly up-regulated by Fe availability. Formation of MA and MA derivatives, as well as the Strategy II mechanism, has been summarized by Hell and Stephan (2003).
A Fe-PS transporter has been identified in Strategy II plants. Uptake of Fe3+-PS complex has been clarified by Curie et al. (2001) who cloned the mutant gene of the transport-defective ys1 (yellow stripel) maize mutant. The wild type YS1 gene complements this maize mutant and is up-regulated in roots and shoots. In addition, Roberts et al. (2004) have shown in yeast cells defective in Fe transport that maize YS1 facilitates the transport of Fe3+-MA; however, YS1 also transports Fe2+-NA. Due to the ubiquitous distribution of NA in the plant kingdom and across Fe-acquisition strategies, NA has been suggested as a substrate for Fe transport within the plant. This hypothesis has been further supported by the identification of eight YS1 -like genes in the MA-free dicot A. thaliana (Curie et al., 2001) and YS1-like genes identified from ESTs (expressed sequence tags) of soybean, Medicago, potato, and tomato (Charlson and Shoemaker, unpublished data). Several YSl-like transporters in Arabidopsis have been shown to transport Fe-NA complexes, suggesting that NA serves as a ligand for the transport of Fe and possibly other metals (E. Walker, University of Massachusetts; personal communication cited by Hell and Stephan, 2003).
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