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the expression of genes involved in heavy metal detoxification and tolerance.
Several different signaling pathways have been implicated in plant responses to
heavy metals, including the MAPK, calcium, and ABA signaling pathways (Jonak
et al.
2004; Chmielowska-Bąk et al. 2013; Carroll et al. 1998; Stroiński et al. 2010 ).
13 Genomic Instability in Medicinal Plants in Response to Heavy Metal Stress 213
The MAPK pathway is
activated in response to heavy metal stress and leads to the
phosphorylation and activation of transcription factors such as MEF2 and WR KY70.
These transcription factors then go on to regulate the expression of genes involved in
heavy metal detoxification, such as metallothioneins. The ABA signaling pathway
also plays a role in plant responses to heavy metals. ABA is produced in response to
stresses such as drought and high temperatures and can also be induced by heavy
metals. ABA binds to its receptor PYR/PYL/RCAR, which leads to the activ ation of
downstream signaling molecules such as SNF1-related protein kinases (SnRKs).
SnRKs phosphorylate and activate transcription factors like MYB2, which then go
on to regulate the expression of genes involved in stress tolerance, including those
encoding for stress-responsive proteins like LEA (late embryogenesis abundant)
proteins (Jonak et al.
2004; Chmielowska-Bąk et al. 2013; Yeh et al. 2007).
Calcium is another important signaling molecule in plants and has been shown to
be involved in responses to a variety of stresses, including heavy metals (Zhang et al.
2016). Calcium is an important player in signaling cascades in plants. It is involved
in the regulation of many processes, including cell division, cell death, and stress
response. Calcium signaling is often mediated by calcium-binding proteins, which
are found in the plasma membrane, cytoplasm, and nucleus (Perfus-Barbeoch et al.
2002). These proteins can act as sensors or regulators of calcium signaling. Calcium-
binding proteins often contain EF-hand motifs, which are responsible for binding
calcium ions. Calcium signaling is important for plant responses to heavy metals.
Heavy metals can induce oxidative stress and cause damage to cells. Calcium
signaling cascades help to protect cells from damage and can promote repair and
regeneration (Wilkins et al.
2016).
The MAPKs (mitogen-activated protein kinases) are a family of proteins that are
involved in signal transduction cascades in response to vario us stimuli, including
heavy metals (Chmielowska-Bąk et al.
2014). They play a key role in mediating the
plant’s response to environmental stressors (Agrawal et al. 2002). MAPKs are
activated by phosphorylation, which is triggered by a variety of upstream kinases.
Once activated, they phosphorylate and activate downstream target proteins, which
leads to the activation of transcription factors and the induction of gene expression
changes. These changes eventually lead to the plant’s physiological response to the
stressor. Heavy metals are potent stressors that can cause damage to plants at high
concentrations. They can disrupt normal cellular processes and cause oxidative
stress, cell death, and other adverse effects. The plant’s response to heavy metal
stress is crucial for its survival (Opdenakker et al. 2012).
In conclusion, signaling cascades in plants play an important role in defending
against stressors like heavy metals. These plants ’ reactions to the presence of toxic
metals help protect their health and ensure they can grow and reproduce success-
fully. Although this process is not completely understood yet, research has provided
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us with a better understanding of how plants use these hormones to respond to
environmental stresses.
214 W. J. Osborne and S. Sharma
13.7 Heavy Metal Transportation, Translocation,
and Sequestration
A proper transport mechanism is necessary for the uptake of heavy metals by plants,
which can be influenced by rhizosphere microbes and root exudates. However, not
many specific or specialized transporters have been reported for heavy metals, but
several hyperaccumulators can take them up in different concentrations.
Factors such as soil pH, water availability, and organic compounds in the
rhizosphere play a critical role in heavy metal disposition by hyperaccumulating
plant species. Ph of soil directly affects the secretion by roots, which can further
acidify the rhizosphere and promote heavy metal dissolution. Moreover, organic
compounds emitted from the rhizosphere of hyperaccumulating plants can promote
the complex formation that leads to the solubility of heavy metals, improving their
uptake by roots.
