1 Introduction

Chromium being the 24th element of the periodic table is having an atomic mass of 51.9961u. The density of Chromium metal is 7.20 g/cm3. As the atomic number is more than 20 and atomic density is more than 5 g cm−3 it is considered to be a heavy metal. Heavy metals are known to be potential pollutants and exhibits certain toxic effects on the ecosystem. They tend to accumulate in the food chain and their concentration might increase accordingly in different trophic levels due to the process of bio-magnification. Chromium being the 7th most rich metal on our planet is rated as the 2nd most plentiful metal responsible for environmental pollution. The sources can be natural as well as anthropogenic like volcanic dust, gases, rain & surface water, fertilisers, pesticides, leather tanning to name a few. Chromium is known to exist in two different valence states Cr [III] & C [VI] i.e., trivalent and hexavalent form. By comparing the toxicological profile it was found that Cr [VI] is 100 fold more harmful than Cr [III] due to its high mobility and easily soluble & reduction capability [1]. Several modifications were observed in the plants. Molecular changes like increase in ROS levels, Ca2+ accumulation, activation of NADPH oxidase & Ca2+ dependent protein kinase is observed. The signaling pathways also showed some modifications like ethylene, JA & AA mediated signaling pathway is activated whereas Gibberellic acid pathway is deactivated [2]. Brassinosteroid, a plant steroid hormone undergoes some alterations to act as a ROS scavenger [3]. Genetic modifications like post transcriptional gene silencing mediated by miRNA is done to reduce stress reaction in plants [4]. MicroRNAs also play a significant role in plants for its adaptation to stressed conditions. Expression analysis concluded up-regulation and down-regulation of certain miRNAs [5]. In plants, two types of miRNAs are found namely, conserved & non-conserved miRNAs. Nearly 45 miRNAs having 833 target genes and 13 miRNAs having 280 target genes are found in roots and leaves respectively. [6]. Chelation, transportation, metabolism and detoxification mechanism are performed by miRNAs to deal with heavy metal stress. miR167a, novel_miR15 and novel_miR22 functions in chelation and transportation of Cr whereas miR165a, miR164, miR396d, novel_miR155 are involved in detoxification and metabolism of Cr [7]. Regulatory proteins like SOLITARY ROOT & MEDIATOR 18, plant hormones like auxin, JA, ethylene, amino acids like cysteine, proline and glutamate provides protection from effects of stress [8]. In between 2000 and 2020 the levels of heavy metals increased from 0.074 to 0.163% in the world. Plants also secretes organic acids that form chelating compounds with the heavy metals and reduces its mobility and activity [9]. Lysine is an essential amino acid which acts as a precursor for glutamate. Glutamate is a signalling amino acid which helps to regulate plant growth & responses to the environment. Zinc is essential for the synthesis of plant growth hormone auxin. So the combination of Zn & Lysine proves to be beneficial for the plant body as it upregulates the growth and maturation of plant by reducing ROS production & oxidative stress [10]. When the micronutrient Zn is chelated with lysine, elevated growth and yield of oil rape crops is recorded. The supplementation of Zn-Lys improved the morphological, physiological and nutrient uptake capability of the crop. By this supplementation, Cr uptake is observed to decrease in wheat plants [11]. Silicon is known to have beneficial impact on the development and maturation of plants. It alters the physiological mechanism and genetic expression of the plant system. It plays a major role in germination of seed and protecting from oxidative stress by improving the anti-oxidant defence system. SiO2 (phytoliths) deposition hardens the cell wall and other tissues and protect the plant against pathogens. Due to this deposition, the translocation of heavy metals is also reduced as it acts as barrier. In turn, the heavy metal ions either form a complex or precipitate along with Silicon. SAR (Systemic Acquire Resistance) is also induced in plants due to the activity of Silicon. Cr stress caused membrane damage and protein breakdown which is compensated by the supplementation of Si [12]. This exposure also damaged the chloroplast of the plants resulting in reduction of growth & yield, intercellular space, stomatal conductance and uptake of micronutrients. The extent of photosynthesis is highly affected by the subjection of plants to Cr6+ [13]. While working upon the rice plants, it was observed that either the Cr ions were immobilised or the activity of antioxidants were enhanced. Some particular antioxidant enzymes like NADP-isocitrate dehydrogenase, ferredoxin-NADP reductase, glutamine synthetase 1 (GS1) & glyoxalase I (Gly I) were found to be activated which helped the plant to fight the stress and eventually made it Cr tolerant [14]. Maximum deposition was seen in root parts and minimum in the shoot areas and least in case of seeds. It was found that deposition of Cr in roots is proportionately 10 times more than leaves [15].The phytotoxic effects of Cr can be depicted from it’s morphological, cellular and physiological response. It causes plasmolysis & wilting in root cells, degradation of 5ALA dehydratase (helps in chlorophyll synthesis), degradation of carotenoids, disintegration of the thylakoids and destruction of the chloroplast membrane as well as uneven swelling of chloroplast was observed. The amount of plastoglobuli and starch granules was increased. In roots, the vacuole size increased and simultaneously the nucleus disrupted and disappeared [16]. A recent technology of using nanoparticles to alter the molecular activity and regulate the genetic expression is beneficial in boosting the suppressed activity of the enzymes and detoxifying the effect of heavy metal stress. Nanoparticles reduce stress impact by interfering with the signalling and transduction pathways. They upregulate the anti-oxidant enzymes that counteracts the oxidative stress. This positive impact of Si nanoparticles has already been recorded in Cucumis sativus and Hordeum vulgare. Morphological, physiological and metabolic attributes of the plants showed an improvement after foliar administration of Si nanoparticles. Roots and shoots of wheat plants under Cr stress showed a significant increase in biomass when treated with Si NPs [17]. The combined effect of two plant growth promoting hormones auxin and cytokinin reduces the production of ROS produced due to stress conditions. When a combination of Si, cytokinin and auxin is used it resulted in increase of the antioxidant activity and photosynthetic efficiency. MDHAR, DHAR, APX and GR genes maintained the balance of Ascorbate–glutathione cycle. Proline metabolism is regulated which results in the maintenance of nutrient homeostasis. Genes like ZIP1, MHX, SULT1, Lsi1, etc. are involved in the improvement of plant growth process [18]. Addressing heavy metal pollution requires comprehensive management strategies to mitigate the release, remediate the contaminated sites and minimize exposure risks.

