Iron in Plants

Abstract

Iron (Fe) is a universal nutritional requirement for virtually all organisms, functional as an electron carrier in respiration and photosynthesis, in the production and detoxification of oxygen radicals, oxygen transport and numerous reduction and monooxygenase reactions. Depending on the redox potential of the environment, Fe occurs in two stable oxidation states, Fe3+ and Fe2+ that differ dramatically in their solubility in water. Plants have evolved two distinct, phylogenetically separate mechanisms driven by an increasing abundance of photosynthesis‐derived atmospheric oxygen, which makes Fe unavailable due to the formation of highly insoluble ferric oxides. Excess Fe is toxic to cells. Therefore, cellular Fe concentrations are tightly regulated by sophisticated mechanisms that control acquisition, distribution and utilisation of Fe. A large proportion of the world's arable land has soil properties that do not allow the uptake of sufficient Fe for optimal plant growth and yield, making an understanding of the mechanisms that control cellular homoeostasis mandatory for the development of Fe‐efficient germplasms.

Key Concepts:

  • With only few exceptions Fe is a universal nutritional requirement by all organisms.

  • Iron occurs in two stable oxidation states, Fe3+ and Fe2+, that differ dramatically in their solubility in water.

  • A large proportion of the world's arable land has soil properties that do not allow the uptake of sufficient Fe for optimal plant growth and yield.

  • Plants have evolved two distinct, phylogenetically separated mechanisms for Fe uptake.

  • The evolution of Fe uptake mechanisms is likely driven by the increasing concentration of photosynthesis‐derived atmospheric oxygen.

  • Cellular Fe concentrations are tightly regulated by sophisticated mechanisms that control acquisition, distribution and utilisation of Fe.

  • The primary sensor for Fe deficiency has yet to be identified.

Keywords: iron acquisition; iron homoeostasis; iron anaemia; malnutrition; rhizosphere; chelators; metalloproteins

Figure 1.

Iron acquisition by plants. Proteins mediating reduction‐based Fe uptake (Strategy I) shown in the left part of the figure are derived from studies using Arabidopsis, whereas those for the chelation‐based Fe uptake (Strategy II) were identified in experiments with rice. Arrows denote the various steps in the phenylpropanoid (a) and PS synthesis (b) pathways, leading to the production of phenolics and mugineic acid (MA)‐type PSs that are secreted into the rhizosphere to mobilise Fe hydroxides in the soil.

Figure 2.

Intracellular and long‐distance transport of Fe. Iron is stored in the vacuole (V), bound to nicotianamine (NA). Import and export of Fe is mediated by the transporters with homology to the yeast Fe/Mn transporter CCC1p, ferroportin (FPN2) and by two NRAMP transporters. For transport across the inner membrane of the chloroplast (C) by PIC1, reduction of Fe(III) may be required.

Figure 3.

Regulation of Fe acquisition. In Strategy I plants (a), a set of Fe‐responsive genes including FRO2, IRT1 and AHA2 are positively regulated by a FIT‐bHLH38/39/100/101 heterodimer. FIT is stabilised by interacting with the ethylene signalling components EIN3 and EIL1. Another heterodimer, PYE‐ILR3/bHLH115, negatively controls a nonoverlapping set of genes, including ZIF1, FRO3 and NAS4. Presumably, PYE competes with BTS for binding to ILR3/bHLH115. The BTS‐ILR3/bHLH115 complex positively regulates the same set of genes, probably to fine‐tune gene activity. PYE also represses the expression of BTS. In Strategy II plants (b), the transcription factors IDEF1 and IDEF2 synergistically regulate the expression of Fe‐responsive genes by binding to their cis‐consensus sequences, IDE1 and IDE2, including the bHLH transcription factor IRO2. IRO2 is induced by Fe deficiency and regulates several Strategy II‐related genes, including IRT1, NAS1 and NAS3. Another bHLH transcription factor, IRO3, negatively affects the expression of this subset of genes. IRO3 also negatively affects the expression of IRO2. Green boxes represent promoters, blue boxes denote genes. Arrows labelled with Fe indicate potential Fe‐sensing sites.

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Further Reading

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Tanaka R, Kobayashi K and Masuda T (2011) Tetrapyrrole metabolism in Arabidopsis thaliana. Arabidopsis Book 9: e0145.

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Buckhout, Thomas J, and Schmidt, Wolfgang(Sep 2013) Iron in Plants. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0023713]