Abiotic stress is a result of several environmental disturbances caused by the continuous encroachment of industrialization, urbanization, and some human interference with the natural ecosystem that influence the quality and quantity of agriculture production per year (Mantri et al., 2012). Abiotic stresses include heat, cold, freezing, drought, salinity, flooding agents, UV, and heavy-metal stresses that have significant impacts on a plant life cycle (Macedo, 2012; Mantri et al., 2012; Singh et al., 2015, 2017; Tripathi et al., 2016c). These stresses not only affect plant biodiversity and productivity but also can interfere with the food web and the ecosystem. Abiotic stresses are worldwide problems as they decline the crop yield (Cramer et al., 2011). Moreover, the presence of heavy metals in food crops causes some severe diseases like cancer and asthma (Brigham et al., 2015). Plants, being the most precious gift of the nature that fulfill several basic requirements of human beings, are severely affected by various abiotic stress factors. Abiotic stress causes significant reduction in growth and yield of plants via inducing oxidative stress through enhanced reactive oxygen species (ROS) production and by lowering the antioxidant activities, level of nutrients, and modification of anatomical structures (Nagajyoti et al., 2010; Nazar et al., 2012; Vaculík et al., 2012; Singh et al., 2015; Tripathi et al., 2016b, 2017a). Hoagland and Arnon (1950) pioneered the most popular and commercial technique for developing plants with their roots in solutions containing mineral nutrients required for the growth of plants.

Various techniques are being used to protect plants from the adverse effects of abiotic stresses, which include exogenous supplementations of silicon, nitric oxide, growth-promoting hormones, enzymes, and nutrient management (Nagajyoti et al., 2010; Nazar et al., 2012; Tripathi et al., 2012, 2016a, 2017a,b,c,d; Vaculík et al., 2012; Saxena and Shekhawat, 2013). Among the remedies for abiotic stress, nutrient regulations or management are considered as the cost effective and eco-friendly techniques (Tripathi et al., 2015; Yadav et al., 2016). It has been reported that plant nutrients (micro and macro) play important roles in growth and development (White and Brown, 2010; Waraich et al., 2011; Tripathi et al., 2014, 2015). Therefore, an adequate and balanced supply of nutrients at the correct time is required for the proper growth and development of plants, maintenance of better soil fertility, and preservation of an intensive cropping system to maintain the global food production, particularly in developing countries (Dordas, 2009; Hansch and Mendel, 2009; Sarwar et al., 2010; Moharana et al., 2012; Waraich et al., 2012). In addition, studies also showed that an exogenous supply of nutrients plays a crucial role in the enhancement of plant tolerance against various abiotic stresses (Pankovic et al., 2000; Hassan et al., 2005; Tlustos et al., 2006; Dheri et al., 2007; Sarwar et al., 2010). Some nutrients such as calcium (Ca), magnesium (Mg), sulfur (S), zinc (Zn), and iron (Fe) have shown significant results when they are examined under salinity, drought, and heavy-metal stresses (Sarwar et al., 2010; Singh et al., 2011; Nazar et al., 2012). Currently, the application of Fe as a nutrient supplement and its role in imparting tolerance to plants against abiotic stresses are gaining attention as an area of research. Some studies suggested that the application of Fe reflects significant and/or potential impacts in alleviating the stress imposed by metal ions (Liu et al., 2007; Yadav et al., 2007; Garnier et al., 2010; Emamverdian et al., 2015).

