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89
Gene Expression in Medicinal Plants
in Stress Conditions
6
Sadashivaiah, L. Sunil, and R. Chandrakanth
Abstract
Medicinal plants have been in use for hundreds of years for the betterment of
mankind against a plethora of ailments in different parts of the world. Different
parts of medicinal plants, like leaf, stem, bark, seeds, flowers, fruits, and so forth,
are reported to exhibit significant therapeutic properties against many diseases
and disorders. Medicinal plants’ prophylactic and therapeuti c potentials are
primarily attributed to the presence of secondary metabolites (SM) secreted in
plants for their defense against several biotic and abiotic stress conditions. Several
factors regulate the production of these bioactive compounds. The genome of an
organism plays an important role in regulating numerous activities. Growth,
development, productivity, and ability to thrive under distressed conditions are
primarily controlled by differential gene expressions. Like any other plant,
medicinal plants are also exposed to constant stress factors, which increases
their ability to produce bioactive SM. Drought, heat, cold, salinity, water logging,
and infection from pathogens are important abiotic and biotic stress factors
affecting medicinal plants and their productivity. In the present chapter, an
attempt has been made to understand the differential regulation of gene expres-
sion in the response of medicinal plants to unfavor able environmental conditions.
Developments in functional genomics, along with advances in techniques like
microarrays, qPCR, and high-throughput next-generation sequencing, have
revolutionized gene expression profiling methods to understand the expression
Sadashivaiah (✉) · R. Chandrakanth
Department of Molecular Biology, Yuvaraja’s College, University of Mysore, Mysuru, Karnataka,
India
L. Sunil
Department of Plant Cell Biotechnology, CSIR-Central Food Technological Research Institute,
Mysuru, 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_6
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profile better. This chapter highlights the ability of medicinal plants to cope under
stress by modulating the expression of key genes involved in the growth, physi-
ology, biochemistry, and metabolism of SM, which are the principle components
of medicinal formulations produced from these plants.
90 Sadashivaiah et al.
Keywords
Medicinal plants · Stress factors · Secondary metabolites · Differential gene
expression
6.1 Introduction
Medicinal plants are known to be utilized from times immemorial. Nature has
offered enormous valuable resources, of which plants are one of the best boons to
mankind. Ever since ancient times, mankind has looked to nature for majority of its
needs. Plants offer essential services to humanity ranging from oxygen to breathe,
food to eat, shelter to live, medicines to survive against an array of infections and
diseases, and several other resources for his well-being (Akerele et al.
1991). From
ancient times to the current day, extracts and formulations from medicinal herbs
remain the chief components of therapeutic agents used by a large section of people
from rural areas. Also, a considerable share of prescribed modern medicines is
derived from medicinal plants. The healing properties of medicinal plants are
primarily attributed to the presence of secondary metabolites (SM). Although they
do not have any fundamental role in plants, they offer significant protection against
various biotic and abiotic stress factors, enabling the plants to withstand and
overcome unfavorable conditions. In addition, they also serve as precursors for
synthesizing multiple flavors, food additives, pharmaceuticals, and other essential
bio-chemicals (Bennett and Wallsgrove
1994; Akula and Ravishankar 2011).The
use of medicinal plants for different therapeutic purposes is known from ancient
times. However, the oldest written evidence was around 5000 years old and
identified on a Sumerian clay slab at Nagpur (Kelly
2009). The use of medicinal
plants for therapeutic purposes has been documented in many ancient civilization
records. Egyptians, Greeks, Romans, Indians, and Chinese are known to have
extensive knowledge of herbal medicines. Medicinal plants have also been men-
tioned in several holy books like the Vedas, the Bible, and the Talmud, in which
spice and aromatic plants have been particularly recorded (Tucakov 1971; Dimitrova
1999).
Although allopathic medicines have found extensive market size, the use of
herbal medicines is regaining the market share because of their minimal side effects,
lower prices, and ease of access even in developed countries besides in majority of
developing countries (Niu et al.