1.1 Chromium uptake and accumulation in plants

Cr comes 17th in the list of the most toxic substances by the ATSDR. By the IARC, it is classified as the topmost carcinogenic substance. When Cr [VI] enters the cells of the plants it gets converted into Cr [V], then gradually into Cr [IV], R-S & O–H radicals and finally into Cr [III]. These different states of Cr is harmful for plant system as it alters the constitution of cell by interfering with the membrane lipids, protein and DNA [19]. Chlorosis and necrosis are the most common symptoms that are observed in plants exposed to stress. Other anatomical and physiological abnormalities are also seen [20]. During seed germination the reduction in seedling growth can be related to defective root development resulting in decreased conduction of H2O and minerals to the shoot. The root exudates bind to the Cr and forms complexes with it and eventually accumulates in the root. This leads to a reduction in the length of roots and overall decrease in its growth. This is probably due to disproportion movement of Ca2+ through the plasma membrane to the cytoplasm which alters the normal activity of calcium modulated binding protein (CAM). Due to the direct impact of Cr on metabolism of shoots, the height of the plant reduced significantly and stunted growth is observed. In leaves, the total dry matter (DM) production is reduced. The photosynthetic pigments were degraded thereby decreasing the photosynthetic activity. Fixation of CO2, electron transport, enzymatic and photophosphorylation reactions were also affected. Due to the presence of metal blockages, uneven transport of water resulted in dehydration of shoots causing wilting, decline in water potential of leaf and diameter of the tracheary vessel. Under Cr stress, Mg uptake is inhibited and Mg2+ transport is increased to the leaves from roots for the proper functioning of leaves. Three metabolic modifications are observed i.e., a significant change in the production of pigments, metabolites and some modification in the metabolic pool leads to the release of certain biochemicals that helps in conferring resistance to Cr stress [21]. Plants vary in their ability to accumulate and tolerate chromium. This accumulation of chromium in edible parts of plants raises concerns about food safety and human health.

1.2 Hormesis effect

It is a biphasic or triphasic dose–response phenomenon which is enhanced at low doses and retarded at higher doses. It serves as a mode of adaptation of biological systems with the changing environmental conditions. So, the heavy metals that the plants are getting exposed to might be fatal at high concentrations whereas beneficial and a growth promoter at lower concentrations. In some plant species, beneficial effects of Cr was observed in terms of increase in biomass and photosynthetic efficiency [22]. Kinases, deacetylases, Nrf-2 and NF-Κb (transcription factors) are found in cells that are involved in hormesis effect. Antioxidant enzymes, protein chaperones, cryoprotective and restorative proteins and growth factor production is increased [23]. Hormetic effect is due to certain factor called hormetins. It strengthens the homeodynamic space by altering the stress response pathway in the cells [24]. The homeodynamic response to stressed conditions occurs in some steps. First, the homeodynamic equilibrium is disturbed due to induction of stressed conditions. Then, signalling pathways are activated respectively in response to it. Effectors are produced as a result of the responses. Homeodynamics is restored with better and enhanced adaptability conditions. The changing impact of stress on homeodynamics is as follows. It first increases and then decreases. And, the plant attains better adaptability features to sustain or adjust to stressed conditions [24]. Exposure of low levels of toxin might improve the stress resistance and improve the repair system for better adaptation of the plant in the environment.

