Introduction

Factors that render ecosystems unfit for survival of life-forms include continuous addition of hazardous contaminants, mainly through anthropogenic activities, and their penetration into diverse aquatic and terrestrial life-forms Kahlon et al. [62, 72]. Amid the sum total of contaminants, HMs, being non-biodegradable and toxic, are remarkably dreadful, inflicting a terrible threat to the existence of different life-forms in view of their chief role in health-related complications [55, 134]. Heavy metal pollution ranks second among the most perilous pollutions and is expected to pull ahead of chief hazardous pollutants notably sulfur dioxide, carbon dioxide and pesticides in near future [22, 72].

HMs are discharged into the ecosystems predominantly through various anthropogenic activities notably smelting, mining of metals, foundries and leaching of metals from diverse sources notably automobiles, landfills, waste dumps, excretion, runoffs, chicken manure and road-works [16, 57, 139, 162]. Natural sources like geological weathering, water and sediment re-suspension, metal corrosion, soil erosion and volcanic eruptions also add to the HM contamination [16, 86, 162]. Besides, agricultural sector, where-in the use of fertilizers, insecticides, pesticides has escalated alarmingly, is considered as secondary source of HM pollution [16, 86, 139]. The concentrations of HMs (Hg, Cd, Pb, Cr, Cu, As) are continuously mounting in the surface water and sediments where-from these find entry into different food chains leaving grave concerns on different life-forms including humans [57, 37, 139]. HMs induce genotoxicity, cytotoxicity, and mutagenicity in humans, animals as well as in plants [24, 114]. Some of the deadly diseases in humans due to HM contaminants include different types of cancers, kidney failure, lung congestion, liver damage and reproductive dysfunction [39]. Prolonged exposure to HMs like As also lead to cardiovascular and neurological disorders, and various types of skin cancers [59, 75, 110]. Millions of people worldwide fall victims to HM contamination chiefly through consumption of contaminated food and drinking water [87, 112].

Plant growth and metabolism are also severely disrupted by HM contamination. Once inside the plant, HMs impede metabolic functions by disrupting protein functioning by forming complexes with their sulfhydryl groups [33], dysfunctioning enzymes cofactors [5, 50], malfunctioning cellular molecules and pigments [50] and severely disrupting the integrity of membranes [5, 33]. These physiological and metabolic disruptions ultimately repress the fundamental primary as well as secondary metabolism pathways in plants including photosynthesis, respiration, nutrient assimilation, etc. [4, 62].

The existing challenges of climate change, diverse environmental constraints, food and energy security, and sustainability compel researchers to explore novel competent technologies to conquer these potential challenges in a proficient manner [131, 146]. Among various such approaches adopted for the purpose, nanotechnology is acknowledged as one of the most imperative, swiftly emerging fields with copious potentialities, contributing to sustainable competitiveness and development in numerous fields including the agricultural field [11, 36, 104, 107, 145, 173]. Application of NPs repairs the perturbed and contaminated ecosystems where traditional agricultural practices have proven unsuccessful [131, 146, 173]. Nanotechnology is proving to be a boon in the agricultural field with copious advantages through the application of nanofertilizers, nanopesticides, and elicitors [19, 78, 151, 155, 173]. Improved performance in plants has been reported due to application of different NPs both under normal as well as perturbed environmental conditions [3, 7, 35, 118, 126, 136, 144]. NPs as elicitors have proven as innovative and efficient resolution to myriads of abiotic environmental pressures including HM toxicity [131, 173]. Application of different NPs (TiO2NPs, SiNPs) alleviates oxidative stress by reducing the content of MDA, H2O2, superoxide radicals by up-regulating the activities of enzymatic and non-enzymatic antioxidants like SOD, CAT, guaiacol and ascorbate peroxidases, GR, GSH [7, 74, 118, 172, 174]. Moreover, NPs also conserve chloroplast structure, improve content of chloroplast pigments, and photosynthetic rate and preserve membrane stability in HM-affected plants [27, 28, 36, 54, 58, 78, 133, 137, 144]. NPs regulate a variety of physiological phenomena in plants notably CO2 fixation, nutrient assimilation, increased activities of imperative primary metabolism enzymes and secondary metabolite production in addition to abiotic stress alleviation [2, 7, 74, 95, 126, 143, 159]. This current review is an attempt to encompass the role of different NPs in regulation of diverse physiological and biochemical phenomena in different plants exposed to HM toxicity.

