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et al. 2012). Likewise, the overexpression of a wheat PIP, TdPIP1;1, in tobacco
exhibited an increased stress resistance coupled with broaden leaf area and length-
ened root size (Ayadi et al. 2011). The overexpression of two exogenous aquaporin
isoforms, CfPIP2;1 (Cucurbita ficifolia) and CsPIP1;1 (C. sativus)in Arabidopsis
significantly inhibit the common expression of other aquaporins, leading to the
modified stress feedback of the plants under several stress situations (Jang et al.
2007a)
.
M
oreover, the greater expression of a BnPIP1 from Brassica napus in
transgenic tobacco leads to enhanced resistance to drought stress, enhanced plant
growth, and greater seed germination rates, while antisense plants revealed develop-
mental abnormality and enhanced susceptibility to drought stress condition.
202 A. Mishra et al.
Despite such resul
ts of overexpressing aquaporins in several transgenic
frameworks, progresses in interactomics have revealed physical interactions in
distinct aquaporin homologs (Di et al.
2013; Chen et al. 2013) and aquaporin
interactions with other many proteins participated in several different activities in
plants, indicating to their complex, integrated function in distinct physiological and
stress responses (Hachez et al.
2014a, b). The interaction of foreign AQP gene with
endogenous genes in plants also increases susceptibility and tolerance toward
different stress condition that may also influence the production of the metabolites
in medicinal plants.
12.7 Conclusions
Since the last decades, comprehensive research has emphasized to unravel the
structure and function of plant AQPs in water movement and stress conditions.
Such reports have attributed to enhance the knowledge on the action of several AQP
isoforms in water and nutrient uptake along with management of leaf and root
hydraulics, plant growth and development, and stress condition. The advancement
has been accomplished by knowing the molecular fundamentals of AQP transport
selectivity and gating. The molecular extent and gene expression has been assessed
by employing RT-PCR or distinct expression methods, indicating distinguished
co-translation and posttranslational event and molecular interactions between AQP
isoforms and functional proteins. Signaling pathways and environmental alteration
might impact the AQP activity and regulation. AQPs might also involve in plant
growth and regulating tissue development. Genetic engineering plays an important
role to study the role of AQP in transgenic plants against several stress conditions.
The interaction of foreign AQP genes and endogenous genes reveals the cumulative
impact in plants under different stress conditions, and it is cumbersome to know the
precise role of AQP genes. Some studies have reported that the overexpression of
foreign AQP genes inhibits the expression of endogenous genes in plants where
AQP genes individually involve to increase plant tolerance under stress condition.
Modifications of AQP gene expression for the increase of plant stress tolerance are
still challenging in terms of metabolite production in medicinal plants.
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12 Aquaporins Gene Expression in Plants Under Stress Condition 203
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207
Genomic Instability in Medicinal Plants
in Response to Heavy Metal Stress
13
W. Jabez Osborne and Shivangi Sharma
Abstract
The effects of heavy metals on plants are still not fully elucidated; hence heavy
metal toxicity is a major concern for plant growth and development. Plants have
developed complex mechanisms to deal with heavy metal stress, including
morphological, biochemical, and molecular adaptations. It is important to under-
stand the molecular mechanisms of plant survival and adaptation to heavy metal
stress, including transcriptional and proteomic patterns and the accumulation of
stress-responsive secondary metabolites. In this chapter, we focus on epigenetic
mechanisms that play an important role in the regulation of gene expression in
plants, including in response to abiotic factor, namely, heavy metals. The study
also discusses the potential of biotechnological approaches such as chromosome
engineering, transcriptome profiling, and targeted gene replacement to develop
crops that are more productive and resilient to stress conditions. This review
focuses on the role of epigenetic mechanisms in the regulation of gene expression
in plants, including in response to environmental stressors such as heavy metals.
Keywords
Genomics · Heavy metals · Signaling cascade · Enzymes
W. J. Osborne · S. Sharma (✉)
Department of Biosciences, School of Bio Sciences and Technology, Vellore Institute of
Technology, Vellore, Tamil Nadu, India
#
The Author(s), under exclusive license to Springer Nature Singapore Pte
Ltd. 2023
D. Singh et al. (eds.), Stress-responsive Factors and Molecular Farming
in Medicinal Plants,
https://doi.org/10.1007/978-981-99-4480-4_13
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208 W. J. Osborne and S. Sharma
13.1 Introduction
Plants are continually subjected to a variety of biotic and abiotic stressors in their
environment. Abiotic stress factors include extreme temperatures (such as heat, cold,
and freezing), ion toxicity (such as heavy metals, salinity), and water availability
(flood, drought). Biotic stress factors mainly include the effect of microbes, namely,
bacteria, fungi, viruses or any other pathogens, and insects on the development of
plant. In response to these stress factors, plants generate reactive oxygen species
(ROS) that can cause severe damage to cells and tissues if not properly regulated.
