Introduction

Heavy metal ions occur naturally in soil environment, and some of these (such as Co, Cu, Fe, Mn, Mo, Ni, and Zn) are required for the maintenance of optimal plant growth, development, and productivity. However, important plant enzyme/protein functions and metabolism and, subsequently, plant growth and development are often limited by both low phytoavailability of previous heavy metal ions, as well as by the presence of their excessive concentrations in the soil solution (Marschner 1995; Mengel et al. 2001; Hänsch and Mendel 2009). Since a range of anthropogenic activities including mining and the establishment of chemical industries are on their high leap, significant elevations in the concentrations of heavy metal ions such as Co, Cu, Fe, Mn, Mo, Ni, and Zn in soil environment and, subsequently, in plant–animal system cannot be ignored that need immediate attention (Adriano 2003; Alloway 2013; Anjum et al. 2013, 2014a). Toxic consequences as a result of excess levels of previous heavy metal ions may include cellular damage, essential functional group displacements, elevated reactive oxygen species (ROS)-led oxidative stress, and cellular metabolic arrest (Schützendübel and Polle 2002; Gill and Tuteja 2010; Anjum et al. 2010, 2012). Nevertheless, Co, Cu, Fe, Mn, Mo, Ni, and Zn may also modulate plant’s ROS-metabolizing/scavenging system comprising enzymatic (such as superoxide dismutase (SOD), catalase (CAT), guaiacol peroxidase (GPX), glutathione sulfotransferase (GST), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and glutathione reductase (GR)) and non-enzymatic (such as ascorbate (AsA), glutathione (GSH), carotenoids, tocopherols, and phenolics) antioxidants (Gill and Tuteja 2010; Anjum et al. 2010, 2012, 2014b). In addition to the significance of the changes in plant growth and development traits, and the status of oxidative stress and its metabolism, the exploitation of plants as cyto/genotoxic bioindicators is well recognized and recommended for screening of potential cyto/genotoxic chemicals (Ma 1999). Despite previous facts, majority of literature available on “plant–metal interactions” has considered either known or potential toxic elements/metals–metalloids (Alloway 2013). Moreover, literature reflects a paucity of information primarily on the discussion of phytotoxicity due to elevated levels of previous heavy metal ions and secondarily, on the facts cross-talking the actions of these metal ions in plants.

Given the above, considering plant-beneficial heavy metal ions (in order: Co, Cu, Fe, Mn, Mo, Ni, and Zn), this paper presents: (a) an overview of their status in soils and uptake, transport, and significance in plants; (b) a critical discussion on their elevated level-mediated phytotoxicity (assessed as their impacts on plant growth and development and modulation of oxidative stress and cyto/genotoxic traits) and underlying potential mechanisms; (c) a brief cross talk on the significance of potential interactions among them; and also (d) a highlight on so far unexplored aspects in the current context. Major information (atomic number, group, occurrence, position in the Earth’s crust, plant-available forms, and general range in plants) related with the discussed herein plant-beneficial heavy metal ions has been summarized in Table 1.

Table 1 Summarized information (atomic number, group, occurrence, position in the Earth’s crust, plant-available forms, and range in plants) related with the discussed herein plant-beneficial heavy metal ions
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Cobalt

Status in soils, plant uptake, transport, and significance

Cobalt (Co) is a transition element generally found in the form of ores (cobaltite, smaltite, and erythrite) and not as a free metal. Co has the atomic number of 27 and is the 31st most abundant element in the Earth’s crust (Krauskopf and Bird 1995). Co is added to the atmosphere in small quantities through various anthropogenic activities such as coal combustion and mining, processing of Co-containing ores, and the production and use of chemicals/fertilizers containing Co salts. Soils near mining region may contain very high amounts of Co that may cause severe illness to living beings. Co cannot be removed easily once it entered into the environment and may also react with other particles or adsorb on soil particles or water sediments. Normally, Co is found in traces and its concentrations in plants are cited to be as low as 0.1–10 μg g−1 dry weight (d.w.) (Jaleel et al. 2009). Co uptake in plants is controlled by different mechanisms. In higher plants, absorption of Co by roots involves active transport (Palit et al. 1994). Transport through cortical cells is by both passive and active diffusion, whereas in the xylem vessels/tissues, Co is poorly loaded, where it is mainly transported by transpirational flow (Page and Feller 2005). The low mobility of Co in plants restricts its transport to leaves from stems (Palit et al. 1994). Nevertheless, Co is considered to have an intermediate mobility in the phloem (Marschner 1995). Apart from absolute content of Co in a soil, a range of other factors such as temperature, pH, fertility, reductive–oxidative conditions, and the content of organic matter in soils may affect the plant Co uptake (Subbiah and Asija 1976). Considering the significance of Co in plants, it is an essential component of several enzymes and co-enzymes. However, Co is not classified as an essential and is usually described as beneficial element (Bakkaus et al. 2005). Since Co is a key constituent of cobalamin (i.e., vitamin B12), it is essential to all animals and microorganisms. However, the physiological importance of Co in higher plants has not been characterized. The available literature hardly shows any important physiological role of Co in higher plants except for its role in symbiotic N2 fixation by leguminous plants (Marschner 1995) and some species of blue-green algae (Cavusoglu and Yalcin 2010). Low concentration of Co has been reported to promote plant growth, development, and yield (Tewari et al. 2002; Jaleel et al. 2009) (Fig. 1).

Fig. 1
figure 1

Summary of major functions of plant-beneficial heavy metal ions in plants

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Toxicity

Literature related with Co toxicity and risk and underlying potential mechanisms in higher plants is scanty (Liu et al. 2000). Also, the predicted no-effect concentration for Co in plants cannot be derived considering too few data on Co toxicity (Li et al. 2009). Excess Co-mediated changes in the plant growth and development, oxidative stress and its metabolism, and cyto/genotoxicity are briefly discussed hereunder.

Plant growth and development

Though it may vary with the plant species, 30–40 μg g−1 of dry matter has been considered as the critical Co level (Kabata-Pendias and Pendias 1992). The concentration of Co above 50 μM in growth medium was reported to cause decrease in growth and dry matter yield, whereas, the exposed plants may exhibit visible effects of Co toxicity beyond the 200-μM Co supply (Tewari et al. 2002). Co may inhibit photosystem II (PS II) activity and, hence, the Hill reaction by inhibiting the reaction center or the component of PS II acceptor by modifying the secondary quinone-binding (QB) site. Co also reduces the export of photo-assimilates and the dark fixation of CO2. In C4 and crassulacean acid metabolism (CAM) plants, Co hinders the fixation of CO2 by inhibiting the activity of enzymes involved (Palit et al. 1994). Co (5 μM)-mediated inhibition in mung bean seedling growth was argued as a result of chlorosis of the younger leaves and to the strongly inhibited Fe transport to the shoot at this concentration (Liu et al. 2000). Li et al. (2009) evidenced 40 to 1708, 7 to 966, and 7 to 733 mg kg−1 as the effective concentrations of added Co causing 50 % inhibition (EC50) in plant shoot biomass for the barley (Hordeum vulgare), oilseed rape (Brassica napus), and tomato (Lycopersicon esculentum), respectively. Excess Co can induce variations of leaf pigmentation, very similar to chlorophyll mutations. To this end, Rancelis et al. (2006) reported a very clear individual polymorphism from normally green to yellowish, yellow, or even white seedlings of Vicia faba. Moreover, specific response of Vicia faba to Co (7.5-mM solutions of Co(NO3)2) exposure was manifested as chlorophyll morphosis induction and a different degree of chlorophyll morphosis phenotypes in individual plants (Rancelis et al. 2012). The real cause of leaf pigmentation variations due to Co exposure has been argued as a result of metal substitution by Co in a range of proteins including iron–sulfur proteins (Ranquet et al. 2007; Thorgersen and Downs 2007) and Rubisco (Van Assche and Clijsters 1986). In pigeon pea (Cajanus cajan), 400 μM Co caused 61 and 75 % reductions in the concentrations of chlorophyll a and chlorophyll b, respectively; however, the ratio of carotenoids to chlorophyll increased with an increase in Co supply from 10 to 400 μM Co (Gopal 2014). Moreover, at the same level of Co (400 μM), the growth reduction (measured as decreased d.w.) was more in aboveground parts (72–78 %) than roots (≈57 %) (Gopal 2014). Excessive amounts of Co within the plants can cause the failure of emergence and unfolding of young emerging leaves, chlorosis of subtending younger leaves, and restricted expansion of lamina (Chatterjee and Chatterjee 2003). In addition, disturbed activity of several enzymes, including Fe-containing enzymes, has also been reported in plants exhibiting excessive load of Co (Chatterjee and Chatterjee 2003). Co is known to cause irreversible damage to a number of vital metabolic processes of cells and cell membranes, and the prevention of Fe incorporation in protoporphyrin molecule resulting in the reduction of chlorophyll pigment (Jayakumar et al. 2009). In fact, excess Co can interfere with proteins and block the synthesis and activities of several enzymes and proteins responsible for chlorophyll biosynthesis (Jayakumar et al. 2009) (Fig. 2).