2014). SM are constituted of complex chemical
components, which can be mainly grouped into various types, namely, alkaloids,
phenolics, terpenoids, steroids, fatty acid-derived substances, and polyketides. These
are a few major types, which, in turn, can be subdivided into numerous subtypes
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(McMurry 2014). Rapid urbanization and uncontrolled environmental changes are
having a heavy toll on the growth and development of food crops and medicinal
plants, particularly leading to unsustainable food supply and decreased SM produc-
tion. Plants adapt to the fluctuating environment by evolving new mechanisms to
overcome the changes resulting from biotic and abiotic stress factors. Medicinal
plants are known to produce elevated levels of SM under distress conditions, which
are,
in turn, used to counter the deleterious effects of stress factors. Plant’s response
to stress depends on several factors like the kind of stress, its amount, duration, mode
of interaction, genotype, and the overall fitness of the plant itself.
6 Gene Expression in Medicinal Plants in Stress Conditions 91
Acute and chronic exposur
e to stress factors will have enormously different
manifestations in plant physiology and health at the cellular and molecular levels
(Tattersall et al.
2007; Pinheiro and Chaves 2011). Understanding the modes of
response in medicinal plants is crucial to devise methods and mechanisms that could
be implemented to increase the adaptability of medicinal plants to different distress
conditions. This helps to develop new cultivars of medicinal plants with tolerance to
unfavorable conditions that could produce elevated levels of important SM, which
will meet the needs of an ever-increasing population.
Technological advancements in genome study techniques, like microarray,
RT-PCR, NGS, and so forth, have led to better understanding of cell and tissue
transcriptome, proteome, and metabolome profiles under different conditions (Pérez-
Torres et al.
2009). Signaling in plants involves perception of stimulus through
receptors of sensitive cells, receptor activation, signal transduction, generation of
secondary messenger, activating transcription factors, and finally, upregulation of
stress-responsive target genes (Xiong et al.
2002).
6.2 Stress Factors and Their Impacts on Medicinal Plants
Any external factor, which can affect the health and well-being of an organism
resulting in the perturbation of growth, development, physiology, and metabolism of
an organism, is termed a stress factor. Organisms constantly try to evolve to cope
against distress condit ions by modulating their gene expression patterns. Few
organisms successfully adapt to the changed environment and become resistant. In
contrast, others who fail to cope with the change become susceptible and finally
succumb to exist in due course of time. Like other plants, medicinal plants are
constantly exposed to many environmental stress factors, which will significantly
affect the quality, quantity, and efficacy of the secondary metabolite they produce
(Gershenzon
1984; Falk et al.
2007). Altered temperature, salinity, drought , water
logging or flooding, heavy metal toxicity, UV radiations, alkalinity, attack from
pests and pathogens, and so forth, are some of the major abiotic and biotic stress
factors faced by a wide variety of medicinal plants (Verma and Shukla 2015).
Productivity and yield in plants decrease significantly under stressful conditions,
which will hugely impact society, constraining the food security system and supply
chain link of medicinal ingredients of various pharmaceutical and herbal drugs.
Therefore, there is an urgent need to develop stress-resistant cultivars of food crops
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and medicinal plants that can adapt to the changed environments and produce
sustainable products. When plants face stress factors, they alter their gene expression
pattern to overcome the harmful damag e caused by external factors by adopting
reversible and irreversible changes in their physical and physiological response.
These changes might be dynamic and complex (Cramer et al.
2011).
92 Sadashivaiah et al.
Drought hinders plant growth by limit
ing the water uptake to rapidly expanding
cells and altering cell walls’ structural and functional properties via ROS-mediated
effect on cell wall enzyme activities (Skirycz and Inzé
2010). It also results in
non-enzymatic modification of cell wall composition by differential interaction
with calcium and pectate, resulting to cause a further decrease in water absorption
(Boyer
2009). In addition, water scarcity also modifies water-conducting aquapor in
proteins and xylem-conducting properties, further decreasing plant growth.
