Selenium causes the restriction of toxic metal ions uptake and their metabolism by rescuing their translocation from lower plant to upper parts (Alyemeni et al., 2018; Hasanuzzaman et al., 2020). For instance, such Se-induced lower Cd uptake by roots had been reported in Cd-stressed tomato seedlings along with reduced translocation into shoots and leaves (Alyemeni et al., 2018). Not only that, Se alters the translocation ability of cultivar through lowering root to stem and stem to grain along with increasing stem to leaves translocation of different rice cultivars (Gao et al., 2018). Supplemented Se is able to enhance the binding of metal ions to the root cell wall by improving pectin and hemicellulose, and also changes the subcellular distribution in roots, for example metal contents are reduced in plastids and mitochondria, whereas enhanced in vacuolar sap and ribosome (Zhao et al., 2019). Moreover, protective role of Se is also involved in improving phytochelatin (PC) synthesis or nontoxic Se-metal complexes formation as well as reducing metals transportation from root to aerial plant parts for alleviating metal toxicity (Hawrylak-Nowak et al., 2014).

In B. campestris sp. Pekinensis, exogenous application of Se reduced the concentration and influx of Cr by altering root morphology (Zhao et al., 2019). Ulhassan et al. (2019) observed a decrease in Cr accumulation in plant tissues with the application of Se in B. napus. Hawrylak-Nowak et al. (2014) reported decrease in Cd accumulation and phytochelatins in roots of cucumber and an increased translocation toward shoot in Se-treated plants. Conversely, some studies have demonstrated that Se supplementation has no significant impact in the accumulation of Pb in the bean grown under field conditions (Mroczek-Zdyrska and Wójcik, 2012; Mroczek-Zdyrska et al., 2017). Pokhrel et al. (2020) reported a decrease in As uptake in rice. Se application influences the uptake of heavy metals in a time-dependent manner. For instance, in rice grown under Cd stress, supplementation of Se enhanced the uptake of Cd, but decreased this uptake on longer Cd exposure (Wan et al., 2016). Further Se-induced decrease in heavy metal uptake in plant varies even among the cultivars of the plant species. Gao et al. (2018) evaluated the influence of Si and Se individually and in combination on the uptake of Cd in three rice cultivars namely WYHZ, NJ5055, and ZF1Y. They observed that all the three forms of protectants applications (Si, Se, and Si + Se) decreased the uptake of Cd in WYHZ, but such influence was not noted in NJ5055 and ZF1Y.

Selenium plays a crucial role in the protection of plants from the toxic impacts of metals/metalloids (Feng et al., 2020; Riaz et al., 2021). It is a common phenomenon that plants under the exposure of metals/metalloids tend to reduce their growth. A number of studies have unveiled that Se in different forms applied at different levels to crop plants grown under heavy metals/metalloids stress boost plant growth and improve physiological functions. Hawrylak-Nowak and Matraszek-Gawron (2020) evaluated the influence of 2 or 6 μM of different forms of Se (selenite IV and selenate VI applied as Na₂SeO₃ or Na₂SeO₄) on Ni (5 and 10 μM)-exposed lettuce plants. The authors observed that Se (IV) at 2 or 6 μM levels caused a reduction in leaf area, shoot (34 and 72%), and root (39 and 73%) FW under 5 μM Ni, whereas a decrease of 82–84% in shoot and root FW was observed in 6 μM Se (IV)-supplemented plants grown under 10 μM Ni. Contrary to this, 2 μM Se(VI) did not influence leaf area and biomass but improved root biomass (23%) under 10 μM Ni stress. Regarding physiological impacts, application of Se (IV) and (VI) improved photosynthetic pigments, except 6 μM Se (IV) (Hawrylak-Nowak and Matraszek-Gawron, 2020). In B. juncea, application of selenite (Se IV, 50 μM) not only enhanced germination by improving germination rate and seedling vigor, but also improved growth by reducing As-induced (As III, 300 μM) adverse effects in shoot and root length, fresh and dry weight by about 9, 12, 5, and 9%, respectively with improvement of root/shoot ratio by 9% under (Sahay et al., 2020). In another work, Handa et al. (2019) reported that in B. juncea grown under Cr (300 μM kg⁻¹), application of Se (4 μM kg⁻¹) recovered root and shoot length by 44 and 18%, respectively. Nano-Se is also reported to provide a relieving effect to crop plants grown in contaminated soils. For instance, supplementation of nano-Se to rice plants grown under Pb and Cd contaminated soil improved plant growth and photosynthesis and its related genes, proteins, and chl concentration (Wang et al., 2021). Alves et al. (2020) also reported that application of 1.0 μM of selenite or selenate resulted in an increase in photosynthesis and biomass of tomato grown under Cd stress (0.5 mM CdCl₂). In wheat, combined application of Se and Zn (10, 20, and 40 mg L⁻¹) alleviated the Cd-induced reduction in growth, photosynthesis, and photosynthetic pigments (Wu et al., 2020). Han et al. (2015) reported an increase in growth of Nicotiana tabacum under As stress (0–5 mg L⁻¹) supplemented with selenite (≤ 1 mg L⁻¹), but it was inhibited under high Se and As (5 mg L⁻¹). In another work, Se (IV; 10 μM) boosted the growth of primary roots, but reduced the growth of lateral roots. Moreover, in the same work, Se application downregulated the expressions of genes of auxin and ethylene biosynthesis, and reduced the levels of these hormones in rice roots (Malheiros et al., 2019). Moulick et al. (2016) evaluated the influence of Se-based seed priming in rice under As stress and reported improved germination by 9%, and shoot-, root-length, and plant biomass by 1.6, 1.3, and 1.4-fold, respectively compared with non-treated As stressed plants.