Fig. 2
figure 2

Simplified schemes highlighting the basic mechanisms underlying excess plant-beneficial heavy metal ion-mediated phytotoxicity (impacts on plant growth and development, oxidative stress, and cyto/genotoxicity) SOD superoxide dismutase, CAT catalase, GPX guaiacol peroxidase, GST glutathione sulfotransferase, APX ascorbate peroxidase, GR glutathione reductase, AsA ascorbate, GSH glutathione

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Oxidative stress and cyto/genotoxicity

Elevated Co can modulate oxidative stress traits and ROS-metabolizing system (Madhava Rao and Sresty 2000; Tewari et al. 2002; Kleizaite et al. 2004; Rancelis et al. 2012; Karuppanapandian and Kim 2013; Gopal 2014). Excess Co was evidenced to elevate the activity of SOD, an enzyme responsible for the dismutation of singlet oxygen (Madhava Rao and Sresty 2000). In Indian mustard (Brassica juncea) leaves, Karuppanapandian and Kim (2013) associated the effect of excess Co (100 μM) with a significant increase in the levels of proline, carbonylated protein, malondialdehyde, superoxide anion (O2·−), and hydrogen peroxide (H2O2). Despite decreased activity of CAT, peroxidase (POX), and SOD, AsA–GSH cycle-related enzymes including MDHAR, DHAR, and GR exhibited remarkable induction under 100-μM Co exposure (Karuppanapandian and Kim 2013). Co can induce polymorphism in SOD (especially for Cu/Zn SOD) most probably as a result of the phenotypic characteristics of the SOD polymorphism and subsequent occurrence of the replacement interactions therein by Co (Kleizaite et al. 2004). However, epigenetic processes may also contribute to SOD polymorphism in Co-exposed Vicia faba plants (Rancelis et al. 2012). Co (50 mg Co kg−1 soil) enhanced the activity of CAT, POX, and polyphenol oxidase (PPO) in horse gram (Macrotyloma uniflorum), whereas, 100–250 mg Co kg−1 soil decreased CAT activity but maintained the increasing activity of both POX and PPO (Jayakumar et al. 2013). In 10.0–400 μM Co-exposed Cajanus cajan, Gopal (2014) reported a lowered activity of CAT and SOD. Co has also been reported earlier to activate recombinant Arabidopsis phytochelatin synthase through modulating GSH and cysteine syntheses (Vatamaniuk et al. 2000). Reports are meagre on Co-accrued cyto/genotoxicity in plants. The differentiation of membrane complexes of the endoplasmic reticulum, as well as the dilation and subsequent destruction of membranes, may be caused by exogeneous Co (Herich and Bobák 1976). Co can inhibit the process of karyokinesis and cytokinesis by acting as a preprophase poison. Co compounds have been reported to act on the mitotic spindle, leading to the formation of chromatin bridges, fragmentation, and sticky bridges at anaphase and binucleate cells (reviewed by Palit et al. 1994). In yeast, Co mutagenicity was observed as mitochondrial respiratory deficiency, whereas an increased level of sulfhydryl compounds was reported in cytokinesis-deficient mutant of Chlamydomonas (reviewed by Palit et al. 1994). Co may also alter the sex of plant species (Wang et al. 1990). Co chloride-mediated significant increase in DNA damage was reported in Allium cepa roots (Yıldız et al. 2009). Co metal ions may also alter conformations of biomolecules (protein or nucleic acid) or break metabolic reactions mainly by interacting with biomolecules and binding them via reactive groups such as hydroxyl, carboxyl, and sulfhydryl (Cavusoglu and Yalcin 2010) (Fig. 2).

Copper

Status in soils, plant uptake, transport, and significance

Copper (Cu), a natural element with the atomic number of 29, and is found at variable concentrations in soil types. In general, soils may exhibit Cu concentrations in the range of 2.0 to 100 mg kg−1 (Lindsay 1979) or 1.0 to 40 mg kg−1 (Havlin et al. 1999). Average of 50 % of the Cu in soils remains insoluble and unavailable, 30 % is bound by organic sites, 15 % is in an oxide form, and 5 % is available for plant uptake (Barber 1995). Cu-deficient soils may exhibit less than 2.0 mg Cu kg−1 soil, where the factors such as pH, organic matter content, and plant species may influence Cu availability (Evans 2007). At pH 7.0, Cu availability may occur 10 to 100 times less than observed at soil pH 6.0. Nevertheless, Cu adsorption is also highly pH dependent and can be correlated with the Cu availability (Evans 2007).

Cu tends to accumulate in the root tissue with little translocated to the shoots (Marschner 1995). The rate of Cu uptake and its concentration in plants depend on environmental, physiological, and biochemical factors (Miotto et al. 2014). Also, the root exudates may influence Cu availability and motility in plants. Once inside cells, Cu chaperons perform Cu homeostasis, where the sequestration of Cu in a nonreactive form and “chaperon–transporter protein interaction” help to deliver Cu at the site of its requirement (Company and González-Bosch 2003). Cu transport in plants involves members of the P-type ATPase and Cu transporter (CTR) families and may also involve members of the natural resistance-associated macrophage protein (NRAMP) and Zn IRT-like protein (ZIP)/Fe-regulated transporter (IRT) families (reviewed by Puig et al. 2007). Cu-transporting P-type ATPases have been identified in the cytoplasmic and thylakoid membrane of the cyanobacterium Synechococcus sp., oxygenic phototrophs that can photolyze either H2O or H2S (Kanamaru et al. 1994). Nevertheless, P-type ATPase of Arabidopsis 1 (PAA1) protein was shown to be involved in Cu transport in chloroplasts and the control of photosynthesis rate (Abdel-Ghany et al. 2005). High-affinity Cu transporters (such as CTR family of transporters) also occur in the plasma membrane and tonoplast (Puig and Thiele 2002). RAN1 in Arabidopsis has been identified and is reported to be involved in sequestering Cu ions into the cytoplasm and post-Golgi vesicle (Himelblau and Amasino 2000). A number of factors such as Cu binding to roots, lack of Cu transport capacity in the xylem, and its poor shoot assimilation of xylem-transported Cu cause the slow translocation of Cu in plants (Liao et al. 2000).

In context with the significance of Cu, it is an essential nutrient for plant growth and development under optimal and stressful environments. Cu at 9.0 mg kg−1 has been considered beneficial for proper plant growth and development (Havlin et al. 1999). It is a significant constituent of protein components of several enzymes, mainly of those participating in electron flow, catalyzing the redox reaction in mitochondria and chloroplasts in the cytoplasm of plant cells (Ouzounidou et al. 1995). In chloroplasts, Cu is an important cofactor for photosynthetic electron transport, where Cu is an integral component of plastocyanin, a blue Cu protein (Raven et al. 1999). Cu is also a cofactor of Cu/Zn SOD enzyme (Bowler et al. 1994). Cu/Zn SOD1 (CSD1), CSD2, Fe SOD1 (FSD1), and Mn SOD1 (MSD1) are the examples of SOD genes identified in Arabidopsis (Kliebenstein et al. 1998). CSD1 encodes a Cu/Zn SOD active in the cytosol, and CSD2 encodes a Cu/Zn SOD active in the chloroplast stroma. The stromal SOD proteins are important to photosynthesis because of their role in the scavenging of ROS in the water–water cycle (Asada 1999) (Fig. 1).

Toxicity

Anthropogenic activities (such as application of sewage sludge, mine slag, industrial wastewaters, pig and poultry slurries, Cu-containing fungicides, and fertilizers) lead to the elevation of Cu to toxic levels in agricultural soils. Adequate Cu level for most plants generally occurs between 2.0 and 20 mg kg−1 d.w. (Mengel et al. 2001) or 5.0 to 30 mg kg−1 (Jones 1991). However, for most crop species, the critical toxicity level of Cu in the leaves is above 20 to 30 μg g−1 d.w. (Robson and Reuter 1981) and 20–100 mg kg−1 (Jones 1991). Since Cu range between adequate and toxic level is very narrow (2.0 to 30 mg kg−1 d.w.), a full care should be taken when supplying fertilizers for Cu deficiency in crop plants. Hereunder, major reports available on the excess Cu-mediated toxicity in terms of changes in the plant growth and development, oxidative stress, and its metabolism and cyto/genotoxicity are briefly discussed.

Plant growth and development

Cu at elevated levels can be a potential toxicant and inhibit root growth/morphology and retard overall plant growth and development by altering leaf chlorosis, chloroplast development, and subsequent loss of photosynthetic activity (reviewed by Yruela 2005; Burkhead et al. 2009; Thounaojam et al. 2014). Compared to shoots, plant root has been considered as the major site of Cu-toxicity and as one of the indicators of Cu status in the aboveground parts (reviewed by Yruela 2005; Michaud et al. 2008; Burkhead et al. 2009; Zhao et al. 2010; Thounaojam et al. 2014). In fact, Cu phytoxicity has been considered as rhizotoxicity, where it is possible to note roots with the following: (a) stunted growth and abnormal branching and (b) thickening and dark coloration (Marschner 1995; Yruela 2005; Michaud et al. 2008). In hydroponically grown durum wheat (Triticum turgidum subsp. durum), Cu2+ concentration above 1.0 μM was reported to cause interveinal chlorosis symptoms and global root growth alteration (Michaud et al. 2008). High Cu concentrations (200 and 500 μM) may also considerably reduce the growth of both root and shoots (Thounaojam et al. 2014) (Fig. 2).