Heavy met als are toxic to plants at high concentrations, and, therefore, the uptake
and transport of these elements through different tissues of plants are strictly
regulated. The plant root system is responsible for the uptake of heavy metals
from the soil, and the proces s involves several steps (Jonak et al.
2004; Yeh et al.
2007; Rao et al. 2011). Initially, the heavy metal ions present in the soil solution bind
to the root surface, and then they enter the root cells via specific transporters or
channels present on the plasma membrane. The concentration of heavy metals in the
soil solution, pH, and other factors can affect the uptake rate of these elements by
plants.
Once inside the root cells, heavy metals can be transported to the shoot through
two pathways: the apoplastic and symplastic routes. In the apoplastic pathway,
heavy metals move through the cell walls and intercellular spaces, while in the
symplastic pathway, they move through the cytoplasm and plasmodesmata. The
symplastic pathway is considered the main route for heavy metal transport, and it
involves a complex network of transporters and channels that are regulated by
different mechanisms.
The transport of heavy metals from root to shoot is regulated by several factors,
including the plant species, the type of heavy metal, and the physiological state of the
plant. Some plants are more tolerant to heavy metals than others, and they can
accumulate these elements in their tissues without showing any harmful effects.
Other plants are sensitive to heavy metals, and their growth and development can be
severely affected by even low concentrations of these elements.
Once in the shoot, heavy metals can be sequestered in different tissues, including
the cell walls, vacuoles, and chloroplasts (Cobbett and Goldsbrough
2002; Stroiński
et al. 2010; Mendoza-Cózatl et al. 2011). The sequestration of heavy metals in these
tissues is considered a detoxification mechanism, as it prevents the toxic elements
from interacting with the metabolic machinery of the plant. The mechanism of heavy
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metal sequestration in plants is complex and involves the action of several
transporters and chelating agents.
13 Genomic Instability in Medicinal Plants in Response to Heavy Metal Stress 215
The uptake, transport, and sequestration of heavy
metals in plants is a complex
and tightly regulated process that involves multiple mechanisms and pathways
(Pandey et al.
2009; Jonak et al. 2004; Yeh et al. 2007). Understanding these
processes is crucial for developing strategies to mitigate the harmful effects of
heavy metal pollution on plant growth and development.
Plasma membrane-based transporters play a key role in homeostasis and metal
uptake. Heavy met als are usually cotransported with soil nutrients across the plasma
membrane in the roots and show specific substrate activity; however, the complete
details regarding the acquisition of heavy metals are limited. Yet several genes
which encode potential heavy metal transporters have recently been identified in
plants (Yuan et al.
2008).
13.8 Strategies to Combat Heavy Metal Stress
Plants have developed complex mechanisms to deal with heavy metal stress, includ-
ing both morphological and biochemical adaptations. Such adaptations help plants
survive and thrive in heavy metal-contaminated environments, making them an
important tool for phytoremediation and environmental restoration.
Plants have evolved different mechanisms to cope with heavy metals in their
environment. The strategies they employ depend on various factors, such as the plant
species and the concentration and duration of heavy metal exposure (Anwar et al.
2012).
The two general mechanisms that plants use to cope with heavy metals are
exclusion and detoxification. In the exclusion mechanism, plants secrete organic
compounds into the rhizosphere (the soil surrounding the roots) to chelate heavy
metal ions. This process transforms the heavy metal ions into nontoxic compounds,
preventing their uptake by the roots. This way, plants avoid the harmful effects of
heavy metals on their cells (Tuteja et al. 2001).
On the other hand, in the detoxification mechanism, plants allow the entry of
heavy metal ions into their cells. However, once inside, they use various biochemical
pathways to detoxify and sequester the heavy metals into nontoxic compounds. This
way, plants can maintain their cellular functions and avoid the toxic effects of heavy
metals.