Iron—the fourth most abundant and essential microelement on the earth's crust—is unavailable to higher plants, largely due to neutral and alkaline soils because of its existence in an insoluble form (Shao et al., 2007). In the plant system, Fe is regarded as an essential element in regulating life-sustaining processes like respiration, photosynthesis, chloroplast development, and chlorophyll biosynthesis where it takes part in electron transportation (Kim and Guerinot, 2007). Inadequate Fe supply in plants causes iron-deficiency, which results in decline in crop yields, interveinal chlorosis in plant leaves, etc. (Kim and Guerinot, 2007; Lan et al., 2011). In addition, excess levels of Fe can be fatal for plant health and productivity (Anjum et al., 2015). Therefore, an appropriate supply of Fe is needed for sustaining plant productivity under stress as well as non-stress conditions. It has been reported that out of the total cultivated areas of the world, one-third has Fe deficiency (Kim and Guerinot, 2007), which is causing significant decline in yield of crops every year. Iron is a chief component of the cell redox systems and also acts as a cofactor of various antioxidant enzymes like catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX) (Marschner, 1995; Sharma et al., 2004; Kumar et al., 2010). Therefore, Fe homeostasis plays a crucial role in the life cycle of plants under stressed conditions (Figure 1) (Gratao et al., 2005; Meda et al., 2007; Sarwar et al., 2010). In addition to overviewing major Fe acquisition strategies in plants, this review briefly appraises the literatures available on the role of Fe in plants exposed to major abiotic stresses like salinity, drought, and heavy metals. Less discussed topics in the current context have also been highlighted.

Despite being categorized as the fourth most abundant element of the earth's crust, the availability of Fe to plants is highly heterogeneous as it has low solubility under aerobic conditions, particularly at high pH and in calcareous soil. Plants firmly regulate the homeostasis of Fe and respond to the shortage as well as the surplus level of Fe (Morrissey and Guerinot, 2009). Therefore, in order to maintain proper ion homeostasis, plants stabilize the uptake, transport, and storage (Grotz and Guerinot, 2006). Hence, for the significant acquisition and agglomeration of Fe, plants have developed two different strategies (strategy I and II) under varied soil conditions (Römheld and Marschner, 1986) (Figure 1). Almost all higher plants except the members of family Gramineae follow Strategy I for Fe uptake (Kobayashi and Nishizawa, 2012) (Figure 1). The uptake and accumulation of Fe is frequently enhanced by three reactions: firstly, the secretion of protons through the plasmalemma P-type ATPase in order to acidify the surrounding solution for augmenting the solubility of Fe³⁺; secondly, by reducing Fe³⁺ to the more soluble Fe²⁺ form through Fe³⁺ chelate reductase; and thirdly, by plasmalemma transport of Fe²⁺ through Fe transporters (Figure 1). The dominant gene essential for these processes was first cloned from Arabidopsis thaliana and peas such as FRO₂, allelic to the frd1 mutation distressing Fe³⁺-chelate reductase activity (Robinson et al., 1999), and FRO1, respectively (Waters et al., 2002). Furthermore, reduced form of iron in the plants transported from the root via IRT1 (divalent cation transporter) (Eide et al., 1996; Eckhardt et al., 2001), which is a member of the ZIP family (Guerinot, 2000). In addition, IRT1 is also reported as an essential gene because, in the case of excess supplementation of exogenous iron, it reduces the lethality of seedlings and suppresses the chlorosis in irt 1 mutants (Henriques et al., 2002; Varotto et al., 2002; Vert et al., 2002). Moreover, Vert et al. (2002) and Dinneny et al. (2008) observed that the acquisition of Fe starts from the epidermal layers of the root, which is proven by the expression pattern of IRT1 and FRO2. The transporter gene IRT1, which is responsible for Fe transport, has been cloned from Arabidopsis (Eide et al., 1996; Dubeaux et al., 2015) and its ortholog RIT1 has been cloned from pea and tomato as well (Cohen et al., 1998; Eckhardt et al., 2001). Fe³⁺-chelate reductases are integral membrane-bound proteins and they belong to the family of proteins that are responsible for the transport of electrons from cytosolic NADPH to FAD and, consequently, through heme groups to electron acceptors located on the outer side of the plasma membrane (Figure 1). In response to Fe deficiency, the up-regulation of the FRO gene has been noted in roots (Robinson et al., 1999; Waters et al., 2002). On the basis of the recent studies, IRT1 has been marked as the major transporter gene responsible for the uptake of Fe from soil solutions (Figure 1). Besides this, IRT1 also plays significant role in the transport of Zn, Mn, Co, and Cd (Rogers et al., 2000). Similarly, PEZ1 is a novel effluxer responsible for the xylem loading of phenolics as well as the remobilization of precipitated apoplasmic Fe in the plant cell. This is due to the fact that FRD3, FRDL1, and PEZ1 are efflux Fe-chelating molecules in their Fe liberated forms and Fe efflux occurs in the xylem sap by more than one transporters (Ishimaru et al., 2011). Morrissey et al. (2009) described the role of ferroportin 1/iron regulated 1 (AtFPN1/AtIREG1) metal efflux protein function in Fe homeostasis in Arabidopsis plants, which is similar to the iron efflux transporter in mammals and are responsible for Fe absorption in the intestines and Fe recycling in macrophages. While the active role of AtFPN1 localized on the plasma membrane has not yet been reported, its promoter activity is prominently seen in the stele region of plants. In addition, its mutant species showed the loss of chlorophyll content in both Fe-sufficient and Fe-deficient media, which signify its indispensable role in acquisition of Fe to shoots (Morrissey et al., 2009) as well as mitochondrial Fe metabolism and trafficking progress via the mitochondrial Fe storage protein (mitochondrial ferritin) and transport (mitoferrin-1 and -2) (Richardson et al., 2010).