Oxidative stress and cyto/genotoxicity

At optimum level, Cu participates as catalytic cofactors in multiple metabolic pathways owing to its redox properties. However, enhanced formation of ROS may also occur as a result of the same redox properties of Cu at its elevated concentration (Halliwell and Gutteridge 1999; Yruela 2005). In turn, nonmetabolized ROS may damage cells at the level of membranes, proteins, and nucleic acids (Halliwell and Gutteridge 1999; Gill and Tuteja 2010). Elevated Cu may cause membrane damage by oxidizing membrane (polyunsaturated) lipids, increasing electrolyte leakage, and enhancing the activity of lipoxygenase, an enzyme known to initiate the process of lipid peroxidation (Weckx and Clijsters 1997; Zhao et al. 2010). Enhanced activity of SOD (i.e., dismutation of superoxides into H2O2) may also be observed in excess Cu-treated plants (Zhao et al. 2010). Modulation of ROS-scavenging enzymes (such as APX, CAT, GPX, POX, thiol-dependent peroxidases, TDPs, peroxiredoxins (PRXs), MDHAR, DHAR, GST, and GPX) has been reported in elevated Cu-exposed plants (Khatun et al. 2008; Rout and Sahoo 2013; Thounaojam et al. 2014). Molecular studies also evidenced a higher expression of stress enzyme genes (such as CAT, RsCat; SOD, Mn SOD) under elevated Cu levels (Rout and Sahoo 2013). Considering elevated Cu-mediated cyto/genotoxicity in plants, elevated Cu-led enhanced ROS may exert damaging effects on biological macromolecules, including DNA (Halliwell and Gutteridge 1999). At the transcriptional level, Cu has been reported to cause the biphasic induction of two ribonucleotide reductase (RNR) genes in Arabidopsis (Mediouni et al. 2008). Toxic effects of Cu on Withania somnifera plants were reflected by the increased level of malondialdehyde (MDA), a decomposition product of polyunsaturated fatty acid hydroperoxides and a usual indicator of membrane lipid peroxidation. The higher lipoxygenase (LOX) activity, coupled with higher H2O2 and superoxide anions (O2 −) content in Cu-treated plants, confirmed that Cu has potential to induce cytotoxicity in plants at elevated levels (Khatun et al. 2008) (Fig. 2).

Iron

Status in soils, plant uptake, transport, and significance

Iron (Fe) is the fourth most abundant metal in the Earth’s crust and has the atomic number of 26. It commonly occurs as metallic element (in Fe+2 or Fe+3 states; predominantly, as Fe+3 oxides) and comprises about 4.6 % of the igneous and 4.4 % of sedimentary rocks (Bodek et al. 1988). Though Fe concentrations in soils vary with the regions, soil types, and presence of other sources, typical soils may exhibit Fe in the range of 20,000 to 550,000 mg kg−1 (Bodek et al. 1988). Soil characteristics such as soil pH and soil organic carbon control Fe availability to plants, where Fe solubility in soil decreases by 1000-fold by each unit increase of soil pH (4–9) (Lindsay 1979; Fageria et al. 2002).

In context with plant Fe uptake and transport, two strategies, namely, strategy I and strategy II are adopted by plant roots for Fe uptake under different soil conditions (Römheld and Marschner 1986). Between the two strategies, reduction-based strategy I for Fe uptake is prevalent in all higher plants, whereas Gramineae plants use the strategy II. Extensive reports are available on the basic mechanisms underlying the strategy I uptake of Fe in plants (Olsen et al. 1981; Henriques et al. 2002; Waters et al. 2002). Basically, the strategy I uptake of Fe in plants involves three reactions: (a) uptake of Fe by release of proton (Olsen et al. 1981), (b) uptake by Fe(III) chelate reduction (Waters et al. 2002), and (c) uptake via plasmalemma-resident Fe2+ transport system (Henriques et al. 2002). On the other hand, species of the Poaceae (grasses) family, such as corn (Zea mays), wheat (Triticum aestivum), and rice (Oryza sativa) use the chelation-based strategy II uptake for Fe (Curie et al. 2001; Ishimaru et al. 2006). Fe transport in plants may involve two major processes such as (a) long-distance Fe transport (von Wirén et al. 1999) and (b) intracellular Fe transport (Thomine et al. 2003). Considering the significance of Fe in plants, it is an essential element for plants and required for respiration, photosynthesis, nitrogen fixation, and many others cellular function such as DNA synthesis and hormone production (Becana et al. 1998; Møller et al. 2007). Compared to other micronutrients, Fe contents in plant tissues are normally higher (≈2.0 μmol g−1 plant d.w.) (Marschner 1995). Important proteins (such as ferredoxin and cytochromes) and antioxidative enzymes (such as CAT, POX, and SOD) are constituted by Fe. Nevertheless, Fe is significantly involved in photosynthetic functions, where its major portion (approx. 80 %) is shared in the photosynthetic cells, the electron transport system, and the construction of Fe–S clusters (reviewed by Hänsch and Mendel 2009) (Fig. 1).

Toxicity

Elevated Fe in soil/plant growth medium may affect cell homeostasis and trigger the processes of toxicity or even nutritional deficiency (Ducic and Polle 2005). Nevertheless, the generation of the highly reactive hydroxyl radical can be contributed by Fe3+ and Fe2+ (Halliwell and Gutteridge 1999; Gill and Tuteja 2010). Thus, the previous processes in isolation or combination may bring several consequences in the form of significant changes in plant growth and development, oxidative stress, and toxicity at cellular and/or genetic levels. Previous aspects are briefly discussed hereunder.

Plant growth and development

Both deficiency and excess of Fe can affect the process of homeostasis and trigger toxic consequences in plants; however, extensive reports are available on the Fe deficiency-mediated consequences in plants (Nenova 2009; Yadavalli et al. 2012). Excess Fe can lead to the appearance of brown spots on the leaves, which often leads to necrosis (Zhang et al. 1999). In O. sativa, excessive Fe2+ absorption was reported to cause Fe toxicity symptoms called “bronzing,” brown spots starting from the leaf tips and spreading toward the leaf base (De Dorlodot et al. 2005). Moreover, excess Fe level may cause stunted root and shoot growth, altered photosynthesis, and decreased chlorophyll concentrations (De Dorlodot et al. 2005; Chatterjee et al. 2006). Fe availability above 200 μM led to a decrease in the growth rate of the cultures of Chlorella vulgaris (Estevez et al. 2001). Cell death in rice roots due to iron toxicity was investigated using inhibitors of signal molecules known to regulate programmed cell death in plants (Tsai and Huang 2006). There is little information about the effects of excess Fe concentrations on the energy flow in PS II and the electron transport chain, which generate ATP and NADPH. Both deficiency (0.001 mM) and excess (>0.1 mM) of iron reduced the potato tuber yield of, by lowering the concentration of sugars, starch and protein nitrogen and increasing the accumulation of nonprotein nitrogen and phenols in potato tubers (Chatterjee et al. 2006) (Fig. 2).

Oxidative stress and cyto/genotoxicity

Uptake of large amounts of Fe2+ by roots, its transport to shoot, and subsequently elevated Fe2+-mediated acceleration in the formation of ROS (such as singlet oxygen, superoxide radicals (·O2), hydrogen peroxide (H2O2), and hydroxyl radical (·OH)) via Fenton reaction have been considered as a major cause of excess Fe2+-mediated oxidative stress in plants (Becana et al. 1998). Fe-accrued oxidative stress may trigger the activation of some ferritin genes (Gaymard et al. 1996). In turn, elevated levels of nonmetabolized ROS may degrade cellular structures, including the plasma membrane, by inducing lipid peroxidation, which may alter its structure and permeability (Connolly and Guerinot 2002). Available reports on Fe-mediated modulation of oxidative stress traits and ROS-metabolizing systems are confined to Fe deficiency (Marschner 1995; Becana et al. 1998; Sharma et al. 2004). However, the reports on previous aspects in excess Fe-exposed plants are scanty. In context with Fe-mediated cyto/genotoxicity in plants, excess Fe-mediated increased cell death was reported in O. sativa roots (Tsai and Huang 2006). Nevertheless, once ROS is produced in cell due to the Fe toxicity, it can directly damage the lipids, proteins, and nucleic acids and lead to cell damages and/or a programmed cell death (reviewed by Connolly and Guerinot 2002). DNA can be modified by ROS in many different ways, mainly on the nucleotide bases. ·OH is the most reactive, 1O2 primarily attacks guanine, and H2O2 and O2·- do not react at all (Wiseman and Halliwell 1996). No comparable information is available in the literature for plant cells in particular on Fe toxicity; however, ROS damage to both mitochondrial and nuclear DNAs was reported as nonrandom and mutation clusters at hot spots were also observed (Halliwell and Gutteridge 1999) (Fig. 2).

Manganese

Status in soils, plant uptake, transport, and significance

Manganese (Mn) is a ubiquitous element in the environment, the 12th most abundant element in the Earth’s crust, and has the atomic number of 25 (CICAD 2004). Though Mn occurs in sufficient amount in the soils to supply the needs of the vegetation, various anthropogenic activities cause Mn enrichment in the soil that in turn may pose a serious threat to plant and animal lives (Pinsino et al. 2012). Among several oxidation states (such as 0, +2, +3, +4, +6, and +7), +2 (Mn2+) is the most soluble state in the soil solution and the most available form for plants (Marschner 1995; Adriano 2003). Nevertheless, only Mn+2, Mn+3, and Mn+4 forms occur in biological systems (Millaleo et al. 2010). Soil pH largely controls Mn solubility and bioavailability in the soil, where a high pH allows Mn adsorption into soil particles and thereby decreases Mn availability (Fageria et al. 2002). The total Mn varies with soil types and the environmental conditions. The soil Mn level may occur in the range of 450–4,000 mg kg−1 soil (Adriano 2003). The background (natural) levels of Mn in soil may occur in the range of 1.0–4000 mg kg−1 d.w. with a mean value in the range of 300–600 mg kg−1 d.w. (CICAD 2004).

