Considering that seed germination is the first physiological activity that Cr affects, a seed’s capacity to germinate in a medium containing Cr would be an indication of how tolerable it is to this metal (Shanker et al., 2005; Rath and Das, 2021). Symptoms of Cr phytotoxicity comprise the early development of seedling or impediment of seed germination, suppressed root growth, and leaf chlorosis. Cr prominently reduced the seed germination of different plants such as vegetables cauliflower (Brassica oleracea L.), citrullus (Citrullus vulgaris), and crops, wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), and maize (Zea mays L.) (Shahid et al., 2017; Ao et al., 2022). It was noted that higher toxicity of Cr in soil reduced the germination rate of jungle rice (Echinochloa colona), bush bean (Phaseolus vulgaris), alfalfa (Medicago sativa), and sugarcane (Saccharum officinarum) by 25%, 48%, 23%, and 57%, respectively as compared with control (Shanker et al., 2005).

According to several investigations, with an increase in Cr concentration in the external medium i.e., soil/nutrients solution, the DNA content of bean seedlings gradually improved and as a result, the DNA content followed a trajectory that was the opposite of the radical expansion (DalCorso, 2012). Higher concentrations of Cr significantly minimized the bean roots by interfering the cell division process in roots (Zeid, 2001; Singh et al., 2013). During seed germination, accumulated reserve materials like proteins and starch are hydrolyzed to produce precursors like sugars and amino acids for the development of embryo axis as well as substrates for different metabolic processes (Ao et al., 2022). Additionally, when the Cr content gradually increased, the activity of the α- and β-amylases of the developing seeds decreased, which may be responsible for the inhibition of seed germination (Oliveira, 2012). Seed germination of black gram (Vigna mungo) was reduced to 50.70% with the presence of Cr(VI) contents (300 µM) in nutrient solution (Rath and Das, 2021). Singh and Sharma (2017), observed that chickpea (Cicer arietinum) and green bean (Phaseolus aureus) seed germination was decreased by 42.60 and 53.53%, respectively, when Cr was present at higher concentrations (100 mg/L). More than 90% of the 45 tomatoes (Solanum lycopersicum) genotypes displayed reduced and delayed germination within 14 days under 78 mg/L Cr(VI) stress, according to a recent study by Hafiz and Ma (2021).

Higher ROS production from Cr treatment may have facilitated the breakdown of stored nutrients in seeds cotyledon, which ultimately leads to changing the characteristics of cell membranes, hence results in reduced seedling germination (Shafiq et al., 2008; Shah et al., 2010). The significant reduction in seedling length under Cr stress might be due to the reduced water potential and secondary stress-causing obstructed nutrient absorption (John et al., 2009). Because there are fewer meristematic cells in root tips than in cotyledons and shoot apex, Cr treatment also results in diminished seedling growth, particularly of roots (Rath and Das, 2021). The hydrolytic enzymes’ activity is impacted by Cr stress, depriving the radical and plumule of seed and ultimately slowing seedling growth (Stambulska et al., 2018). According to Sundaramoorthy et al. (2009), hexavalent Cr concentration even results in chromosomal abnormalities in the roots of seedling, which stimulate c-mitosis and result in extremely reduced root growth. The amylase activity of seeds under Cr stress may be inhibited, which would lead to a reduction in the transfer of carbohydrates to the germ (Stambulska et al., 2018). Additionally, Cr treatment stimulates protease activity, which results in a lower rate of seed germination or possibly seed death (Khan et al., 2020; Ao et al., 2022).

By altering the soil’s nutritional composition and controlling plant nutrient absorption, distribution, and transport, Cr have a significant impact on the metabolism of minerals and causes phytotoxicity in soil-plant systems (Chen et al., 2018). Cr can alter the mineral nutrition of plants in a complex way because of its structural resemblance to some critical elements (Ding et al., 2019). Researchers have focused most of their emphasis on how Cr affects the absorption and accumulation of other inorganic nutrients. Different processes are used by plants to absorb Cr (Ao et al., 2022). Both forms, i.e., Cr(III) and Cr(VI), have the potential to obstruct the uptake of several other ionically related ions, including Fe and S. Both Cr(III) and Cr(VI) have been reported to interfere with macronutrient elements (Ca, K, Mg, N, P, and S) and trace elements (Cu, Fe, Mn, Si, and Zn) through competitive uptake, even though the methods and pathways by which plants absorb Cr(III) and Cr(VI) differ (Ding et al., 2019; Askari et al., 2021; Ashraf et al., 2022a). Complex barriers caused by Cr prevent plants from absorbing essential minerals. According to Shahid et al. (2017), the existence of Cr and critical nutrients in soil and plant cells may be the cause of their antagonistic interactions and competitive absorption. Recent studies reported that excessive Cr toxicity minimizes adsorption sites and forms insoluble/low-bioavailable compounds in rhizosphere soil, which prevents the accumulation of vital nutrients including Ca, Cu, Fe, Mg, P, S, and Zn (De-Oliveira et al., 2016; Chen et al., 2018). The absorption of essential nutrients (such as N, P, K) in paddy irrigation reduced with an elevation in the level of Cr(VI) (Sundaramoorthy et al., 2010). In addition to Cr toxicity, a reduction in Fe content in leaf tissue indicates Cr(VI) involvement in Fe supply, leading to instability in Fe metabolism instability (Gopal et al., 2009).