Strategy II is exclusively specific to the Gramineae family and is based on the biosynthesis and secretion of mugineic acids (MAs) (Figure 1). The roots of graminaceous plant release phytosiderophores (PSs), which are responsible for the chelation of Fe³⁺ in the rhizosphere. Specific transporter proteins located on the plasma membrane transport Fe³⁺ PS complexes in the cytosol (Römheld and Marschner, 1986). In response to Fe deficiency, both processes are enhanced through up-regulation of the underlying genes (Figure 1). Until now, nine different kinds of MAs have been recognized, which have been produced via a conserved pathway from S-adenosyl-L-methionine (Bashir et al., 2006; Ueno et al., 2007; Kobayashi and Nishizawa, 2012). Through sequential enzymatic reaction mediated by the precursors of MAs such as NAS, NAAT, DMAS, and DMA, strategy II enhanced acquisition of Fe in graminaceous plant (Figure 1). Normally, the transportation of Fe in plants may be seen in the anatomical structures of roots when observed under the Phase-contrast microscopy. It can be well observed that the deposition percentage of Fe in root cells is increased by the increased level of Fe concentration.

After the acquisition of Fe inside the root, its translocation occurs across the plasma membrane through several different members of transporter families that are responsible for the transportations of intra- and intercellular ions (Figure 2). Such transporters include Nramp and YSL families (Grotz and Guerinot, 2006). The Nramp family leads the translocation of divalent cations, whereas the members of the YSL family are probably only responsible for the translocation of metal chelates (Conte and Walker, 2011). Members of the NRAMP family are responsible for the transportation of an array of metals across the plasma membranes including Mn²⁺, Zn²⁺, Cu²⁺, Fe²⁺, Cd²⁺, Ni²⁺, and Co²⁺ in microorganisms, plants, and animals (Nevo and Nelson, 2006). In plants, the expression of Nramp transporters has been located in roots and shoots and is responsible for the transportation of metal ions across plasma membranes and the tonoplast (Krämer et al., 2007). In A. thaliana, NRAMPs facilitate the transport of Fe and Cd. NRAMP1 displays a significant role in the transport and homeostasis of Fe. Besides this, members of the YSL family mediate the acquisition of Fe through the translocation of Fe³⁺ phytosiderophore complexes (Curie et al., 2001). In addition, CDF transporters have also been recognized recently that lead to the transportation of metal ions (in divalent form) in prokaryotes (Nies, 1992) and also in several eukaryotes (Montanini et al., 2007). The apoplast sometime plays an important role in ion uptake into the cells and also performs some important physiological roles in nutrient transport and storage (Sattelmacher and Horst, 2007).

