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11 Signaling Molecules in Medicinal Plants Response to Cold Stress 171
Fig. 11.1 Catastrophic effects of cold stress on plants leading to termination of growth and
development
Under cold stress, the relative proportion of saturated fatty acids escalates the
unsaturated fatty acids corresponding to the membrane transformation from a semi-
fluid to a semi-crystalline state (Yadav
2010; Solanke and Sharma 2008). Frost
build-up in plant tissues predominantly in the apoplasts establishes a vapor pressure
gradient due to which the water from the cytosol moves down the gradient to the apo
plastic space. Also, stomata closure and debilitated uptake of water by the roots
(Solanke and Sharma
2008) are paramount to severe dehydration and puncture of
cell membranes due to lamellar-to hexagonal-II phase transition, and/or fracture
jump lesion, expansion-induced lysis limiting water availability for photosynthesis.
Although dehydration is a key for membrane damage, freeze-driven release and
buildup of ROS accelerate the impairment (Thomashow 1999). Enhanced perme-
ability, solute leakage, disrupted ion balance, an increased expression followed by
activation of proteins and energy-bound enzymes, and modification of the ultrastruc-
ture of plastids, thylakoids, mitochondria, and a few others are further ameliorating
effects on the integrity of the cell membrane (Fig. 11.1) (Theocharis et al. 2012).
Low-temperature stress also prompts the abortion of flowers as a result of
functional abnormalities of reproductive organs, sterile pollen, shrinkage of the
style and ovaries, damage to embryo sac, callose deposition in style, lessened
receptivity of the stigma, etc. thereby slackening fertilization, grain filling and
grain yield (Thakur et al.
2010). The cold injury also results in reactive oxygen
species (ROS) generation which imbalances the electron transfer reactions and their
associated biochemical pathways primarily the PS-II reaction center ensuing in plant
death (Solanke and Sharma
2008). Another principal effect of cold stress on plants is
the undulation of phytohormones and their associated functions. Auxins,
gibberellins (GA), and cytokinins display a significant decrease due to low
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temperatures leading to a ceased flow of photo-assimilates from source-to-sink,
incomplete filling of kernels, seed abortion, delay or inhibition of inflorescence
development, fruit drop, stunted development of embryo and endosperm, arrested
cell division, degradation of complex proteins into simple forms (soluble), leaf
necrosis, premature leaf senescence and others (Thakur et al.
2010; Shi et al.
2015; Tian et al. 2022; Tiwari et al. 2020).
172 G. K. Dugganaboyana et al.
Though
low
temperat
u
re effectuates constant distress on plants, the intriguing
tailoring response in plants is quite worthy of note. Plants derive either an unremit-
ting evolutionary, phonological, morphological remodeling or transitory avoidance
or acclimation responses to cold stress (Ruelland et al.
2009; Terletskaya et al.
2021). Several observable physiological, morphological, biochemical, and molecu-
lar adaptations triggered by the alterations in gene expressions collectively induce
cold/freeze tolerance in plants (Theocharis et al. 2012) . Translation of certain
proteins (anti-freeze proteins, heat shock proteins, dehydrins, cold regulated
proteins), uptake and storage of several sugars (trehalose, Saccharose, stachyose,
raffinose), sugar alcohols (inositol, ribitol, sorbitol), increased synthesis of second-
ary metabolites (sesquiterpenes, phenols, lactones, and/or alkaloids), the
unsaturation of cell membrane, upregulation of several antioxidant enzymes (Glu-
cose oxidase, Catalase, peroxidase glutathione reductase, superoxide dismutase and
others), stomatal closure and water retention, prevention of ice nucleation by
osmotic regulation and uptake of small nitrogenous compounds like glycine betaine
(GB), and proline, increased biosynthesis of phytohormones and their associated
compounds include ethylene, abscisic acid (ABA), melatonin, jasmonates,
brassinosteroids, salicylic acid (SA), and various other compounds. Respectively,
micro and macronutrients, escalated gene expressions leading to altered
transcriptomics, proteomics and metabolomics (Theocharis et al. 2012; Thakur
et al. 2010; Shi et al. 2015; Tiwari et al. 2020). These complex multigenic processes
require a tailored genetic capacity conducive to activating the right mechanisms
necessary for cold acclimation (Fig. 11.2).