1.3 Hormesis effect of Cr at varied concentrations

An experiment on alfalfa plants showed that lower levels of K2Cr2O7 (0.5 mgL−1) promoted growth by increasing biomass and area of leaf. But increasing K2Cr2O7 concentration up to 1 mgL−1 reversed the impact by decreasing the biomass, leaf area and photosynthetic yield. Further increase in concentration up to 5 and 10 mgL−1 caused NO stress [25]. At low concentration the net rate of photosynthesis is increased. Electron transport activity of PS-II is enhanced. An increase in the medulla & outer skin tissue of the root is seen. It also promotes root & root hair growth. But in the elongation region of the roots, a reduction in the cell division & cell size is observed. Growth of lateral roots is also reduced. The root tips turn yellow & necrotic. A reduction in the number of root cells, length & volume of roots is observed. [9]. In the shoot region, a reduction in the biomass & dry matter is observed. Photosynthesis is negatively affected. Several enzymatic reactions & transport of water is disrupted. Dwarfing of the plants & yellowing of leaves is marked. A reduction in growth & yield is observed [9]. Under Cr stress internal damage to the cell is also observed. Plastoglobuli, starch and chloroplast basal granules increased in number ultimately it ruptured. The number of thylakoids are decreased and their membrane is also damaged [9]. A decline in the number of leaves, size of the leaves and reduction of biomass is observed. Leaves were affected by chlorosis and necrosis. As a result, photosynthesis is highly affected [26]. Anatomical change in epidermis, cortex & stele is observed. Stomatal pore became wider and wax deposition is reduced. Palisade and spongy parenchyma cells also showed a decline in its number. Pith & cortical tissue increased in roots. In some, the root cap, root hairs & lateral roots are found to be damaged [26].

1.4 Physiological and biochemical responses of plants to chromium stress

Higher content of ROS promotes degradation of nutrients in the seed. Cr also triggers protease activity, resulting in the suppression of hypocotyl transport. Hence, it inhibits seed germination activity [9]. The heavy metals can damage the cell membrane and alters their selective permeability. The abnormal increase in permeability causes lipid peroxidation. To measure the extent of damage two commonly used indicators are Relative Conductivity & Malondialdehyde [9]. Grain weight and production was reduced. No or less harvestable yield was reported causing an overall loss in productivity and yield [26]. The seedlings that we treated with 1 mM Cr showed an elevated nitrate reductase activity and in higher concentrations it was found to be toxic and reduced enzymatic activity. In dicot plants its effect is observed in terms of Fe uptake by preventing reduction of Fe [III] into Fe [II] or by racing with Fe [II] at the site of intake. At high concentrations of Cr, ATPase activity was increased. The retardation in functioning of ATPase caused a decline in proton expulsion. As a result, the uptake of most of the nutrients were reduced due to reduction in the transportation activities of the plasma membrane of the roots. Plants needed to remove the harmful ROS which are produced due to the stress. The antioxidant system functions here in scavenging the ROS and H2O2 [21]. The catalytic function of anti-oxidant enzymes decreased. Deteriorating function of the following enzymes SOD, CAT, POX & GR is observed. It results in oxidative stress in the plants. Several organic acids & metallothionein are known to nullify the effect and help in detoxification of the heavy metals [27]. It is observed that various number of enzymes, organic acids and metallothionein to perform in scavenging the ROS efficiently and act as a first line of defence in plant systems (Table 1). These catalytic activities exhibit that, the toxic species are ultimately converted into a harmless product i.e., water. Some phenolic compounds, carotenoids, flavonoids, alkaloids, free amino acids & tocopherols also aids in the catalysing activity in removing the ROS. ROS exists in two forms i.e., radical and non-radical forms. The radical form comprises of the following O2−, OH & HO2−. Whereas, the non-radical form includes H2O2 & 1O2. The radical and non-radical form of particle comprises the composition of ROS. In plants, mitochondria have a carbohydrate-rich environment and act as vital site for production of ROS. Chlorophyll, endoplasmic reticulum, cell wall, apoplast & cell membrane also contributes in the production of ROS [28]. In response to the stress imposed by heavy metal, plant species exhibit different physiological responses to mitigate the harmful effect of these stressors (Table 2).