In addition to this, Deiana et al. (1992) studied homeostasis, kinetics, and the consequences of mechanisms of Fe uptake in plants. Römheld and Marschner (1983) reported that when Fe was found in very high concentrations, caffeic acid plays a very active role in Fe mobilization at the interface of the soil-root. Caffeic acid is the phenolic compound produced from plants (Whitehead et al., 1981, 1983; Olsen et al., 1982; Wei Jin et al., 2008) that functions in the reduction of Fe³⁺ and takes part in redox reactions near rhizosphere and free spaces (Brown and Ambler, 1973; Olsen et al., 1981; Deiana et al., 1992). Deiana et al. (1992) reported that the concentration of Fe³⁺ is directly proportional to the yield of Fe²⁺, i.e., on increasing the Fe³⁺ concentration. The yield of Fe²⁺ also increased the value higher than 9 for Fe(III)/CAF molar ratios. Deiana et al. (1992) also found that one molecule of the phenolic plant product (CAF) is capable of reducing the nine electrons of the Fe³⁺ ion to the Fe²⁺ ion, and they also demonstrated that the whole reaction mechanism involves two steps. The first one includes the formation of the Fe³⁺ ion from organic molecules. This reaction is faster and involves the transfer of five electrons, whereas the second one includes the transfer of four electrons with a comparatively slower reaction (Deiana et al., 1992).

Plants may undergo Fe toxicity in two conditions: either excessive amount of Fe or Fe deficiency. Iron plays key roles in plant metabolism like electron transport systems (redox reactions of Fe-S proteins and cytochromes), respiration, photosynthesis, and nitrogen fixation processes (Soyder and Schmidt, 1974; Carrow et al., 1988; Taiz and Zeiger, 1991). Furthermore, the deficiency of Fe can affect about one-third of the cultivated land. Deficiency of Fe also causes the declination of photosynthetic components, especially Fad (Fe-S protein ferredoxin) of the chloroplast (Tognetti et al., 2007). Fe deficiency in plants causes interveinal chlorosis (Bienfait and Van der Mark, 1983), poor root development, growth retardation, and the eventual death of the plant (Kobayashi et al., 2003). In addition, Fe deficiency also leads to the alteration in expression of chlorophyll-binding proteins and the down-regulation of many photosynthetic pigment levels (Thimm et al., 2001; Rout and Sahoo, 2015). In the agricultural soils, Fe deficiency may also occur either at extremely high pH or at extremely low pH levels. López Jiménez et al. (1985) demonstrated that the increased levels of chlorosis in young Avocado leaves was correlated with a decline in the level of Fe in leaf parts, chloroplast numbers, and activity of catalase (Rout and Sahoo, 2015). The Fe deficiency is also characterized by interveinal chlorosis in young leaves rather than main veins, a type of symptom known as “iron deficiency chlorosis” (Rout and Sahoo, 2015). Similarly, excessive amounts of Fe also cause toxicity in plants. Tanaka et al. (1966) demonstrated that upon increasing the level of Fe, roots are affected more than leaves. It shows that in the presence of an excess amount of Fe, plant roots increase their capability and translocate the Fe in younger leaves. High level of Fe disturb the basal level of magnesium and potassium in plants and cause nutritional disturbances (Tanaka et al., 1966).

