Differential Regulation of nramp and irt Metal Transporter Genes in Wild Type and Iron Uptake Mutants of Tomato

Abstract

Metal transporters regulated by iron can transport a variety of divalent metals, suggesting that iron regulation is important for specificity of iron transport. In plants, the iron-regulated broad-range metal transporter IRT1 is required for uptake of iron into the root epidermis. Functions of other iron-regulated plant metal transporters are not yet established. To deduce novel plant iron transport functions we studied the regulation of four tomato metal transporter genes belonging to the nramp and irt families with respect to environmental and genetic factors influencing iron uptake. We isolated Lenramp1 and Lenramp3 from tomato and demonstrate that these genes encode functional NRAMP metal transporters in yeast, where they were iron-regulated and localized mainly to intracellular vesicles. Lenramp1 and Leirt1 revealed both root-specific expression and up-regulation by iron deficiency, respectively, in contrast to Leirt2 and Lenramp3. Lenramp1 and Leirt1, but not Lenramp3 and Leirt2, were down-regulated in the roots of fer mutant plants deficient in a bHLH gene regulating iron uptake. In chloronerva mutant plants lacking the functional enzyme for synthesis of the plant-specific metal chelator nicotianamine Leirt1 and Lenramp1 were up-regulated despite sufficient iron supply independent of a functional fer gene. Lenramp1 was expressed in the vascular root parenchyma in a similar cellular pattern as the fer gene. However, the fer gene was not sufficient for inducing Lenramp1 and Leirt1 when ectopically expressed. Based on our results, we suggest a novel function for NRAMP1 in mobilizing iron in the vascular parenchyma upon iron deficiency in plants. We discuss fer/nicotianamine synthase-dependent and -independent regulatory pathways for metal transporter gene regulation.

The Essential Basic Helix-Loop-Helix Protein FIT1 Is Required for the Iron Deficiency Response

“…This is in contrast with reports that Leirt1 mRNA abundance is dependent on the fer gene (Ling et al, 2002;Bereczky et al, 2003). The FIT1 and fer genes share similarities and differences in terms of their mRNA localization pattern.…”

Iron deficiency-mediated stress regulation of four subgroup Ib BHLH genes in Arabidopsis thaliana

“…Such functions could involve import or export of iron from the vascular system. In this respect it is noteworthy that a number of genes involved in aspects of iron transport were found expressed near the vasculature suggesting that these tissues are indeed involved in iron homeostasis (for example Bereczky et al 2003;Green and Rogers 2004;Jakoby et al 2004;Jean et al 2005).…”

“…Then the plants were incubated in 100% ethanol for de-staining, and kept in 70% ethanol before observation. Ten micro meter sections were obtained after embedding into paraplast as described in Bereczky et al (2003). Fluorimetric GUS activity tests were performed on protein extracts using 2 mM 4-methylumbeliferyl--D-glucuronide as substrate as described by Jakoby et al (2004).…”

“…FER and FIT share higher sequence similarity with each other rather than with any other known bHLH protein from the two species. FER is required for induction of iron reductase and iron transporter genes in tomato (Ling et al 2002;Bereczky et al 2003;Bauer et al 2004;Li et al 2004). FIT was found essential for the induction of FRO2 upon iron deWciency in Arabidopsis (Colangelo and Guerinot 2004;Jakoby et al 2004) as well as for induction of IRT1 (Jakoby et al 2004).…”

Enhanced plant tolerance to iron starvation by functional substitution of chloroplast ferredoxin with a bacterial flavodoxin

“…Most of them rely on optimization of metal uptake from scarcely available sources. Dicotyledoneous plants and nongraminaceous monocots display the so-called strategy I response, characterized by development of root hairs and transfer cells, increased proton extrusion to the rizosphere to improve Fe 3ϩ solubility, and enhanced accumulation of ferric-chelate reductases and broad-range metal transporters in the root cell plasma membrane to favor Fe 2ϩ intake (14)(15)(16)(17)(18). Grasses, on the other hand, have developed a different response, releasing phytosiderophores of the mugineic acid family into the surrounding soil (5).…”