This reduced uptake of nutrients might be occurred because of the decrease in root development and restriction of root penetration under Cr stress, or because of the reduction in translocation of essential elements (Shahzad et al., 2018; Sharma et al., 2020). Therefore, Cr(VI) competitive binding to common carriers may decrease the absorption of several nutrients. The suppression of plasma membrane H+ ATPase could be a possible explanation for the lower absorption of many of these elements in Cr stressed plants (Shanker et al., 2005). Additionally, the significant Cr buildup in the plant cell wall may harm the plasmodesmata, which serve as crucial channels for the transport of mineral nutrients, resulting in an imbalance in their metabolism (Kitagawa et al., 2015).

The detrimental consequences of Cr concentrations cannot be precisely predicted in soil and surface water (Waseem et al., 2014). Plant roots serve the primary purposes of absorbing inorganic and organic nutrients, and water, protecting and anchoring the plant body to the ground, storing nutrients, and promoting vegetative reproduction (Rucińska-Sobkowiak, 2016). These organs typically contain higher Cr concentrations than in the above-ground plant and are known to be the first points of contact with harmful metals like Cr ions (Shanker et al., 2005; Burkhead et al., 2009). Accumulation of Cr ions in tissues may influence soil water absorption and tends to lower the water content in plant roots (Kumar et al., 2016). The direct involvement of Cr ions with the guard cells or the early effects of Cr buildup on plant parts (such as stems and roots) are what induce stomata to close (Ahmed et al., 2016). It is believed that Cr’s effects on water supply in soils, root development, reduced water absorption, and other harmful effects are distinct from its influence on the connection between plants and soil water (Chow et al., 2018). The osmotic ability of soil solution in Cr-enriched soils may be less than that of root cell sap (Vernay et al., 2007). In these circumstances, osmotic pressure, and soil solution will significantly restrict plant water absorption levels (Vernay et al., 2007; Rucińska-Sobkowiak, 2016). When the toxic metal i.e., Cr concentration hits the 10^-3 M threshold level, osmotic pressure is thought to exist (Levitt, 1972). Adjustments to endogenous factors, such as root structure and morphology, are more likely to influence plant water absorption indirectly (Kumar et al., 2016). After being exposed to Cr, green amaranth (Amaranthus viridis) showed a substantial decrease in total root area (Sampanpanish et al., 2006). Reduced root hair surface, primary root elongation, increased root dieback, and poor secondary development is abnormalities in Cr-stressed plants that affected how water and plants interacted in the soil (Shanker et al., 2005; Chow et al., 2018). In epidermal and cortical cells of bush bean plants unveiled to Cr, there was impaired turgor and plasmolysis (Vazques et al., 1987). According to Gopal et al. (2009), Cr(VI) inhibits the physiological water supply, as evidenced by a drop in leaf water capability and elevation in diffusional stiffness in spinach leaves, implying that they are growing under water stress. Cr-induced structural changes reduce plant ability to acquire water in the soil and cause insufficient root-soil interaction (Ertani et al., 2017). A broad range of water-related changes is brought about by Cr exposure throughout the entire plant. Reduced water absorption and restriction of short-distance water transport in the apoplast and symplast pathways are effects of Cr toxicity in roots (Srivastava et al., 2021). Additionally, the apoplast’s resistance to water flow is increased by the thickening of the cell wall brought on by Cr ions or other incrusting substances within cell walls (Bhalerao and Sharma, 2015). The inhibition of aquaporin functions and variations in protein expression is most likely to blame for the impaired water transport through the membrane (Ullah et al., 2019a). Such changes affect the flow of water via the vascular system and reduce root sap exudation (Chen et al., 2010). Long-distance water transfer is reluctant, which causes a reduction in leaf water and, as a result, a water deficit in leaves (Shahid et al., 2017). Events that enhance plants’ capacity to retain water include a quick fall in root vacuolization, osmotic ability, and alternation in the tissues of stems and leaves (Srivastava et al., 2021; Ao et al., 2022).

Cr has a significant impact on root growth and development in addition to seed germination (Shahid et al., 2017). The roots, which are a major organ for nutrient uptake and are consequently linked to Cr uptake, act as a major source of Cr toxicity in plants (Srivastava et al., 2021). A considerable reduction in root length of sour orange (Citrus aurantium) seedlings was discovered while conducting an experiment in a greenhouse experiment, under doses of 200 mg/kg Cr(III), (Shiyab, 2019). In water lettuce (Pistia stratiotes), Cr promotes root length, width, and laminal length at low concentrations (0.25 mg L^-1) when compared to controls, but at higher concentrations (2.5 mg L^-1), the root length was observed to be reduced (Kakkalameli et al., 2018). Similarly, it was observed that Cr toxicity minimized the shoot length of oats (Avena sativa) by 41% as compared to control (Shanker et al., 2005). The growth of lateral roots and the quantity of secondary roots are further effects of Cr (Mallick et al., 2010; Srivastava et al., 2021). Root cell division may have decreased because of the Cr-induced reduction in root length. Cr(VI) prevents plants from absorbing nutrients and water, which shortens roots and reduces cell division (Shahid et al., 2017). Treatment with Cr(VI) in maize (Zea mays) resulted in shorter and fewer root hairs, as well as a brownish color (Mallick et al., 2010). Even various studies claimed that the cell cycle extended when exposed to Cr toxicity (Sundaramoorthy et al., 2010). According to Zou et al. (2006), green amaranth (Amaranthus viridis) root tip cells had their mitotic index reduced because of exposure to Cr.