6.3 Gene Expression Study Methods and Their Uses
Functional genomics (gene expression profile) is primarily carried out to annotate
the functions of different genes expressed under a given condition. Genome expres-
sion is highly complex; understating its relative expression profile under a different
set of conditions provides precious information about the genes involved, their
functions, the proteins they produce, their efficacy, whether they are structural,
functional, or regulatory, will tremendously facilitate the research community to
understand, modulate, and exploit the genetic information according to the desired
requirement.
An organism’s global gene expression profile is analyzed by measuring the total
mRNA produced in the organism under normal conditions and is defined as an
organism’s transcriptome. Different cells, tissues, and organs express their genome
differently under other conditions. They will produce unique transcriptome data
under different situations. Thus, having the transcriptome of a cell, tissue, or organ
under our desired conditions will provide huge information about its respective
functional state.
Complementary DNA amplified fragment length polymorphism (cDNA-AFLP),
serial analysis of gene expression (SAGE), and massively parallel signature sequenc-
ing are a few frequently used techniques to understand gene expression patterns
when the transcriptome sequence data is not required (Guo et al. 2018; Higuchi et al.
1993). However, more recent and advanced methods like microarray, RT-PCR, and
NGS are currently used when previous sequence know ledge is known (Pérez-Torres
et al. 2009). Microarray techniques allow one to study simultaneous gene expression
profiles under different experimental conditions, which helps to understand the
response strategy against a given condition (Mm 2004).
Results from gene expression studies indicate which set of genes are expressed
under given conditions indicating the co-expression of gene sets. This enables
annotating genes functionally under a given set of conditions. However, due to the
complex nature of the biological system, processing huge information is quite
challenging. The technological advancements in sequencing resulted in the
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evolution of a novel, sensitive, reliable, and high-throughput method, called NGS,
which has taken over sequencing studies to a new level, thereby making gene
expression studies more efficient (Jain 2012). Analyzing gene expression profiles
against a particular stress factor results in identifying a set of activated genes to
tackle a specific stress.
6 Gene Expression in Medicinal Plants in Stress Conditions 93
Similarly, studying the
expression profile of gens against the second, third, and
subsequent kinds of stress factors the plant will encounter more frequently results in
obtaining a large set of gene expression patterns. Careful analysis of these expression
profiles results in understanding what kind of gene expression takes place under
different stress challenges, are these genes the same, different, or belong to a
common gene family, or if is there any overlapping pattern occurring under different
conditions, can be identified. Analyzing a particular set of genes involved in the
fundamental process like osmoregulation, elicitor or stress sensors, signal transduc-
tion pathway, metabolism, detoxification, transcriptional regulations, and so forth,
will give better information to understand the tolerance mechanism. Such studies
will help breeders and biotechnologists to develop novel resistant cultivars with
increased production capacity.
6.4 Studies on Gene Expressions in Medicinal Plants Under
Stress Conditions
Both abiotic and biotic stress factors are reported to negatively influence the growth
and development of crop plants and medicinal herbs, resulting in a significant
reduction in their production and yield. However, reports indicate the increased
production of SM in medicinal plants subjected to stress. This is attributed to the
protective role exhibited by various SM, which enable the plant to overcome
stressful conditions. The quality and quantity of these SM are, intern, determined
by the kind and amount of stress factor the plant is exposed to. Medicinal plants have
adapted tolerance to stress factors by altering their gene expression profile to be
suitable for better survival. Gene expression studies are gaining importance in
understanding the mechanism of tolerance in medicinal plants against stress factors,
ultimately leading to the development of novel resistant cultivars of medicinal plants
with the potential of increased secondary metabolite production.