Table 1 List of enzymes, organic acids and metallothionein and their catalytic activity
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Table 2 Physiological responses exhibited by different plant species in response to Cr stress
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1.5 Genetic and molecular approaches to enhance Cr tolerance in plants

Heavy metals cause DNA damage, arrest the cell cycle, changes the transcripts, proteins & metabolites or eventually cause cell death. Approximately around 3000 genes were found to be affected by Cr stress, of which 1138 were up-regulated and 1610 were down-regulated. Different anti-oxidant enzymes like APX, SOD, POD & GPX coding genes were also found to be activated [29]. It is reported from the study that various genes & proteins plays the role to neutralize the stress (Table 3). Proteomic analysis of the rice roots revealed 20 proteins (R6, R9, R11, R12, R14, R20, R23, R25, R26, R27, R28, R29, R35, R39, R41, R42, R44, R45, R46, R47) and their physiological roles in cell wall synthesis, detoxification, primary metabolic reactions, signalling molecules and molecular chaperones and some other proteins with unknown functions are found [14].

Table 3 List of genes/proteins & their functions in plants
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1.6 Remediation approaches to remove pollutants from environment

The process of detoxifying and removing the pollutants from our environment by the help of plants is known as phytoremediation. The heavy metal contamination of soil can be mitigated by use of certain plants which is achieved by increased antioxidant activity, hyperaccumulation, phytoextraction or precipitation. BCF & TF values are calculated values that act as parameters to determine the detoxification ability of the plants. Depending upon the BCF value a plant can be categorised as a hyperaccumulator or an excluder. BCF > 1 indicates that it is an hyperaccumulator and the plant can serve as a potential phytoremediation agent. Different plant species like Convolvulus arvensis L. [67], Leersia hexandra Swartz (Gramineae) [68], Helianthus annuus [69], Salix matsudana [70], Prosopis laevigata [71], Spartina argentinensis [72] Nopalea cochenillifera [73], Pennisetum sinese [74], Pteridium aquilinum (Dennstaedtiaceae) [75], Allium griffithianum, Colocasia esculenta, Pistia stratiotes, Spirodela polyrrhiza, Hydrocotyle umbellate, Rumex dentatus, Parthenium hysterophorus, Cannabis sativa, Canna indica, Euphorbia helioscopia, Arachis hypogea, Vigna unguiculata, Origanum vulgare, Callitriche cophocarpa, Arundo donax, Chrysopogon zizanioides, Leersia hexandra, Miscanthus sinensis, Oryza sativa, Spartina argentinensis, Vetiveria zizanoides, Eichhornia crassipes, Pteris vittate, Genipa americana, Solanum viarum [26] function as hyperaccumulator plants and aid in phytoremediation activity. These plants can be cultivated on contaminated soils for the neutralisation or reduction of the toxicity of heavy metals. Biochar is a nutrient enriched organic substance obtained by the pyrolysis of biomass residues. When biochar was applied to stressed plants an improvement in their overall growth and development was observed. A significant decrease in the levels of abscisic acid is also recorded. Biochar also improved the soil texture and increased its water holding capability [76]. Biochar can be produced by two processes namely, torrefaction & pyrolysis [77]. Torrefaction is done in an oxygen-deficit environment, where a thermo-chemical process is conducted causing loss of moisture content, resulting in the formation of a brittle end product which is the char. The most commonly known biochar available is charcoal. The production quantity can be estimated from criteria such as temperature & residence time. Here residence time indicates the lifetime of the substance i.e., the total amount of time spent by a substance in the reactor. Chars are considered toxic when they contain harmful substances like polycyclic aromatic hydrocarbons. Several physical and chemical properties are taken into consideration which confers mechanical stability to the char. It’s fibrous structure, elemental constituents, energy value, pH, reactivity and degradation is recorded for further analysis [78].  The soil horizon has a crucial role in water absorption by roots. The physical properties of biochar determine the soil texture and quality. For the optimum growth of plant, it’s roots must be healthy with a high penetrating power for better availability of water and minerals. The pore & particle size, density, depth, texture and structure plays an important role in determining soil quality. Due to its porous nature, it aids the growth and survival of certain beneficial microorganisms which in turn provides ecosystem services. The soil ecology can be maintained and balanced by the use of biochar. It also helps in carbon sequestration & improving soil fertility. The natural carbon reservoirs might get converted into biochar within an estimated span of some 100,000 years. Upon comparison between normal soil and biochar it was observed that the pH and cation exchange capacity were increased. It resulted in the formation of hydrophobic sites that act as adsorber of microorganisms, minerals and even chemical fertilisers [79]. Rice Husk Biochar (RHB) an agricultural waste rich in K and Si reduced chlorosis and stunting growth in the stress affected plants and increased their biomass and grain yield. In wheat plants, when the subcellular fractions of shoots were compared before and after application of RHB, the extent of accumulation of heavy metals varied in different compartments. In stressed plants heavy metal content is highest in soluble fraction followed by cell wall and organelles. After application of RHB, the accumulation is highest in cell wall and minimal in soluble fraction and organelles [80]. The heavy metals interact with the functional groups present on the surface of biochar and forms a complex ultimately immobilizing the heavy metals which reduces its availability for uptake by plants [81]. Bioremediation measures can prevent leaching of heavy metals deeper into the soil and convert it into less toxic complexes. It is an effective natural solution with least side effects.

