Being a non-essential trace element, the accumulation of Cd in agricultural soils is an onerous threat to plants (Haider et al., 2021). Thus, eliminating Cd from the soil is crucial to sustaining food security and environmental safety. SiNPs-mediated amelioration of Cd has been reported in several plant species, including Phyllostachys edulis (Emamverdian et al., 2021), Satureja hortensis (Memari-Tabrizi et al., 2021), Oryza sativa (Cui et al., 2017; Hussain et al., 2020), and Triticum aestivum (Hussain et al., 2019). Applying SiNPs against Cd stress has been considered more efficient than regular fertilisers (Chen et al., 2018). The experiment on Phyllostachys edulis suggested that SiNPs make a complex with Cd ions via adsorption and reduce the accumulation of Cd in roots and leaves (Emamverdian et al., 2021). It subsequently enhances the germination and growth parameters. Soil-applied SiNPs can alleviate Cd stress in Triticum aestivum plants with improved growth and chlorophyll content (Ali et al., 2019; Khan et al., 2020a). Further, SiNPs minimise Cd accumulation and oxidative stress while improving nutrient uptake and antioxidant defence system in Triticum aestivum (Thind et al., 2021). Roots treated with SiNPs have increased the xylem cell wall lignification in Trigonella foenum-graceum (Nazaralian et al., 2017). The increased cell wall lignification was coupled with an improved xylem cell wall thickness. Such cell wall adjustments improve nutrient transport and silicon for faster growth (Asgari et al., 2018). SiNPs engage the Cd ions on the root surface to terminate their translocation in the aerial parts or immobilise them in the soil (Silva et al., 2017). Cui et al. (2017) also reported the downregulation of OsLCT1 and OsNramp5, genes with SiNPs application involved in Cd uptake and transport, respectively, in Oryza sativa. At the same time, SiNPs upregulated genes involved in Cd transport into the vacuole (OsHMA3) and Si uptake (OsLsi1). Higher silicon uptake can further restrict Cd uptake and transport and, thus, Cd toxicity.

Pb is a non-essential element and a detrimental contaminant for agricultural soils. It hampers plant metabolism, cell adhesion, and signalling by accumulating ROS in the cell wall (Küpper, 2017; Aslam et al., 2021). Foliar application of SiNPs boosted photosynthetic machinery and antioxidant enzymes in Coriandrum sativum and restricted Pb toxicity (Fatemi et al., 2020). A tissue culture experiment on Pb stress mitigation via silicon dioxide nanoparticles in Pleioblastus pygmaeus showed a reduction in the soluble protein content assimilated in the cell membrane while maintaining the cell membrane vitality (Emamverdian et al., 2019).

Arsenic contaminates groundwater globally, and irrigation with As-rich water amplifies its bioaccumulation and toxicity in several crops (Finnegan and Chen, 2012; Farooq et al., 2016; Vaculík and Vaculíková, 2017; Abbas et al., 2018; Abedi and Mojiri, 2020). Nonetheless, SiNPs application arrests As uptake and translocation to aerial parts of tissues and alleviates phytotoxicity in Solanum lycopersicum (González-Moscoso et al., 2019, González-Moscoso et al., 2022). SiNPs were reported to lower oxidative stress via improving antioxidative defence (SOD, APX, GR, and DHAR) in Zea mays seedlings (Tripathi et al., 2016). Further, SiNPs increase the mechanical strength of the cell wall in rice suspension cells under As toxicity via increasing pectin content, cation exchange capacity, and pectin methyl-esterase activity, reducing pectin methyl-esterification. SiNPs also blocked the uptake of As by inhibiting the expression of genes encoding As uptake (OsLSi 1, low silicon 1; OsLSi 2, low silicon 2) (Cui et al., 2020) since As and Si share a common transport system. Thus, adding SiNPs into the As medium causes a direct competition for the transport proteins (Cui et al., 2020). Moreover, SiNPs treatment enhances the expression levels of plasma-membrane localised NIP aquaporin family proteins, OsNIP1;1 and OsNIP3;3, which are permeable to arsenite (Mitani-Ueno et al., 2011; Sun et al., 2018). The overexpressed OsNIP1;1 and OsNIP3;3 are reported to reduce the As accumulation in Oryza sativa plants (Sun et al., 2018).