Unlike the general trend of decreased yield and vigor in plants exposed to
drought, few studies have reported increased production of SM in medicinal plants
cultivated under stress conditions. For instance, phytochemical investigations
reported elevated synthesis and accumulation of SM in medicinal plants cultivated
in arid and semi-arid conditions compared with those plants grown under normal
conditions (Kleinwächter and Selmar
2015). Examination of this phenomenon
revealed the fact to be due to metabolic reprogramming induced under externally
modified conditions. It was found that elevated levels of reduced compounds like
NADPH accumulated under drought stress resulted in stomatal closure and lowered
photosynthesis and biomass accumulation. Stressed plants use this strategy to
reallocate their metabolic resources from primary oxidative pathways toward
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synthetic pathways, producing key metabolites that can confer protection against
many distress situations. Few metabolites like isoprenoids, alkaloids, or phenolics
have been identified to pose such capacities. Several reports mention elevated SM
production under stress conditions, and their gene expression profiles have also been
reported to be upregulated under such conditions, which further justifies their timely
interventions.
94 Sadashivaiah et al.
Satur
eja
mutica,
a
lso
ca
lled white savory, belongs to the mint family, a delicate
perennial plant growing on the calcareous rocks in the northeastern parts of Iran. The
plant contains high carvacrol content in its essential oil, which is known for its high
aromatic properties. The fresh and dried parts of the plant find their use in many
dietary applications because of its rich aromatic flavor. High concentrations of
carvacrol and phenols in its oils render it to be used against a plethora of medical
ailments. Studies have reported elevated biosy nthesis of these essential oils under
drought conditions. It has been demonstrated that the expression pattern of three key
genes, namely, geranyldiphosphate synthase, terpene synthase, and limonene
hydroxylase (lh), involved in the production of essential oils were found to be
increased significantly under the externally induced drought stress (Davari
Shalamzari et al. 2022).
Gene expression analysis was conducted on Ziziphorapersica, an Iranian medici-
nal plant belonging to the mint genus. This study demonstrated the upregulation of
key genes responsible for elevated essential oil production under drought stress.
Limonene synthase, lh, isopiperitenon dehydrogenase, and isopiperitenon reductase
are the genes found to be upregulated, resulting in enhanced production of essential
oil in the medicinal plant (Jalilifar et al.
2022).
Menthapiperita L., peppermint, another important medicinal plant belonging to
the Lamiaceae family is known to be used in multiple fields ranging from ancient
herbal therapy to modern pharmaceutical products. It also finds its application in
food processing and flavor industries (Gedif and Hahn
2002; Badal et al. 2011).
Terpenoids like menthol, menthone, limonene, isomenthone, menthofuran,
pulegone, neomenthol, β-caryo phyllene, 1,8-cineole, and sabinene are the chief
components of essential oil produced from this plant (Rohloff 1999). Gene expres-
sion studies under water stress conditions revealed the upregulation of ipd, ipr, and
mfs genes and the downregulation of mdeh involved in the menthol biosynthesis.
This modulation resulted in causing the desired effect with an increase in essential
oil quality and yield, thereby contributing to its enhanced medicinal functionalities
(Rahimi et al. 2017).
Withania somnifera Dunal (Ashwagandha), a well-known ayurvedic herb grown
predominantly in the Indian subcontinent, is from the Solanaceae family (Kaileh
et al.
2007). Chemical moieties, like alkaloids and steroidal lactones, in this herbal
extract are known for their vivid pharmaceutical properties (Rigano et al.
2013).
Withanolides are reported to exhibit multiple therapeutic properties against various
diseases. Gene expression studies demonstrated that gene coding for squalene
synthase (SQS) a key enzyme involved in the biosynthesis of withanolides through
the isoprenoid pathway was significantly upregulated during drought stress. This is
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achieved by fluxing the carbon metabolites toward biosynthetic pathways instead of
oxidative fates (Singh et al. 2018).
6 Gene Expression in Medicinal Plants in Stress Conditions 95
Similarly, gene expression
studies in basil (Ocimum
basilicum) an important
medicinal plant of the Lamiaceae family, have revealed the upregulation of essential
genes involved in the biosynthesis of phenylpropanoids, methyl chavicol, and
methyl eugenol. These changes were brought out by the corresponding increased
expression of EVMT and CVOMT genes with simultaneous decreased expression of
4CL and C4H genes under drought stress (Mandoulakani et al.
