Understanding the Complexity of Iron Sensing and Signaling Cascades in Plants

Under iron-deficient conditions, plants induce the expression of a set of genes involved in iron uptake and translocation. This response to iron deficiency is regulated by transcriptional networks mediated by transcription factors (TFs) and protein-level modification of key factors by ubiquitin ligases. Several of the basic helix-loop-helix TFs and the HRZ/BTS ubiquitin ligases are conserved across graminaceous and non-graminaceous plants. Other regulators are specific, such as IDEF1 and IDEF2 in graminaceous plants and FIT/FER and MYB10/72 in non-graminaceous plants. IMA/FEP peptides positively regulate the iron-deficiency responses in a wide range of plants by unknown mechanisms. Direct binding of iron or other metals to some key regulators, including HRZ/BTS and IDEF1, may be responsible for intracellular iron-sensing and -signaling events. In addition, key TFs such as FIT and IDEF1 interact with various proteins involved in signaling pathways of plant hormones, oxidative stress and metal abundance. Thus, FIT and IDEF1 might function as hubs for the integration of environmental signals tomodulate the responses to iron deficiency. In addition to local iron signaling, root iron responses are modulated by shoot-derived long-distance signaling potentially mediated by phloem-mobile substances such as iron, iron chelates and IMA/FEP peptides.