Another growth metric that is frequently impacted by Cr exposure is plant stem growth (Ding et al., 2019). The shoot length of sunflower (Helianthus annus) was observed to decrease when Cr(VI) content increased (Fozia et al., 2008). Similarly, when the soil’s Cr(III) concentration was raised in sour orange (Citrus aurantium) the shoot length decreased by 90.4% at 200 mg kg^-1 of Cr (Shiyab, 2019). After being exposed to 600 mg kg^-1 Cr(III), tea (Camellia sinensis) developed a short stem that grew slowly (Tang et al., 2012). According to Lukina et al. (2016), Cr(VI) toxicity (1000 mg kg^-1) in 32 species had a negative impact on 94% of the species’ stem growth. The Cr-reduced root growth and development, which results in decreased water and nutrient transfer to the above-ground plant components, may be the cause of the decreased stem growth and height (Srivastava et al., 2021). Additionally, increased Cr transport to shoot tissues may directly interact with delicate plant tissues (leaves) and functions (photosynthesis), affecting shoot cellular metabolism and resulting in a shorter plant (Sharma et al., 2020).

In general, trace metal stress plants by oxidizing them either directly or indirectly by producing reactive oxygen species (ROS) (Qianqian et al., 2022). Cr toxicity causes oxidative damage in plants through overproduction of ROS such as O^2-, H₂O₂, and OH^- (Shahid et al., 2017; Basit et al., 2022). The process of reducing Cr(VI) to lower oxidation states is the root cause of Cr toxicity, where only ROS are produced (Shanker et al., 2005; Shahzad et al., 2018). Wakeel et al. (2020) reported that when Cr(VI) is radical reduced, the unstable intermediates i.e., Cr(IV) and Cr(V), which contribute to the generation of ROS, are created. Various plant organelles, such as mitochondria, peroxisomes, and chloroplasts, create these ROS as by-products of diverse metabolic activities (Srivastava et al., 2021). The primary causes of ROS generation in plant organelles i.e., mitochondria and chloroplasts are the inhibition of CO₂ fixation and excessive decrease of the electron transport chain (Ao et al., 2022). Furthermore, the production of ROS is caused by the leakage of electrons from O₂ caused by electron transport activity in mitochondria, peroxisomes, and chloroplasts (Anjum et al., 2016b). Cr toxicity in plants tends to share electrons, sulfhydryl groups in proteins establish covalent interactions with redox-inactive minerals (Anjum et al., 2017). Numerous studies have been reported showing a dramatic escalation in ROS (Sharma et al., 2019) with an increase in malondialdehyde (MDA) content with Cr toxicity (Adrees et al., 2015b). A pivotal part as signaling pathways molecules and mediators of responses to cellular metabolic disturbance, environmental stimuli, pathogen infection, various developmental stimuli, and a variety of biological and physiological responses are played by plants under normal circumstances when appropriate concentrations of ROS are present (Waszczak et al., 2018). However, the overproduction of ROS in plants results in disruption of cell homeostasis, cell membrane or protein fragmentation, DNA strand breaks, deactivation and degradation of genetic material, and harm to photosynthetic pigments (Srivastava et al., 2021; Ao et al., 2022). Similar findings were observed by Ullah et al. (2019b), who reported that increased ROS generation in plants with Cr toxicity results in oxidative damage, inflicting damage to DNA, lipids, pigments, and proteins, and stimulating the lipid peroxidation functions. These effects inhibit plant growth by preventing cell division or inducing cell death, which lowers biomass production (Wakeel et al., 2020). According to Shahid et al. (2014), the duration of exposure, Cr content, plant species, stage of development, level of stress, and particular organs all affect how hazardous Cr-induced ROS are for plants

Complex defense approaches, including non-enzymatic and enzymatic antioxidants, have evolved to prevent oxidative damage to plant cells (Semchuk et al., 2009; Ashraf et al., 2021; Zulfiqar et al., 2021). As with many other metals, excess Cr can promote the development of ROS and generally increases the activity of anti-oxidative enzymes ( Table 2 ). Activities of enzymatic antioxidants such as catalase (CAT), glutathione peroxidase (GPX), ascorbate peroxidase (APX), peroxidase (POD), glutathione reductase (GR), superoxide dismutase (SOD), dehydroascorbate reductase (DHAR), glutathione-S-transferases (GST), monodehydroascorbate reductase (MDHAR), and non-enzymatic antioxidants such as glutathione (reduced form, GSH, and oxidized type, GSSG), ascorbic acid (AsA), and phenolic metabolites were significantly increased under Cr toxicity to minimized the counter effects of ROS production in plant metabolic processes (Shahid et al., 2017; Jan et al., 2020). Cr exposure was found to increase the content of GSH and AsA, while the concentration of phenolic contents was depleted (Panda, 2007). Moreover, non-enzymatic antioxidants that control the levels of ROS in cells, such as tocopherols, carotenoids, GSH, proline, and AsA are regarded as moderators of oxidative damage (Adrees et al., 2015a). Antioxidant capabilities can also be found in other low-molecular-weight substances such as tocopherols, carotenoids, and phenols (Akyol et al., 2020; Ao et al., 2022). However, their antioxidants’ activity and availability are dependent on secondary metabolites’ capacity to synthesize specific compounds, which varies widely among different plant species (Akyol et al., 2020).