Impact of nanomaterials on plants

Adequate literature is available on the subject of NPs-mediated impact on plants and studies reveal both positive as well as deleterious effects [26, 102, 144, 145]. For example, exogenously sourced titanium dioxide nanoparticles (TiO2NPs) reduce the oxidative stress imposed by UV-B radiations [74], improve net photosynthesis [46], enrich acquisition of nitrate and assimilation by incorporating inorganic nitrogen into organic molecules in spinach plants [165]. Moreover, TiO2NPs-treated spinach chloroplasts show improved light absorption by chlorophyll a molecules and enhanced electron transfer efficiencies, PSII fluorescence quantum yield and rate of evolution of oxygen [89, 166]. Tomato and spinach plants exposed to TiO2NPs have been reported to improve transfer efficiency and light absorption capacity of PSII [73, 111]. TiO2NPs-triggered enhancement in photosynthesis is attributed to large specific surface area, high photo-catalytic capacity and high thermal conductivity of these NPs [73, 89, 166]. Application of TiO2NPs also serves an essential role in kidney bean in the modulation of enzymatic antioxidant gadgets [56]. Likewise, MnNPs improve activity of PSII by boosting the photolysis of water and evolution of oxygen and also bring an enhancement in the photophosphorylation activity of the electron transport chain in mung-bean [110]. Carbon nanotubes, another important type of NPs, have been observed to potentially enter the seed coat of tomato plants thereby facilitating the water acquisition required during germination [63]. However, no reports of apparent carbon nanotubes induced toxicity in plants are present, except in rice plants in which flowering time has been observed to be delayed by one month [80, 142]. Moreover, carbon nanotubes up-regulate the expression of stress-related genes in tomato plants [66] and increase activity of POX in the seedlings of sainfoin [135]. This feature of carbon nanotubes regulating the expression of stress-related genes in plants can prove handy in the regulation of plant growth and development [71].

Besides the positive effects, deleterious effects of some of the NPs are also documented. Asli and Neumann [10] showed that transpiration is blocked by TiO2NPs and bentonite NPs declining the pace of hydraulic conductivities in maize. Musante and White [97] reported that exogenous application of AgNPs decreased the transpiration rate in Cucurbita pepo. Mukherjee et al. [96] established that ZnONPs application resulted in decreased chlorophyll content in pea. Higher concentration of NPs proves inhibitory to the plants as revealed from the application of CeO2NPs at the concentrations of 1000 and 2000 mg L−1 which recorded a decrease of 60 and 85%, respectively, in the chlorophyll biosynthesis [85]. Likewise, contents of chlorophyll and carotenoids were reported to be significantly decreased by increased concentrations of AgNPs in rice seedlings [100]. TiO2NPs have also been observed to reduce the content of chlorophyll in kidney bean and tobacco [56, 125]. Exogenous supplementation of AlO2NPs reduce the activities of two important enzymes (dehydrogenase and oxido-reductase) in tobacco [108]. ZnONPs application has also been observed to hinder the translocation process in plants like cowpea [153]. Moreover, TiO2NPs have been known to markedly reduce the water absorption and transpiration rate [10]. There are also reports of TiO2NPs-induced cytotoxic impacts in plants and such cytotoxicity in diverse plant cell systems has been endorsed to ROS overproduction [167]. Likewise, ZnONPs are also known to trigger phytotoxic effects in ryegrass which is attributed to membrane lipid peroxidation and ROS production [81]. Improved antioxidant enzymes activities like catalase, SOD have been shown by NiONPs application in tomato [34]. NPs have the ability to impair different growth and developmental aspects in plants as a number of events like flowering, fruiting, timing of senescence, abscission and dormancy are influenced by them [141, 149]. Manufactured NPs can lead to membrane lipid peroxidation by ROS generation [18]. Engineered NPs can significantly impinge on the membrane permeability and fluidity and as a result will affect the acquisition kinetics of the nutrients. So it evolves that the existing literature gives us mixed prospectus as far as the responses of plants to NPs is concerned. Nonetheless, the NMs mediated phytotoxic effects are the focus of most of the prevailing literature, the present review has, therefore, shed-light especially to uncover the NPs-induced remediation of HMs from aquatic ecosystems and HM stress tolerance in plants and the associated underlying mechanisms.

