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Iron is an essential nutrient that plays a critical role in life-sustaining processes. Due to its ability to gain and lose electrons, iron works as a cofactor for enzymes involved in a wide variety of oxidation-reduction reactions (i.e., photosynthesis, respiration, hormone synthesis, DNA synthesis, etc.). This function makes iron an essential nutrient, and its deficiency causes iron chlorosis, which seriously constrains normal plant development. Iron toxicity in plants is indicated by bronzing characteristics, which have been observed in plants grown in greater than 100 mM iron solutions. Higher iron uptake by plants reduces protein synthesis in leaves. Ferritin is considered crucial for iron homeostasis, and it consists of a multimeric spherical protein called phytoferritin, which is able to store up to 4500 iron atoms inside its cavity in non-toxic form. A resistant variety may accumulate a larger amount of phytoferritin, which forms a complex that reduces iron toxicity. Large amounts of iron in plants are responsible for protein degradation, because iron may lead to the formation of reactive oxygen radicals that are highly phytotoxic and cause protein degradation. Iron chlorosis is a widespread problem, especially for regions where the bioavailability of iron in the soil is low. Usual remediation strategies consist of amending iron to soil, which is an expensive practice. Thus, genetically improved chlorosis-resistant rootstocks and new cultivar combinations offer the best solution to iron chlorosis, and it is one of the most important lines of investigation needed to prevent this nutritional problem. Therefore, there is a need for new methods to diagnose and correct this nutritional disorder. The use of microarray techniques revealed the changes in gene expression levels due to iron deficiency, and it has allowed insights into the transcriptional regulation of some functions. These studies have extended our knowledge of stress responses to iron deficiency, and have identified candidate genes and processes for further experimentation to increase our understanding of iron deficiency stress. It is likely that more extensive microarray analyses, coupled with suitable annotations of completely sequenced genomes, will prove valuable in future studies of iron stress responses in plants. Novel gene regulation networks for iron deficiency responses include iron deficiency-inducible basic helix-loop-helix transcription factors (OsIRO2), iron deficiency-responsive cis-acting elements (IDEs), and the two transcription factors. The identification of the trans-acting factors responsible for the pathways will contribute greatly to an understanding of the regulatory mechanisms, and these factors will also prove beneficial to plants under iron-deficient conditions.
Iron is an essential micronutrient, and the expression of genes involved in iron transport is regulated by iron availability and demand. Most previous studies have compared the expression of genes in response to iron deficiency (Mori, 1999; Kobayashi and Nishizawa, 2012; Bashir et al., 2013). However, the expression of genes involved in iron transport is also regulated in response to high iron demand at different developmental stages. Among the abiotic stresses, iron deficiency constitutes a major factor leading to a reduction in crop yield due to the high pH, especially in calcareous soils with extremely low iron solubility (Kobayashi et al., 2005). They reported that the expression of genes encoding every step in the methionine cycle was thoroughly induced by iron deficiency in roots, and almost thoroughly induced in the leaves. The rice genes involved in iron acquisition are coordinately regulated by conserved mechanisms in response to iron deficiency, in which iron deficiency inducible (IDE)-mediated regulation plays a significant role. A promoter search revealed that iron deficiency-induced genes associated with iron uptake possessed sequences that were homologous to the iron deficiency-responsive cis-acting elements IDE1 and IDE2 in their promoter regions, which were identified at a higher rate than those showing no induction under iron deficiency. Kobayashi et al. (2005) and Paz et al. (2007) reported that two major mechanisms of iron acquisition studied in plants are based on either the reduction of organic ferric iron chelates via membrane-associated ferrireductase or the release of phytosiderophores that bind ferric ions and introduce them into the cell. Plant ferritin is synthesized as a precursor containing a unique amino acid sequence composed of over 70 residues at the N-terminal, followed by a region that is relatively conserved among other ferritins. The first part of this region is known as transit peptide (TP), which is responsible for plastid targeting and is processed upon entry into the plastid. The second part is known as the extension peptide, which may be lost in the germination process (Ragland et al., 1990). The first and second ferritins consist of about 40 and 30 amino acid residues, respectively. The conserved region neighboring the extension peptide has been shown to have 40–50% sequence identity with mammalian ferritins.