Mercury (Hg) is a highly toxic pollutant that contaminates cropland to different extents worldwide (Liu et al., 2020a). Li et al. (2020) demonstrated that exogenously applied SiNPs ameliorate the adverse effects of Hg in Glycine max seedlings. SiNPs significantly reduced Hg uptake, accumulation, and translocation in these seedlings. Further analysis with synchrotron radiation X-ray fluorescence showed lower Hg accumulation in the epidermis and pericycle of roots and stems of Glycine max plants treated with SiNPs.

SiNPs-mediated alleviation of Cr toxicity has been reported in Pisum sativum via reduced Cr uptake and accumulation in plant tissues (Tripathi et al., 2015). It was proposed that SiNPs facilitated mineral nutrient uptake and downplayed ROS synthesis by triggering antioxidant enzymes. SiNPs protected leaf ultrastructure under Cr toxicity in Triticum aestivum (Manzoor et al., 2022). While Cr deteriorated cellular organelles, SiNPs protected the cell walls, cell membranes, mitochondria, granal lamellae, thylakoids, nucleoli, and nuclear membrane. In a hydroponic study with Brassica napus, Huang et al., 2024 witnessed 100 um SiNPs (20 nm) boosted Si content in leaves increased by 169%, mostly restricted to intercellular spaces, chloroplasts, guard cells, and stomata. This can upgrade PSII biochemistry (NPQ, ETR, and quantum yield of PSII) and photosynthetic productivity. In the same study, SiNPs hampered the expression of Cr (and related TEs) transporter genes such as ST1, ST8, ABCG37, HMA, and MT, resulting in decreased Cr uptake (by 92% in roots and 76% in leaves). In Oryza sativa seedlings, SiNPs reversed the Cr-induced cell cycle arrest at the G2/M phase along with IAA application (Sharma et al., 2022). Similarly, endogenous NO levels in root tips were improved which could assist in ROS scavenging and upregulated antioxidant activity as was reported in the study.

Emamverdian et al. (2020) demonstrated the mitigative effect of SiNPs on three different TEs stresses: Mn, Cu, and Cd on Arudinaria pygmaea. Enhanced localisation of Cu and Mn by SiNPs was observed in the root surface, which could minimise TEs accumulation in the stem and leaves. Also, SiNPs treatment enhanced the photosynthetic capacity, biomass, and overall growth, which authors correlated with the reduced TEs uptake and accumulation in the plant shoot. Similarly, Riaz et al. (2022) suggested that SiNPs can relieve Cu²⁺ toxicity in wheat seedlings. SiNPs treated plants showed increased root length and plant height and enhanced antioxidant defence system. It was manifested by decreased malondialdehyde (MDA) and H₂O₂ contents and Cu²⁺ concentrations in shoots.

Si-induced alleviation of Mn toxicity has been reported in several studies, suggesting Si could contribute to the depression of Mn uptake and transport (Li et al., 2015). It could restrict lipid peroxidation by upregulating non-enzymatic and enzymatic antioxidants (Shi et al., 2005; Li et al., 2015). The reduced ·OH accumulation was also detected in the leaf apoplast (Dragišić Maksimović et al., 2007, Dragišić Maksimović et al., 2012). Moreover, Doncheva et al. (2009) observed a substantial thickening of epidermal layers after the Si treatment in the Mn-sensitive maize variety over a tolerant one. It suggests that Si could induce Mn storage in non-photosynthetic tissue to prevent Mn-toxicity effects on chloroplast functions. Similarly, Iwasaki and Matsumura (1999) assume that Si could assist in displacing and storing Mn in a metabolically inactive form around the base of the trichomes on the leaf surface. The mitigative effect of Si in the form of nanoparticles on Mn-toxicity was also described in the aforementioned study (Emamverdian et al., 2020; see Chapter 5.6).