Alves et al. (2020) demonstrated that Se improved photosynthesis and caused alteration in anatomical traits. In B. napus, Se application induced reduction in phytotoxicity, resulting in enhanced growth, biomass, photosynthetic pigments, leaf gas exchange, and F_v/F_m (Ulhassan et al., 2019). Khan et al. (2015) reported a decrease in Cd-induced oxidative stress following the application of Se in wheat plants. Sun et al. (2016) demonstrated that Se application caused a reduction in Cd-induced phytotoxic effects on cucumber plants by regulating stress response related proteins and pathways, such as glycolysis pathway and nitrate assimilation pathway (i.e., fructose bisphosphate aldolase 2, NiR), which may increase Se-induced Cd tolerance. Chauhan et al. (2020) reported a boosted activity of glutathione S-transferases (GST), peroxiredoxin, glutaredoxins, and heat shock proteins resulting in amelioration of As-induced oxidative stress in rice. In another work, combined application of 24-epibrassinolide (EBL) and Se under Cu stress ameliorated the negative impacts of Cu in B. juncea via altering Pro metabolism with increasing Pro accumulation (87%) through stimulating proline synthesizing enzymatic activity of pyrroline-5-carboxylate synthase (Yusuf et al., 2016). In addition, foliar application of both Se and EBL caused the higher activities of RuBisCO and carbonic anhydrase and also improved net photosynthetic rate and stomatal conductance under Cu toxicity.

Although studies related to ion homeostasis in response to Se application under heavy metal stress are available, but the available ones have shown positive results regarding said trait in plants. For instance, Zhao et al. (2019) examined the impact of Se supplementation in in B. campestris sp. Pekinensis grown under Cr stress and observed that 0.1 mg L⁻¹ of Se-induced enhanced uptake of nutrients including Na by 40%, Ca by 32%, Fe by 63%, Mn by 21%, Cu by 24%, and Zn by 30% under 1 mg L⁻¹ of Cr stress due to alterations in root morphology. In another work, Ulhassan et al. (2019) tested the effect of 5 and 10 μM of Se supplementation on 100 and 200 μM of Cr in B. napus. The authors noted a decline in major (N, P, K) and micronutrients (Zn, Fe, Mn) uptake under Cr stress. Application of Se (5 and 10 μM) notably improved the mineral transportation under Cr contamination condition which was restricted in only stress condition (Ulhassan et al., 2019). The above discussed studies demonstrates the positive impact of Se application on ion homeostasis under heavy metals stress (Table 1). However, more research is needed to unveil the mechanisms involved in Se induced maintaining of ions homeostasis in plants.