Both exclusion and detoxification mechanisms are essential for plants to survive
in heavy metal-contaminated environments. However, the effectiveness of each
mechanism may depend on the specific plant species and the level and duration of
heavy metals exposure.
The external morphological structures (such as epidermal layer, leaf trichomes,
lignification of cell walls) serve as the first defense against heavy metals stress by
restricting their entry into plant cells. Also, trichomes serves the very crucial role of
detoxification of heavy metals by releasing the secondary metabolites which reduces
the adverse effects of heavy metals (Harada et al.
2010). However, if heavy metals
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do penetrate plant cells, the plant cells engage a number of interconnected metabolic
pathways to counteract the toxicity caused by the heavy metals. Low-molecular-
weight proteins, metallochaperones or chelators, metallothioneins, phenylpropanoid
compounds, amino acids, stress-responsive phytohormones, and even heat shock
proteins are just a few of the diverse groups of metabolites that are biosynthesized
and accumulated through these pathways (Sharma and
Dietz 2006; Dalvi and
Bhalerao 2013).
216 W. J. Osborne and S. Sharma
These metabolites chelate or bind wi
th heavy metals and transport them to the
vacuole or out of the cell, where they are sequestered or detoxified. The biosynthesis
and accumulation of these metabolites are regulated by various transcription factors
and signaling pathways, such as ROS signaling pathway, the mitogen-activated
protein kinase (MAPK) pathway, and phytohormone signaling pathways
(Hasanuzzaman et al.
2020; Apel and Hirt 2004).
MTs are synth esized in the cytosol and can be induced by various stress
conditions, including heavy metal exposure. Expression of MT genes has been
shown to increase in response to heavy metal exposur e in different plant species,
including Arabidopsis, rice, soybean, and tobacco. Overexpression of MT genes has
also been shown to improve heavy metal tolerance in transgenic plants, suggesting
their potential use in bioremediation of heavy metal-contaminated soils
(Chmielowska-Bąk et al. 2013; Opdenakker et al. 2012; Harada et al. 2010).
Several studies have shown that plants can tolerate or hyperaccumulate heavy
metals by managing the expression of genes involved in metal chelation or transport.
These genes are potentially regulated by epigenetic mechanisms, which can modify
the expression patterns of genes without altering the key DNA sequence. Epigenetic
modifications, such as histone modification, DNA methylation, histone
modifications, and small RNA-mediated gene silencing, are reversible and can
respond rapidly to environmental stressors, allowing plants to adapt to changing
conditions. This makes epigenetic mechanisms a promising target for improving
plant tolerance and hyperaccumulation of heavy metals, which has important
implications for phytoremediation and sustainable agriculture (Gallo-Franco et al.
2020; Song and Chen 2015).
Overall, the production and accumulation of metal chelating compounds such as
metallothioneins (MTs) and phytochelatins play an essential role in the management
of metal homeostasis and detoxification of heavy metals, ultimately leading to
improved resistance to heavy metal stress (Cobbett and Goldsbrough 2002).
Additionally, amino acids, such as cysteine, methionine, and glutathione, have
been shown to play important roles in heavy metal detoxification through their
ability to chelate metals and participate in redox reactions (Sharma and Dietz
2006; Castillejo et al. 2023
). Glutathione, a tripeptide composed of glutamate,
cysteine, and glycine, is a key antioxidant molecule that is involved in the detoxifi-
cation of heavy metals and reactive oxygen species. Glutathione also plays a role in
the synthesis of phytochelatins, which are important metal chelators in plants.
Methionine and cysteine are sulfur-containing amino acids that are involved in the
biosynthesis of phytochelatins and metallothioneins and have been shown to be
important in the detoxification of heavy metals. Overall, a variety of small molecules
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and proteins are involved in the detoxification of heavy metals in plants, and their
activities are tightly regulated to maintain metal homeostasis and prevent cellular
damage.
13 Genomic Instability in Medicinal Plants in Response to Heavy Metal Stress 217
Organic acid (OA) efflux
from the root apex is a common mechanism in plants for
excluding aluminum (Al) ions from the root zone and reducing Al toxicity (Zhang
et al.