Generally, Zn is a vital element for plant growth, as it is imperative in numerous metabolic pathways. Its deficiency is one of the plant’s most widespread micronutrient deficiencies (Anwaar et al., 2015; Kaur and Garg, 2021). However, as was reported by Long et al. (2003), a concentration above 3000 mg kg⁻¹ Zn in dry soil can have a noxious effect on plant yield and growth as it alters the ionome of plants through the inhibition of nutrients’ uptake and translocation (Bokor et al., 2015). The potential utilisation of Si in alleviating Zn toxicity has been studied, for example, in cotton or bamboo species (Anwaar et al., 2015; Emamverdian et al., 2018a). Both studies suggested that Si can limit Zn bioavailability and instigate the plant defence system by increasing antioxidant capacity and non-enzymatic activity, thus alleviating cellular oxidative damage. Similarly, Song et al. (2014) described that Si activated and regulated some photosynthesis-related genes in Oryza sativa with high-Zn exposure, improving photosynthesis over Zn-stressed plants that lacked Si treatments. These studies operated with bulk-Si, whereas using the SiNPs could potentially support its beneficial effects. However, in highly Zn-polluted soils, Zn can coexist with silica in the form of a Zn-silicate complex (Goswami et al., 2022), and Bokor et al. (2015) revealed such a complex can have a comparable negative effect on Lsi gene expression and mineral nutrition homeostasis as high concentration of Zn alone.

Sb is a non-essential metalloid with noxious effects for plants. However, studies to manage Sb-toxicity with SiNPs are rare. Still, there are several reports of Si-induced mitigation of Sb-toxicity in higher plants that could point in the right direction. Vaculíková et al. (2014, 2016) described Si-induced alleviation of Sb-toxicity on root growth and architecture in maize seedlings. Si supported the antioxidant defence system and thus reduced oxidative stress symptoms, and although Si did not reduce Sb content in roots, it considerably restricted Sb translocation to shoot. Shetty et al. (2021) attributed the blocked Sb translocation to root lignification, which was observed to a greater extent in Si-treated plants of Arundo donax. Moreover, enhanced photosynthetic pigments and overall photosynthetic yield were described. In poplar callus exposed to Sb-stress, Si declined the content of Sb in the calli and supported overall callus growth and nutrient uptake as well as the content of photosynthetic pigments. The improved Sb tolerance was secured via the Si-induced modification of antioxidant enzyme activity (Labancová et al., 2023). These findings also correspond to results obtained from the wild-type and the low-silica rice mutant cultivated with 10 or 30 μmol L⁻¹ Sb (Huang et al., 2012). Si treatment promoted growth and decreased Sb content in the shoots of both mutants by regulating the Sb distribution between the roots and shoots.

Although Ni is an essential component of several metalloenzymes of plants, it could be very toxic at supraoptimal concentrations. Like Sb, Ni-toxicity management is something least discussed in the literature with SiNPs. Exogenous Si was investigated in rice as a possible mitigative driver for Ni stress. Si protected the seedlings by upregulating the antioxidant defence components and glyoxalase systems, helping the ROS scavenge and detoxify cytotoxic methylglyoxal (Hasanuzzaman et al., 2013). The Si-induced recovery of growth, gas exchange, and pigment contents in cotton seedlings under the Ni stress was observed. It was secured by decreasing the Ni uptake and accumulation in the leaf, stem, and roots. It also increased antioxidant enzyme activities to restrict MDA, H₂O₂, and electrolyte leakage in leaves and roots (Khaliq et al., 2016). The enhanced antioxidant system, improved integrity of cell membranes and averted Ni-induced root anatomy alteration were also denoted as the mechanisms of Si-induced mitigation of Ni toxicity in maize plants (Vaculík et al., 2021). Improved leaf water status, enzymatic and non-enzymatic defence systems, and increased content of assimilatory pigments and leaf area were also described by Fiala et al. (2021).

Exogenous SiNPs ameliorate toxic TEs effects and improve overall plant growth (Tripathi et al., 2016; de Sousa et al., 2019; El-Saadony et al., 2021). These overcome responses have been correlated with soil physical properties, including TEs speciation, changes in soil pH, or deprivation of TEs due to co-precipitation with silicon (Bhat et al., 2019). Pretreatment of bulk-Si enhances phenolic root exudation (Chen et al., 2016) and organic acid exudation (Fan et al., 2016), which might be critical in TEs mitigation. Silicon immobilises TEs on the root’s outer surface by increasing the soil’s pH or altering TEs speciation by forming silicate complexes in soil solution. He et al. (2013) observed the complexification of most Cd with wall-bound silicon in rice root cells, leading to reduced Cd uptake and translocation. Kopittke et al. (2017) performed synchrotron studies and confined most of the Al complexed with Si in the Sorghum bicolor root cap. They evinced that this Si-Al complex formation in the root periphery could minimise the metal accumulation in the cell wall. Thus, the Si-mediated extrinsic defence mechanism through organic acid or phenolic exudation could be crucial in TEs toxicity mitigation.