Root epidermal cells adsorb Mn2+ via active diffusion (Gherardi and Rengel 2003). However, H+-ATPase largely controls the Mn2+ uptake process by creating an electrochemical gradient across root cell plasma membrane and subsequently modulating the ion channels embedded therein (Rengel 2000). Plant Mn uptake may occur in two phases: (a) the initial uptake of Mn2+ occurs in apoplast of root cells; involves adsorption by the negatively charged cell wall constituents; and is very rapid, reversible, and nonmetabolic, and (b) the second phase is slow and metabolic process, where Mn2+ is taken by the symplast (Rengel 2000). Not much is known about the molecular mechanism potentially involved in Mn uptake in plant cells. However, the studies on several bacterial and yeast species have yielded significant outcomes in this context. A number of transporter gene families (such as cation/H+ antiporters, zinc-regulated transporter/iron-regulated transporter (ZRT/IRT1) and its related protein (ZIP) transporters, natural resistance-associated macrophage protein (NRAMP), cation diffusion facilitator (CDF), and P-type ATPases) have been implicated in plant Mn2+ transport (Thomine et al. 2003; Pittman 2005; Ishimaru et al. 2010; Chen et al. 2013). Once in the plant system, Mn can be distributed at organ and cellular levels. Compared to belowground organ (roots), Mn tends to accumulate in aerial tissues (Page and Feller 2005), where the leaves may exhibit the highest Mn level (Lidon 2001). Owing to its high mobility in the xylem, Mn is transported from root to shoot and leaves through the transpiration stream via xylem sap (Le Bot et al. 1990). Though Mn is relatively phloem immobile (Page et al. 2006), the plant species and stage of development-dependent remobilization of leaf-associated Mn into other organs are reported in different plant species (Rengel 2000; Marschner and Rengel 2007). At cellular level, the highest Mn2+ accumulation may occur in vacuoles (González and Lynch 1999) followed by chloroplast, cell wall (Pittman 2005), and endoplasmic reticulum (Wu et al. 2002). Considering Mn significance in plants, it is one of the beneficial/essential heavy metal ions for plant growth and development. Mn is involved in the structure of several photosynthetic proteins and enzymes, influences the biosynthesis of growth substances, chlorophyll, and secondary metabolites. It modulates metabolism of carbohydrates and lipids, stress tolerance, redistribution of trace elements, and other heavy metals in soils. Mn also affects the nucleic acid biosynthesis and the gene expression (Marschner and Rengel 2007) (Fig. 1).

Toxicity

Though Mn is an essential micronutrient, elevated levels of Mn can be toxic and bring a number of damaging consequences in plants including the arrest of plant metabolic processes and the distortion in photosynthetic apparatus (Ducic and Polle 2005; Millaleo et al. 2010). Excess Mn-mediated changes in the plant growth and development, oxidative stress traits, and cyto/genotoxicity are briefly discussed hereunder.

Plant growth and development

Excess Mn can lead to loss of chlorophyll (via chlorosis; interveinal and marginal), necrotic brown leaf spots (particularly on older leaves), leaf cupping, crumbled, distorted, small-sized leaves, reduced chloroplast size, and chloroplast disintegration (Kitao et al. 2001). Decreased photosynthetic rate, CO2 assimilation, photosynthetic electron transport chain and stomatal conductance (Li et al. 2010), and inhibited chlorophyll synthesis as a result of blocked Fe-concerning process (Clairmont et al. 1986) have also been reported due to elevated Mn level. Between the photosystems I and II, Mn toxicity mainly targets PS I (Millaleo et al. 2013). Elevated Mn can bring a number of changes at histological/ultrastructural level. Elevated Mn can lead to decreased leaf cell volume, cell disintegration and collapsed mesophyll and epidermal cells (Dienelt and Lawson 1991), and alterations in the structure of endoplasmic reticulum, Golgi bodies (Izaguirre-Mayoral and Sinclair 2005), and mitochondria (Santandrea et al. 1998a). Damages in the structure of cell membranes (measured as ruptured and detached plasma membrane from cell wall) as a result of excess Mn can also be found (Punz and Sieghardt 1993; Santandrea et al. 1998a). As reported in Citrus volkameriana, elevated Mn can also cause increases in the length of palisade parenchyma cells and thickness of spongy parenchyma and induction in the swelling of thylakoids (both granal and stromal) in chloroplasts (Papadakis et al. 2007). Elevated Mn can also cause a complete disorganization of chloroplast with underdeveloped grana, disordered thylakoids, and scarce starch granules (dos Reis and Lavres 2011), cytoplasm disorganization and increased vacuolization (José et al. 2010), and reduction in length of root cap and apical meristem, particularly in the peripheral cells (Punz and Sieghardt 1993) (Fig. 2).

Oxidative stress and cyto/genotoxicity

Altered metabolic processes and disrupted cell homeostasis have been credibly reported as a result of excess Mn (reviewed by Millaleo et al. 2010). Excess Mn2+-mediated stimulation in the generation of ROS (such as H2O2, O2 −·, and OH·) has been reported in plants (Demirevska-Kepova et al. 2004; Li et al. 2010; Maksimović et al. 2012). Elevated Mn-mediated modulation of plant antioxidant machinery has been reported (Fecht-Christoffers et al. 2003; Li et al. 2010; Zhao et al. 2012). Excess Mn may enhance the activity of apoplastic guaiacol peroxidase, NADH peroxidase (Fecht-Christoffers et al. 2003), and peroxidase (Maksimović et al. 2012). Excess Mn may also upregulate or downregulate SOD activity in plants. In particular, increased activity of Mn SOD type was reported in a number of plants including Pisum sativum (del Río et al. 1985) and Glycine max (Leidi et al. 1987). In contrast, Demirevska-Kepova et al. (2004) evidenced diminished SOD activity in elevated Mn-exposed Hordeum vulgare. In addition to SOD, excess Mn enhanced the activity of other antioxidant enzymes such as APX (Demirevska-Kepova et al. 2004; Zhao et al. 2012), peroxidase (Zhao et al. 2012), CAT (Li et al. 2010; Zhao et al. 2012), GPX, GR, and MDHAR (Li et al. 2010).

Very few reports are available in the literature on the excess Mn-mediated cyto/genotoxicity in plants. Elevated Mn is known to induce chromosomal and mitotic alterations (Fiskesjö 1988) and also to increase the frequency of chromosomal aberrations (Mukhopadhayay and Sharma 1990). Mn (5.2 mM) was reported to induce chromosomal and mitotic alterations in Allium cepa root tip cells (Fiskesjö 1988). In Vallisneria spiralis, Allium cepa, and Pisum sativum, Mukhopadhayay and Sharma (1990) evidenced clastogenic effects and increased frequency of chromosomal aberrations in terms of breaks, gaps, translocations, and spindle disturbances as a result of both cationic and anionic salts of Mn. Santandrea et al. (1998b) reported 2.0-mM Mn-mediated increase in prophase and anaphases, whereas Mn at 5.0 mM increased the number of telophase chromosomal aberrations (such as sticky metaphase, micronuclei formation and even inhibition of spindle formation) in in vitro-cultured Nicotiana tabacum. In green gram (Vigna radiata), excess Mn (≥25 mg L−1) was evidenced to cause a number of mitotic abnormalities such as bridges and laggard formation and chromosome stickiness and increased the frequency of abnormal cells; decreased total, absolute, and average chromosome length; and increased the number of binucleate cells (Mumthas et al. 2010) (Fig. 2).

Molybdenum

Status in soils, plant uptake, transport, and significance

Molybdenum (Mo), a rare transition element with the atomic number of 42, is the 54th most abundant element in the Earth’s crust (constituting 1.2 mg kg−1) and the least abundant trace element in the soil (Purvis 1955). Plants (and also animals, humans and microbes) require Mo for the maintenance of their optimum growth and development (McBride et al. 2000; Kaiser et al. 2005). However, Mo is the heaviest of all elements required by the plants (Purvis 1955). Mo exists in rocks in mineral forms such as molybdenite (MoS2; molybdenum disulfide), wulfenite (PbMoO4; lead molybdate), and ferrimolybdenite [Fe2(MoO4)3; ferric molybdate] (Reddy et al. 1997). Mo is released and enters soil upon weathering previous rock-associated mineral forms of Mo (Lindsay 1979). Though oxidation states of Mo range from −2 to +6 in soil, molybdate [Mo(VI); MoO4 2−] is the predominant form in agricultural soils. The background concentration of Mo in soils may range from 0.2 to 6.0 mg kg−1, whereas metal-rich soils may exhibit 10 to 100 mg Mo kg−1 (He et al. 2005). Soil properties including pH, organic compounds, water drainage, and amount of F -oxides were reported to control the Mo availability for plants (Kaiser et al. 2005; McGrath et al. 2010a). Very few reports are available on the Mo uptake and transport in higher plants, and also the mechanism of uptake of Mo in plants is poorly understood. Nonetheless, the most of the Mo transport systems have been characterized in prokaryotes and some lower-order eukaryotes (Self et al. 2001; Hänsch and Mendel 2009; Mendel 2013). Owing to its unique chemical properties, the MoO4 2− (rather than its other complex forms) is the major transportable form of Mo (Marschner 1995). The long-distance transport of Mo is facilitated via both xylem and phloem. Notably, high-affinity Mo transporter or nonspecific sulfate/phosphate transporters (via plasma membrane) have been reported to facilitate Mo (MoO4 2−) uptake (Mendel 2013). ModABC systems were reported to perform Mo transport in prokaryotes (Self et al. 2001). Arabidopsis thaliana (Tomatsu et al. 2007; Baxter et al. 2008) was evidenced to exhibit molybdenum transporter 1 (MOT1), a transport protein and relative of sulfate transport superfamily, and has been identified as a high-affinity molybdate transporter. MOT1 protein is localized in plasma membrane and vesicles in Arabidopsis thaliana (Tomatsu et al. 2007). In context with the significance of Mo in plants, Mo itself is biologically inactive; however, it is a constituent/cofactor (molybdopterin cofactor (Moco)) of more than 60 metalloenzymes and proteins, including nitrogenase (nitrogen-fixing enzyme) and nitrate reductase (NR, the primary nitrogen assimilation enzyme) (Kaiser et al. 2005; Mendel and Schwarz 2011; Mendel 2013). Mo has been reported as a major constituent/cofactor in as a host of enzymes including NR and nitrogenase enzymes (found in bacteroids of legume nodules), xanthine dehydrogenase/oxidase (involved in purine catabolism and ureide biosynthesis in legumes), aldehyde oxidase (AO, involved in ABA biosynthesis), and sulfite oxidase (convert sulfite to sulfate, an important step in the catabolism of sulfur-containing amino acids) (Kaiser et al. 2005; Hänsch and Mendel 2009; Mendel 2013) (Fig. 1).