The Se application in stressed plants results in a decrease in ROS viz. OH^•, O2•-, and H₂O₂ levels, which protects the plants from oxidative stress (Hasanuzzaman et al., 2020). One of the main functions of Se is the enhancement of GPX activity because selenocysteine is present at the catalytic site of this enzyme. Under metal/metalloids stress, plants suffer several damages in different organelles like chloroplast, which suffer modifications in their shape and organization. Nevertheless, the supply of appropriate levels of Se reduces the level of damages in chloroplast and enhances the photosynthetic process, which has been disrupted (Feng et al., 2013). There are several works reporting the positive effect of Se application in the photosynthetic apparatus (Chauhan et al., 2017), which can be associated with a higher capacity of Fe uptake and which have a crucial role in the electron transport chain (ETC), resulting in the generation of substrates to remain organized under high excitations of electronic levels (Feng et al., 2013). Moreover, Se is involved in a higher synthesis of PC or the generation of nontoxic Se metal complexes, which are responsible for the reduction of metals/metalloids resulting in toxicity (Hawrylak-Nowak et al., 2014). The Se application also has a positive effect in DNA structure avoiding methylation changes, which occurs in species grown under metal stress conditions (Figure 2; Filek et al., 2008).

Ulhassan et al. (2019) noted a reduction in the generation of ROS species such as H₂O₂, O2•-, and MDA, a higher membrane integrity capacity associated with the increase of GSH and AsA levels, and lower activities of SOD and CAT and enzymes involved in AsA–GSH cycle (ascorbate peroxidase, APX; monodehydroascorbate reductase, MDHAR; dehydroascorbate reductase, DHAR; and glutathione reductase, GR) under Cr stress. Moreover, the application of Se also resulted in a reduction of glyoxalase I (Gly I) and glyoxalase II (Gly II) enzyme activities, and consequently in a decline of MG contents. Hawrylak-Nowak et al. (2014) reported decrease in MDA in Se-treated cucumber plants. Qutab et al. (2017) demonstrated that Se supplementation to root system reduced Cd stress triggering a decrease in protein concentrations in the roots of maize exposed to different Cd levels. Sun et al. (2013) found that the application of exogenous Se reduced the toxicity to the different Cd levels. Similarly, in Cd-stressed rice, supply of Se significantly reduced Cd accumulation in rice grains (Hu et al., 2013). Wu et al. (2020) investigated the combined impact of Se and Zn on wheat grown under Cd stress. They observed an increase in photosynthesis and antioxidant enzyme activity, but a decrease in MDA in plant tissues. Zhang et al. (2019) discerned the interaction of different sites with natural presence of Se and Cd and reported that the high ratio of Se to Cd in soil has a beneficial effect on mitigation of Cd stress and its uptake in maize plant.

Reviewing previous literature, there are several references focused on the ameliorative effects of the supply of Se on the antioxidant defense of different species grown under metal/metalloid toxicity (Table 2). The effects of increasing supplies of Se as Na₂SeO₄ amendments (0, 2, 4 and 6 μM kg⁻¹) in B. juncea plants were tested in a pot experiment with soils containing increasing Cr concentrations (0 and 300 μM kg⁻¹). The levels of H₂O₂ and O2•- were reduced and there was a significant increase in SOD, CAT, MDHAR, DHAR, GR, and GPX activities as well as in non-enzymatic antioxidant viz. AsA and GSH due to the external supply of Se (Handa et al., 2019).

There are also studies about the ameliorative effects of Se in several species under Cd stress. The foliar spraying of Se (Na₂SeO₄) in the different growth stages in wheat plants grown under Cd-contaminated soil resulted in higher biomass accumulation and antioxidant enzyme activity. Lipid peroxidation decreased whereas leaf SOD, POD, CAT, and APX increased significantly with the supplies of Se (Wu et al., 2020). Presoaking sunflower seeds with Se (5, 10, and 20 μM) resulted in a reduction of the oxidative damage associated with Cd (20 μM of Cd) toxicity. Presoaking with Se induced an enhancement of CAT, APX, and GR activities (Saidi et al., 2014). Two varieties of wheat (soft and durum) showed a reduction of Cd toxicity due to the external supply of Se via root and foliar, where a reduction of lipid peroxidation and an increase in SOD and POD activities were observed (Zhou et al., 2021).