Non-controlled silica deposition can harm a plant; hence, plants evolved effective mechanisms of cell walls’ silicification (Kumar et al., 2017). Silicon binds to lignin in the cell wall to form a Si-TE ion complex that subsequently arrests ion translocation from the root to the other plant organs (He et al., 2013; Sheng and Chen, 2020; Soukup et al., 2020). Most Si-mediated fruitful effects are reported to be linked with the accumulation of Si in roots, stems, leaves, and hulls, which acts as a physical barrier by enhancing the mechanical strength of plant tissues (Ma et al., 2006; Emamverdian et al., 2018b). Si, along with lignin, can deposit in dermal regions of the cell walls, thickening the Casparian strips and blocking the TE transport in plants (Kim et al., 2014). Si-induced changes in the cell wall binding properties might be essential in mitigating TEs toxicity. Si reduced Cd accumulation in roots and grains of rice (Chen et al., 2019) and maize (Liu et al., 2020b) which could result from Si deposition in root cells apoplast hindering Cd uptake (Song et al., 2009; Wang et al., 2015). It is stipulated that Si enhances Lsi expression while suppressing Nramp 5 (Cd transporter gene) in rice roots, suggesting that upregulated Si transporters resist Cd toxicity under ample silicon supply (da Cunha and do Nascimento, 2009; Ma et al., 2015). Considerable amounts of covalent-bound Si are also complexed with hydroxyl groups of pectin contained in the cell wall (Schwarz, 1973; Sheng and Chen, 2020). He et al. (2015) suggests that hemicellulose, rather than pectin and cellulose, is the primary ligand bound to Si complexes in rice. Si accumulated in the cell walls in hemicellulose-bound organosilicon compounds can improve cell wall mechanical properties and regeneration and inhibit the Cd uptake by a mechanism of Cd complexation and subsequent co-deposition (He et al., 2015; Ma et al., 2015).

TEs overaccumulation stages the ROS-induced oxidative emergency, threatening many vital processes. Thus, the plant’s top priority for survival under such scenarios is scavenging ROS. This goal is facilitated by an antioxidant system comprising several enzymatic (SOD, CAT, APX, POD, and GR) and non-enzymatic (ascorbic acid, α-tocopherol, proline, carotenoids, flavonoids, and reduced glutathione) antioxidants. In this context, pretreatment of SiNPs was reported to stimulate the enzymatic antioxidants against TEs toxicity in Solanum lycopersicum under As stress (González-Moscoso et al., 2019, González-Moscoso et al., 2022), Glycine max under Hg stress (Li et al., 2020), and Satureja hortensis (Memari-Tabrizi et al., 2021) and Triticum aestivum under Cd toxicity (Ali et al., 2019). The formation of free radicals under TEs toxicity directly damages the cell membrane permeability and stability that, in time, would cause the homeostasis collapse of cells and tissues. However, silicon counteracts it by enhancing the stability of the plasma membrane under TEs stress (Vaculík et al., 2020). Another TEs detoxifying mechanism is the synthesis of various chelating agents, i.e., flavonoids, phenolics, and organic acids (Bhat et al., 2019). Silicon reportedly influences the synthesis of several chelating compounds, including cysteine, glutathione, and phytochelatins, under TEs toxicity (Rahman et al., 2017).