2017).
Liquorice (Glycyrrhiza uralensis Fisch), an important medicinal plant, produces
Glycyrrhizin, an important bioactive triterpenoid, saponin, and liquiriti. These con-
stitute the chief metabolites of the flavonoid group in its roots and stolons. They are
known to impart health benefits; hence, liquorice is considered an important herbal
plant with huge pharmaceutical and economic value. It is a perennial plant belonging
to leguminous species (Kitagawa
2002; Hayashi and Sudo 2009). Analysis study
revealed that inoculation of liquorice with arbuscular mycorrhizal (AM) with simul-
taneous drought stress induction resulted in increased production of key metabolites
glycyrrhizin and liquiritin. Enhanced production of active components was in
agreement with the corresponding increased expression of key genes SQS1, β-AS,
CYP88D6, and CYP72A154 of the glycyrrhizin biosynthetic pathway. Similarly,
elevated levels of the CHS gene were in agreement with increased production of
liquiritin (Xie et al. 2018 ).
Soil salinity is another severe threat imp eding the growth and development of
plants, including medicinal herbs (Miransari 2017; Miransari and Smith 2019).
Increased salts elevate the osmotic potential, thereby limiting the water and nutrient
uptake resulting in altered plant metabolism and physiology and enhanced ROS
production accompanied by a decrease in the absorption of micro and macronutrient
(El-Esawi et al.
2018). The development of salt-resistance varieties would be an
ideal and attractive method for addressing the problem. The development of resistant
cultivars is always a challenging task for a plant breeder. The challenges faced by the
plant are varied, and it might be one or more or a combination of different stress
factors.
Further, salt tolerance, a complex trait, is frequently influenced and controlled by
multiple genes, complicating the understanding of signa ling and metabolic
pathways. Hence, compressive knowledge about the gene expression profile under
vivid environmental and genetic setups is essential for successfully developing salt-
resistant cultivars. Because of this, several gene expression studies have been
conducted in different medicinal plants resulting in the accumulation of vast
amounts of information with respect to the kind of genes expressed, key enzymes
involved, and transcription factors activated under varied saline environments. This
information is crucial for genetic breeders to develop saline-resistant cultivars,
which can withstand and grow successfully under salt stress. This will enable the
tolerant plants to produce enhanced amounts of SM, which serves as an important
precursor of several herbal and pharmaceutical formulations in addition to food and
flavor industrial requirements (Mishra et al.
2018
).
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96 Sadashivaiah et al.
Phytochemical analysis of root and leaf tissues of the licorice plant, an important
medicinal plant grown under saline conditions, revealed enhanced amounts of
SM. The main types of chemical moieties present were identified to be glycyrrhizin,
betulinic acid, saponins, and phytosterols. The results agreed with many studies that
indicated increased concentration of these SM under identical stress conditions
(Stutte
2006). These metabolites were reported to protect cells and tissues from
oxidative and ionic stress induced under saline environments (Wahi d and Ghazanfar
2006).
Gene expression analysis in the above experiments revealed that the important
genes involved in stress modulations were expressed differentially to suit the plant’s
requirement against suitable environmental conditions. Key genes like squalene
synthase (SQS1 and SQS2), β-amyrin synthase (bAS), lupeol synthase (LUS),
cycloartenol synthase (CAS), β-amyrin 11-oxidase (CYP88D6) and β-amyrin
24-hydroxylase (CYP93E6) involved in the biosynthesis of phytosterols, oleananes,
and lupine saponins were found to be upregulated in plant root tissues. Similarly, the
genes producing glycyrrhizin and total phenols having anti-oxidant functions were
also upregulated under salt-induced stress. However, salt-induced gene expression
was not uniform throughout the response period. Also, the expression was different
at root and leaf tissues. Further, a few genes were upregulated at the start of the stress
induction, while others were induced at a later time of salt stress. This clearly
explains the differential expression of genes involved in the process of licorice
plant acclimatization to salt stress (Shirazi et al.