Toxicity

Since Mo requirement for the growth of plants is in traces, there exists a very narrow range between its deficiency and toxicity (McBride et al. 2000). Compared to other trace elements, the range between Mo excess and Mo deficiency has been suggested to be 50 times greater (Hewitt 1951). In general, the plant requirement for Mo occurs in the range of 0.1–1.0 ppm. However, Brassicaceae plants exhibit a have greater demand for Mo. Moreover, high Mo accumulation was reported in the leguminous plants when compared to the cereals (Shivashankar and Hagstrom 1991). Plants can tolerate high levels of Mo (up to 1000 ppm) in the medium (McGrath et al. 2010a). Very few studies have been undertaken to investigate Mo toxicity in plants.

Plant growth and development

Toxicity of Mo hardly occurs in field conditions; rather, it can be induced under experimental conditions (Brune and Dietz 1995). Higher Mo doses may impact plant growth as well as yield (Rout and Das 2002; McGrath et al. 2010b). Elevated Mo uptake can interfere with metabolic processes and cause a number of physiological disorders (Kabata-Pendias and Pendias 1992; Rout and Das 2002). Mo toxicity is a major factor limiting plant growth and yield in poorly drained acidic soils, because these conditions support Mo availability (Rout and Das 2002). Reduction in growth traits such as root and shoot length was reported in Cicer arietinum due to excess Mo (>7.5 ppm) (Datta et al. 2011). Mo (≥1.5 ppm) can also alter anatomy of leaves, root, and stem (Datta et al. 2011). Nevertheless, these authors also observed a complete distortion of mesophyll tissue in leaves due to 3.0 ppm of Mo and an undifferentiated mass of cells under exposure of 10 ppm Mo. Altered lamina morphology, disorganized thylakoids in chloroplasts, and impaired PS II and photosynthetic activity were found in Mo (50–600 μM)-exposed metal-tolerant hydrophyte, Trapa natans (Baldisserotto et al. 2013). Reduced plant growth in terms of decreased root and hypocotyl length, and cotyledon length and width was recently reported in 10-mM Mo-exposed cabbage (Brassica oleracea var. capitata) (Kumchai et al. 2013). The presence of excess Mo was correlated by these authors with the enhanced levels of blue/purple anthocyanin in their cotyledons and hypocotyls. Excess Mo can also cause increase in the accumulation of total chlorophyll and that of chlorophylls a and b (Datta et al. 2011; Kumchai et al. 2013). Enhancement in the accumulation of anthocyanin can also be found in elevated Mo-exposed plants (Stroud et al. 2010). Excess Mo has also been reported earlier to cause the deficiency of other mineral nutrients such as Mn and Mg (Brune and Dietz 1995) (Fig. 2).

Oxidative stress and cyto/genotoxicity

The studies related to Mo-induced alterations in plant oxidative metabolism and genotoxicity are rare. Excess Mo-mediated alterations in plant antioxidant enzymatic system has been reported (Rout and Das 2002; Stroud et al. 2010; Kumchai et al. 2013). Higher levels of Mo have been correlated positively with increased activity of POX and CAT in Oryza sativa that were considered important for screening of plants for their Mo tolerance (Rout and Das 2002). Significantly induced activity of antioxidant enzymes can be found in plants under elevated Mo exposure (Stroud et al. 2010; Kumchai et al. 2013). Upregulation of genes coding for phenylalanine ammonia lyase, chalcone synthase, flavonone 3-hydroxylase, leucoanthocyanidin dioxygenase, and GST and that of involved in anthocyanin biosynthesis and accumulation pathways was reported in elevated Mo-exposed plants (Kumchai et al. 2013). Considering the genotoxicity of excess of Mo in plants, an exhaustive literature search revealed no study report in this regard. However, extensive reports are available on nano/bulk-Mo-mediated significant cyto/genotoxicity in mammalian models (Braydich-Stolle et al. 2005; Sun et al. 2007). Hence, efforts should be made to explore these aspects also in plant systems.

Nickel

Status in soils, plant uptake, transport, and significance

Nickel (Ni), a naturally occurring 22nd most abundant element in the Earth’s crust, has the atomic number of 28 and is also present in soil environment (Sunderman and Oskarsson 1991). Ni occurs either as a free metal (a mixture of five stable isotopes) in igneous rocks or in combination with Fe. There occur various valence states of Ni (from +1 to +4), but Ni is present in the environments predominantly in its divalent form [Ni(II); Ni2+] that is the form most prevalent in soils and available to plants (Coogan et al. 1989). Anthropogenic sources may contribute 43 × 106 kg (Schmidt and Andren 1980) or 55.6 × 106 kg (Nriagu and Pancyna 1988). Nearly 66 % of anthropogenic-sourced Ni enters into the soil system (Nriagu and Pancyna 1988). Agricultural soils may exhibit Ni in the range of 3.0–1000 mg kg−1 (Brown 2006), with a mean range between 20.0 and 40.0 mg kg−1 (Uren 1992). However, polluted soils may exhibit Ni concentrations in the range of 200 to 26,000 mg kg−1 (20- to 30-fold higher than the overall range, i.e., 10–1000 mg kg−1) (Izosimova 2005).

Considering Ni uptake, transport, and distribution in plants, much less information is available on Ni uptake, as compared with other micronutrients (reviewed by He et al. 2012). Ni is taken up by plants predominately via roots through passive diffusion as well as active transport (reviewed by Seregin and Kozhevnikova 2006). However, the relative uptake of Ni through passive or active transport mechanism varies with plant species, oxidative state, soil pH, presence of other metals, and concentration of Ni in the soil, or nutrient solution (Dan et al. 2000; Podar et al. 2004). Nevertheless, cation transport system has also been reported to facilitate the absorption of soluble Ni (Weng et al. 2004). Ni2+ uptake may involve two phases: an initial rapid phase followed by a slow linear phase (Tumquist et al. 1990). A number of factors such as Ni concentration in the growth medium, speciation (form and origin), pH (acidity), the presence of other metals and organic matter, cation exchange capacity, plant species, plant organ, and plant metabolism have been credibly reported to modulate Ni absorption (Nishida et al. 2011). Additionally, the soil pH, organic matter content, CEC, and Fe oxides/hydroxides control the bioavailability of Ni2+ for plants (Weng et al. 2004). Once in plant root, the translocation of root-associated Ni to other plant parts takes place (Peralta-Videa et al. 2002). Ni can be readily retranslocated from older to younger leaves since Ni2+ is highly mobile in plants (Gray and Mclaren 2006). The process of the Ni2+ mobility (for example, to seeds, buds, and fruits) is intensely controlled by metal–ligand complexes (Rauser 1999) and transporter proteins (Colpas and Hausinger 2000). Particularly in hyperaccumulators, the flow of xylem sap can rapidly and efficiently transport xylem-loaded Ni to the shoots (Maestri et al. 2010). Nevertheless, the role of organic acids (Boominathan and Doran 2002) and transport proteins (Nishida et al. 2011; Merlot et al. 2014) has been reported in the control of root-to-shoot Ni2+ transport.

In context with Ni significance in plants, Ni, an essential micronutrient for plants (at 0.01–5.0 mg kg−1 d.w.), in small amounts enhances the growth and yield of plants (Ahmad and Ashraf 2011; López and Magnitskiy 2011). Ni is an important constituent of many metalloenzymes (such as urease, methyl-coenzyme M reductase, Ni–Fe hydrogenase, Ni–SOD, acetyl-coenzyme A synthase, RNase-A carbon monoxide dehydrogenase, and hydrogenases (Marschner 1995; Negi et al. 2014). It is also essential for synthesis of anthocyanins (López and Magnitskiy 2011). Particularly in nodulated legumes, Ni is required for biological N2 fixation and activation of enzyme hydrogenases (López and Magnitskiy 2011) (Fig. 1).

Toxicity

A range of anthropogenic activities can cause significant elevations in soil Ni (Kukier and Chaney 2004; Maestri et al. 2010). Excess Ni can be toxic to plants and cause a number of morphological and physiological/biochemical consequences and subsequently limitation to plant growth, development, and productivity. A brief description is presented hereunder on the excess-Ni-mediated changes in plant growth and development, oxidative stress, and toxicity at cell and genetic levels.

