Iron is an essential micronutrient with numerous cellular functions, and its deficiency represents one of the most serious problems in human nutrition worldwide. Plants have two major problems with iron as a free ion: its insolubility and its toxicity. To ensure iron acquisition from soil and to avoid iron excess in the cells, uptake and homeostasis are tightly controlled. Plants meet the extreme insolubility of oxidized iron at neutral pH values by deficiency-inducible chelation and reduction systems at the root surface that facilitate uptake. Inside the cells the generation of highly toxic hydroxyl radicals by iron redox changes is avoided by intricate chelation mechanisms. Organic acids, most notably nicotianamine, and specialized proteins bind iron before it can be inserted into target molecules for biological function. Uptake and trafficking of iron throughout the plant is therefore a highly integrated process of membrane transport and reduction, trafficking between chelator species, whole-plant allocation and genetic regulation. The improvement of crop plants with respect to iron efficiency on iron-limiting soils and to iron fortification for human nutrition has been initiated by breeding and biotechnology. These efforts have to consider molecular and physiological evidence to overcome the inherent barriers and problems of iron metabolism.

The transport of metal micronutrients to developing organs in a plant is mediated primarily by the sieve elements. Ligands are thought to form complexes with the free ions in order to prevent cellular damage, but no binding partners have been unequivocally identified from plants so far. This study has used the phloemmediated transport of micronutrients during the germination of the castor bean seedling to identify an iron transport protein (ITP

A mutation in the Arabidopsis gene STARIK leads to dwarfism and chlorosis of plants with an altered morphology of leaf and cell nuclei. We show that the STARIK gene encodes the mitochondrial ABC transporter Sta1 that belongs to a subfamily of Arabidopsis half-ABC transporters. The severity of the starik phenotype is suppressed by the ectopic expression of the STA2 homolog; thus, Sta1 function is partially redundant. Sta1 supports the maturation of cytosolic Fe/S protein in ⌬ atm1 yeast, substituting for the ABC transporter Atm1p. Similar to Atm1p-deficient yeast, mitochondria of the starik mutant accumulated more nonheme, nonprotein iron than did wild-type organelles. We further show that plant mitochondria contain a putative L -cysteine desulfurase. Taken together, our results suggest that plant mitochondria possess an evolutionarily conserved Fe/S cluster biosynthesis pathway, which is linked to the intracellular iron homeostasis by the function of Atm1p-like ABC transporters. INTRODUCTIONABC transporters constitute one of the largest protein families with diverse functions in membrane transport. The designation of ABC transporter recognizes a highly conserved ATP binding cassette, which is the most characteristic feature of this superfamily. These proteins mediate the relatively specific, active transmembrane transport of molecules that can range in size from small ions to proteins (reviewed in Higgins, 1992) and utilize the energy of ATP hydrolysis to pump substrates across membranes, usually against a concentration gradient. The typical ABC transporter consists of four domains. Two of these domains are hydrophobic, and each comprises six membrane-spanning segments. The transmembrane domains are believed to form a channel and to determine the specificity of the transporter. The other two domains are located at the periphery of the membrane and couple ATP hydrolysis to the transport process.The ABC transporter Atm1p of budding yeast mitochondria represents a "half-transporter" in which one transmembrane and one ATP binding domain are expressed in a single polypeptide (Leighton and Schatz, 1995). Atm1p is localized in the mitochondrial inner membrane and is believed to function as an exporter, because its ABC domains face the mitochondrial matrix. The ATM1 loss-of-function results in respiration-deficient mitochondria that lack cytochromes (Leighton and Schatz, 1995; Kispal et al., 1997) and that tend to loose their DNA (Leighton and Schatz, 1995). These pleiotropic effects of the Atm1p deficiency are generally attributed to the fact that in ⌬ atm1 cells, mitochondria accumulate up to 30-fold higher levels of iron than do wild-type organelles (Kispal et al., 1997;Mitsuhashi et al., 2000). The Atm1p transport function appears to be conserved in eukaryotes, and the human mitochondrial ABC transporters Abc7 and MTABC3 have been shown to be true functional orthologs of the Atm1p (Csere et al., 1998;Mitsuhashi et al., 2000). The human ABC7 gene has been implicated in hereditary X-linked sideroblastic anemia and a...

Basic cellular processes such as electron transport in photosynthesis and respiration require the precise control of iron homeostasis. To mobilize iron, plants have evolved at least two different strategies. The nonproteinogenous amino acid nicotianamine which is synthesized from three molecules of S-adenosyl-l-methionine, is an essential component of both pathways. This compound is missing in the tomato mutant chloronerva, which exhibits severe defects in the regulation of iron metabolism. We report the purification and partial characterization of the nicotianamine synthase from barley roots as well as the cloning of two corresponding gene sequences. The function of the gene sequence has been verified by overexpression in Escherichia coli. Further confirmation comes from reduction of the nicotianamine content and the exhibition of a chloronerva-like phenotype due to the expression of heterologous antisense constructs in transgenic tobacco plants. The native enzyme with an apparent M r of < 105 000 probably represents a trimer of S-adenosyl-l-methionine-binding subunits. A comparison with the recently cloned chloronerva gene of tomato reveals striking sequence homology, providing support for the suggestion that the destruction of the nicotianamine synthase encoding gene is the molecular basis of the tomato mutation.Keywords: antisense constructs; chloronerva mutation; gene isolation; Hordeum vulgare; iron metabolism.Iron is essential for fundamental cellular processes such as electron transfer in photosynthesis, respiration, nitrogen fixation as well as DNA synthesis [1]. Excessive accumulation causes severe damage to cellular components due to the formation of highly reactive hydroxyl radicals by the Fenton reaction [2]. Thus, the precise control of iron homeostasis is a basic prerequisite for cellular function. According to WHO data the health of more than three billion people worldwide is affected by iron deficient diet. Crop plants with a higher iron content, for example in the endosperm of cereals, could contribute to the improvement of this situation. In soil iron is mainly found as stable Fe(III) compounds with low solubility at neutral pH [1,3]. Therefore, plants have evolved special mechanisms of iron acquisition, classified into two strategies [4]. Strategy I plants, including dicots and nongraminaceous monocots, facilitate iron uptake mainly by increased acidification of the rhizosphere due to enhanced proton extrusion and the reduction of Fe(III) to Fe(II) by an inducible plasma membrane-bound reductase. In contrast, graminaceous monocots (strategy II plants) release phytosiderophores of the mugineic acid family into the rhizosphere. These compounds act as chelators of ferric ions and are taken up by root cells as Fe(III)-phytosiderophore complexes.The nonproteinogenous amino acid nicotianamine (NA) is found in all multicellular plants [5] and is considered to be a key component for both strategies of iron acquisition (Fig.1). In strategy I plants NA might function as a chelator of iron in symplastic...