Regarding As toxicity, Singh et al. (2018) assessed the effects of Se application in rice plants. The chemical analysis reported an antagonism between both nutrients reducing the uptake of As under increasing Se concentrations. This antagonism also resulted in lower values of MDA concentration as well as the consequent rise of antioxidant enzymes such as SOD, CAT, and APX. Similarly, in wheat plants the supply of 10 μM of selenate in the nutrient solution resulted in lower oxidative damage in seedlings exposed to increasing As concentrations. Selenium has improved the growth of wheat plants, thereby increasing the activity of APX as well as GPX and GR (Ghosh and Biswas, 2017). In case of rice plants, Pandey and Gupta (2018) reported a clear reduction in the accumulation of H₂O₂ as well as O2•- and the increase in the activity of several antioxidant enzymes (CAT, SOD, POD, and GR) with an exogenous supply of Se in rice plants under As stress.

The damages caused by Pb stress can be ameliorated by external supplies of Se. For instance, Mroczek-Zdyrska and Wójcik (2012) reported that the supply of Se at concentrations of 1.5 and 6 μM as sodium selenite in a Hoagland nutrient solution alleviated Pb toxicity (50 μM) in V. faba roots. The alleviation was based on the reduction of O2•- generation in the apical parts of roots and enhanced the total sulfhydryl content and GPX activity. Moreover, the supply at low dosage enhanced cell viability, whereas at high levels it improved both lipid peroxidation and cell membrane injury. Zhou et al. (2017) investigated the positive role of Se in alleviating Hg stress. The supplies of increasing concentrations of Se (1, 2, 3, and 4 μM) under 0.5 mg L⁻¹ Hg stress resulted in enhanced antioxidant activities (SOD, POD, CAT, and APX). Consequently, there is an increase in the scavenging of ROS and a reduction in lipid peroxidation.

Dai et al. (2019) assessed the positive role of Se in B. campestris sp. Pekinensis grown under Zn stress. They worked with two treatments (0.5 mg kg⁻¹ Se + 30 mg kg⁻¹ Zn and 1.0 mg kg⁻¹ Se + 30 mg kg⁻¹ Zn), and the data obtained reported that the antioxidant enzyme activities increased (POD, SOD, CAT, APX, GR) as well as Pro concentration under Se supplies. Cartes et al. (2010) tested the effects of Se supplies in ryegrass grown under Al stress in hydroponic conditions. They applied 0.2 mM Al and six levels of Se (Na₂SeO₃·5H₂O) (0, 1.0, 1.5, 2.0, 5.0, and 10 μM) for 20 days. Selenium application increased POD activity whereas SOD showed a significant decrease. Hawrylak-Nowak and Matraszek-Gawron (2020) tested if there were differences between selenite and selenate application in lettuce exposed to Ni stress for 14 days. The data reported that selenate had a higher ameliorative effect compared with selenite and also a dose-dependent response. Zhu and Ma (2018) studied the influence of sodium selenite (Na₂SeO₃) on physiological characteristics of wine grape seedlings under Cu stress in a greenhouse pot experiment. Selenium treatment (1.0 mg kg⁻¹ soil) enhanced pigment concentration, POD, and CAT activities and reduced the level of lipid peroxidation.

With respect to yield, only limited number of studies have demonstrated the yield potential under Se-supplemented heavy metal stressed crop plants. Shekari et al. (2019) evaluated the impact of Se supplemented at different growth stages on cucumber (Cucumis sativus L.) grown under Cd and Pb stress. The authors observed that 4 and 6 mg L⁻¹ of Se application induced an increase in flowering, reduction in flowering time, improved female to male flowers ratio, and induced significant enhancement in fruit yield and production time under 60 mM Pb and 100 mM Cd stress compared with the non-treated heavy metals stressed plants (Shekari et al., 2019). In a pot experiment under greenhouse conditions, Huang et al. (2017) tested the effect of Se application in three rice cultivars (Zhongjiu A (female parent) and Huazhan R (male parent), and their F1 hybrid) grown under Cd stress and observed that Se enhanced dry weight of the male parent of rice grain and F1 hybrid in a concentration-dependent manner at the same level of Cd stress. Mozafariyan et al. (2014) examined the impact of Se (0, 3 or 7 μM) under Cd (0, 0.25 or 0.50 mM) on peppers. The authors noted that under Cd stress (0.25 mM), Se applications at 3 or 7 μM while at 0.5 mM Cd level, 3 μM of Se enhanced fruit yield per plant through increasing photosynthetic pigments and improving antioxidant (Mozafariyan et al., 2014).