2019).
In another study, Apocynum venetum, a Chinese medicinal plant, was subjected to
salt stress, and the gene expression profile was analyzed. Here, as expected, the
overall growth and development of the plant were compromised under salt stress.
However, the key metabolites quercetin and kaempferol, known to possess signifi-
cant medicinal properties, were found to be secreted in higher concentrations.
Further studies confirmed the upregulated genes to be AvF3H, and AvFLS, primar-
ily expressed in the flavonol biosynthetic pathway. This explains that plants adapt
favorably to stressful conditions by reprogramming their genome to suit their
requirements by controlling and diverting the primary metabolic pathways toward
secondary metabolite productions to effectively counteract stress-induced
complications (Xu et al. 2020).
Metal and metalloids are natural components of soil systems that support the
growth and development of numerous organisms. Due to indiscriminate anthropo-
genic activities, rapid industrialization, and unscientific use of land resources, the
soil composition is being altered rapidly toward an unfavorable state (Rahman and
Singh
2019). Whenever the concentration of these metals exceeds their safe limits, it
contaminates the land, ultimately resulting in their toxic effect in many tissues of
plants and animals connected to them directly or indirectly by disturbing normal
cellular homeostasis (Gong et al. 2020). Different metals have a differential effect on
the plant and animal systems. Some metals are crucial; others have no harm.
In contrast, like heavy metals, few are known to cause deleterious effects and are
thus termed toxic heavy metals, particularly at their elevated concentration (Marg
2011). Many of these metals are crucial for normal biochemical and physiological
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functions in most organisms, including medicinal plants. Some are critical
components of metalloenzymes and prote ins, including transcription factors
(Ricachenevsky et al. 2013). Although plants have evolved mechanisms to limit
their uptake and utilization, they are limited to specific concentrations. Beyond that
threshold, they fail to control their absorption. Only a small number of plants, about
700 out of 300,000 vascular plants, are highly adapted to elevated metal ion
concentrations through the mechanism called hyperaccumulation (Pollard et al.
2014). Plants adapted to heavy metal toxicity are generally endemic to places of
high metal ion concentration, either occurring naturally or produced because of
anthropogenesis activities (Reeves et al.
2018). In addition to their effect on p lant
health and productivity, they also have a significant toll on the health of humans and
animals when they feed on the plant and plant products grown on such contaminated
land, thereby causing mild to fatal diseases. Heavy metal toxicity has been reported
to cause significant loss in crop yield, leading to a severe decrease in global food
production and a proportional reduction in SM concentration in medicinal plants.
However, few plants a
dapted to such stress are known to produce increased SM.
6 Gene Expression in Medicinal Plants in Stress Conditions 97
Limiting heavy m
etal absorption, preventing their long-distance transport, and
increasing the efflux of metal ions by membrane-embedded transporter pumps and
proteins are some primary mechanisms involved in imparting heavy metal tolerance
in plants (Silver and Ji 1994). Confining the metal ions by forming a complex with
specialized proteins similar to heat shock proteins is another way to protect from
heavy metal toxicity (Neumann et al.
1994). Elevated synthesis of heavy metal
binding proteins or small chelating peptides called phytochelatins, which are
synthesized in cells through non-translational mode using a few molecules like
glutathione as substrate, are additional strategies employed (Goldsbrough
2020).
The use of metallothioneins, another class of metal-binding peptides synthesized by
gene translation method is a few more ways to deal with metal toxicity (Rauser 1995;
Clemens 2001). Increased secretion of organic acids like carboxylic acids and amino
acids, such as citric, malic, and histidine, which serve as potential ligands for heavy
metals binding, are some additional modes available to counter metal ion-induced
stress. Another way of heavy metal tolerance is achieved by the sequestration of
ionic metals in specialized organelles like vacuoles, thereby decreasing interference
with other sensitive biomolecules within the cell. This strategy is mainly observed in
the case of Cu and Cd contamination (Ernst et al. 1992).