Remediation of HMs from aquatic ecosystems using nanomaterials

Rational water resources’ utilization has emerged as one of the most critical environmental crisis, the resolution to which chiefly lies in the efficient treatment of the contagious wastewater from varied sources. One of the efficient treatment methods is to control the contents of different HMs [9] which are included among the most biologically hazardous and noxious components of the wastewater effluents. Trace elements (metals and metalloids) having atomic density greater than 4 ± 1 g cm−3 are included in the list of HMs and are believed to be the most prevalent toxic soil and water mineral contaminants [17, 92]. Heavy metal accumulation into different soil and water ecosystems signifies a massive threat to the living systems and their bioaccumulation at consecutive trophic positions via biomagnification is adding to its severity [5]. Contrary to the organic pollutants, bulk of the metal/metalloid contaminants are incessantly accumulated in the soil as they are not decomposed and/or degraded chemically or microbially ending in long-term soil eco-toxicity [1]. The invariable enhancement of such toxic contaminants in soil and water ecosystems is a principal global disquiet [106, 129], and the expedition of inventive technological advances has amplified its severity leading to extinction of various living beings and questioning the sustained existence of others dwelling in such contaminated ecosystems by causing noxious ailments [31, 70]. So, it becomes indispensable to find resourceful means of detoxifying/remediating such toxic contaminants. Nevertheless, nanotechnology owns immense potential as environmental cleaner including mitigation of diverse HM toxicities [30]. Several studies of the kind have been spotted in literature dealing with the metal NPs-induced amelioration of heavy metal/metalloid toxicity [152]. A variety of chemical technologies have also been utilized for the remediation purpose, among which, adsorption is incredibly common as well as efficient in view of its cost-efficiency, treatment stability and simplicity [13].

A few techniques are currently accessible to sequester the heavy metals/metalloids from different ecosystems; however, employing nanomaterials is sprouting as a promising option in view of substantial efficiency. Amid the diverse application of NMs in different fields, purification of water so as to trim down the concentration of the toxic contaminants has opened new doors of anticipation toward developing a comparatively feasible environment to thrive in [122]. NPs of different elements are also promisingly effective in remediation of different toxic metal ions from soil ecosystems as well. Fe3O4 NPs impregnated with silica have successfully removed bulk of the toxic contaminants from different ecosystems [161]. Additionally, nanosilica alone has also been applied to eliminate heavy metal ions from the contaminated wastewater (Xin Rong et al. 2001). Zero-valent iron NPs (nZVFe NPs) effectively trims down the concentration of chromium (Cr-VI) and arsenic (As-III) contaminants from waste water [13, 20, 38, 109, 130, 156]. The recent decade has witnessed comprehensive application of ZVFe NPs for the remediating different noxious environmental contaminants [38, 130, 169, 168]. The properties like high surface area and reaction activity permits their employment for rapid decontamination of various aquatic contaminants including toxic metals/metalloids [14]. Furthermore, literature supports that FeONPs scavenge various toxic HMs notably As(III), Cd(II), Cr(VI), Cu(II), Pb(II) and Zn(II) [23, 60, 67, 79, 121, 147, 169]. Moreover ZVFeNPs have been documented to improve plant biomass by decreasing heavy metal contamination of the soil [101, 138]. Environmental engineers, nevertheless, are making relentless efforts to remediate the soil and water ecosystem contaminants, particularly the toxic metal/metalloids through the application of nanomaterials though much more is needed in this regard [41, 84].

Among the most successful NMs widely utilized for the remediation of noxious HMs from the industrial wastewater are single and multi-walled carbon nanotubes, the fullerenes, and graphene oxide [17]. Large surface area and fairly low aggregation capacity of fullerenes makes them ideal to serve as adsorbents for the removal of HMs from industrial wastewater [83, 123]. Significant sorption capacity and competence of oxidized CNTs for [Cd(II), Pb(II)] and [Cr(VI)] ions makes them ideal for removal of these contaminants [17, 76, 119]. In view of their high specific surface area, increased functional groups and active sites on their surface, and reasonably superior chemical stability, the last decade has witnessed a manifold increase in the use of graphene and graphene-based materials for treatment of wastewater [43]. Moreover, graphene is oxidized to add hydrophilic groups for the effective remediation of HMs [14]. Graphene-based materials have a very strong sorption capacity for different metals/metalloids [25, 52, 171]. Heavy metal remediation by ZVFeNPs is predominantly dogged by redox potential of the metal pollutant. Compared to Fe, metal contaminants having more negative or analogous standard redox potential (e.g., Zn and Cd) are eliminated by adsorption capacity with ZVFeNPs; contrarily, metals having positive standard redox potential (e.g., Ni and Pb) are eradicated through reduction as well as adsorption. Cd-spiked soil can be reclaimed using ZVFeNPs (0.01% and Cd accumulation in seeds and leaves of rice grown on such soil gets reduced [158]. Reduced Cd toxicity is endorsed to reduced bioavailability due to the adsorption of the metal contaminant to the nanoparticle surface. Moreover, Liu et al. [82] reported that supplementation of FeNPs under Cd-spiked soils immobilizes Cd leading to reduced bioavailability for the plants. Further support comes from the study of Houben and Sonnet [51] which reveals 45–63% reduction in the amounts of Cd and Zn after the application of powdered FeNPs to the soil.