Plant roots with high levels of Cr(III) content, SOD increased primarily, while the quality of H₂O₂ displayed a discontinuous pattern for the various Cr(III) absorption, which was assumed because of heterogeneity in the activity of various peroxidases (Kováčik et al., 2013). Plant resistance may have surpassed the innate immune level for high doses of Cr in this case, resulting in the observed declines in enzyme activity. With increasing Cr(III) content, there was an increase in proline content. Usman et al. (2020), reported that giant milkweed (Calotropis procera) treated with Cr(VI) (20 mg L^-1) showed enhanced activity of CAT, GR, and SOD with SOD activity being the greatest (up to 12.2 U mg^-1). The formation of reducing agents (GSH and AsA metabolites) that catalyze the dismutation of H₂O₂ to O₂ ^- and H₂O is aided by the synergistic effects of GR, CAT, and APX and which all play critical roles in scavenging ROS (Bashir et al., 2020). When Cr metal binds to proteins, whether in the catalytic domain or elsewhere, it inhibits enzyme reactants by attaching unique functional groups to proteins, resulting in enzymatic function modifications (Gupta et al., 2010). In addition, from the enzyme, dislocation of essential cations the equilibrium of ROS in cells is disrupted by binding sites, and consequently, ROS is produced in dramatic amounts (Shahzad et al., 2016). The oxidation number of glutathione (GSH) and its constituents appear to bind and utilize Cr metal, which is important for reducing ROS (Lee et al., 2003). In addition, NADPH oxidase contributes to oxidative damage as it is associated with Cr (Pourrut et al., 2013). NADPH oxidases can consume cytosolic NADPH in the existence of Cr metal and generate free radical O₂; it is quickly converted to H₂O₂ through SOD enzyme (Shahid et al., 2017). In the presence of NADPH oxidase, Cr-generated free radicals are external to the plasma membrane, where the pH is generally lower than on the interior side of the membrane (Sagi and Fluhr, 2006). The transporter membrane promotes Cr ingestion and affects the plasma membrane’s ability to produce ROS (Maiti et al., 2012). However, the underlying molecular mechanisms of scavenging ROS by antioxidants and non-enzymatic antioxidants are yet unknown and need more research.

Phytotoxicity of Cr adversely affects various metabolic processes i.e., CO₂ fixations, electron transfer, photophosphorylation, and enzyme concentration, which directly impairs photosynthesis (Anjum et al., 2017; Sharma et al., 2020; Ashraf et al., 2022b). Taken to be critical indices that measure plant photosynthesis under Cr stress are photosynthetic rate, photosynthetic pigments, and photochemical efficiency (Ma et al., 2016). Cr is a potent inhibitor of plant photosynthesis, according to numerous studies (Shanker et al., 2005; Shahzad et al., 2016; Bashir et al., 2020). According to Mathur et al. (2016), Cr toxicity prevents CO₂ fixation, electron transfer, enzyme activity, and photophosphorylation in plants. This destroys the photosynthetic apparatus, specifically light-harvesting complex II, PSI, and PSII, and prevents the production of Calvin cycle enzymes (responsible for ATP production) (Sinha et al., 2018). In a study, Anjum et al. (2016a) found that maize plants exposed to Cr stress had significantly lower the levels of net photosynthesis, chlorophyll contents, gas exchange capacity, transpiration rate, water use efficiency, and stomatal conductance. The degradation of photosynthetic pigments caused by exposure to the high concentration of Cr leads to reduction in light-harvesting capacity (Handa et al., 2018b; Srivastava et al., 2021). Net photosynthetic rate (Pn) and chlorophyll content in wheat (Triticum aestivum) were decreased as Cr exposure period gradually increased (Srivastava et al., 2021). Cr prevents mitochondrial electron transport in higher plants, which increases the production of ROS and causes chloroplast modifications, pigment changes, and oxidative stress (Sharma et al., 2016). One of the crucial plant parts involved in photosynthesis is the leaf and total leaf area (Srivastava et al., 2021). In rice (Oryza sativa) the Cr(VI) toxicity reduced the number of leaves per plant by 50% while significantly affecting the overall leaf area and photosynthesis activity of plant (Sundaramoorthy et al., 2010). Under 3.4 mM Cr(VI) toxicity in nutritional media, smooth mesquite (Prosopis laevigatar) was shown to have fewer leaves that significantly affect the chlorophyll content and photosynthesis activity of plant (Buendía-González et al., 2010). Furthermore, it was shown that Cr toxicity significantly decreased the leaf’s net photosynthetic rate, transpiration rate, stomatal conductance, and intercellular CO₂ concentration, of sunflower with reductions of 36%, 71%, 57%, and 25%, respectively (Sharma et al., 2020).The first requirement for large plant yields is high plant biomass (Shahid et al., 2017). Cr is known to have negative impacts on several physiological and metabolic processes, which compromises plant production and yield equally (Ali et al., 2015). Various studies highlighted that Cr phototoxicity results to minimize plant biomass and yield of melon (Cucumis melo) (Akinci and Akinci, 2010), wheat (Triticum aestivum) (Adrees et al., 2015a), french bean (Phaseolus vulgaris) (Sharma et al., 2016), okra (Hibiscus esculentus) (Amin et al., 2013), turnip mustard (Brassica campestris) (Qing et al., 2015), Arabidopsis (Arabidopsis thaliana) (Ding et al., 2019), common duckweed (Lemna minor) (Reale et al., 2016), wheat (Triticum aestivum) (Ali et al., 2015), barley (Hordeum vulgare) (Ali S. et al., 2013) maize (Zea mays) (Anjum et al., 2017), cotton (Gossypium hirsutum) (Farooq et al., 2016), makoi (Solanum nigrum) (UdDin et al., 2015). In plants, higher concentration of Cr significantly affects various biochemical and morphological parameters i.e., minimized nutrient and water uptake, reduction in cell division, nutrients imbalance (translocation and uptake), the inefficiency of inorganic nutrient uptake by plant, higher oxidative stress, and ROS formation, oxidative stress damage to sensitive cell organelles such as chlorophyll, mitochondria, lipids, proteins, and reduction in photosynthesis activity that results to minimize the growth, biomass, yield of plant (Shanker et al., 2005; Shahid et al., 2017; Ao et al., 2022). At the cellular, molecular, organ, and plant levels, each of these elements, alone or in combination, have an impact on plant growth, development, and yield (Shahid et al., 2017). However, the type of plant and chemical speciation of Cr will determine which of these factors will be more severely impacted. The impact of Cr on plant development, however, differs depending on the variety of plants. In general, transgenic and hyperaccumulator plants have a lot of potential for Cr tolerance and selective accumulation (Sarangi et al., 2009).