Ocimum tenuiflorum, an important medicinal plant with potential health benefits
belongs to the Lamiaceae family and is grown worldwide (Dev et al.
2011). Many
studies have been carried out on the plant with respect to its genome composition and
pathway genes involved in SM biosynthesis (Rastogi et al.
2013, 2014). Gene
expression analysis revealed the key genes involved in phenylpropanoids and
essential oil components biosyntheses, like PAL, C4H, 4CL, COMT, C3H,
CS3’H, and CCR were downregulated under external stress factors, including
drought, salt, and flood. In contrast, other genes like CCOMT and CAD showed
elevated expression in all three stress conditions except drought (Rastogi et al.
2019). This clearly explains the differential pattern of gene expression under
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different stress factors. However, few genes expression remains the same under most
of the stress factors showing common response phenomena.
98 Sadashivaiah et al.
Artemisinin, an importan
t anti-malarial drug produced by the Artemisia annua is
a sesquiterpene lactone produced via the MEP pathway. Researchers have put in
significant efforts to increase the production of arteminisin by various approaches.
Several studies had reported increased production of arteminisin when the plant was
subjected to different abiotic stress factors (Yadav et al. 2017). They have reported a
substantial increase in the production of artemisinin and other essential oil
components when the plant was subjected to salt stress.
Similarly, another research found the increased synthesis of the active SM
artemisinin when the plant was treated separately with salt and heavy metal stress.
However, the heavy metal treatment resulted in a higher concentration of artemisinin
than that was produced under salt stress (Qureshi et al. 2005). The plants were
subjected to waterlogging and chilling stress in another study. Here also, the
expression of key genes involved in the biosynthesis of artemisinin pathways like
HMGR, FPPS, DXS, DXR, CYP71AV1, and ADS was elevated significantly (Yin
et al. 2008 ). Along with these results, the transcriptome analysis of Artemisia annua
was carried out to understand and analyze differential expression of various key
genes involved in the biosynthesis of important SM mainly under four different
abiotic stress factors: drought, salinity, waterlogging, and cold. Most of the genes
upregulated under stress conditions were associated with SM production pathways
indicating the profound role played by these SM in protecting the plants from
harmful effects. The gene expression pattern from such studies will give crucial
information for the genetic breeders to develop resistant cultivars of important
medicinal plants.
Sunlight contains a spectrum of electromagnetic radiation differing in wavelength
and energy. They are classified into different types like gamma rays, X-rays, UV,
visible rays, IR, microwaves, and radio waves. Each of these is known to exhibit
other properties. The ozone layer in the stratosphere absorbs most of the damaging
rays, incl uding UV light, by limiting its penetration of the earth’s surface
(Frohnmeyer and Staiger
2003). However, due to swift urbanization, industrializa-
tion, and anthropogenic activities, harmful chemicals like chlorofluorocarbon are
released continuously. The ozone layer is under constant threat because of such
disruptive chemicals resulting in increased amounts of harmful UV light penetrating
to earth’s surface in different parts (Inostroza-Blancheteau et al.
2016). A small
section of the spectrum from 290 to 315 nm is termed UV-B rays and is known to
cause more damage (Neugart and Schreiner
2018). UV-B light is reported to alter the
physiological health of plants by damaging photosynthetic apparatus, disrupting
thylakoid membrane integrity, limiting oxygen evolution and carbon fixation, and
many more functions (Swarna et al. 2012; Rojas-Lillo et al. 2013). Plants, including
medicinal herbs, are evolving mechanisms to bear UV-B-induced stress by adjusting
their genetic and biochemical response strategies. They are known to induce
increased oxidative stress by producing increased amounts of free radicals (Nawkar
et al.
2013). Reactive species, if not neutralized or rendered inactive, are reported to
cause increased lipid peroxidation, a decrease in lipid-soluble antioxidants, and
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