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Review
. 2021 Mar 17;72(6):2045-2055.
doi: 10.1093/jxb/erab003.

All together now: regulation of the iron deficiency response

Nabila Riaz  1 , Mary Lou Guerinot  1
Affiliations
Review

All together now: regulation of the iron deficiency response

Nabila Riaz et al. J Exp Bot. .

Abstract

Iron (Fe) is one of the essential micronutrients required by both plants and animals. In humans, Fe deficiency causes anemia, the most prevalent nutritional disorder. Most people rely on plant-based foods as their major Fe source, but plants are a poor source of dietary Fe. Therefore, there is a critical need to better understand the mechanisms involved in the uptake and trafficking of Fe and how plants adapt to Fe deficiency. Fe participates in key cellular functions such as photosynthesis and respiration. Perturbations of Fe uptake, transport, or storage affect plant growth as well as crop yield and plant product quality. Excess Fe has toxic effects due to its high redox activity. Plants, therefore, tightly regulate Fe uptake, distribution, and allocation. Here, we review the regulatory mechanisms involved at the transcriptional and post-translational levels that are critical to prevent Fe uptake except when plants experience Fe deficiency. We discuss the key regulatory network of basic helix-loop-helix (bHLH) transcription factors, including FIT, subgroup Ib, subgroup IVc, and URI (bHLH121), crucial for regulating Fe uptake in Arabidopsis thaliana. Furthermore, we describe the regulators of these transcription factors that either activate or inhibit their function, ensuring optimal Fe uptake that is essential for plant growth.

Keywords: Arabidopsis; E3 ligases; bHLH transcription factors; iron deficiency; iron homeostasis; iron uptake.

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Figures

Fig. 1.
Fig. 1.
Fe uptake mechanism in Arabidopsis thaliana. Arabidopsis employs a three-step reduction-based strategy to facilitate Fe uptake from the environment. First, insoluble Fe3+ is solubilized by acidification of the rhizosphere by AHA2, a proton ATPase. Pleiotropic drug resistance 9 (PDR9) protein exports phenolics which chelate Fe3+. The second step involves the reduction of solubilized chelated Fe3+ to Fe2+ by the ferric chelate reductase FRO2. The last step involves transport of Fe2+ into the plant by IRT1, a high-affinity Fe2+ transporter.
Fig. 2.
Fig. 2.
Regulation of the Fe deficiency response via FIT. The model depicts proteins that directly interact with FIT via protein–protein interactions to regulate Fe uptake (indicated by solid black lines). Binding of FIT with subgroup Ib bHLH transcription factors, MED16 and EIN3/EIL (MED25 interacts with MED16 and EIN3/EIL1), and phosphorylation by CIPK11 leads to positive regulation of the Fe deficiency response and activation of Fe uptake genes such as FRO2 and IRT1 during Fe-deficient conditions (indicated by blue arrows). Binding of FIT with DELLA and ZAT12 prevents the binding of subgroup Ib bHLHs to FIT and prevents transcription of Fe uptake genes (indicated by the green arrow), whereas FIT interactions with subgroup IVa bHLH transcription factors lead to proteasomal degradation of FIT (indicated by red arrows). Abbreviations used are Fe, iron; Ca2+, calcium; ET, ethylene; GA, gibberellic acid; ROS, reactive oxygen species; JA, jasmonic acid; red circle, calcium ion; P, phosphorylation. Red faded lines, signaling pathway; rounded rectangles, proteins; dashed circle, nucleus.
Fig. 3.
Fig. 3.
bHLH-dependent transcriptional regulation of Fe deficiency in Arabidopsis thaliana. During Fe deficiency, URI (bHLH121) is phosphorylated and interacts with subgroup IVc bHLH transcription factors, bHLH34, bHLH104, bHLH115, and bHLH105. These heterodimers transcriptionally regulate expression of genes including subgroup Ib bHLH transcription factors, BTS, BTSL, and PYE (indicated by blue arrows). Heterodimerization of FIT with subgroup Ib bHLHs induces expression of Fe uptake genes IRT1 and FRO2. PYE is a known transcriptional repressor that inhibits expression of genes involved in Fe transport and sequestration. BTS and BTSL are also negative regulators of Fe deficiency whose expression is induced by Fe deficiency. During Fe-sufficient conditions, BTS and BTSL bind to available Fe and interact with members of bHLH subgroups IVc and Ib, and URI and FIT transcription factors, and cause their proteasomal degradation (indicated by red arrows). Fe-dependent turnover by BTS and BTSL is crucial for tight regulation of Fe deficiency response to prevent toxic effects of excess Fe. Abbreviations used are –Fe, Fe deficiency; +Fe, Fe sufficiency; P, phosphorylation; Fe, iron metal. Rounded rectangles and circle, proteins; rectangles, genes; solid black arrow, transcription and translation; blue arrows, transcriptional regulation; red arrows; proteasomal degradation; purple cylinder, BTS and BTSL; red X, transcriptional repression.
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