Expanding populations, climate change (biotic and abiotic stress), chemical
exposure, pollution, compromised sanitation, and hygiene have collectively
increased the mortality and morbidity estimates worldwide. Several
non-communicable diseases and communicable diseases contribute to nearly 74%
and 26% of Global mortality rates respectively (WHO report, dated 16th September
2022). The hectic lifestyle of individuals drives them to take up convenient allo-
pathic therapies and drugs which have collaborative benefits like ready availability,
cost, instant relief and symptomatic relief, effortless drug administration methods.
Meanwhile, traditional Herbal therapies are derogated as their manuscripts have
become relics of a bygone era and have relatively low applicability in the existing
scenario (Kala et al.
2006). However, in recent times faith in herbal medicines is
once again escalating as it aims at uprooting the ailment from its roots with a holistic,
natural, and safe approach along with marginal side effects. It is therefore a need of
the day to increase the cultivation of herbal/medicinal plants along with the periodi-
cally grown commercial crops such that it is accessible to the expanding population
(Kanso et al.
2022). Secondary metabolites released during various biochemical
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pathways mentioned in the Kyoto Encyclopedia of Genes and Genomes (KEGG)
pathway database are of prime importance and have an important role in increasing
the pharmacological benefits of herbal plants, such as antimalarial, antidiabetic,
hepatoprotective, anti-ulcerative, anti-inflammatory, and antimicrobial activities
(Sharma 2018
). These pathways are consequentially influenced by environmental
stress. Therefore, it is a prerequisite to understand various inherent tolerance
mechanisms and novel approaches adopted by medicinal plants to combat cold
injury, attain acclimation, minimize the risk of their extinction, and ensure their
quality and safety. With this chapter we address the underlying mechanisms and
signaling molecules involved in various plants to overcome the repercussions of
p
rimary cold stress injury, oxidative stress, and osmotic secondary stresses.
11 Signaling Molecules in Medicinal Plants Response to Cold Stress 173
Fig. 11.2 Signaling molecules and strategies adopted by medicinal plants to combat cold stress
and achieve cold acclimation
11.2 Cold Acclimation Mechanisms Acquired by Medicinal
Plants
11.2.1 Biotic and Abiotic Elicitors: Rudiments Guiding a Catena
of Events during the Cold Stress
In nature, corresponding to the freezing and chilling temperatures, primarily the
plants liberate several biotic and abiotic elicitors that regulate the expression of
several freeze tolerance genes and anti-freeze proteins via a chain of signal transduc-
tion mechanisms followed by transcriptional activation of the corresponding fight or
flight responsive genes (Sharma
2018). The biotic and abiotic elicitors, such as
osmolytes, amino acids, photosynthetic pigments, molecular chaperones, anti-freeze
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proteins, cytosolic calcium, ROS, phytohormones, etc. are collectively contemplated
to be the primary signaling molecules in plants that trigger a concatenation of
adaptive responses against cold stress (Ghassemi et al. 2021; Solanke and Sharma
2008; Theocharis et al. 2012; Sharma 2018).