Cr stress can stimulate potentially three forms of metabolic changes in plants: (i) modification in the synthesis of organic pigments facilitates the growth and development of plants (e.g., anthocyanin, and chlorophyll (Shanker et al., 2005; Shahid et al., 2017); (ii) enhanced the synthesis of metabolites (e.g., ascorbic acid, and glutathione) as a direct reaction to Cr stress that will affect the plants (Srivastava et al., 2021); and (iii) modifications in the metabolic-pool to channelize the synthesis of new biochemically associated metabolites that will confer tolerance or resistance to Cr stress (e.g., histidine and phytochelatins) (Shanker et al., 2005; Ao et al., 2022). Initially at germination stage, toxicity of Cr significantly reduced the activity of gibberellin (GA) and enhanced the activity of abscisic acid (ABA) (major factor of seed dormancy), which lead to seed imbibition and reduced germination rate (Seneviratne et al., 2019). Similarly, according to Yan et al. (2014) hydrolyzing enzymes secreted by the aleurone layer of seeds are crucial for seed germination. By releasing food reserves from the endosperm, enzymes i.e., acid phosphatases (ACPs), α-amylases, and proteases promote effective seedling establishment and growth (see section 5.1). Acid phosphatase, α-amylase, and alkaline phosphatase activity were decreased in the endosperm of cereals i.e., wheat, oat, barley, and maize seeds when Cr was present (Seneviratne et al., 2019). In addition, the enzymes involved in the assimilation of important nutrient nitrogen i.e., nitrogenase, nitrate reductase, nitrite reductase, glutamine synthetase, glutamate synthase, glutamate dehydrogenase were significantly reduced with the contamination of Cr in plants (Sangwan et al., 2014). Deficiency of nutrients in plants due to Cr toxicity results into degradation of various amines, alkaloids, pigments, vitamins, coenzymes, nucleic acids, and nucleotides as nutrients are structural component of these organelles (Shanker et al., 2005; Sangwan et al., 2014). Similarly, the activities of enzymes involved in photosynthesis NADP-malic enzyme (NADP-ME), pyruvate, phosphate dikinase (PPDK), and Phosphoenolpyruvate carboxylase (PEPC), plant respiration i.e., α-ketoglutarate dehydrogenase and isocitrate dehydrogenase, and gene transcription i.e., RNA polymerase are significantly reduced in various plants due to phototoxicity of Cr.

Phytoremediation is a process in which plants are used for remediation of polluted soils and considered an eco-friendly and green approach (Ali H. et al., 2013; Srivastava et al., 2021). There are various strategies associated with phytoremediation techniques including phytoextraction, rhizofiltration, phytovolatilization, biotransformation, rhizdegradation, phytostabilization, and phytorestoration (Yan et al., 2020). Phytoextraction is focused on the ‘hyperaccumulation’ process, and phytostabilization is focused on the surface complexation mechanism and both are involved in metal affinity phenomena (Xu et al., 2012). Phytoextraction and phytostabilization are two of those practically and economically viable solutions for treating metal-polluted soils (Kuiper et al., 2004). Biotransformation is another term for phyto-transformation. That is the separation of pollutants absorbed by plants via internal metabolic pathways or the segmentation of pollutants just outside of the plant because of plant-generated chemicals (such as enzymes). Plant absorption and metabolism are the primary components, which result in plant deterioration. The uptake of contaminants by plant roots and its conversion to a gaseous state, and release into the atmosphere is referred as phytovolatilization. Volatilization through leaves (ITRC, 2009) is the phytovolatilization process. Degradation by plant rhizospheric microorganisms is the method referred as rhizodegradation (Mench et al., 2009). This ecologically accepted technology is successfully used to fix soils that are polluted by various contaminants. Furthermore, phytoremediation is increasingly used as a technical alternative to treat contaminated water in various forms of wetland treatment (Zhang et al., 2010). In crux, phytoremediation is a feasible, socially, and economically suitable, and eco-friendly solution for the soils polluted with Cr. Nonetheless, to counteract the health risks due to Cr concentration in edible parts of food crops, the proportion of Cr in edible parts of food crops should be closely scrutinized.