Plants evolved several responsive manoeuvers against TEs toxicity, such as minimising TEs bioavailability and uptake, enriching plants with nutrients, and stimulating the antioxidant system and the biosynthesis of protective agents (osmolytes, organic acids, metallothioneins, and phytochelatins) (Tripathi et al., 2015; Moharem et al., 2019; Cao et al., 2020; Lian et al., 2020; Wang et al., 2020a, Wang et al., 2020b). Among the different NPs applied, SiNPs have proven to be quite promising (Tripathi et al., 2015; Wang et al., 2015; Tripathi et al., 2017; Khan et al., 2020a). Though not an exclusively effective metal/metalloid barricade, the apoplasmic barrier also fulfils essential defensive functions in plant roots by regulating the flow of ions, oxygen and water (Lux et al., 2004; Chao et al., 2013). The efficiency of apoplasmic barriers as contaminant barricades can be enhanced by NPs application (Rossi et al., 2017). NPs attach to the TEs in the root cell walls, making stable complexes and rendering them unavailable. NPs-TEs complexes, once adsorbed, become immobile, obstructing the mobility of the TEs inside the plants and reducing their biological activity (Cui et al., 2017; Wang et al., 2021; Zhou et al., 2021). Accumulating organic acids (behaving as metal chelators) and chelating TE contaminants are necessary adaptations to TEs tolerance. The biosynthesis of such protective organic acids is improved by SiNPs, reducing the damage caused by TEs like Cd and As (Cui et al., 2017; Tripathi et al., 2017; Zhou et al., 2021). The interaction of SiNPs with the TEs is crucial while studying the different characteristics of TEs stress alleviation. SiNPs can also reduce the mobility and bioavailability of TE contaminants in the soil (Tripathi et al., 2015; Wang et al., 2015; Tripathi et al., 2017; Khan et al., 2020a). The application of mercapto SiNPs increased the stability of Cd and, thus, decreased its mobility (Wang et al., 2020b). Alternatively, Si may co-precipitate with metals/metalloids as silicates in the roots and leaves of different plants. Co-precipitation of Zn as Zn silicates or Si–Zn complexes in the cell walls of leaf epidermal cells was seen in Minuartia verna and Cardaminopsis halleri (Neumann et al., 1997; Neumann and Zur Nieden, 2001). Gu et al. (2011, 2012) suggested the sequestration of Zn–Si and Zn-Cd precipitates in rice to the cell walls of less bioactive tissues. Similarly, Mn, Cu, and Cd might co-precipitate with Si to restrict their accumulation in shoot phytoliths, but it is not fully confirmed yet (Iwasaki et al., 2002; Zhang et al., 2008; Oliva et al., 2011; Emamverdian et al., 2020).

SiNPs regulate a variety of physiological phenomena in plants, notably nutrient assimilation, CO₂ fixation, accretion of secondary metabolism products and activities of different enzymes under normal as well as perturbed environmental conditions (Tripathi et al., 2015, Tripathi et al., 2017; Ahmad et al., 2019; Khan et al., 2020a; Mukarram et al., 2021). Al-toxicity alleviation in barley and maize after Si supplementation has been endorsed to accumulate phenolic compounds (Adrees et al., 2015; Vega-Mas et al., 2019). SiNPs have also been observed to modulate the regulatory enzymes of the shikimic acid pathway, leading to increased accretion of phenols in the leaves, as has been reported in Mentha piperita (Ahmad et al., 2019). Because of their metal-chelation capacity with flavonoid-phenolics, phenols play a critical role in TE toxicity mitigation by reducing the uptake and translocation of toxic TEs, as has been reported for Al and Mn (Kidd et al., 2001; Dragišić Maksimović et al., 2007; Shahnaz et al., 2011). Bulk-Si facilitates phenol overaccumulation by triggering phenylalanine ammonia-lyase (PAL), a critical regulatory enzyme of the phenylpropanoid pathway (Rahman et al., 2015; Ahanger et al., 2020). It also upregulated PAL, cinnamyl alcohol dehydrogenase, and chalcone synthase in Rosa hybrida (Shetty et al., 2011). Likewise, Si-induced PAL activity also assisted in managing Cu stress in Arabidopsis thaliana roots (Li et al., 2008).