2016; Sharma and Dubey 2007). This mechanism involves the exudation of
malate and citrate from root cells to form stable complexes with Al ions in the
rhizosphere. The first genes linked to Al tolerance were identified as malate and
citrate transporters in wheat, sorghum, and barley. Later, two transporter families,
the Al-activated malate transporter (ALMT) and the organic acid/H
+
transport
channel (MATE), were found to be responsible for exuding malate and citrate,
respectively, in response to Al. However, other transporters such as ABC carriers
and aquaporins have also been shown to play an important role in OA transport.
Understanding the molecular mechanisms of Al exclusion in plants can have impor-
tant implications for breeding crops with enhanced Al tolerance and improving food
security in acidic soils.
MicroRNAs (miRNAs) are small, noncoding RNA molecules that regulate gene
expression posttranscriptionally. They are involved in various cellular processes
including development, differentiation, and stress response. Stress-induced miRNAs
have been shown to play a crucial role in conferring tolerance against heavy metals
by regulating the expression of target genes involved in stress response pathways
(Ghosh et al.
2022).
By targeting the negative regulators of stress tolerance, stress-induced miRNAs
can enhance the expression of positive regulators, thereby conferring tolerance
against heavy metals. Advances in next-generation sequencing (NGS) technology
have made it possible to identify and characterize large numbers of stress-responsive
miRNAs through miRNAomic analysis, which is a fast, sensitive, and reliable
technique. The identification of stress-responsive miRNAs through miRNAomic
analysis can provide insights into the molecular mechanisms underlying stress
tolerance and can aid in the development of new strategies for enhancing stress
tolerance in plants and other organisms (Ghosh et al.
2022).
Several studies have reported the differential expression of miRNAs in plants
exposed to heavy metal stress, indicating their involvement in the regulation of stress
responses. For example, in Arabidopsis thaliana, exposure to cadmium stress leads
to the upregulation of miR398, which targets the copper/zinc superoxide dismutase
(CSD1) and CSD2 genes, involved in scavenging reactive oxygen species (ROS)
(Cho et al. 2006). Similarly, in rice, exposure to arsenic stress leads to the
downregulation of miR528, which targets the ABC transporter gene OsABCG18,
involved in arsenic detoxification.
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218 W. J. Osborne and S. Sharma
13.9 Chemical Toxicity from Heavy Metals Causes Oxidative
and Genotoxic Stress
Due to exploitation of chemical fertilizers and human activities, heavy metals like
chromium, mercury, etc. pollute groundwater, water supplies, and soil. Inducing
both oxidative and genotoxic stress responses in plants, heavy metals cause cytotox-
icity and disruption to several cellular constituents (Pandey et al.
2009). The two
primary antioxidant defense mechanisms that plants have evolved are the activation
of antioxidant enzymes and the deposition of nonenzymatic antioxidant molecules. It
has been reported that the primary causes of heavy metal-induced oxidative stress
response in plants are overproduction of reactive oxygen species (ROS) and alter-
ation of the cellular redox environment (Anwar et al. 2012;Jańczak-Pieniążek et al.
2022). ROS serve a key role in the production of oxidative damage in living cells and
are formed as byproducts of endogenous metabolic activities. Several studies have
confirmed the cruci al role of ROS in plants. A series of essential processes in plants,
including cell growth and differentiation, programmed cell death, adaptations to
abiotic and biotic stress, and activation of systemic signaling, have been linked to the
dual activities of ROS. Yet, it has been discovered that the main factor for heavy
metal-induced oxidative stress response in plants is excessive ROS generation and
the associated disruption of the cellular redox environment and can result in DNA
damage, mutagenesis, and genome instability (Lin et al.
2008; Cho and Seo 2005;
Gratão et al.
2005).