1.7 A case study in Odisha

The Sukinda chromite mines located in the Jajpur district of Odisha, India, represents one of the largest chromite ore reserves globally. Extensive open-cast mining and inefficient waste management practices led to a hazardous condition. In the year 2007, the Blacksmith Institute reported it as the world’s 4th most polluted place. Approximately 70% of the surface water and 60% of the drinking water contains Cr [VI]. While the standard for Cr [VI] is 0.005 ppm for India, at several water sources of Sukinda it was up to 2.5 ppm which is around 50 times the standard value. While the tribal people residing there still rely upon agriculture and forests for a living, the contaminated soil and water is directly and indirectly affecting their health and livelihood. Bacterial species like Micrococcus luteus, Serratia marcescens & Acinetobacter caloaceticus have been introduced to the contaminated soils for its detoxification. Out of them, Acinetobacter caloaceticus showed a reduction of 70.53% Cr (VI) at pH 8.0 & temperature 30 °C/24 h [82].  Bioleaching, biosorption and bioreduction processes are performed by microbes. Some microbes are capable of secreting the enzyme chromate reductase which converts hexavalent chromium into less toxic trivalent chromium [83]. Some, also act as an adsorbing surface by adsorbing the heavy metals on their cell walls [84].  Sukinda is one of the most polluted places in Odisha and its impact reflects in the deteriorating health conditions of the ecosystem and the people residing there.

2 Discussion

The physiological responses of plants to heavy metal stress are intricate and multifaceted involving complex signalling pathways. It has both positive and negative impact on the growth and development of plants. This phenomenon is discussed under the hormesis effect i.e., biphasic or triphasic dose–response phenomenon. The impact varies with the varied concentrations of Chromium. At low concentrations, it proves to be beneficial whereas at high concentrations the effects are detrimental. Due to Cr stress, an overall physical, biochemical, anatomical, molecular and physiological alteration is seen in different species of plants. miRNA sequencing and their expression were analysed in different plants of the plant. A declined photosynthetic rate, yield, symptoms of chlorosis & necrosis are some of the impacts. And different plant species responded differently to the same stress conditions. As a response to the stress, some biochemical activities were upregulated while some were downregulated. The activity and response of different genes and proteins are listed above. The toxic substances that were produced by the plants as a result of the defence mechanism were either detoxified or removed from the plant body. This mechanism infers that some plants can act as a phytoremediation agent for the heavy metals. In mining areas, where the soil is highly contaminated, the plantation of suitable plants can prove to be beneficial for our environment. Another organic substance namely biochar, possessing the property of bioremediation can also be used. Biochar is obtained from Biomass energy crops. So, the plantation of such crops in the contaminated areas can help to restore and enhance the overall quality of soil and the ecosystem. And silicon is also known to mitigate the harmful effects of chromium. So, addition of Si-supplements to soil can also neutralise the harmful effect up to an extent.

3 Conclusion

This review highlights the physiological, molecular and biochemical alterations that occurs in a plant in response to heavy metal stress. Through an extensive analysis of the recent literature, we conclude that bioremediation can be a sustainable solution for reducing the levels of heavy metals from our environment. Despite treating chemicals with chemicals and worsening the environmental conditions, the use of plants, organic substances and environmental-friendly processes should always be encouraged. This is a theoretical review of the work done till time. The future depends upon its practical implementations and necessary awareness. Morphological, physiological, biochemical and ecological studies can be done upon this to find out what actually happens when the individual components that had positive impacts individually, act altogether. By utilising the collective knowledge from this review, we can pave a way towards a cleaner and greener environment.