174 G. K. Dugganaboyana et al.
11.2.1.1 Osmolytes/Osmoprotectants: Combatants of Cell Wall Damage
A knee-jerk response of plants towards low temperatures is an increase in the degree
of fatty acid unsaturation, and a decrease in PC, PE, and PG constituents leading to
the loss of membrane elasticity (Solanke and Sharma
2008; Welti et al. 2002; Wang
et al. 2006). This triggers the accumulation of Osmolytes, low-molecular weight
compatible polar solutes, instantaneously protect the membrane disruption due to
dehydration and membrane crystallization in response to cold injury. Most signifi-
cant among them are polyols (like mannitol,
D-ononitol, galactinol, and sorbitol),
oligosaccharides, and soluble sugars (like trehalose, raffinose, glucosides, fructans,
and arabinoxylans), nitrogenous solutes like proline, polyamines (like putrescine,
spermine, and spermidine), betaines (GB), etc. that distinctively play a vital role in
cold acclimation via osmotic regulation, cell compatibly and membrane homeostasis
(Pollock and Lloyd 1987; Penna 2003; Hekneby et al. 2006). Upregulation in the
expression of native endogenous or ectopic genes responsible for the biogenesis of
antecedent osmolytes has been identified in various plants during LT stresses. These
genes could be engineered in transgenic plants so that they can be groomed to
improve their cold tolerance.
Sugars
As cryoprotectants sugars accumulate in excess in the apoplastic space and prevent
the loss of water from the cells thereby increasing the turgidity and reducing the
flaccidity. Since, water is unavailable the heterogenous ice-nucleation or crystal
formation in the apoplastic space could be intercepted (Theocharis et al.
2012). In
addition, in response to cold stress starch produced during photosynthesis
diminishes to free sugars as a result of incessant hydrolysis. Raffinose, Stachyose,
Trehalose, and Sucrose (Pollock and Lloyd
1987; Bohnert and Sheveleva
1998)
accumulate in leaves and promote the increment of starch by regulating post-
translational modifications and activating vital enzymes, such as sucrose phosphate
synthase, fructose-1,6-bisphosphatase and sucrose synthase in Calvin cycle thereby
counteracting the effects of stress on photosynthesis (Fernandez et al. 2010).
Echinocactus grusonii, Euphorbia resinifera, Crassula lacteal, Yucca aloifolia,
Bryophyllum pinnatum, Aloe vera, Sansevieria trifasciata, and a few others are
some of the classically reported examples of herbal plants accumulating sugars, such
as
D-Glucose, D-Glucose-6 Phosphate, starch, amylose, maltose, and their biogenetic
enzymes to stabilize the cell membrane from excessive dehydration and promote
natural photosynthesis mechanisms. Under experimental conditions, these plants
have shown several folds of increase in sugar levels when compared to their controls
indicating that sugar accumulation would drive the plants a step ahead during cold
tolerance (Sasaki et al.
1996; Khan et al.
2015; Latif et al. 2019). Another study on
Menthe piperita and Catharanthus roseus have proved that the sugar accumulating
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under stress serves as a signaling molecule and protect different parts of the plants
from osmotic shock, and mitigate the oxidative damage induced by reactive oxygen
species (Alhaithloul et al. 2019).
11 Signaling Molecules in Medicinal Plants Response to Cold Stress 175
Proline
The dramatic accumulation of
Proline, is another compatible osmoticum facilitates
the plants to develop tolerance to temperature and drought stresses. Upregulation of
the genes encoding enzymes like γ-glutamyl kinase, glutamic- γ- semialdehyde
dehydrogenase, P5C reductase (P5CR), proline methyltransf erase, pyrroline
5-carboxylate synthetase (P5CS) and proline permease ensures uncurbed biogenesis
and restrained degradation of proline under stress conditions (Taylor
1996; Kishor
et al. 2005). This immediate accession of proline has conglomerative effects like a
decrease in cytosolic pH, stabilization of the membrane protein degradation,
downregulation of lipid peroxidation, upregulation of stress-responsive genes and
radical scavenging enzymes, increased photochemical reactions in the PSII and
protection against photoinhibition by stabilizing the activity of Rubisco (Verbruggen
and Hermans 2008; Szabados and Savouré 2010). Experimental analysis of Menthe
piperita, Yuca aloifolia and Catharanthus roseus leaves demonstrate a 32.8%,
34.13%, and 42.34% increase in proline content respectively. In addition to a
manifold of functions, proline also regulates respiration in plants by increasing the
NADP+/NAD+ ratio. Increased NADP+ consecutively regulates the PPP pathway
resulting in the release of both NADPH and pentose sugars for the biogenesis of
nucleic acids (Alhaithloul et al. 2019).
