NewBioWorld A Journal of Alumni Association
of Biotechnology (2020) 2(2):5-7
Heavy Metal Pollution and
its Impact on Plants
Jipsi Chandra1, Apurva
Mishra2, S. Keshavkant1*
of Studies in Biotechnology, Pt. Ravishankar Shukla University, Raipur 492 010,
2Amity Institute of
Biotechnology, Amity University, Gwalior 474 005, India
author email: firstname.lastname@example.org
Pollution of heavy metal imposes a serious
threat to environment. Heavy metals could be present in environment naturally
as its part or added by enormous human activities like usage of fertilizers
and pesticides, dumping of solid wastes, mining, industrial effluents,
burning of fossil fuels, etc. Heavy
metal such as zinc, lead, cadmium, mercury, copper, arsenic, etc., are absorbed by the plants from
natural environments and through them get entered into the food chain. In
plants, above threshold limit of many of these heavy metals show adverse
effects and imposes deleterious effects on their growth and development. The
impact of heavy metals could be noticed by numerous morphological,
biochemical and molecular alterations in plants. These changes are brought
disturbance in normal cellular metabolism. Thus, the major by-products of
normal cellular metabolism, reactive oxygen species (ROS) could also been
thought to be involved in damaging reactions. Excessive generation and
accumulation of these ROS creates oxidative stress condition and
cytotoxicity. To conquer this situation, plants have various defensive
mechanisms including enzymatic and non-enzymatic antioxidants, which detoxify
ROS molecules by converting them into non-toxic products.
Keywords: Antioxidant; Heavy metal; Lipid;
Protein; Nucleic acid; Reactive oxygen species
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).
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.
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
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).
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.
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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