Plant growth and development

High doses of Ni negatively affect plant growth and physiological mechanisms and also induce visible toxicity symptoms (Negi et al. 2014). General symptoms/responses linked with Ni toxicity in plants may include the following: inhibited germination, reduced root and shoot growth and biomass, poor branching system, distorted various plant parts, irregular flower shape, decreased leaf area, leaf spotting, disturbed (root tip) mitosis, increased Fe deficiency, severe chlorosis and foliar necrosis, and significantly decreased crop yield (Seregin and Kozhevnikova 2006; Ahmad and Ashraf 2011; Negi et al. 2014). Excess Ni can interfere several physiological and metabolic processes, including mineral absorption by roots, photosynthesis, stomatal conductance and transpiration, plant water relations, nitrogen metabolism, symbiotic association of Rhizobium with legumes/N2 fixation, activity of plasma membrane-bound H+-ATPase, carbohydrate metabolism, and enzyme activities (Sheoran et al. 1990; Pandey and Sharma 2002; Gajewska and Sklodowska 2009; reviewed by Yusuf et al. 2011; Negi et al. 2014). A high dose of Ni may also decrease photosynthetic pigments (Sheoran et al. 1990); block chlorophyll synthesis (Seregin and Kozhevnikova 2006); interfere with the process of photosynthesis such as light-harvesting complex II, Hill’s reaction, and electron transport chain (Ghasemi et al. 2012); and inhibit the activities of enzymes of Calvin cycle (Dan et al. 2000) (Fig. 2).

Oxidative stress and cyto/genotoxicity

Having relatively high oxidation/reduction potential, Ni does not effectively catalyze the reaction (Fenton/Haber–Weiss reaction) leading to the direct production of OH· (Kehrer 2000). However, chelating agent-catalyzed Ni-mediated production of H2O2 and OH· is possible (Torreilles and Guérin 1990). Moreover, Ni-induced oxidative stress in plants was argued as a result of the occurrence of a competition between Ni and Fe in biochemical and physiological processes and also due to Ni-mediated modulation of the activities of antioxidant Fe enzymes (e.g., Fe SOD and CAT) (Seregin and Kozhevnikova 2006; Pandey and Sharma 2002; Gajewska and Sklodowska 2007; Nishida et al. 2011). Plasma membrane-bound NADPH oxidase has been shown to be involved in O2 −· generation under Ni toxicity and induced oxidative stress (Hao et al. 2006). Ni in excess may also induce the accumulation of H2O2 (Gajewska and Sklodowska 2007; Gajewska et al. 2012; Nasibi et al. 2013). Higher dose of Ni can induce lipid peroxidation (measured as MDA accumulation) (Madhava Rao and Sresty 2000; Cardoso et al. 2005; Lu et al. 2010; Gajewska et al. 2012) and electrolyte leakage and can also alter fatty acid composition in membranes (Gajewska et al. 2012). Excess Ni can also modulate plant antioxidant defense components. However, it depends upon the Ni content in the medium, period of exposure, genotype, and plant tissue (reviewed by Seregin and Kozhevnikova 2006; Yusuf et al. 2011). In 6-day-old seedlings of Cajanus cajan, Madhava Rao and Sresty (2000) reported elevated activity SOD, peroxidase, and GR and declined CAT activity and the level of GSH and AsA. Upon Ni exposure (0.075 and 0.75 mM) to Nicotiana tabacum cells, CAT activity was found to play a significant role in early response to Ni-induced oxidative, particularly at low Ni concentration (0.075 mM), whereas at higher Ni concentration (0.75 mM), GR is involved (Pompeu et al. 2008). Gajewska and Sklodowska (2005) found an increased activity of APX in leaves and that of GST in roots of 14-day-old Pisum sativum plants under Ni stress (10–200 μM). Studies have also reported a significantly enhanced activity of SOD, peroxidase, and APX in the roots of 30-day-old Zygophyllum xanthoxylon growing in soils contaminated with 50–450 mg Ni kg−1 (Lu et al. 2010). Nasibi et al. (2013) reported Ni toxicity-accrued increase in the activity of APX, GPX, and CAT. Studies have found increased activity of SOD and peroxidase in the seedlings of Cajanus cajan (Madhava Rao and Sresty 2000) and Zea mays (Mishra et al. 2010). In contrast, Ni stress has been observed to decline the activities of CAT and peroxidase in Brassica spp. (Pandey and Sharma 2002), SOD and CAT in Alyssum bertolonii (Boominathan and Doran 2002), APX and SOD in the leaves and roots of 14-day-old Pisum aestivum (Gajewska and Sklodowska 2005), and CAT activity in Cajanus cajan (Madhava Rao and Sresty 2000) and T. aestivum (Gajewska and Sklodowska 2007). Proline accumulation has also been observed in a number of excess Ni-exposed plants including Pisum sativum (Gajewska and Sklodowska 2005) and Hyoscyamus niger (Nasibi et al. 2013).

Considering Ni-accrued cyto/genotoxicity in plants, the presence of even very low Ni levels that do not normally cause phytotoxicity can cause genotoxicity (Mičieta and Murín 1998). Extensive reports are available on excess Ni-mediated cyto/genotoxicity in plants. Excess Ni can depress root meristem mitotic activity (El-Ghamery et al. 2002; Demchenko et al. 2010). Ni (100 μM) was reported to suppress cell proliferation (DNA synthesis and division) in root meristem including cap, main meristem, and quiescent center region of T. aestivum (Demchenko et al. 2010). Induced chromosomal aberration (clastogenic effects) (Fargašová 2012) and reduced genomic template stability and increased polymorphism (Erturk et al. 2013) have also been evidenced in plants under elevated Ni concentrations. Nevertheless, elevated Ni-mediated enhanced ROS can cause point mutations and strand breakages, thereby can interfere directly or indirectly with the DNA repair system (Hartwig et al. 1994). As observed in Nigella sativa and T. aestivum, high levels of Ni (10–25 ppm) can reduce DNA and RNA content (El-Ghamery et al. 2002). Excess Ni was reported to induce DNA polymorphism through point mutations and cause alterations in DNA sequence in Jatropha curcas (Sarkar et al. 2010). Ni ions can also change cytosine methylation patterns, causing hypomethylation or hypermethylation of DNA (Kovalchuk et al. 2001). Ni ion-mediated DNA damage is important to discuss since hypermethylation can cause chromosomal instability and hypomethylation can increase the susceptibility of the chromosomes to breakages (Kovalchuk et al. 2001). Ni ions have greater affinity for proteins than for the DNA and that Ni ions can significantly interact with proteins and other intracellular molecules and prevent proteins from binding to DNA (Lee et al. 1998). Ni compounds may induce single-strand DNA breaks and also form DNA–protein cross-links (Knasmüller et al. 1998). Thus, Ni-induced genotoxicity could be due to interactions of Ni ions with the chromatin proteins, where Ni ions may affect heterochromatic regions that exhibit a compact protein–DNA structure. Ni-accrued mutations were reported earlier as a result of Ni-induced DNA damage and the formation of DNA–protein cross-links (Rossman 1995) (Fig. 2).

Zinc

Status in soils, plant uptake, transport, and significance

Zinc (Zn), a transition metal of atomic number 30, is the 24th most abundant element on the Earth and is also essential for plants, animals, humans, and microorganisms (Alloway 2008b, 2013). Though Zn occurs naturally in air, water, and soil, anthropogenic sources (such as fossil fuel combustion and mine waste), excessive use of fertilizers, limestone, manures, and sewage sludge and use of agrochemicals trigger steady elevations in its concentration in the environment (Alloway 2008b). In particular, urban refuse, agricultural food wastes, and fertilizers were estimated earlier to contribute respectively 22–97, 12–150, and 0.26–1.1 106 kg Zn year−1 (Nriagu and Pancyna 1988). Weathering of parent rocks has been considered as the major contributor of Zn in soils, where the geochemical composition of the parent rocks largely controls the total soil Zn content. Zn may occur in the range of 70–80 μg Zn kg−1 and 10–120 μg Zn g−1, respectively, in the lithosphere and sedimentary rocks (reviewed by Broadley et al. 2007; Alloway 2008b). Agricultural soils may exhibit Zn in the range of 10–300 μg Zn kg−1 (Barber 1995). The world average of background total concentration of Zn in soils has been estimated as 55 mg Zn kg−1 (Alloway 2008b). Moreover, the amount of Zn in the soil varies with the type of soil texture (Kabata-Pendias and Pendias 1992). Zn in soil mainly occurs in three forms: (i) water-soluble Zn (free ions: Zn2+ and ZnOH+ and soluble organic fractions), (ii) adsorbed and exchangeable Zn in colloids (clay particles, humic compounds, Al and Fe hydroxides), and (iii) insoluble Zn complexes and secondary minerals (Lindsay 1979; Barber 1995). The soil-specific precipitation, complexation, and adsorption reactions of Zn are the major factors that determine the distribution of Zn in soils (Alloway 2008b). Soil pH largely governs Zn speciation and distribution in the soil (Alloway 2008b).

