Introduction

Increasing environmental pollution enforce greater pressure on the flora and fauna (Bhaduri et al. 2012). Alterations in the environment due to anthropogenic actions are main reason to cause pollution in water and soil (Schutzendubel et al. 2002). Soil pollution is the existence of unwanted/ xenobiotic compounds by various human activities such as inappropriate dumping of wastes, industrial activities, mining, construction, and usage of pesticides and fertilizers in agricultural fields, etc. In the mining industries, abstraction of chemicals from ores, and their purification and processing added various metal pollutants in water and soil. These pollutants badly influence the living beings including plants (Babu et al. 2014). Usual soil contaminants are petroleum hydrocarbons, aromatic hydrocarbons, pesticides, fertilizers, solvents, and heavy metals like cadmium (Cd), lead (Pb), zinc (Zn), mercury (Hg), copper (Cu), arsenic (As), chromium (Cr), etc. These pollutants are persistent and resistant to microbial or other degradation processes thus stays for relatively longer duration and difficult to be removed from the soil resulting into their accumulation (Saini et al. 2018). The plant absorbed these xenobiotic compounds along with micro and macronutrients, in this way it enters into food chain and can be damaging for the animals and human beings (Babu et al. 2014). 

Plants usually exposed to various abiotic and biotic stresses like heat, salinity, cold, osmotic shock, dehydration, etc. They have acquired well developed defensive mechanisms against all the stressors including heavy metals. Thus, the plant becomes resistant to several environmental disturbances and not much affected. Such adaptations against stress factors are brought by morphological and physiological alterations in plants. Moreover, biochemical processes in their developmental pattern also bring other transformations. All the adaptations that are imposed because of stress conditions are done with the help of adjustments in metabolism that results in assembling of diverse organic molecules for instance proline, polyols, sugars, etc. (Saini et al. 2018).

 Heavy Metal Contamination

Rapid urbanization, industrialization and various human activities directly threat environment by addition of heavy metals in the soil and water. Due to heavy metal contamination, plants experiences stress, as it threatens their growth and development. Sometime heavy metals plays important role in plant life cycle as stimulates several metabolic activities and helps in production of secondary metabolites. However, above threshold limit of heavy metals can cause genotoxic and cytotoxic effects in plants, resulting into instability in their genome. In the periodic table, D-block elements have been recognized as heavy metals because of their densities. Naturally, heavy metals are found in the form of cations and found to be harmful to crops in several ways. This ionic form of metals competes with essential nutrients of plants and interferes in their absorption processes by binding to the root surface. Once heavy metal enters into cells, it causes many disturbances in structure and functions of cellular molecules and hampers their growth and development by causing nutrient deficiency (Dutta et al. 2018). Elements like cobalt (Co), magnesium (Mg), phosphorus (P), Zn and Cu are essential micro- and macro-nutrients for various cellular processes, like metabolism of nucleic acids, synthesis of chlorophylls, photosynthesis, synthesis of carbohydrate, modification in proteins, and fixation of nitrogen. However, many other metals like Hg, Cd, Cr, aluminium (Al), Pb, etc., are responsible to pose damaging effects in plants, like lesser production of biomass, stunted growth, chlorosis, decrease in the rate of photosynthesis, etc.

Impact of Heavy Metals on Plants

In the present scenario, plenty of researchers are studying the deleterious effects of heavy metals in crop plants as major part of agricultural land are affected by these pollutants (Babu et al. 2014, Dutta et al. 2018). Growing root tips/ radicles of germinating seeds are first plant organ which initially gets exposed to heavy metals resulting in suppression of the mitotic activity inside meristems. Moreover, due to root growth inhibition, transportation of auxin also gets disturbed (Dutta et al. 2018). Heavy metals get entered inside cells through membrane receptors. Thus, heavy metal tolerance can be correlated with lower depolarization rates of the membrane and, a quick change in the voltage of membrane have been seen in Arabidopsis arenosa and Arabidopsis halleri (Dutta et al. 2018). These heavy metals interferes with reactions going on inside the cells, like assimilation of CO₂, metabolism of nitrogen  by hampering the stability and activity of important enzymes, ribulose biphosphate (RUBP) carboxylase glutamine dehyrogenase, glutamine oxoglutarate, nitrate reductase, and glutamine synthetase.