Several methods of metal remediation have been used to address the harmful impacts of metal contamination, including physical, chemical, and biological processes, to inactive specific hazardous metals from the atmosphere (Marques et al., 2011). Microbial remediation has gained significant attention among different biological remediation methods because of its cost-effectiveness, higher efficacy, and non-expendable technologies (Malaviya and Singh, 2014; Fernandez et al., 2018). Some of the microbes that tolerate Cr establish ability to minimize the toxicity of Cr(VI) concentration from the atmosphere and thus play a prominent role in the remediation of Cr(VI) ( Table 3 ; Figure 3 ). Many investigations on the collection and profiling of distinct Cr-lowering microbial strains of bacteria have been published in last few years (Pseudomonas spp., Bacillus spp., Enterobacter spp., Acinetobacter spp,.), fungi (Aspergillus spp., Penicillium spp., Rhizopus spp.), and yeast (Candida spp., Saccharomyces spp.) (Chen et al., 2016).

The use of plant growth-promoting bacteria (PGPB) in plants, is also regarded as a significant and environmentally acceptable method for the removal of heavy metals from soil (Fahad et al., 2014). These bacteria encourage plants to endure extreme stress and improve plant nutrition to stimulate plant growth (N, P, Fe) and release different metabolites related to stress, such as the production of phytohormones, solubilization of phosphates, and production of siderophores (Dodd and Perez-Alfocea, 2012). Several studies have documented the use of plant growth promotion (PGP) rhizobacteria for heavy metal bioremediation, like Bacillus sp., Pseudomonas sp., etc. (Ndeddy-Aka and Babalola, 2016). Microorganisms have been found to reduce hexavalent Cr through various means, either by using hexavalent Cr as the final acceptor of electrons or by releasing some dissolving enzymes ( Table 4 ; Ahemad, 2015). In an experiment, Karthik and Arulselvi (2017) evaluate the effect of Cr(VI) on the plant growth-promoting properties of potential rhizobacterial strain isolated from the rhizosphere of a common bean (Phaseolus vulgaris). The strain AR8 was chosen from 36 rhizobacterial strains when compared to uninoculated Cr(VI) treated plants, the inoculation of Cellulosimicrobium funkei strain AR8 significantly improved the root length of test crops in both the presence and absence of Cr(VI). Strain AR8 could be used for growth stimulation as well as for the removal of Cr in Cr-contaminated soil because of these exceptional characteristics ( Figure 3 ).

According to Caravelli et al. (2008) the Sphaerotilus natans CSCr-3, a filamentous bacterium obtained from activated sludge, reduced Cr concentration up to 1.5 mM in the presence of a carbon source. This removal efficiency is significant because S. natans was originally recognized for its biosorption capability. Under alkaline medium, another bacterium, Ochrobactrum sp., was able to decrease Cr(VI). This isolate substantially tolerated and reduced Cr(VI) up to 15.4 mM. The inclusion of glucose generated a significant improvement in Cr(VI)-reduction, while the availability of sulphate or nitrate had no effect (He et al., 2009). Five Cr resistant bacterial strains with auxin biosynthesis abilities were used by Arshad and Ahmed (2017). Halomonas venusta APA and Arthrobacter mysorens AHA were determined to be the most effective isolates in terms of phytostimulatory effects on green gram (Vigna radiata). A huge proportion of microbial variants have been recorded for remediation of Cr(VI) using biosorption and bioaccumulation methods, such as Paecilomyces lilacinus (Sharma and Adholeya, 2011), Aspergillus niger (Srivastava and Thakur, 2006a, b), Bacillus cereus IST105 (Naik et al., 2012), Zobellella denitrificans (He et al., 2016), and Bacillus mycoides 200AsB1 (Wang et al., 2016). In conclusion, the use of a suitable microbial inoculum might become useful in effectively altering the soil infected with Cr.