Substantial attention has been focused on the potential benefits of different nanomaterials in water treatment processes. However, concerns with regard to their looming effects on humans and other ecosystems have arisen. If these concerns are addressed cautiously, NMs can possibly play a cardinal role to ascertain excellent soil and water quality to congregate the escalating demand for clean and safe water and soil for agricultural practices [12, 140].

Nanoparticles-induced heavy metal stress alleviation in plants and the underlying mechanism

Plant resistance against HM stress can be improved through application of NPs which can be applied in the form of aqueous solutions through foliage in addition to application through soil; for example, alleviation of Cd and Pb stress in oryza sativa L. through leaf-applied selenium and silicon NPs [54]. Foliar application of NPs has been reported to be more efficient in HM stress alleviation than their soil application [78]. Although to minimize the detrimental trajectories caused due to HM stress, plants have established varied homeostatic mechanisms that regulate the accumulation and uptake of metals/metalloids, besides, managing their detoxification as well as trafficking; nevertheless, the ability of HM detoxification varies and can be improved. The critical approaches adopted to improve HM resistance in plants comprise; decrease in the quantity of bioavailable metal contaminants, regulation of expression of genes involved in metal/metalloid transport, recuperating the capability of apoplastic barricade to intercept metal contaminants, supplying more nutrients to the plant under stress, fortifying the enzymatic and non-enzymatic antioxidant gadgets and amplified biosynthesis of defensive agents (organic acids, osmolytes, phytochelatins and root exudates) [19, 54, 78, 94, 151, 155]. Apoplastic barrier, though not a complete contaminant blockade, serves vital protective functions in plant roots, controlling the flow ions, oxygen and water [21, 36, 54]. Entry of HMs in plant roots is checked by the apoplastic barriers and their efficiency can be enhanced by the NPs [54, 120]. NPs stick to the HMs in the cell walls forming complexes, thereby making them unavailable. Interaction of NPs with the HMs is crucial while studying the different characteristics of HM stress alleviation. Reduced mobility and, therefore, bioavailability of metal contaminants in the soil has been endorsed to NP application. For example, application of mercapto SiNPs and Fe3O4 NPs increases the stability of Cd, thus, decreasing its mobility [66, 124, 155]. NP-HM complexes, once adsorbed, become immobile, obstructing the mobility of the HMs inside the plants which in-turn reduces their biological activity [28, 154, 173]. Moreover, certain organic acids functioning as metal chelators are bio-concentrated in the cell walls which trim-down the HM-induced damage by chelating the metal contaminants. Biosynthesis of such protective organic acids is known to be improved by NPs as has been reported in case of SiNPs-application reducing the damage caused due to Cd [7, 28, 36, 54, 118, 173]. Moreover, NPs improve soil characteristics; for example, release of phosphate and increased soil pH as a result of hydroxyapatite NPs application in-turn reduces HM toxicity [29, 54]. Furthermore, NPs having high surface to volume ratio are capable of interacting with certain cell biomolecules and elicit different biochemical pathways [30].

ROS production being an indispensable phenomenon of various plant metabolic processes like photosynthesis and respiration, have been reported to act as a defense signal regulating miscellaneous aspects of growth and development. Nevertheless, disproportionate ROS accretion during stressful conditions, damages cell membranes, impairs the structure and functioning of different cellular components as well as proteins [160, 173]. Up-regulation of genes related to various primary physiological phenomena including antioxidative metabolism as well as genes involved in HM stress tolerance requires an optimum concentration of NPs [154, 173]. NP-induced activation of the antioxidative defense gadget of the plant reduces and/or mitigates the HM-induced excessive production of ROS [160, 173].