In context with the Zn uptake and transport, soil Zn occurs mainly in +2 oxidation state (Lindsay 1979) and is the prime form taken up by plant roots (Alloway 2008b). The availability of soil-associated Zn to plants is controlled by a range of factors associated with soil (soil pH, clay content, redox status, soil moisture content, organic matter content, total Zn, and level of other trace elements and macronutrient), activity of microbes, and climate (Alloway 2008b; Cakmak 2008a). Zn availability to plants can be limited by a low level of total Zn (sandy soils), high pH (calcareous and heavy limed soils), high organic matter, high P levels, low soil temperature, and high salt concentration (Na, Ca, Mg, biocarbonates, and phosphates) and flooding/waterlogging in the soil (Lindsay 1979; Alloway 2008b). Zn is mainly accumulated in roots and is translocated to shoot as and when required. Zn translocation may occur from old leaves to developing organs, and the translocation may increase with Mn supply (Alloway 2008b). The Zn2+ influx across plasma membrane is a passive process and occurs toward negative electrical potential (Kochian 1993). Extensive reports are available on the role of apoplastic and symplastic fluxes in Zn flux to the shoots (Lasat and Kochian 2000; Sattelmacher 2001). The role of depolarization-activated nonselective cation channel (DA-NSCC) in the passive Zn2+ influx (Piñeros and Kochian 2003) and that of voltage-independent NSCC (VI-NSCC) in the Zn2+ transport (reviewed by Demidchik and Maathuis 2007) has also been reported. Nevertheless, nicotinamide may also facilitate Zn mobility/translocation in roots (Bouain et al. 2014). Proteins having strong affinity for Zn have been reported to mediate Zn uptake in plants. To this end, Zn uptake and translocation in plants can be assisted by several protein families including the zinc-regulated, iron-regulated proteins (ZIP), metal tolerance proteins (MTP), heavy metal ATPases (HMA), and cation diffusion facilitator (CDF) and cation antiporters (Grotz et al. 1998; Hussain et al. 2004; Bouain et al. 2014). Within plants, several members of the ZIP protein family have been found to be involved in uptake, translocation, and homeostasis of Zn2+ (Grotz and Guerinot 2006). A range of Zn transport proteins have been reported in a number of plants including Arabidopsis thaliana (ZIP1, ZIP3, and ZIP4, Grotz et al. 1998; AtHMA2 and AtHMA4, Hussain et al. 2004; AtZIF1 and AtZIFL1 and AtZIFL2, Haydon and Cobbett 2007), O. sativa (OsZIP1, OsZIP3, OsZIP4, and OsZIP5i, Ishimaru et al. 2011), and Zea mays (ZmZIP, Li et al. 2013). Despite previous facts, the exact molecular mechanism underlying the regulation of Zn by ZIP proteins is not well understood. Earlier, the significance of basic region leucine zipper (bZIP) 19 and bZIP23, transcription factors of the bZIP, and that of a zinc deficiency response element (ZDRE) in the control of the expression of ZIP4 gene has been explored in plants (Deppmann et al. 2006).

Considering the significance of Zn in plants, Zn is one of the well-known essential micronutrients required by plants, animals, and humans (Broadley et al. 2007; Alloway 2008a, b). It is a structural component of regulatory proteins (Berg and Shi 1996). It plays important roles in carbohydrate, auxin and protein metabolism, pollen formation, maintenance of structural and functional integrity of membranes, and resistance to pathogenic infections and is also involved in flowering and seed production (Marschner 1995; Alloway 2008b, 2009). Zn plays a significant role in protein synthesis and gene expression (Alloway 2008b). Zn is an important constituent and cofactor of various enzymes (such as carbonic anhydrases and SOD) and proteins involved in metabolic activities within the plants (Broadley et al. 2007; Alloway 2009). In contrast to other metals/metalloids, Zn is required by the largest number of proteins (Cakmak 2008b). Around 10 % of proteome of eukaryotic cells has been considered to be represented by Zn-binding proteins, and the involvement of 36 % of Zn proteins in gene expression was evidenced (Andreini et al. 2006) (Fig. 1).

Toxicity

Though plants may contain Zn in the range of 30–100 mg kg−1 d.w. and it is an essential micronutrient in plants, the concentration >300 mg Zn kg−1 d.w. is considered phytotoxic (Marschner 1995). Excess Zn may inhibit plant growth and development and cause oxidative stress and several toxic consequences at cellular and genetic levels. These aspects are briefly discussed hereunder in separate subsections.

Plant growth and development

The usual symptoms of Zn toxicity include the following: stunting of shoot, inhibition of root and shoot growth, curling and rolling of young leaves, death of leaf tips, and chlorosis/necrosis (Jin et al. 2008; Wang et al. 2009a). Zn has been found to induce chlorosis in Typha latifolia seedlings (at 80 μM; Ye et al. 1997) and in the leaves of Thlaspi ochroleucum after 4 days of culture (at 1.0 mM Zn; Shen et al. 1997). Excess Zn (2.5–7.5 mM) can cause reductions in seed germination, seedling growth, and the dry matter of root and shoot (Madhava Rao and Sresty 2000). Plant growth inhibition, chlorosis, and the loss of chlorophyll pigments were observed in B. juncea leaves under 0.07–1.12 mM Zn exposure (Wang et al. 2009a). Though Zn-treated soils (200, 450, and 900 mg Zn kg−1) caused inhibition in the emergence of T. aestivum and the growth of Zea mays, significantly increased levels of chlorophyll were observed only in T. aestivum (Alonso-Blázquez et al. 2014). Excess Zn can alter the leaf morphology and anatomy (Todeschini et al. 2011) and increase water potential (Tewari et al. 2008). Impaired PS II activity, decreased rates of photosynthesis and transpiration, inhibited stomatal conductance, and decreased pigment content can also be found in elevated Zn exposure (Shi and Cai 2009; Bonnet et al. 2000; Todeschini et al. 2011). Impaired nutrient uptake (Wang et al. 2009a, b), altered gene expression (van de Mortel et al. 2006), disrupted electron transport chain in chloroplasts and mitochondria (Sensi et al. 2003; Di Baccio et al. 2009), and decreased ATP content (Xu et al. 2013) have also been observed at elevated Zn levels. Zn damages D1 and D2 proteins in reaction center of PS II (Todeschini et al. 2011). Zn (1.0, 5.0, and 10 mM)-mediated decrease in growth; leaf fresh weight; leaf water content; and the contents of leaf Cu, Mg, K, and Ca were also reported (Bonnet et al. 2000). Zn stress-accrued modulation of plant elemental status may vary with plant species. To this end, Zn stress was reported to increase the uptake of Cu, Fe, Mn, and Mg and decrease P uptake in Hydrilla verticillata (Wang et al. 2009b). In a similar recent work, Alonso-Blázquez et al. (2014) reported an increased accumulated of Mg and Mn with increasing Zn concentration in T. aestivum and Z. mays. In contrast, a decrease in concentration of P, Cu, Fe, Mn, and Mg was noticed in roots of B. juncea under Zn stress (Wang et al. 2009a). Zn accumulation was evidenced to cause increased accumulation of free Ca in the cell walls of 300 mg Zn kg−1-treated Populus alba cv. villafranca (Todeschini et al. 2011). Excess Zn has also been reported to hasten plant senescence via disintegrating chloroplast, nuclei, and mitochondria in H. verticillata (Xu et al. 2013). Zn stimulated chlorophyll synthesis and suppressed activity of NADH oxidase at 0.05–0.5 mg L−1, whereas the activity of NADH oxidase was enhanced at 30 mg L−1 and the synthesis of chlorophyll was inhibited at >5.0 mg L−1 (Wang et al. 2009b).

Elevated Zn-mediated impact at anatomical and ultrastructural levels may include breakdown of cells in cortex, decrease in size of cells in roots, breakdown of vascular bundles in stem, decrease in intercellular space, and reduction in epidermal and palisade cell size (Maruthi Sridhar et al. 2007). Induction of xerophytic structures such as thick lamina, upper epidermis, and palisade mesophyll and increased number of small stomata can also been observed under elevated Zn exposure (Shi and Cai 2009). Nevertheless, excess Zn has also been reported to cause significant thickening in leaf lamina, palisade and spongy tissue, and upper epidermis; increase in the palisade cell width; and large intracellular spaces in palisade tissue (Todeschini et al. 2011). Excess Zn (at ≥50 μM) was reported to cause plasmolysis, disruption of plasma membrane, thickened cell walls, increased cell vacuolation, and induced structural disorganization of chloroplasts (Jin et al. 2008). To the other, Zn at ≥250 μM resulted in less compact grana with smaller number of thylakoids and even breakdown of chloroplast membrane, enlarged starch grains, and numerous plastoglobules (Jin et al. 2008).

Oxidative stress and cyto/genotoxicity

Though Zn is a nonredox metal, high concentrations of Zn induce ROS generation, cause oxidative stress, and disturb cellular redox homeostasis (Weckx and Clijsters 1997; Madhava Rao and Sresty 2000; Bonnet et al. 2000; Cuypers et al. 2001; Tewari et al. 2008; Wang et al. 2009a, b; Xu et al. 2013). Elevated Zn levels in the plant growth medium can cause membrane peroxidation (Jin et al. 2008; Wang et al. 2009b; Xu et al. 2013). Zn (500 μM) caused accumulation of H2O2 and O2 −· in leaves of nonhyperaccumulating ecotype of Sedum alfredii (Jin et al. 2008) and also enhanced H2O2 content in primary leaves of Phaseolus vulgaris (Weckx and Clijsters 1997) and leaves of mulberry (Tewari et al. 2008). Excess Zn2+-accrued enhanced membrane lipid peroxidation (measured as MDA or TBARS accumulation) has been reported in Mentha pulegium leaves (Candan and Tarhan 2003), primary leaves of Phaseolus vulgaris (Weckx and Clijsters 1997), shoots of B. juncea (≥5.0 mM; Prasad et al. 1999), roots and leaves of Phaseolus aureus (at 100 μM; Chaoui et al. 1997), in Hydrilla verticillata (>5.0 mg L−1, Wang et al. 2009b; 15–150 μM, Xu et al. 2013), roots and leaves of B. juncea (Wang et al. 2009a), and roots and shoots of Cajanus cajan (2.5–7.5 mM, Madhava Rao and Sresty 2000).