13.10 Transcriptional Response of Heavy Metal Toxicity in Plants
Abiotic stresses can lead to a wide range of phenotypic changes in plants, which
result from complex biochemical, molecular, and physiological modificati ons. These
changes can occur at different levels and time scales, ranging from short-term
metabolic and gene expression alterations to long-term genetic and epigenetic
modifications. Epigenetic marks, such as DNA methylation and histone
modifications, have been reported to perform a c rucial role in regulating gene
expression in response to stress conditions. These epigenetic marks can be heritable
and lead to the establishment of epialleles, which are genomic sequences with
maintained methylation patterns across generations. Transposons can also contribute
to epigenetic variations that affect gene modulation by suppressing gene expression
through the methylation state of transposons located near or within genes (He et al.
2010; Galindo-Gonzá lez et al. 2018). The understanding of the molecular
mechanisms underlying plant responses to abiotic stresses and the role of epigenetic
modifications in these responses can provide insights into plant adaptation to
changing environmental conditions and can be used to develop new strategies to
improve plant resistance to stress (Zhang et al. 2022).
Various families of transcription factors, such as MYB, WRKY, bZIP, bHLH,
and ERF, have been demonstrated to be essential in controlling the expression of
stress-responsive genes. The activation of numerous transcription factors, which
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control downstream targets implicated in protection against heavy metal toxicity,
occurs as part of the transcriptional response to heavy metal stress, which is typically
brief and short. Several studies has been done to understand the transcriptional
control of plant response to heavy metal stress for Cd stress in numerous plant
species (Chmielowska-Bąk et al.
2014; Perfus-Barbeoch et al. 2002; Cho and Seo
2005). The fact that additional abiotic stresses, such as salt, drought, and low
temperature, share signaling pathways with the transcriptional response to
Cd-stress suggests that Cd-responsive transcription variables feature in intricate,
interconnected networks that respond to abiotic stress. However, there is insufficient
available data on the transcriptional response to other heavy metals such as Hg, Pb,
As, etc. in developing nations (Li et al.
2006; Singh et al. 2009; Rao et al. 2011;
Cargnelutti et al. 2006). Consequently, more studies are necessary to acquire more
insight about the transcriptional response in plants to these widespread heavy metals.
13 Genomic Instability in Medicinal Plants in Response to Heavy Metal Stress 219
Stress-induced methylation in plants
can be faithfully inherited by offspring,
leading to increased stress tolerance and even changes in economically important
traits. The studies also suggest that epigenetic variation and inheritance may be an
important evolutionary process in crops.
According to one study by Boyko et al. (2010), A. thaliana plants exposed to
numerous stressors displayed an overall rise in DNA methylation, which was linked
to a higher level of stress tolerance in the plant progeny. Similarly, in Polygonum
persicaria, Herman and Sultan (2016), found that DNA methylation enhanced
offspring tolerance to drought when parental plants were subjected to this stress.
Additionally, other studies have found specific examples of heritable methylation
changes induced by stressors such as nitrogen deficiency, heavy metal toxicity, and
drought in rice. These changes were associated with stress-responsive genes and
reported to alter rice adaptation to long-term drought conditions.
Recent studies have revealed that epigenetic mechan isms also play a crucial role
in the response of plants to heavy metal stresses. There are two main epigenetic
approaches that are significant in heavy metal stress: (1) epigenetic marks are
utilized to regulate transposon and stress-responsive genes, and (2) epigenetic
alterations serve as a way to protect plants from potential DNA damage induced
by metal ions. The patterns of DNA methylation and histone modifications which
occur in specific genome regions must be evaluated in order to understand the
potential epigenetic mechanisms concerning the regulation of complex gene
networks involved in Al tolerance responses, even though some studies have
suggested that heavy metal exposure may result in putative epigenetic changes
(Tuteja et al. 2001).
Also, this research reveals that an Al-tolerant rice variety had a higher concentra-
tion of methylated cytosines, demonstrating a distinct methylation pattern associated
with upregulated Al-responsive genes. This provides support to the theory that DNA
methylation plays a fundamental role in the rice plants’ reaction to aluminum
exposure.