However, ROS is also important signaling molecules that activate various cellular
responses to stress. Anthropogenic factors, such as pollution and climate change, can
exacerbate these stress factors and put additional pressure on plants. For example,
exposure to UV light, infrared, and other types of radiation, as well as air- and
soilborne pollutants, can further stress plants and increase the production of ROS.
So, it’s crucial to comprehend the mechanisms underlying plant reactions to stress
and to create plans to reduce the environmental stress effects on plant growth and
yield.
Heavy metals build up in the soil and water systems which often leads to their
uptake by plants, resulting in various physiological and biochemical alterations. For
instance, heavy metal toxicity inhibits photosynthesis, respiration, and other meta-
bolic activities, which ultimately reduce plant growth and yield (Zhang et al.
2002;
Li et al. 2006). Additionally, heavy metals can disrupt the uptake and translocation
of essential nutrients, leading to nutrient imbalances and deficiencies. Moreover,
heavy metal contamination can also result in genotoxic stress, leading to DNA
damage and mutation (Singh et al.
2009). Overall, the various abiotic and biotic
factors mentioned above can significantly impact plant growth and development,
leading to reduced crop yields and economic losses for farmers. Therefore, it is
essential to implement appropriate mitigation strategies, including the use of resis-
tant plant varieties, soil amendments, and other agricultural practices to help plants
adapt and cope with these stresses. Additionally, efforts to reduce anthropogenic
activities and environmental pollution can also help minimize the impact of these
stressors on plant health and productivity.
Heavy metals can compete with nutrient cations such as calcium, magnesium, and
potassium for absorption and uptake by roots, which can lead to nutrient deficiencies
and reduced growth (Zhu
2002). Heavy metals can also bind with functional
proteins, such as enzymes and receptors, which can interfere with normal cellular
functions (Zhu 2016). Additionally, heavy metals can displace specific cations from
essential binding sites, which can lead to enzyme dysfunction and metabolic disrup-
tion (Gwóźdź et al. 1997; Samac and Tesfaye 2003; Das et al. 1997). Finally, heavy
metals can induce the overproduction of reactive oxygen species (ROS), which can
cause oxidative stress and damage to cellular membranes, proteins, and DNA,
resulting in heavy metal toxicity in plants (Nada et al. 2007; Pandey et al. 2009).
Plants have generated a number of defense mechanisms to combat the toxicity of
heavy metals, and they can be broadly categorized into excluders, indicators, and
hyperaccumulators based on their ability to tolerate and accumulate heavy metals in
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their tissues (Cobbett and Goldsbrough 2002; Clemens 2001). Excluders avoid metal
toxicity by actively expelling heavy metals from their tissues, while indicators keep
on accumulating heavy metals with the concentration without implementing the
metal avoidance approach. However, due to their high bioaccumulation ability,
hyperaccumulators can accumulate significant levels of metals in their shoots with-
out displaying any harmful symptoms.
13 Genomic Instability in Medicinal Plants in Response to Heavy Metal Stress 209
Althoug
h
heavy
m
etals
play
a
crucial function in the growth and development of
plants, their excessive presence can have cytotoxic and genotoxic effects that cause
instability in the genomes of plants (Rascio and Navari-Izzo
2011). The presence of
heavy metals like cadmium and lead in the soil causes the plants to experience severe
stress (Das et al.
1997). It is well recognized that heavy metal stress inhibits plant
growth, particularly during seed germination and the first few weeks of seedling
development (Zhang et al. 2002; Li et al. 2006; Nada et al. 2007). Protein thiol
groups are attacked by heavy metals, changing the shape of the protein structure
(Singh et al.
2009). Moreover, oxidative damage to cellular macromolecules and the
photosynthetic system can be brought on by heavy metal-induced ROS generation.
All mentioned stress factors are responsible for the majority of crop losses
worldwide, with average yield reductions of over 50% for most major crops. This
not only affects food security and economic stability but also has significant envi-
ronmental implications, as the loss of crops can lead to increased pressure on natural
resources and ecosystems. Therefore, understanding the mechanisms of survival and
adaptation of plants under abiotic stress is crucial for developing strategies to
improve crop productivity and ensure sustainable agriculture.