Reactive oxygen species are by-products of normal cellular metabolism generated inside the peroxisomes, mitochondria and chloroplasts (Shahid et al. 2014). With that, NADPH oxidases that are bounded with plasma membranes are associated in inducing oxidative stress via heavy metal contamination. Detoxification of such ROS is achieved by various enzymatic antioxidants like superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), glutathione redutase (GR), guaiacol peroxidase (POD), etc., (Emamverdian et al. 2015). However, during heavy metal stress condition, excessive generation and accumulation of ROS imbalances the detoxification process of ROS and promotes challenges in plants drastically (Rughani et al. 2016). This is the first biochemical alteration inside plants and is known as “oxidative stress situation”. Generally, superoxide (O₂^•−) and hydrogen peroxide (H₂O₂) are formed and their levels are maintained by antioxidants. However, in stress condition due to improper detoxification process Haber–Weiss or Fenton reaction take place (Kehrer et al. 2000). In these reactions, H₂O₂ is converted into •OH, which is again dangerous to cellular macromolecules. Moreover, redox-inactive metals like Pb, Cd, Hg, Ni and Zn directly affect the enzymatic activities of antioxidants because of their tendency towards sulfhydryl (–SH) groups. The excessively accumulated ROS can cause cytotoxicity by damaging the cellular macromolecules like lipid, protein, enzymes and nucleic acids (RNA and DNA) by various oxidation/ peroxidation/ inhibition reactions (Shahid et al. 2014).

Lipid is the major component of the cell membrane and plays vital role in organelle maintenance and energy generation for metabolism. Moreover, membrane lipid is the prime target for ROS (Rughani et al. 2016), causing peroxidation of membrane lipids and hence deleterious reactions (Shahid et al. 2014). Along with lipids, ROS are also known to amend quality and quantity of proteins like displacement of Zn and more importantly metal ions, combining free thiols to metal ions and functional groups, modification in gene expression, increase in the activity of ribonucleases, reduction in content of free amino acids which are linked with metabolism of nitrogen (Rughani et al. 2016). Heavy metals directly forms complexes with proteins by binding at functional groups such as –SH, –NH2, –COOH and cause structural modification in proteins and malfunctioning in the cells. Reactive oxygen species inactivated the catalytic properties of enzymes by oxidizing the side groups of amino acids like lysine, cystein, histidine, methionin, tyrosine, arginine, tryptophan and proline (Shahid et al. 2014).

Genotoxicity caused by heavy metal stress usually occurs indirectly due to ROS. Among many forms of ROS, •OH is highly reactive to DNA, which make cross linkages with nucleic acids. It affects DNA by deletion or modification of bases, strand breakage and formation of pyrimidine dimers. Nitrogenous bases, hydroxyl of ribose sugar, phosphate group and keto groups of the exocyclic bases are major sites on DNA which have high potential to combine with heavy metals. Heavy metals associate themselves with purines at N7 atom or with pyrimidines at N3 atom directly, and bind with phosphate groups indirectly. Metals like Cr, Hg, Zn, Cd, Cu and Pb get involved with DNA, at mainly -SH groups and at backbone made up of phosphate. Besides this, gene expression also gets altered. These metals may also interfere with dividing cells spindle apparatus to damage the DNA (Shahid et al. 2014).

Plant Defense System

Experimental results of various studies showed that various enzymatic antioxidants like SOD, CAT, APX and GPX are involved with ROS detoxification during oxidative stress condition. Exposure of heavy metals affects the gene expression and activities of these enzymes by both positive and negative means (Alaraidh et al. 2018). Among all of these enzymes, SOD is the prime agent in preventing plants from ROS (Rughani et al. 2016). The very first ROS produced is O₂^•−, which detoxified by SOD into H2O2. Further, H₂O₂ is converted into H2O and O2 by CAT, APX and POD in the peroxisomes, chloroplasts and mitochondria, respectively. In APX detoxification pathway, ascorbate is oxidized into monodehydroascorbate (MDHA) and then reduced to ascorbate with the aid of monodehydroascorbate reductase (MDHAR). Another pathway for this is the transformation of ascorbate into dehydroascorbate (DHA) and dehydroascorbate reductase (DHAR). The reductant used in this process is glutathione (GSH), which gets oxidized into GSSG (oxidized glutathione) (Shahid et al. 2014). There should be fine balance between ROS generation and antioxidant metabolism for normal cellular functioning (Bhaduri et al. 2012).

Conclusions

Environmental pollution caused by various activities like mining, industrialization, etc., poses heavy metal contamination in soil and water, which entered inside plants and cause serious threats on their growth and development. The effects of heavy metals can be observed in plants by numerous morphological, biochemical and molecular indicators like stunted growth, chlorosis, necrosis, reduced metabolism, etc,. These alterations are due to disturbances in normal cellular metabolism and excessive generation of ROS. Generation and accumulation of ROS during stress condition creates oxidative stress situation, which is cope up by the action of various enzymatic and non-enzymatic antioxidants by detoxifying ROS molecules. However, if the ROS generation exceeds the detoxification process of antioxidants it causes deleterious reactions on plants by attacking over cellular macromolecules viz.; lipid, protein and nucleic acids.

Acknowledgements The authors would like to thank Pt. Ravishankar Shukla University, Raipur, and University Grants Commission, New Delhi, for awarding fellowship to Jipsi Chandra under Research Fellowship (No. 79/8/Fin.Sch/2014, dated 16.04.14) and National Fellowship for students of Other Backward Classes (F./2016-17/NFO-2015-17-OBC-CHH-27902) respectively.

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