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220 W. J. Osborne and S. Sharma
13.11 Genetic Mechanisms Upholding Heavy Metal Tolerance
Epigenetic modifications play a crucial aspect in regulating gene expression in
stressed plants. The various epigenetic mechanisms (such as microRNA expression,
histone modifications, DNA methylation) involve stress regulation and genome
modulation. For example, after exposure to stress, plants may develop tolerance
through adaptive techniques acquired by epigenetic modifications, such as DNA
methylation, which can affect chromatin structure, chromosome stability, and tran-
scription (Song and Chen
2015; Mallory and Vaucheret 2006).
Furthermore, the progeny of stressed plants may exhibit improved stress tolerance
due to transgenerational adaptation, which involves changes in DNA methylation
patterns and genome stability that are regulated by small RNA molecules. These
small RNA molecules function as reversible and sequence-specific transacting
epigenetic signals for the transcriptional level changes of gene expression (Chen
2012).
Studies have shown that heavy metal stress can induce alteration in gene expres-
sion and DNA, including stress-responsive transcription factor genes (such as MYB,
bZIP, AP2/ERF, F-box proteins, and serine-threonine/tyrosine-proteins). The
components of the DNA methylation apparatus, including NRPE1, NRPD1,
ROS1, MET1, DRM2, and CMT3, can respond strongly to heavy metal toxicity
and alter DNA methylation patterns at the genome-wide level or in a sequence-
specific manner, resulting in higher stress tolerance in progeny population (Apel and
Hirt 2004).
Regarding the regulation of cell cycle gene expression under heavy stress in
plants, information is still inadequate, and more studies are needed to interpret the
relationship between epigenetic modifications and cell cycle regulation in stress
responses.
Histone alterations, in addition to DNA methylation, are crucial in the control of
gene expression through the alteration of chromatin structure. The N-terminal tails of
histone proteins can be modified by a variety of enzymatic processes, including
acetylation, methylation, phosphorylation, and ubiquitination, which can result in
changes in chromatin structure and accessibility to transcriptional channel (Song and
Chen
2015; Grewal and Moazed 2003; Reik et al. 2001). For example, histone
acetylation is generally connected with gene activation, while histone methylation
can either activate or repress gene expression, depending on the specific residue and
degree of methylation.
Small RNAs, including microRNAs (miRNAs) and small interfering RNAs
(siRNAs), are another epigenetic mechanism involved in gene expression regulation.
These small RNA molecules are involved in posttranscriptional gene silencing and
are processed by enzymes from longer double-stranded RNA precursors. They act
by binding to complementary sequences in messenger RNA (mRNA) molecules,
resulting in mRNA cleavage or translational inhibition. The RNA-directed DNA
methylation (RdDM) pathway is a small RNA-dependent epigenetic mechanism that
is involved in DNA methylation and gene silencing in plants. In this pathway,
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siRNAs guide DNA methylation at target loci, resulting in transcriptional gene
silencing (Mallory and Vaucheret 2006; Chen 2012).
13 Genomic Instability in Medicinal Plants in Response to Heavy Metal Stress 221
Overall, epigenetic mechanisms play
an important role in the regulation of gene
expression in plants, including in response to environmental stressors such as heavy
metal toxicity. The reversible nature of epigenetic modifications makes them an
attractive target for plant breeding and genetic engineering approaches aimed at
improving plant tolerance to environmental stressors.
In addition to histone modifications, various other mechanisms also contribute to
the regulation of chromatin structure and gene expression under abiotic stress in
plants. For example, DNA methylation, a covalent modification of DNA, also plays
a critical role in stress response. Abiotic stress-induced changes in DNA methylation
patterns can affect chromati n structure and gene expression, leading to adaptation or
acclimation to stress conditions. Moreover, noncoding RNAs, such as microRNAs
and long noncoding RNAs, also regulate gene expression by targeting chromatin-
associated proteins or by modulating chromatin structure directly. For example, in
Arabidopsis, histone acetylation at H3K9 and H3K14 was found to be increased in
response to salt stress, which resulted in the upregulation of salt-responsive genes
(Godfray et al.
