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Understanding the regulation of key genes involved in plant iron acquisition is of crucial importance for breeding of micronutrient-enriched crops. The basic helix-loop-helix protein FER-LIKE FE DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT), a central regulator of Fe acquisition in roots, is regulated by environmental cues and internal requirements for iron at the transcriptional and posttranscriptional levels. The plant stress hormone ethylene promotes iron acquisition, but the molecular basis for this remained unknown. Here, we demonstrate a direct molecular link between ethylene signaling and FIT. We identified ETHYLENE INSENSITIVE3 (EIN3) and ETHYLENE INSENSITIVE3-LIKE1 (EIL1) in a screen for direct FIT interaction partners and validated their physical interaction in planta. We demonstrate that the ein3 eil1 transcriptome was affected to a greater extent upon iron deficiency than normal iron compared with the wild type. Ethylene signaling by way of EIN3/EIL1 was required for full-level FIT accumulation. FIT levels were reduced upon application of aminoethoxyvinylglycine and in the ein3 eil1 background. MG132 could restore FIT levels. We propose that upon ethylene signaling, FIT is less susceptible to proteasomal degradation, presumably due to a physical interaction between FIT and EIN3/EIL1. Increased FIT abundance then leads to the high level of expression of genes required for Fe acquisition. This way, ethylene is one of the signals that triggers Fe deficiency responses at the transcriptional and posttranscriptional levels.

Iron is an essential element for life on Earth and its shortage, or excess, in the living organism may lead to severe health disorders. Plants serve as the primary source of dietary iron and improving crop iron content is an important step towards a better public health. Our review focuses on the control of iron acquisition in dicotyledonous plants and monocots that apply a reduction-based strategy in order to mobilize and import iron from the rhizosphere. Achieving a balance between shortage and excess of iron requires a tight regulation of the activity of the iron uptake system. A number of studies, ranging from single gene characterization to systems biology analyses, have led to the rapid expansion of our knowledge on iron uptake in recent years. Here, we summarize the novel insights into the regulation of iron acquisition and internal mobilization from intracellular stores. We present a detailed view of the main known regulatory networks defined by the Arabidopsis regulators FIT and POPEYE (PYE). Additionally, we analyze the root and leaf iron-responsive regulatory networks, revealing novel potential gene interactions and reliable iron-deficiency marker genes. We discuss perspectives and open questions with regard to iron sensing and post-translational regulation.

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Highlights d Iron-regulated and calcium-dependent protein kinase CIPK11 interacts with FIT d CIPK11 and calcium sensors CBL1/9 promote FIT-dependent Fe deficiency responses d FIT is phosphorylated in plants, and CIPK11 phosphorylates FIT at Ser272 d Mutation at Ser272 modulates FIT activity, affecting seed iron content