In-situ or ex-situ complex formation through chelating substances has been used for metal extraction (Di-Palma et al., 2005; Finzgar and Lestan, 2007). The efficacy of extraction depends upon the availability of readily exchangeable ions in the soil matrix capable of forming strong complexes with minimum specific chelating agents (Di-Palma, 2009). For removal of maximum amounts of metals found in polluted soils, phytoextraction may be used, with some portion of the soil metal content freely available to plants. There are various synthetic chelating components, such as EDTA (ethylene diamine tetra acetic acid), diethylene trinitrile pentaacetic acid (DTPA), nitrile triacetic acid (NTA), pyridine-2,6-dicarboxylic acid (PDA), trans-l,2-diaminocyclohexane-N,N,N0,N0-tetraacetate (CDTA), or ethylenediamine disuccinate (EDDS) used for remediation of soil polluted with organic and inorganic contaminants. To increase the accessibility of metals in soil and the transference of metals from root to shoot, several ideas have been proposed (Meers et al., 2005). Application of chelating agents substantially improved the Cr uptake in above-ground biomass of many crops ( Table 5 ). Patra et al. (2018) revealed that in Cr(VI) polluted soil, application of chelators including EDTA, DTPA, citric acid, and salicylic acid, along with metal ions, enhanced the growth of lemongrass (Cymbopogon citratus) and enhanced Cr bioavailability. Chigbo and Batty (2013) analyzed that the application of EDTA and citric acid reduced alfalfa (Medicago sativa) shoot dry matter by 55%, decreasing the soil Cr removal efficiency. The removal of Cr increased to 54.28% when the polluted soil was pre-treated with 0.01M EDTA-2Na (Xu Y. et al., 2019). Mohanty and Patra (2012) revealed the total accumulation rate for Cr was improved with the application of DTPA to rice (Oryza sativa) and wheat (Triticum aestivum), While the use of EDDHA was proven to be useful in accelerating the process of Cr accumulation in green gram (Vigna radiata) seedlings. The role of chelating substances in reducing the harmful impact of Cr(VI) is demonstrated in this study. The chelating agents in the culture medium augmented with Cr(VI) improved the bioavailability of Cr in plants. In another study, EDTA application in Cr contaminated soil resulted in higher endogenous levels of Cr(III) in plants. Moreover, EDTA addition improved the growth by regulating Cr species, ion homeostasis and accumulation of secondary metabolites in castor bean (Ricinus communis L.) (Qureshi et al., 2020).

Nano-remediation is an eco-friendly and cost-effective method of detoxifying heavy metals in soil and other environments using nanoparticles (NPs) (Ahmed et al., 2021a; Wei et al., 2022a; Table 6 ). By absorbing heavy metals, lowering the hazardous valence to a stable metallic state, and accelerating the reaction, this unique remediation strategy has been demonstrated to be efficient in the removal of toxic heavy metals (Mondal et al., 2020). Synthesis of nZVI NPs in colloidal solution using green tea extract having an average particle diameter of 5-10 nm with polyphenol coating (which served as a capping and reducing agent) was significantly effective in remediating Cr(VI) from groundwater passing through porous soil beds (Mystrioti et al., 2014). Synthesis of NPs by using various rose apple (Syzgium jambos L.), candlenut tree (Aleurites moluccanus L.), and oolong-tea leaves extracts were significantly remediate Cr(VI) from aqueous medium up to 90% at initial 5 minutes, due to its maximum NPs antioxidant property, but complete removal took after 60 minutes (Xiao et al., 2016). The removal effectiveness of Cr(VI) was greatly influenced by factors i.e., Cr(VI) initial concentration, NPs dosage, solution pH, and temperature. For a constant concentration of Cr, the availability of active sites rises with increasing NPs dosage, which improves the removal rate (Xiao Z. et al., 2017). Various probable processes for effective decontamination of inorganic pollutants by NPs have already been hypothesized throughout the application, including precipitation, adsorption, complexation, and reduction (Mondal et al., 2020; Ahmed et al., 2021b). The most well-known method for eliminating Cr is called as reduction, which is followed by adsorption. According to Li and Zhang (2007), whenever the trace-metal ions already had a greater negatively standard-redox strength (E⁰ ) as compared to, or were like Fe⁰ (-0.41 V), the method for decontamination of Cr via green-synthesized iron-NPs was largely regulated by surface complexation/adsorption. However, whenever the Cr ions already had substantially higher positive E⁰ as compared to Fe⁰, precipitation and reduction of Cr ions predominate (Lin et al., 2019). When the Cr cations had somewhat more positive E⁰ as compared to Fe⁰, both reduction and adsorption happened (Ahmed et al., 2021b). Other possibilities included co-precipitation and Fe-hydroxide oxidation (Sebastian et al., 2018; Figure 4 ). In addition, bimetallic Fe-NPs and Fe-oxide remove pollutants through catalytic degradation and adsorption respectively (Ahmed et al., 2021b). However, there are significant limitations and knowledge gaps that must be addressed to ensure social acceptance and safe usage of green synthesized NPs for toxic heavy metals remediation. As a result, more field studies are required to assess the application’s safety, reliability, efficacy, fate, intrinsic toxicity of NPs, and long-term impacts of NPs on Cr bioavailability and absorption in contaminated soils. To attain its promised implications in the environmental sector, future research should focus on doze optimization and safe targeted delivery of NPs.

Organic material is facilitated in soil deposition of Cr, according to Branzini and Zubillaga (2012). We postulated that soil comprising most of the humidified organic material had a lesser Cr accessibility, which would minimize Cr deposition in plants. The use of organic modifications in Cr polluted soils and their impact on reducing Cr absorption in plants have been reported in several studies.