2010). Similarly, histone acetylation at H3K9 and H3K14 was also
shown to be involved in the regulation of drought-responsive genes in maize (FAO
2012).
The regulation of chromatin structure through histone modifications and the
associated histone chaperones plays a fundamental role in plant growth, develop-
ment, and adaptation to environmental stresses. Additionally, changes in histone
modifications have also been observed in response to other abiotic stresses, such as
drought, salinity, and cold. These modi fications are thought to play critical roles in
regulating stress-responsive gene expression and adaptation (Zhao et al. 2014).
In summary, chromatin structure and organization are crucial for maintaining
genome stability and regulating gene expression in plants. Abiotic stresses can alter
chromatin structure and modify histone and DNA methylation patterns, leading to
changes in gene expression that help plants to adapt or acclimate to stress conditions
(Zhao et al.
2014; Grewal and Moazed 2003). Further studies are needed to fully
understand the complex regulatory mechanisms that govern chromatin structure and
function under abiotic stress in plants.
Several studies suggest that DNA methylation plays a role in the regulation of
plant responses to heavy metal stress. Two main mechanisms have been proposed: a
protective effect against heavy metal-induced DNA damage and gene expression
control. Hypermethylation has been observed in plants with increased tolerance to
heavy metals, and changes in DNA methylation have been associated with variations
in gene expression. Some studies have also shown the heritability and stability of
heavy metal stress-induced methylation changes across plant generations. Overall,
these findings suggest that epigenetic mechanisms, including DNA methylation,
contribute to plant adaptation to heavy metals stress.
The relationship between DNA methylation and heavy metal exposure in plants is
complex and species-dependent. Pour et al. (
2019), study on wheat plants suggested
that DNA hypermethylation may be a protective response to higher concentrations of
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Al (30 mM), but hypomethylation may occur at lower concentrations (7.5 mM),
which can lead to cellular damage. It will be interesting to see further studies on
different plant species and their responses to heavy metal exposure, as this can help
us better understand the mechanisms of plant adaptation and potential applications in
phytoremediation.
222 W. J. Osborne and S. Sharma
Transposable elements have also
been reported to play a crucial role in heavy
metal responses in plants. The study by Kashino-Fujii and colleagues (
2018)
demonstrated that a multi-retrotransposon-like (MRL) insertion located upstream
of the coding region of the HvAACT1 gene in barley significantly enhanced the
expression of the gene responsible for citrate efflux in roots, a mechanism involved
in Al detoxification. This study also revealed that the MRL insertion and gene
expression were due to demethylation processes and were necessary for Al tolerance
in barley.
Transposon insertions close to genes have also been proposed as a source of
epialleles, which can affect the transcriptional regulation of specific genes. These
findings suggest that transposable elements and epigenetic modifications may play a
crucial role in the adaptation of plants to environmental stress, such as Al toxicity
(Gallo-Franco et al. 2020).
13.12 Conclusion
The increase in global food grain production due to improvements in agricultural
practices and research has been reported, but it also highlights the persistent issue of
undernourishment in rural areas of developing countries. It suggests that increasing
global food production by 70% by 2050 may be necessary to meet the needs of a
growing populatio n but also notes the challenges posed by competition for resources
and environmental stressors, such as heavy metal pollution. The passage suggests
that genetic engineering approaches can be used to develop heavy metal stress
tolerance in crop plants but acknowledges the need for further research to transfer
these technologies to field conditions. The passage also mentions the potential for
using natural plant species that are better adapted to heavy metal stress, as well as
biotechnological approaches such as chromosome engineering, transcriptome
profiling, targeted gene replacement, and nanotechnology, to develop crops that
are more productive and resilient to stress conditions.
Future research should concentrate on assessing the stability of epigenetic
alterations over generations, as epialleles can permanently alter genotypes and
phenotypic responses. The development of bioinformatics applications for epige-
netic assessments is indeed significant.
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