Elevated Zn has been extensively reported to effect plant defense system through modulating enzymatic and non-enzymatic antioxidants. Nevertheless, the generation and dismutation of ROS and its metabolism (via enzymatic and non-enzymatic antioxidants) were extensively reported to depend on Zn concentration, exposure period, plant species, and plant organs (Chaoui et al. 1997; Prasad et al. 1999; Bonnet et al. 2000; Madhava Rao and Sresty 2000; Cuypers et al. 2001; Jin et al. 2008; Tewari et al. 2008; Wang et al. 2009b). Zn (≥5.0 mM) caused an increase in the activity of SOD, CAT, GPX, APX, MDHAR, DHAR, and GR in B. juncea shoots (Prasad et al. 1999). SOD and APX activity was enhanced in rye grass (Lolium perenne) under Zn toxicity (50 mM; Bonnet et al. 2000). Zn (500 μM) increased the activity of SOD, CAT, GPX, APX, DHAR, and GR in leaves of Sedum alfredii (Jin et al. 2008). Chaoui et al. (1997) found increased activity of GPX, GR, and APX in leaves of bean (Phaseolus vulgaris), whereas the activity of CAT declined in both roots and leaves in response to Zn toxicity (100 μM). In Hydrilla verticillata, there was an increase activity of SOD (at >0.05 mg L−1 Zn), APX and GPX (at 0.5–30 mg L−1 Zn), and GR (at 0.05–5.0 mg L−1 Zn) under Zn stress, whereas a decrease was noticed in GR activity at 5–30 mg L−1 Zn (Wang et al. 2009b). Cuypers et al. (2001) observed an increase in the activity of peroxidases in roots and primary leaves of Phaseolus vulgaris under Zn stress (50 μmol L−1). However, Zn did not cause any change in the activity of SOD and CAT in primary leaves of Phaseolus vulgaris, whereas an increase was noticed in APX activity (Weckx and Clijsters 1997). In contrast, a decrease in APX and GR activity was noticed in Phaseolus vulgaris roots under Zn stress (50 μM; Cuypers et al. 2001). Under Zn concentrations between 0.28 and 1.12 mM, SOD and CAT activity decreased, whereas peroxidase and NADH oxidase activity increased roots and leaves of B. juncea (Wang et al. 2009a). Madhava Rao and Sresty (2000) reported elevated SOD, peroxidase, and GR activity and a declined CAT activity under 2.5–7.5 mM Zn. Zn (250 μM) enhanced APX activity and declined CAT activity in leaves of mulberry (Tewari et al. 2008). Higher concentration of Zn also enhanced the content of antioxidant metabolites such as AsA and GSH in the roots of nonhyperaccumulating ecotype of Sedum alfredii (at 500 μM; Jin et al. 2008), leaves of Hydrilla verticillata (at 10–30 mg L−1; Wang et al. 2009b), shoots of B. juncea (at ≥5.0 mM; Prasad et al. 1999), leaves of mulberry (at 250 μM; Madhava Rao and Sresty 2000), leaves of Phaseolus vulgaris (at 50 μM Cuypers et al. 2001), and the roots and leaves of B. juncea (0.14–1.12 mM; Wang et al. 2009a). Likewise, Zn stress caused an increase in dehydroascorbate (DHA) and nonprotein thiol content in B. juncea (at 0.14–1.12 mM) and DHA/AsA ratio (redox couple) in mulberry leaves (Tewari et al. 2008). In contrast, a decrease in AsA and total GSH content was reported in Cajanus cajan under Zn stress (at 250 μM; Madhava Rao and Sresty 2000). van de Mortel et al. (2006) demonstrated that Zn homeostasis in plants involves a complex mechanism, and there exists large differences in expression of genes for Zn homeostasis in Thlaspi caerulescens (a Zn accumulator) and Arabidopsis thaliana (a non Zn-accumulator). In a recent work, Zn-accrued oxidative stress generation was argued as a cause of its interference with the major enzymatic antioxidants such as peroxidases (APX and GPX), CAT, and SOD (Alonso-Blázquez et al. 2014). Notably, the responses of APX and the reduced pool of GSH were advocated to be considered as good biomarkers of serious stress by Zn in soils.

In context with excess Zn-accrued cyto/genotoxicity in plants, excess Zn can interfere with cell division (mitosis and meiosis) and induce chromosomal aberrations and genotoxicity (Tripathi and Girjesh 2010; Oladele et al. 2013; Truta et al. 2013). However, there are conflicting reports regarding clastogenicity and genotoxicity of Zn in plants. Marcato-Romain et al. (2009) and Steinkellner et al. (1998) reported that Zn is not genotoxic even at high concentrations, whereas others have reported clastogenic and genotoxic effects of Zn (Tripathi and Girjesh 2010; Truta et al. 2013). Steinkellner et al. (1998) reported that Zn alone is not able to induce micronuclei (MN) formation in Vicia faba, whereas binary solution of Cu and Zn causes significant MN formation (Marcato-Romain et al. 2009). The differential response depends upon the plant species, Zn concentration, exposure period, type of Zn compound, length of diploid complement, and the number of somatic and metacentric chromosomes (Ma et al. 1995). Zn can extend the length of cell cycle in root meristems (Powell et al. 1986). Zn exposure (at 7.5 mM) induced condensation of chromatin, increased number of nucleoli, and disruption and dilation of nuclear membrane in root tip cells of Cajanus cajan (Sresty and Madhava Rao 1999). Zn (25–300 ppm; as ZnCl2) caused chromosome stickiness, abnormal spindles, irregular chromosomal segregation, pycnotic nuclei, and chromosome degeneration during meiosis in pollen mother cells in grass pea (Tripathi and Girjesh 2010). Zn at 25–100 mg l−1) can inhibit MI and induce stickiness and bridge formation (Oladele et al. 2013). Zn (100, 250, 500 μM)-mediated clastogenic and aneugenic effects have also been reported in Hordeum vulgare (Truta et al. 2013). Zn can induce metaphase disorders (stickiness, c-metaphase, expulsed chromosomes) and anatelophase aberrations (bridges and laggards). Thus, Zn-accrued high aberration levels are indicative of clastogenic effects of Zn and its interference with nucleic acids, whereas Zn interference with mitotic spindle indicates its aneugenic action (Truta et al. 2013) (Fig. 2).

Conclusions, cross-talks, and perspectives

Plants require a differential level of several heavy metal ions such as Co, Cu, Fe, Mn, Mo, Ni, and Zn to maintain their optimal growth and development. However, the previous ions may readily impact function of many enzymes and proteins, halt metabolism, and exhibit phytotoxicity at supra-optimum supply (Marschner 1995; Schützendübel and Polle 2002; Hänsch and Mendel 2009; Krämer 2010; Anjum et al. 2014b). Studies assessing the potential risk of metals/metalloids in plants are extensive. However, least reports are available in this context on the interaction of higher plants with previous heavy metal ions. Also, information is meagre on potential interaction among previous heavy metal ions that indeed may induce/modulate oxidative stress and cyto/genotoxicity that eventually may affect overall plant growth, development, and productivity (Fig. 2). Fe and Zn levels can be increased with excess Mn (Zhao et al. 2012). Apart from performing other significant physiological roles, Cu is also involved in the biosynthesis of Mo cofactor (Kuper et al. 2004). Plants may coordinately regulate the alternative use of Cu- versus Fe-containing enzymes (such as cytochrome oxidase vs. di-iron oxidase, Cu vs. heme nitrite reductases, and Cu/Zn SOD vs. Fe SOD) to catalyze the same biochemical reaction with completely different apoproteins (reviewed by Puig et al. 2007). Owing to the similar radii of hydrated Zn+2 and Fe+2 ions, Fe deficiency may cause chlorosis (Marschner 1995). Plant shoots may exhibit Mn and Cu deficiency symptoms due to excess Zn-mediated hindrance in root-to-shoot transport of Mn and Cu (Ebbs and Kochian 1997). Excess Co (0.5 mM) can induce Fe deficiency symptoms on young leaves and also decrease Fe concentration in different plant parts (Chatterjee and Chatterjee 2003). However, addition of Fe along with excess Co may ameliorate the effects of excess Co by partially increasing the biomass, concentration of chlorophylls a and b, Fe, proteins, activity of Hill reaction, and CAT and by elevating the accumulation of carbohydrates (Chatterjee and Chatterjee 2003). In context with Ni, least reports are available on the physiological insights into “Ni-interaction with other essential/beneficial micronutrients” (Wood 2010). The study of Wood (2013) suggested the mutual antagonism of Fe and Ni with tissue toxicity by one being reversible by increasing tissue concentration of the other. Fe transport system in roots can competitively uptake Ni (Pandey and Sharma 2002), and the Fe-deficient treatments can cause elevation in root Ni (Schaaf et al. 2006). Nevertheless, the deficiency of one may be enhanced by increasing the tissue concentrations of the other. The deficiency and/or reduced physiological availability of Ni may be caused by excessive tissue Zn or Cu and/or by high Zn/Ni or Cu/Ni ratio (Wood et al. 2003; Wood 2010). In addition, Ni has been reported to compete with Fe in biochemical and physiological processes in plants (Seregin and Kozhevnikova 2006). Extensive reports are available in support of excess Ni-mediated interruption in the Fe transport system and inhibition in the configuration of Fe2+–ligand complexes (Krämer et al. 2000; Callahan et al. 2007; Saito et al. 2010). The association of Ni-induced oxidative stress with the competition of Ni with Fe has also been reported, where excess Ni was evidenced to modulate the activities of antioxidant Fe enzymes (such as Fe SOD and CAT) (Pandey and Sharma 2002; Gajewska and Sklodowska 2007). Considering future perspectives, employing biological toxicity assays in plants, efforts should be made to systematically estimate the predicted no-effect concentration of the heavy metal ions discussed herein. Efforts should also be made to unveil the molecular mechanisms of potential antagonisms and synergisms among the discussed herein plant-beneficial heavy metal ions, as well as to determine both participation (and also the coordination and interaction) of one heavy metal ion in the transport and/or homeostasis of the other(s), and the role of previous processes on the overall potential impact of the heavy metal ion discussed herein.