13.2 Heavy Metals
Heavy metals are categorized based on their density (5 g/cm
3
), and some of them are
essential micronutrients for plants, while other s can be toxic and cause senescence,
growth inhibition, and major yield loss. The first organs that come into contact with
heavy metals are the roots, and their growth can be inhibited due to the disruption of
mitotic activity and auxin transport. Plant cells essentially need 19 macroelements
(carbon (C), nitrogen (N), oxygen (O), hydrogen (H), magnesium (Mg), potassium
(K), sulfur (S), cadmium (Cd), phosphorus (P)) and microelements (iron (Fe), zinc
(Zn), copper (Cu), molybdenum (Mo), manganese (Mn), nickel (Ni), chlorine (Cl),
boron (B), bromine (Br), and cobalt (Co)) for their essential metabolic activities.
Heavy metal stress directly affects the respiration and photosynthesis rates by
interfering with the light reaction and nitrogen metabolism process.
However, several studies (Singh et al.
2009; Samac and Tesfaye 2003; Nada et al.
2007; Gallo-Franco et al. 2020) have reported the toxic effect of mercury (Hg), lead
(Pb), chromium (Cr), arsenic (Ar), Cd, and Al which includes imbalance in water
and nutrients uptake, reduced photosynthesis and hence chlorosis ultimately leading
to reduced biomass production. Plants react to heavy metal toxicity via intricately
interconnected systems, including both immediate and long-term reactions and
alterations at the physiological, metabolic, and genetic levels. Management of
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gene expression remains fundamental part in stress response and includes exclusive
modification of transcripts levels of responsive genes (Singh et al. 2016).
210 W. J. Osborne and S. Sharma
Geologic plant materials, such
as rocks and sediments, can contain elevated
concentrations of heavy metal elements. The presence of specific heavy metals and
their concentrations varies based on the type of sediments or rock. Igneous rocks
(hornblende, augite, olivine, etc.) are known to enhance the significant amount of
Zn, Cu, Ni, Co, and Mn in the soil. However, sedimentary rocks (shale crust, etc.)
contribute a significant amount of Ni, Cu, Zn, Co, Cr, Pb, Mn, Hg, and Sn
(Kavamura and Esposito
2010; Wuana and Okieimen 2011). Limestone and sand-
stone are crucial source of heavy metals. Additionally, geologic sources, forest fires,
and volcanic eruptions can also lead to the emission of heavy metals in the atmo-
sphere and later accumulation in the soil.
Living organisms cannot readily metabolize heavy metal elements; therefore they
gradually build up in the environment over time. Additionally, human activities such
as domes tic waste disposal and agricultural and industrial practices further cause
accumulation of heavy metals. Agricultural practices such as the use of fertilizers,
fungicides, and pesticides can contribute to the accumulation of heavy metals in the
soil. In addition, various industrial activities casting, smelting, mining, and metal
processing can drastically raise the release of heavy metals into the environment,
which can eventually contaminate soil and water bodies. Furthermore, improper
disposal of electronic waste, including batteries and electronic equipment, can
release heavy metals into the environment, including soil. Domestic effluents can
also contribute to the contamination of rivers and lakes with heavy metals and other
chemical components.
Plants, as primary producers in the food chain, can take up heavy metal ions
through their roots from the soil or through their leaves from atmospheric deposition.
Once accumulated in plants, these metals can then be transferred to animals that
consume them, leading to biomagnification of the heavy metals up the food chain.
However, the situation could be more severe in aquatic systems, as the entire plant
body gets dependent on polluted water and absorbs higher concentration of metal
ions due to particle deposition (Emamverdian et al. 2015).
13.3 Effect of Heavy Metals on Plants
Plant development and production can be significantly impacted by abiotic stressors
such as heavy metal toxicity. Heavy metals can disrupt cellular processes and severe
damage to fundamental biomolecules such as membrane lipids, proteins, and nucleic
acids, leadi ng to a range of physiological and metabolic responses. This can result in
reduced seed germination, stunted growth, leaf chlorosis, and other negative effects
on plant development (Nada et al. 2007; Aslam et al.
2014
; Dutta et al. 2018).
Additionally, heavy metal toxicity can cause oxidative stress and the formation of
reactive oxygen species (ROS) that leads to severe damage to cellular components
and early senescence (Pandey et al. 2009). Furthermore, heavy metal toxicity can
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also affect the absorption and utilization of principal nutrients by plants, leading to
nutrient deficiency and further hampering plant growth and development.
13 Genomic Instability in Medicinal Plants in Response to Heavy Metal Stress 211
Plants have developed numerous strategies to
archive heavy metal stress, such as
sequestration, chelation, compartmentalization, and exclusion. All of these
mechanisms involve the synthesis and accumulation of various compounds such
as organic acids, amino acids, phytochelatins, and metallothioneins which bind and
detoxify heavy metals, preventing their toxicity to the plant cell (Cobbett and
Goldsbrough 2002; Clemens 2001).