Genetic improvement of iron content and stress adaptation in plants using the ferritin gene has been reviewed (Goto et al., 2001), and the gene transfer technique has been a revolutionary tool for quickly creating novel varieties. The plants that have had the ferritin gene introduced possess two particular advantages: 1) the storage of excess iron and other elements and 2) the cancellation of oxidative stress. There are two possibilities to create useful plants using the exogenous ferritin gene in this review. One is iron-fortified plants used to overcome iron deficiency in humans, and the other is plants that have resistance to heavy metals, pathogens, and other stresses or those that exhibit enhanced growth. These genetic improvements could lead to increased iron content or stimulated growth. However, the mechanism of iron storage and the scavenger system for oxidative stress using ferritin gene expression have not been determined. An understanding at a physical and molecular level is now necessary to take advantage of the ferritin gene at an advanced stage. In non-graminaceous plants, the Fe II -transporter gene and ferric-chelate reductase gene have been cloned from A. thaliana, but Fe+2-reductase has not been cloned. In graminaceous monocots, the genes for mugineic acid (MAs) synthesis, nicotianamine synthase (nas) and nicotianamine aminotransferase (naat), have been cloned form barley, whereas the Fe+3-MAs transporter gene has yet to be cloned. Transferrin absorption in Dunaliella has been reported, suggesting a phagocytic (endocytic) iron-acquisition mechanism. In rice, the overexpression of OsIRO2 resulted in increased MAs secretion, but the repression of OsIRO2 resulted in lower MAs secretion and hypersensitivity to iron deficiency. Northern blots revealed that the expression of genes involved in the Fe (III)-MAs transport system was dependent on OsIRO2 (Mori, 1999; Kobayashi et al., 2005; Ogo et al., 2009). The expression of the genes for NAS, a key enzyme in MAs synthesis, was notably affected by the level of OsIRO2 expression. Furthermore, a microarray analysis demonstrated that OsIRO2 regulates 59 iron-deficiency-induced genes in roots. Some of these genes, including two transcription factors that were upregulated by iron deficiency, possessed the OsIRO2 binding sequence in their upstream regions. OsIRO2 possesses a homologous sequence of the iron deficiency-responsive cis-acting element in its upstream region (Ogo et al., 2009). Three NAS genes have been identified (OsNAS1-3) in rice, and they are reported to catalyze the formation of NA (Inoue et al., 2003). OsNAAT1 converts NA to a 3'-keto intermediate (Inoue et al., 2008), which is further converted to deoxymugineic acid (DMA) by OsDMAS1 (Bashir and Nishizawa, 2006; Bashir et al., 2006). In rice, this is secreted to the rhizosphere by TOM1 (Nozoye et al., 2011), where it binds to Fe+3 and is absorbed by OsYSL15 (Inoue et al., 2009). The role of YSL family transporters in iron transport is well characterized. The rice genome contains 18 putative YSL family genes (Koike et al., 2004), among which OsYSL2, OsYSL15, OsYSL16, and OsYSL18 have been characterized (Aoyama et al., 2009; Inoue et al., 2009; Kakei et al., 2012; Zheng et al., 2012). OsYSL2 is a Mn-NA and Fe-NA transporter (Koike et al., 2004). These genes will contribute greatly to an understanding of the regulatory mechanisms.
Mandal AB and Roy B (2000) Development of Fe-tolerant rice somaclone through in vitro screening. Paper presented at the 4th Int. Rice Genet. Symp., International Rice Research Institute, Manila, Philippines, October 2000, pp. 22-27.