Iron is one of the most important nutrients for all eukaryotes. However, it is also one of the most dangerous elements. Because of its redox properties, iron is a critical element for such basic processes as DNA and hormone synthesis, respiration and photosynthesis (Briat et al. 1995). Although iron is the fourth most abundant element in the Earth crust, it is not easily available. This is because of low solubility of iron-containing minerals, especially in aerobic and neutral pH environments (Guerinot, Yi 1994). In order to cope with this problem, plants have developed several mechanisms of iron acquisition. Except for morphological changes leading to the extension of active root area, these mechanisms include proton pumping (acidification), secretion of organic acids and phenolics (chelation) and induction of the membrane bound reductase (dicots and nongraminaceous monocots). In addition, high-affinity chelators are used to dissolve Fe(III) oxides (Fox, Guerinot 1998; Hinsinger 1998; Jones 1998; Marschner, Rornheld 1996; Schmidt 1999; Thoiron et al. 1997). However, in the presence of active oxygen species the same desirable iron may catalyze the generation of hydroxyl radicals (OH~.) - the most powerful oxidizing agents known thus far (Fentonreaction) (Cadenas 1989; Meneghini et al. 1995; Nappi, Vass 2000). Attack of toxic oxygen species usually leads to severe results like lipid peroxidation, protein and DNA oxidation and eventually cell disintegration. Plant antioxidant defenses include such compounds like glutathione, ascorbate, carotenoids, tocopherols, etc. Antioxidant enzymes like catalase, peroxidases, dismutases and enzymes of the ascorbate-glutathione cycle are also activated (Becana et al. 1998; Larson 1995). It is obvious that there has to be a strict control of "free" iron in the cell, just to prevent generation of reactive oxygen species (ROS) in the first place. Limited generation of ROS, however, may be a part of plant defense systems against pathogens (Wojtaszek 1997). Predominant portion of cell iron is bound in proteins and enzyme cofactors, but it may easily be "set free" under stress situations.
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