Revisiting the Metal-Binding Chemistry of Nicotianamine and 2′-Deoxymugineic Acid. Implications for Iron Nutrition in Strategy II Plants

Recently, von Wirén et al. (1999) published a study of the metal-binding properties of two ligands important for Fe physiology in higher plants, nicotianamine (NA) and 2′-deoxymugineic acid (DMA; Fig. 1). However, they failed to address two key aspects of the Fe ligand chemistry that we will address here, and that have important implications for Fe physiology and transport in plants, particularly “Strategy II” species (Ma and Nomoto, 1996). ... NA is ubiquitous throughout the plant kingdom (Scholz et al., 1992). It acts as a general internal chelator rather than by stereospecific binding (Ripperger et al., 1982), and has been implicated in cellular Fe homeostasis and regulation (Liu et al., 1998; Pich et al., 2001). Although most evidence argues against a major role for NA in long-distance transport of Fe within plants, NA appears to be important for Cu translocation in Strategy I species (Pich et al., 1994). In Strategy II species, NA has been implicated as a possible precursor in the synthesis of phytosiderophores, but conclusive evidence is lacking (Mori, 1994). Deoxymugineic acid is representative of the family of phytosiderophores that are excreted by Strategy II species into the rhizosphere in response to Fe deficiency (Ma and Nomoto, 1996). Phytosiderophores increase the solubility of FeIII by chelation, and are absorbed as the intact FeIII ligand by Strategy II plants (Romheld and Marschner, 1986; Grusak et al., 1999).