Although high concentrations of Fe can lead to toxic consequences in plants (Anjum et al., 2015). Excess free Fe ions like Fe²⁺ and Fe³⁺ can cause ROS generation by participating in the Fenton reaction (Fenton, 1894; Haber and Weiss, 1934; Kehrer, 2000; Mai and Bauer, 2016) and leading to oxidative stress (Mai and Bauer, 2016). Down regulated detoxifying proteins in Fe-deprived conditions, viz. CAT2 (CATALASE 2; AT4G35090) proteins and PA2 (PEROXIDASE 2; AT5G06720) proteins, were reported in some plants (Donnini et al., 2010; Rodríguez-Celma et al., 2011). These are enzymes that help in the conversion of H₂O₂ (hydrogen peroxide) to H₂O (water) and oxygen. In addition, Fe deficiency in the roots of M. truncatula (Rodríguez-Celma et al., 2011) and P. dulcis × P. persica (Rodriguez-Celma et al., 2013) was characterized by the superoxide dismutase, i.e., ATMSD1 (ARABIDOP-SIS MANGANESE SUPEROXIDE DISMUTASE 1; AT3G10920) (Mai and Bauer, 2016). Furthermore, non-enzymatic ROS generation also occurs due to Fe deficiency. In this process, two enzymes reported in the roots (Mai et al., 2015) and shoots (Zargar et al., 2013) of Arabidopsis thaliana were GST1 (ARABIDOPSIS GLUTATHIONE S-TRANSFERASE 1; AT1G02930) and MDAR1 (MONODEHYDROASCORBATE REDUCTASE 1; AT3G52880) (Rodríguez-Celma et al., 2011; Mai et al., 2015). In the case of Fe deficiency, two heme-binding proteins (CAT2 and PA2) were found to decrease the level of peroxidases. Moreover, ATMSD1 is a ROS-eliminating enzyme formed after the up-regulation of ROS-eliminating enzyme and beneficial in compensation of deprivation of Fe-dependent peroxidases and stimulated the ascorbate-glutathione cycle from GPX3 (GLUTATHIONE PEROXIDASE3; AT2G43350) (Mai et al., 2015). GST1 and MDAR1 were reported to detoxify the ROS generated due to Fe deficiency (Apel and Hirt, 2004).

Increased anthropogenic activities and technologies have rendered a polluted environment. Furthermore, the situation is likely to be much worsened by an immense increase in human population that is expected to reach about 9.1 billion by 2050 according to the report of the FAO (2008), which would result in decreased areas of arable land for future agricultural practices. It is well documented that approximately 50% of the world's agricultural land suffers from adverse effects of complex environmental stress factors such as salinity, drought, and heavy metal. Since plants are restricted in their movements, they are inevitably exposed to such stressful conditions that would cause reduction in their yields. To counteract inhibitory/adverse effects of such stresses, plants employ several efficient and sophisticated biochemical defense mechanisms at multiple levels (morphological, anatomical, biochemical, and molecular). Data analyses based on previous findings showed tremendous progress at various levels against stress factors in plants. Of these strategies, management of mineral nutrient status in plants was found to be the most promising. Among mineral nutrients, Fe is regarded as one of the most efficient and essential micronutrients that acts significantly in a number of cellular processes such as metal detoxification, metabolism of secondary metabolites, and maintenance of the cell redox cycle. Previous findings suggested that Fe regulates adverse effects of salinity, drought, and heavy metal by controlling the redox status of the cell and antioxidant defense system. In addition, the formation of Fe plaque also facilitates the sequestration of a large number of metals on the root surface. Although not much is known about the relevancy of Fe nutrition in the mitigation of different stresses, its exogenous application, however, definitely regulates toxicity of several stresses. Despite major progress achieved in the field of Fe nutrition-mediated alleviation of stress, there are several questions still awaiting for the answers. In conclusion, it may be stated that uptake, accumulation, and metabolism of Fe inside the plant still deserve attention and may shed insight into the process of modifying agricultural productivity in stressful environments.

DKT, ShwetaS, and SwatiS: designed the manuscript; DKT, ShwetaS, SG, and SwatiS: wrote the manuscript; VS, DC, PS, ShiveshS, VY, ND, SL, ShivendraS and SP: critically evaluated the manuscript.