Among the various types of NPs, role of TiO2NPs has been comprehensively studied in plants. Improved plants’ performance in terms of growth, photosynthesis (net photosynthetic rate, stomatal conductance, rubisco activity), enzyme activities, nutrient status and yield has been worked out in response to the TiO2NPs application both under normal as well as non-biotic pressures [3, 35, 40, 133, 170]. TiO2NPs have been reported to enhance the activities of different antioxidative enzymes (SOD, CAT, APOX, GPOX) in S. olereacea and L. minor (Song et al. 2012); [74]. Moreover, TiO2NPs reduce Cd and free radical accretion and lipid peroxidation by improving enzymatic and non-enzymatic antioxidants and relative water content, conserve chloroplast structure, improve content of chloroplast pigments, and photosynthetic rate and preserve membrane stability in HM affected plants [46, 58, 78, 133] (Table 1). Lei et al. [74], tested the efficacy of TiO2NPs on Spinach and reported the significant decrease in O2•− and H2O2 accumulation, and consequently lipid peroxidation in chloroplasts under oxidative stress. Application of TiO2NPs prevents electrolyte leakage in the chickpea cultivars [74, 91]. Similar results have been validated by Sharma et al. [128] in mustard due to the application of AgNPs (25 and 50 mg L–1). Studies on Spinach have disclosed that TiO2NPs limit oxidative stress by reducing the content of MDA and H2O2, and superoxide radicals and fortify antioxidant enzymatic activities (SOD, APOX, GPOX and CAT) [74, 172]. Application of ZnONPs alleviated Cd-toxicity in Leucaena leucocephala [148]. Likewise, Cd toxicity in mustard by nano-scale hydroxyapatite has been reported [77]. Venkatachalam et al. [148] suggested that ZnONPs-induced mitigation of Cd toxicity is accredited to decreased ROS production which prevents membrane damage as can be confirmed by reduced MDA content thereby increasing plant growth rate, mineral accretion and biomass accumulation. The study also suggested that ZnONPs lead to increased activities of the antioxidative enzymes by enhancing the level of isoenzyme pattern and improved genomic alterations to conquer the heavy metal-induced genotoxicity.

Table 1 showing the effect of different nanoparticles on antioxidant enzymes of different plants exposed to heavy metal stress
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SiNPs alleviate Cr and As toxicity in wheat and maize cultivars [27, 144, 143]. Compared to the organic Si, SiNPs were observed to be more efficient in the study and the authors’ report more efficient protective impact of SiNPs in maize seedlings under AsV stress [27, 137, 144], blockade of As entry leading to reduced AsV accumulation due to blockade in the root endodermis, improved activities of plant antioxidants (SOD, APX, GR, DHAR, GSH) which in-turn re-establishing the redox status and reduces MDA content in As-treated maize [27, 144]. A similar type of protective mechanism has been observed in SiNPs-treated pea seedlings affected with Cr toxicity and the study revealed that SiNPs successfully alleviated the toxic effects induced by Cr [27, 143]. The up-regulated activities of enzymes and the reduced MDA content in the HM-affected plants after their treatment with NPs, is noteworthy [143, 148] (Table 1). SiNPs-induced alleviation of As, Cd, Cr(VI), Pb toxicity has also been reported in different plants [27, 36, 42, 54, 66]. The studies unveil that SiNPs improve the activities of antioxidant enzymes, nitrogen assimilation and maintain the cytosolic homeostasis by optimizing K+/Na+ ratio which is imperative for the stimulation of essential ROS detoxifying enzymes (Alsaeedi et al. 2018; [61, 132, 143]. In a similar study, increased expression of POX, CAT, and Cu/ZnSOD mRNA has been observed in Arabidopsis treated with CuONPs [99]. Likewise, TiO2NPs-induced increase in the activities of CAT and GR on has been reported in an aquatic macrophyte, Hydrilla verticillata, [105]. Similarly, enhancement in the SOD activity has been reported in A. cepa after exposing the plant to AlO2NPs [113].

Reduced oxidative damage in response to CeO2NPs has been validated in rice seedlings by the free radical scavenging ability of the NPs at lower concentrations; however, at higher concentrations, H2O2 concentration has been reported to increase steadily which is endorsed to the SOD mimetic activity of CeO2NPs [44, 47, 116, 117, 163]. Lower CeO2NPs concentrations (200 and 100 mg/L) enhance cellular resistance to metal-induced oxidative stress by reduced oxidative stress suppression of ROS [41]. The ROS scavenging capability of CeO2NPs has been studied in detail compared to other NPs. The surface lattice of CeO2NPs encloses unoccupied oxygen sites which aids them to change their oxidation states [+ 4 (Ce4+) and + 3 (Ce3+], which in-turn facilitates them to trap the membrane damaging toxic free radicals (O2•− and HO•−) [15]. Fascinatingly, the ability of various NPs to mimic the activity of natural antioxidant enzymes has been previously reported (reviewed by [159]. For example, CuONPs and AuNPs mimic POX activity; Fe3O4NPs, Co3O4 NPs and CeO2NPs, imitate CAT and POX activities; CeO2NPs, fullerene and Pt NPs, displays the mimetic activity of SOD [159]. Effect of various NPs on the activities of various antioxidative enzymes is presented in tabulated form (Table 1).