Bulk-Si can influence the biosynthesis of H₂O₂, nitric oxide (NO), and hydrogen sulphide (H₂S) to govern Ag and Cd toxicity in mustard and pepper (Soundararajan et al., 2018; Vishwakarma et al., 2020; Kaya et al., 2020a). Si decreases electrolyte leakage and H₂O₂ and MDA content by boosting the antioxidant system, possibly with NO involvement, and regulates plant growth and development under TE toxicity (Tripathi et al., 2021). SiNPs-induced NR biosynthesis in Mentha piperita and restricted H₂O₂ production in Cymbopogon flexuosus suggests SiNPs crosstalk with H₂O₂ and NO (Ahmad et al., 2019; Mukarram et al., 2023). Thus, it does not seem hasty speculation that SiNPs could interact with gaseous signalling molecules in a similar fashion to bulk-Si. It will hold relevance during TEs stress alleviation as well. The nitrate reductase (NR) pathway is the best-characterised NO biosynthetic pathway (Planchet and Kaiser, 2006). In addition to the general function of nitrate-to-nitrite reduction, NR also performs a crucial part in plants by transferring an electron to nitrite using NAD(P)H as a source of electrons, ultimately resulting in NO biosynthesis (Planchet and Kaiser, 2006). The synergistic interaction of NO and Si can discourage As uptake and increase phytochelatin biosynthesis, reducing As translocation in mustard (Ahmad et al., 2021). In a similar study, Liu et al. (2020b) established the collegial effect of Si and NO in mitigating Cd toxicity in Triticum aestivum seedlings. Si stimulates endogenous H₂S accretion that upregulates antioxidants’ activity in Capsicum annuum raised on Cd- and B-spiked soils (Kaya et al., 2020a, 2020b).

Given the scanty literature concerning the SiNPs-mediated TEs tolerance in plants, the proteomic approach is comparatively novel to gaining insights into the expression of various stress-related enzymes and proteins. TEs-stimulated oxidative stress leads to altered protein expression and structure, leading to loss of protein activity or its content. Nevertheless, silicon and SiNPs supplementation regulates the expression of several proteins and enzymes of signal transduction cascades of the antioxidant defence system (Tripathi et al., 2015; Muneer and Jeong, 2015a; Mukarram et al., 2022). Once inside the cells, silicon plays an imperative role in stress alleviation by maintaining ion homeostasis and structural rigidity, upregulating antioxidant metabolism, and increasing the expression of genes and proteins involved in stress alleviation (Ma, 2004). SiNPs-induced upregulation of the different primary and secondary metabolic enzymes is well-reported. Mukarram et al. (2021) reported that the expression of terpene (neral, geranial) and NR enzyme activity was upregulated in SiNPs-treated Cymbopogon flexuosus. Si application improves PSII polyprotein expression under Zn toxicity (Song et al., 2014). Further, the bulk-silicon enhanced the protein content related to stress (17%), hormones (11%), and other cellular biosynthesis (11%), and many others associated with gene expression and secondary metabolism in 25 mM salt-stressed Lycopersicon esculentum plants (Muneer and Jeong, 2015b). The stress-related proteins included zinc finger A20, COPINE 1 family protein, caffeoyl-CoA O-methyltransferase, and others. Down-regulation of Zn transporter (OsZIP1) protein after Si supplementation decreases Zn uptake in Oryza sativa (Huang and Ma, 2020). Si accumulation involves both influx and efflux transporters. The SiNPs application has been endorsed for upregulating the menthol-reductase enzyme to proliferate menthol in mint oil (Ahmad et al., 2019). Similarly, geraniol dehydrogenase enzyme activity was positively influenced by SiNPs foliar application in Cymbopogon flexuosus under 160 mM and 240 mM salt stress (Mukarram et al., 2023). SiNPs-induced antioxidants can improve isoenzyme patterns and genomic alterations to restrict TE toxicity in Pisum sativum and UV-B stress in Triticum aestivum (Tripathi et al., 2015, Tripathi et al., 2017). Further, Si can delay chlorophyll-protein complex degradation such as supercomplexes, PSI core binding LHCI, PSI core, F1-ATPase binding Cytb6/f complex, PSII core, and trimeric and monomeric LHCII (Wang et al., 2019). Si can also improve photosynthetic performance, given its observed benefits on absorption, transformation, and transfer of light energy through optimising thylakoid membrane proteins in water-deprived Oryza sativa seedlings (Wang et al., 2019).

Studies on TEs-stressed plants have indicated that ROS-induced DNA damage was more pronounced in the Cd and Pb-exposed plants, as indicated by the disappearance of several normal bands in the RAPD pattern of the DNA. In contrast, new DNA amplicons could be located in TE-exposed plants treated with different NPs. Moreover, oxidation of proteins is a common TEs toxicity symptom as TEs ions directly interact with protein molecules due to their strong affinity with carboxyl- thionyl- and histidyl groups (Hossain et al., 2015). 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 similarly to the metal ions (Hossain et al., 2015). As discussed, different NPs upregulate the expression of various genes in plants, speeding up the biosynthesis of several primary and secondary metabolism products (Večeřová et al., 2016; Marslin et al., 2017).

