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Fe UPTAKE MECHANISMS AND TRANSCRIPTIONAL CONTROL Iron (Fe) is an essential micronutrient for almost all living organisms and represents one of the most versatile metals in biology, being involved in many ubiquitous metabolic processes such as respiration and photosynthesis, and required as a cofactor for numerous enzymes (Sperotto et al., 2010; Grillet et al., 2014a). In plants, Fe deficiency can cause severe chlorosis, growth arrest, and even plant death. Although highly abundant in the earth’s crust, Fe phytoavailability is usually low, mainly because Fe3+ forms insoluble Fe oxides (Lemanceau et al., 2009). To circumvent this problem, plants developed mechanisms to acquire Fe from the rhizosphere (Sperotto et al., 2012; Grillet et al., 2014a). Fe uptake mechanisms were classically separate into two strategies. Strategy I, or reduction strategy, is carried out by all plants except those from Poaceae family, and consists of: (a) H+ extrusion by P-type ATPases to acidify the rhizosphere and increase Fe3+ solubility (e.g., AtAHA2, Arabidopsis H+-pump ATPase); (b) reduction of Fe3+ by a plasma membrane (PM)-bound ferric chelate reductase to more soluble Fe2+ (e.g., AtFRO2, Ferric Reductase Oxidase); and (c) Fe2+ absorption into root epidermal cells by transmembrane transporters (e.g., AtIRT1, Iron-Regulated Transporter) (Hindt and Guerinot, 2012; Ivanov et al., 2012). All three components of this strategy increase their activities during Fe deficiency. Strategy II, or chelation strategy, is used by plants from Poaceae family, and involves: (a) synthesis and release of small molecular weight compounds of the mugineic acid family called phytosiderophores (PS) into the rhizosphere, which bind Fe3+ with high affinity, via TOM1/OsZIFL4 (Nozoye et al., 2011; Ricachenevsky et al., 2011); and (b) Fe(III)-PS complex uptake into root cells by a Yellow Stripe/Yellow Stripe-Like (YSL) transporters. Both processes (PS excretion and Fe(III)-PS transport) are increased in response to Fe deficiency. Since both maize (Zea mays) ys1 mutant (defective for Fe(III)-PS transport) and Arabidopsis thaliana irt1 mutant (defective for Fe2+ transport) cannot survive under Fe deficiency conditions, it was first widely accepted that these two Fe uptake strategies were the main mechanisms for Fe acquisition in each plant group. However, later work on rice (Oryza sativa) showed that two functional Fe2+ transporters, OsIRT1 and OsIRT2, were expressed in roots upon Fe deficiency (Ishimaru et al., 2006; Walker and Connolly, 2008). It was proposed that rice uses a combined strategy, which has all features of a strategy II plant (PS release through TOM1/OsZIFL4 and Fe(III)-PS uptake through OsYSL15, the YS1 ortholog— Inoue et al., 2009; Lee et al., 2009) and some features of a strategy I plant (Fe2+ uptake using IRT transporters). The other two components of strategy I plants, proton extrusion, and Fe(III)-chelate reductase activity, were not detected in Fedeficient rice roots (Ishimaru et al., 2006). Further evidence for combined strategy was provided by rice plants carrying a mutation in the NICOTIANAMINE AMINOTRANSFERASE (NAAT) gene, a key enzyme in PS synthesis. This mutant, which lacks PS, is able to grow if Fe2+ is supplied as Fe source (Cheng et al., 2007). Based on these findings, it was proposed that the ability to absorb Fe2+ evolved in rice as an adaptation to the soil conditions in flooded paddies, where Fe2+ is more abundant than Fe3+ (Ishimaru et al., 2006; Walker and Connolly, 2008; Hindt and Guerinot, 2012). So far, rice is the only plant described to use the combined strategy mechanism. A number of studies described key players and major transcriptional networks that control Fe homeostasis in both grasses and non-grasses (Hindt and Guerinot, 2012; Ivanov et al., 2012). Interestingly, orthologous genes have been described in rice and Arabidopsis thaliana, showing similar roles. The bHLH transcription factor FIT (FER-like irondeficiency-induced transcription factor) from A. thaliana interacts with bHLH038 and bHLH039 to regulate IRT1 and FRO2 under Fe deficiency (Yuan et al., 2008). FIT has no ortholog in rice, but bHLH38/39 are highly similar to OsIRO2 (Hindt and Guerinot, 2012), a known downstream regulator of Fe deficiencyresponsive genes. OsIRO2 regulates the Fe(III)-PS transport-related genes, but not OsIRT1 (Ogo et al., 2007). OsIDEF1, acting upstream of OsIRO2, and OsIDEF2, are transcriptional regulators of distinct but partially overlapping branches of Fe deficiency response in rice (Ogo et al., 2008; Kobayashi et al., 2009) However,

There and back again, or always there? The evolution of rice combined strategy for Fe uptake