Trends in Plant Science
ReviewMolecular mechanisms governing Arabidopsis iron uptake
Section snippets
Irony of Fe availability
Owing to their sessile nature, plants use complex cellular and developmental strategies to adapt to their changing environment. The acquisition of soil nutrients, such as metal ions, is important for plant survival. Among these, Fe has special importance because of its capability to change redox states, making it an indispensable cofactor responsible for the function of electron transport chains and catalytic processes 1, 2. However, Fe overaccumulation may lead to the overproduction of
Strategy I Fe acquisition
Arabidopsis acquires Fe in a three-step process and depends on the action of plasma membrane (PM) proteins present in the cells of the root epidermis (Figure 1A). Under Fe deficiency, the solubilization of Fe is mainly mediated by H+-ATPase AHA2-mediated proton extrusion, which results in local rhizosphere acidification [8]. Solubilized Fe may freely enter the apoplast (the cell-wall space of the outer root cell layers); however, the next steps in the Fe acquisition process are greatly
Transcriptional regulation of Fe acquisition
Expression of the genes encoding the major strategy I Fe-acquisition proteins is upregulated under conditions of Fe deficiency, making transcription a key regulatory factor for the performance of the whole system. The central transcriptional regulator of Fe uptake is the basic helix–loop–helix (bHLH) protein FIT. FIT was identified based on its homology to the previously identified tomato (Solanum lycopersicum) protein FER 21, 22, 23. Under Fe deficiency, FIT was shown to be essential for the
Protein networks regulating Fe-dependent transcription
There is abundant evidence showing that the Fe-uptake system receives regulatory inputs from various plant signaling pathways that modulate its activity (Box 1). As a central element in the transcriptional regulation of Fe uptake, FIT is a target for many of these pathways. The production of nitric oxide (NO) is essential for upregulating FIT gene expression 38, 39 (Figure 1B) as well as the tomato homolog FER [40]. GROWTH REGULATING FACTOR 11 (GRF11), which encodes a 14-3-3 protein, is part of
Analysis of IRT1 gene expression: going beyond Fe uptake
Being the main Fe importer and owing to its affinity for other divalent metals 59, 60, 61, IRT1 is subject to tight regulation at the transcriptional and post-transcriptional levels. As discussed above, under conditions of Fe limitation the activity of the FIT regulatory system leads to increased IRT1 gene expression. Interestingly, the presence of low numbers of IRT1 transcripts in fit mutants 26, 62 suggests that the basic IRT1 expression is also under the control of other transcriptional
IRT1 trafficking: cellular events involved in Fe import into roots
Characterization of Fe-uptake components and regulators led to the conclusion that the strategy I transcriptional machinery was not sufficient to control Fe uptake on its own. Many post-transcriptional events are responsible for regulating the activity, localization, and stability of the major Fe-uptake components in various organisms 25, 28, 79, 80, 81, 82. As a transmembrane protein functioning at the PM, it is surprising to find that IRT1 localizes predominantly to intracellular compartments
Concluding remarks and future outlook
The findings reported by recent studies have revealed new avenues for further exploration of the Fe-uptake mechanism and have raised many questions that require further research. One challenge will be to identify novel transcriptional regulators acting in complex with or upstream of FIT, to bridge the gap in our understanding between the known inputs into Fe-dependent transcription and the specific signaling pathways involved, as has been done for the FIT–EIN3/EIL1 interaction [42]. In
Acknowledgments
The authors thank the members of the Institute of Botany at Heinrich-Heine University for their valuable comments on the manuscript.
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