br Acknowledgements This work was supported
Acknowledgements This work was supported by a grant from the BBSRC (BB/D011809/1) to GJL and The Royal Society to SHS (Uf090321). SIM and AH were the recipients of scholarships from HEC Pakistan.
Introduction Iron (Fe) is the fourth most abundant element in the earth’s crust. However, Fe is often a limiting nutrient for crop production owing to the low solubility of hydroxides and phosphates of Fe3+ that predominate in calcareous soils (Haydon et al., 2012). In plants, Fe has a direct physiological role in photosynthesis, respiration, and the fixation of nitrogen, phosphorus, and sulfate, and it is involved in enzyme activation and electron transfer (Briat et al., 2015; Sanaz et al., 2018). In addition, Fe-deficiency stress is signaled by a broad range of basic redox reactions and plant hormones, including reactive oxygen species (ROS), reactive nitrogen species (RNS), ethylene, auxin, gibberellins, cytokinins, and brassinosteroids (Kobayashi and Nishizawa, 2012.). Plants have evolved sophisticated mechanisms to cope with Fe limitations, which are grouped into Strategy I, found in non-graminaceous monocots and dicots, and Strategy II, found in graminaceous monocots. As the most widespread Fe-acquisition mechanism of plants, Strategy I consists of the induction of three processes under low-Fe conditions (López-Millán et al., 2000): An H+-ATPase extrudes H+ into the rhizosphere to lower the pH of the soil, thus making Fe3+ more soluble. The inducible ferric-chelate reductase (FCR) activity reduces Fe3+ to Fe2+ (Robinson et al., 1999), which is the rate-limiting step for Fe acquisition under Fe-limiting conditions in dicots (Connolly et al., 2003). Fe2+ is then transported into the plant by Fe-regulated transporter 1 (IRT1), which is the major Fe transporter in roots (Barberon et al., 2014). With the help of IRT1 and other zinc/iron permeases (ZIPs), the reduced Fe is loaded into IOX4 (Vert et al., 2002). After the acquisition, oligopeptide transporter (OPT) is required in both shoot-to-root Fe signaling and redistribution (Zhai et al., 2014). Then, Fe is transported to the appropriate organelles. The vacuole is an important subcellular structure for Fe storage. VIT, a vacuolar iron transporter, imports Fe into the vacuole (Sun et al., 2006). On the contrary, natural resistance-associated macrophage protein (NRAMP) has a function of limiting Fe storage in vacuoles (Lanquar et al., 2005). The major biochemical reactions in which Fe participates are completed in chloroplasts. Fe deficiency affects photosynthetic electron transport, which is a big problem for photosynthetic plants. Permease in chloroplasts (PIC) is a chloroplast iron transporter that plays an important role in maintaining ion balance (Duy et al., 2007). The mitochondrion, another subcellular structure, requires Fe and the transport depends on ABC transporter ATP-binding/permease protein (STA) in the membrane (Kushnir et al., 2001). Fe limitations can also regulate root development, e.g. by inducing the formation of subapical root hairs and increasing root branching to enhance Fe uptake (Jin et al., 2011). Understanding how plants resist Fe deficiencies and maintain cellular Fe homeostasis provides a basis for engineering plants with increased Fe-deficiency tolerance levels to soils having a low Fe solubility and plants with enhanced Fe concentrations in edible parts, which may help to counteract malnutrition when plants are the major dietary nutrient source. Thus, a large number of genes have been documented as being involved with Fe regulation in various plants. The basic helix-loop-helix transcription factor, a Fe-deficiency-induced transcription factor (FIT), known as FER in tomato (Solanum lycopersicum L.) (Ling et al., 2002), is essential for the expression of ferric reduction oxidase (FRO) and metal transporter (IRT) (Yuan et al., 2005), and influences Fe homeostasis and stress responses (Shaista et al., 2018). The reduction in Fe3+ and uptake of Fe2+ can be separately regulated by FRO (Robinson et al., 1999) and IRT (Vert et al., 2002) in Arabidopsis thaliana. Fe deficiency has significant effects on stimulating the transcriptional level of FIT, suggesting that a trans-acting factor is interacting with the promoter of FIT to mediate its transcription (Colangelo and Guerinot, 2004). However, little information is available on the sensing of some other signals of the aforementioned genes, especially those involved in the cross-talk between Fe-deficiency signals and defense-related signals.