A significant problem is avoiding and reducing the harmful effects of heavy metals contamination in soil (Zeeshan et al., 2021). Genetic engineering can significantly improve a plant’s ability to transform, translocate, and lessen the adverse impacts of heavy metals (Raza et al., 2021). Omic tools have gained a lot of interest recently for their use in plant development and programs to mitigate agricultural production challenges, specially to mitigate heavy metal stress (Khan et al., 2021). To identify target genes, proteins, and metabolites linked to Cr detoxification and stress tolerance responses in plants, genomics, proteomics, and metabolomics have become effective methods (Chaudhary et al., 2019). It is possible to modify the Cr stress-responsive genes, proteins, and metabolites to either increase plant tolerance to Cr stress or decrease Cr accumulation (Thakur et al., 2019). Tools for genetic engineering that are particularly effective at changing the genes involved in the acquisition, transport, and accumulation of Cr inside the plant are necessary for this type of manipulation (Khan et al., 2021). The main goal of genetic engineering is the creation of tolerant varieties using either a transgenic approach or genome editing (Raza et al., 2021). Anwar and Kim (2020) reported that through genome editing active participation in the control of plant metabolism, essential genes important for increased metal tolerance have been developed into transgenics, which provide insights into how to understand and improve the tolerance capacity of plants. A successful method for creating resistant cultivars is to transfer candidate genes from plants with a high tendency for HM hyper-accumulation (Rahman et al., 2022).

The best way to reduce metal toxicity within cellular locations is to use transgenic plants with altered efficiencies for metal transport into vacuoles (Khan et al., 2021). Heavy metals (HM) transporter genes are thought to be potential candidates for genetic engineering to improve metal tolerance in plants (Zhang et al., 2018). OsMTP1 in cultivated tobacco (Nicotiana tabacum) and PgIREG1 in Arabidopsis are two examples of metal transporter genes that have been genetically modified (Merlot et al., 2014; Das et al., 2016). Other metal transporter genes include those that encode metal chelators, metallothioneins (MTs) (Peng et al., 2017), and genes associated with antioxidant machinery (Peng et al., 2017; Raza et al., 2021). The use of transgenic techniques to increase resistance to metal oxidation has also been documented. Transgenic hyperaccumulators may be created by manipulating the antioxidant system to maintain redox equilibrium to avoid the destruction of biomolecules such as DNA, proteins, and lipids and to maintain the structural and functional stability of cellular structures of plant under Cr stress (Du et al., 2019). Transgenic plants that overexpress antioxidant genes for SOD, CAT, and APX with reduced ROS generation under Cr stress have been created to prevent metal toxicity-induced oxidative stress (Gao et al., 2016). Additionally, enhanced antioxidant systems in transgenic lines are associated with higher growth performance in terms of photosynthesis, mineral uptake, maintenance of redox homeostasis, and enzyme activity (Khan et al., 2021). Although transgenic lines created for over-expression traits do not always show the expected benefits, they can nevertheless have positive consequences by influencing the alternative tolerance mechanisms.

The phytochelatins (PCs), which contain hazardous metal ions and are enzymatically generated from GSH, amino acids, organic acids, or MTs, are another crucial area for improving the Cr stress tolerance in plants (Yadav, 2020). It should be noted that only MTs have coding genes, but the production of other compounds (such as GSH, amino acids, and organic acids) is controlled by the actions of the enzymes involved. Better physiological and biochemical characteristics, including membrane function and antioxidant activity, are displayed by transformed plants (Khan et al., 2021). According to Ai et al. (2018), overexpression of MYB1 from grown radish improved PC and anthocyanin synthesis, giving transgenic Petunia higher resistance against several metal toxicities, including Cr. Improved growth and stomatal density were seen in MYB1 over-expressing lines mainly due to the maintenance of relative water content (RWC), chlorophyll, and antioxidant activity. Therefore, it can be concluded that transgenic research aimed at creating cultivars with improved metal tolerance will have a considerable impact on crop production in the future (Ai et al., 2018).

The engineering of transcription factors (TFs) that control the synthesis of important metabolic chemicals also has an impact on the Cr stress tolerance in addition to the previously described essential regulators of metal tolerance. Many TF gene families play a vital role in the ability of HMs to withstand stress, including R2R3-type MYB, ZAT6, Zinc-Finger type, bZIP, GeBP-LIKE 4 (GPL4), and NAC (Khan et al., 2021; Raza et al., 2021). It was noted that transgenic rice that overexpresses OsMYB-R1 has a noticeable increase in lateral roots, which was assumed to be related to improved tolerance to Cr (Tiwari et al., 2020). Further supporting the role of lateral roots in Cr tolerance is the correlation between the increase in lateral roots and a corresponding increase in auxin accumulation in transgenic lines as compared to wild type plants. Along with that, it was also thought that the OsMYB-R1 over-expressing lines had significantly higher antioxidant activity and proline accumulation, which were likely mediated by salicylic acid (SA) signaling and contributed to the transgenic rice’s ability to tolerate Cr (Tiwari et al., 2020). As a result, TFs are essential molecular regulators that help plants tolerate Cr stress and lessen the negative effects of exposure to metals, which supports plant growth and development. However, the identification and functional confirmation of several additional TFs from diverse TF families, many of which are still mostly unknown, could, therefore, be helpful in creating enhanced plant types with high HM tolerance.