In addition, plants also employ various antioxidant defense mecha nisms to
combat heavy metal-induced oxidative stress, including synthesis and accumulation
of vario us antioxidants (such as flavonoids, carotenoids, glutathione, ascorbate, etc.)
followed by activation of numerous antioxidant enzymes (such as peroxidase,
catalase, superoxide dismutase; Masood et al.
2016; Bücker-Neto et al. 2017).
These defense mechanisms help to scavenge and neutralize the excess ROS
generated by heavy metal toxicity, preventing damage to cellular components and
maintaining cellular redox homeostasis.
Overall, heavy metal toxicity is a significant abiotic stress that interferes with
plant growth and development and generates serious concerns for the safety of the
world’s food supply (Zhu
2016).
13.4 Effects of Heavy Metals on Plant Enzymes
Plants have incredible abilities to adapt and respond to their environment. One such
response is the signaling cascade that occurs in plants when they encounter heavy
metals. These cascades involve a complex network of cellular signaling pathways
that allow plants to survive and continue growing despite exposure to toxic
compounds (Jonak et al. 2004; Chmielowska-Bąk et al. 2013; Yeh et al. 2007).
Plants have developed complex strategies to sense and respond to heavy metal
toxicity. When plants are subjected to heavy metals, they activate several signaling
pathways to counteract the negative effects of these metals.
The first response to heavy metal stress is the induction of oxidative stress. The
stress leads to the generation of ROS which can cause severe damage to the cellular
components such as lipids, protein, and DNA, leading to the production of
antioxidants, which scavenge ROS and prevent oxidative damage. Also, plant
regulates gene expression to combat heavy metal stress. Plants activate specific
transcription factors which control the expression of stress-responsive genes.
These genes encode proteins involved in numerous functions such as metal chela-
tion, detoxification, and sequestration.
In addition to these responses, plants also accumulate stress-responsive secondary
metabolites such as flavonoids. Flavonoids are known to have antioxidant properties
and can scavenge ROS, which provides protection from oxidative damage (Singh
et al. 2016; Zhao et al. 2014).
Overall, monitoring oxidative stress induction, transcriptional and proteomic
patterns, and accumulation of stress-responsive secondary metabolites can provide
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valuable information on the stress response associated with heavy metal toxicity in
plants. These details can be used to develop techniques for increasing plant resis-
tance to heavy metals.
212 W. J. Osborne and S. Sharma
13.5 Signaling Cascade in Plants in Response to Heavy Metals
When plants are subjected to higher concentrations of heavy metals, they undergo a
series of changes in gene expression known as the metal stress response. This
response is mediated by a complex signaling cascade that involves multiple tran-
scription factors and other proteins.
The first step in metal stress response is the activation of a receptor protein called
MTF1. MTF1 binds to metal ions and triggers a signaling cascade that leads to the
activation of several transcription factors, including ZnF1, Cys2/His2-type zinc
finger proteins, and bZIP transcription factors. These transcription factors then
activate genes that encode proteins involved in metal detoxification, such as
metallothioneins and ferritins (Wong et al.
2004; Kohler et al. 2004).
In addition to metal detoxification proteins, the metal stress response also leads to
the production of enzymes that help plants cope with oxidative stress. Oxidative
stress is a common side effect of heavy metal exposure, and it can damage plant cells
and lead to cell death (Inzé and Montagu 1995). Enzymes such as peroxidases and
superoxide dismutases provide protection from oxidative damage by scavenging
ROS (Anwar et al.
2012;Jańczak-Pieniążek et al. 2022; Hasanuzzaman et al. 2020).
The metal stress response is a complex and coordinated biological process that
helps plants survival in high concentration of heavy metals.
13.6 Signaling Cascades and Response to Heavy Metals
Plants have often survived extreme environments, and heavy metal pollution is
widespread form of contamination across the globe. One such defense mechanism
involves signaling cascades—a series of molecular events that work together like a
domino effect to activate specific responses within the plant. Signaling cascades are
important for plants to respond to their environment. They help the plant regulate its
growth and development in response to alterations in the external environment, such
as temperature, light, water availability, and nutrients.
One way that plants use signaling cascades is in response to heavy metals. Heavy
metals are toxic to plants and can damage their cell membranes, disturb their
metabolism, and interfere with their growth. When a plant is exposed to heavy
metals, it activates a series of signaling pathways that help the plant protect itself
(Yeh et al. 2007; Chmielowska-Bąk et al. 2014).
These signaling pathways help the plant to detoxify the heavy metals, repair any
damage that has been done, and ultimately survive the exposure. The plant’s
response to heavy metals is essential for its survival in polluted environments.
These cascades involve the activation of transcription factors, which in turn regulate
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