Proteomic study of AgNPs on rice has been an important breakthrough as it has revealed the up-regulation of about twenty-eight responsive proteins including those involved in oxidative stress tolerance, Ca2+ signaling, protection of nucleic acid (DNA/RNA) damage and proteins contributing to the regulation of gene-expression [49, 90]. The enhanced activities have been accredited to the NP-induced de-novo biosynthesis of proteins or the expression of enzyme isoforms, as reveals from different studies. For example, enhanced protein content in Bacopa monnieri and Pisum sativum seedlings due to the application of AgNPs and SiNPs have been reported by Krishnaraj et al. [68] and Tripathi et al. [143], respectively. Venkatachalam et al. [148] have reported that ZnONPs treatment to Cd and Pb-affected plants leads to the over-expression of a POX isoform as revealed from the peroxidase isoenzyme pattern in the study. Over-expression and in-turn over-production of such isoforms are believed to be involved in the alleviation of the heavy metal-induced oxidative stress in crop plants. The molecular study of Venkatachalam et al. [148] clearly indicated that the ROS-induced DNA damage was more pronounced in the Cd and Pb-treated plants, as indicated by the disappearance of several normal bands in the RAPD pattern of the DNA, whereas new DNA amplicons could be located in metal-exposed plants treated with ZnONPs. Moreover, oxidation of proteins is a common HM toxicity symptom as ions of HM directly interact with proteins molecules due to their high binding affinity with carboxyl- thionyl- and histidyl-, groups [48]. Studies have revealed that the NPs within the plant cell systems may interact with these sulfhydryl and carboxyl groups eventually altering the protein activity by acting and reacting similar to the metal ions [48]. As discussed, different NPs up-regulate the expression of different genes in plants speeding-up the biosynthesis of certain secondary metabolism products like essential oils and phenols in addition to the primary metabolic products [95], Ahmad et al. 2019; [126]. A comprehensive survey of the available literature reveals that secondary metabolite accumulation, particularly phenolics, constitutes a crucial adaptive response of plants against HM toxicity [62, 69, 98]. Some additional mechanisms like the NPs-induced biosynthesis of abiotic stress regulators (nitric oxide, methyl jasmonate, salicylic acid) may also be responsible metal stress alleviation; however, scientists are leaving no stone unturned to elucidate such mechanism which may possibly be discovered in future. A pictorial representation of the mechanism of HM stress alleviation induced by NPs is presented in Fig. 1.

Fig. 1
figure 1

showing the nanoparticles-induced mechanism of heavy metal stress alleviation. NPs detoxify the excessive ROS produced due to heavy metal stress by up-regulating the expression of antioxidative enzymes. NPs improve the activity of chelators and help the plant to sequester the toxic metals/metalloids more efficiently. Moreover, improved biosynthesis of phenols due to NP application is accredited to HM stress alleviation

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Conclusion and future prospects

Adequate literature is in conformity with the elicitor effect of NPs on different plants. Alleviation of HMs-induced toxicity by the soil and foliage applied NPs has been attributed to the up-regulated activities of imperative primary and secondary metabolic enzymes. The mimetic activities of NPs as antioxidant enzymes, NPs-induced up-regulation of different oxidative metabolism enzymes, osmolytes and chelators, sequestration of HM contaminants into vacuoles, detoxification of HM-induced ROS, increased accretion of secondary metabolites particularly phenols have been endorsed to the elicitor effect of NPs. However, in-depth understanding at the molecular level is the need of the hour to gain insights regarding the absolute mechanism of mitigation of the HM toxicity in plants, and advanced research in this regard is advocated.

The mimetic activities of certain NPs to some antioxidant enzymes as well as their increased expression have opened new doors of anticipation in oxidative stress alleviation. The defense system of the sensitive plants can, therefore, be fortified by exogenous sourcing of NPs and the less tolerant can be reinforced, though further studies are needed in this regard. Further research needs to be conducted at the molecular level on the effect of NPs on phytochelatins and metallothioneins in the plants and the endogenous concentrations of other phytohormones particularly related to stress.