The role of SiNPs exemplifies regulating an array of transcription factors (TFs) implicated in abiotic mitigation, notably DREB2, NAC, NAM, and CUC. These TFs overexpress genes associated with scavenging free radicals and maintaining osmotic potential and ionic homeostasis (Manivannan and Ahn, 2017). Moreover, silicon can induce regulatory proteins coupled to gene expression under stress, particularly TFs for transcription elongation (SPT4), ribosomal protein L16, RNA polymerase mediator, tRNA-lysidine synthase, MADS-box, ribosome-recycling factor and reverse transcriptase (Muneer and Jeong, 2015b; Al Murad et al., 2020).

Exogenous application of SiNPs modifies plant’s nutrient status, facilitating N, Fe, Mg, Zn, and Si absorption (Wang et al., 2015; Ahmad et al., 2019; Mukarram et al., 2021). SiNPs-mediated increase in Si uptake leads to a decrease in the Cd uptake, facilitating the growth of Oryza sativa seedlings raised on Cd-rich soils. Cui et al. (2017), in their study on rice, observed that SiNPs upregulate the expression of Si transporter (OsLsi1) while the expression of Cd-transporters (OsLCT1, OsNramp5) is down-regulated. Down-regulation of essential abiotic stress tolerance genes, notably ERF5 (ethylene response factor 5, RBOH1 (respiratory burst oxidase), MAPK2, and MAPK3 (mitogen-activated protein kinases), by the application of SiNPs, is well-reported (Almutairi, 2016). SiNPs-mediated regulation of primary metabolism, biosynthesis and modifications of secondary metabolism products, particularly phenols, possibly enhances the tolerance against stress (Rahman et al., 2015; Tripathi et al., 2015, Tripathi et al., 2017; Ahmad et al., 2019; Ahanger et al., 2020). Furthermore, SiNPs were suggested to induce different transcripts (CfADH2a-b, CfADH1, CfAKR2b, CfAAT3, and CfALDH) to regulate the constitutional makeup of plant essential oil (Mukarram et al., 2021).

Despite growing studies on SiNPs, most publications need more genomic insights. Here we introduce significant recent findings dealing with the influence of Si on gene activity, generally to indicate a possible role with SiNPs. Different studies have suggested the positive regulation of gene transcripts of various metabolic processes by Si application (Brunings et al., 2009; Chain et al., 2009; Debona et al., 2017). The role of Si in upregulating photosynthetic genes has been studied in detail. For example, Zn-induced damage in PsbY expression was overcome by Si supplementation (Song et al., 2014). Moreover, increased PSII activity and electron transfer rate by upregulating PsbY mRNA transcripts are endorsed for Si supplementation in rice under Zn stress. The other Si-upregulated genes under Zn-toxicity include PetC, PsaH, PetH encoding chloroplast, cytochrome proteins, and ferredoxin NADP⁺ reductases, respectively (Song et al., 2014). Moreover, Si upregulates several genes encoding for electron transport chain proteins and light-harvesting complex viz., PetE, PetF, PsbQ, PsbP, PsbW, and Psb28. Furthermore, increased expression of gene transcripts (PsbW, Psb28, PsbQ, and PsbP) involved in the photolysis of water has been attributed to Si application (Zhang et al., 2018). PetH, Os03g57120 and Os09g26810 genes involved in stress mitigation, NAD(P)H and glutathione biosynthesis are also upregulated by Si (Manivannan and Ahn, 2017). Kaushik and Saini (2019) have reported the upregulation of LeGR (glutathione reductase gene) in Solanum lycopersicum after Si supplementation. In Triticum aestivum, Si attenuates TEs toxicity by upregulating metallothionein and phytochelatin synthase gene expression (TaMT1, TaPCS1) (Hossain et al., 2018). Genes coding for enzymatic oxidants (SlCAT, SlGR, SlGST, SlSOD, SlPOD, SlGPX) have been observed to be upregulated by exogenous sourcing of Si attenuating stress response in Solanum lycopersicum (Khan et al., 2020b).