Glycine Betaine
Calendula officinalis is one of the prospective herbal plants with numerous
ethnopharmacological applications in both the Ayurveda and Unani system of
medicine. The concoctions extracted from various plant parts are prominently used
as antipyretic, anti-inflammatory, anti-fungal, and anti-tumor drugs. Under cold
stress, the seeds, leaves, and roots of C. officinalis accumulate GB as an important
osmolyte to protect and thrive the cold-induced injury. Experimental proofs
suggested that the levels of GB progressively augment starting from 24 h
(40.81%), 48 h (75.88%), and 72 h (115.15%) respectively (Arora et al.
2013; Jan
et al. 2018 ). GB plays a vital role in stabilizing the post-transcriptional and post-
translational mechanisms and activates peroxide-mediated signaling pathways
ensuring to curb of both osmotic and oxidative stresses. Several therapeutically
significant transgenic plants are often deprived of the natural tolerance mechanisms
adopted by the wild varieties against environmental stresses. That being the case,
their genetic makeup is most commonly engineered with genes encoding GB, which
is considered a potential osmolyte to confront LT stresses (Theocharis et al. 2012).
Polyamines
Polyamines are aliphatic nitrogenous bio stimulants produced during different
metabolic pathways in every tissue of the plant. Endogenously synthesized or
exogenously applied polyamines are know n to enhance the development and growth
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of plants especially, under environmental stresses (Chen et al. 2019). Their role
during cold tolerance is remarkable as they subside the oxidative damages of ROS
by reducing the Malondialdehyde (MDA) biosynthesis, enhancing the bioavailabil-
ity of photosynthetic pigments and enzymes, such as magnesium and iron-chelatase,
protoporphyrinogen IX oxidase and so on (Aarti et al.
2006) and thereby promote
cold acclimation.
176 G. K. Dugganaboyana et al.
Stevia
rebaudi
ana
B
er
toni (Aka. Candy leaf) is one of the crucial herbal plants
renowned for its analeptic properties pertaining to hypertension, fungal infections,
acute and chronic inflammations, cancer, lipidemia, diabetes, and others.
Dulcoside A, Rebaudioside B, C, D, E, and F, rubusoside, steviolbioside in tandem
referred to as Steviol glycosides are prime SMs that impart the pharmacological
benefits (Carrera-Lanestosa et al.
2017). Kamran Moradi Peynevandi et al. reported
that polyamine biosynthesis riveted cold tolerance/acclimation in Stevia rebaudiana
Bertoni (Moradi Peynevandi et al.
2018). This bioactive molecule accumulates in
response to the effects of cold injury, such as the generation of ROS, paralyzed
photosynthetic machinery, intracellular metabolic downturn, and others. It is well
understood that the biogenesis of polyamines primarily surges the synthesis and
accumulation of cryoprotectants like sugars, GB, proline, amino acids, Secondary
Metabolites, and others as well as, and they act as nitrogen and carbon sources for
the biosynthesis of the latter (Alcázar et al. 2011). Further, polyamines upregulate
the genes encoding the enzyme desaturase and catalase, where the former is condu-
cive to converting saturated to unsaturated fatty acids promote membrane fluidity
and intercepting membrane crystallization while the latter combat the accumulated
peroxides and defending the plant from oxidative stress. Aggregation of LHC II is
promoted by polyamines enabling photosynthetic stability during cold stress
(Tsiavos et al. 2012). At last, polyamines also activate proteases in due to cold stress
and promote the degradation of complex proteins to amino acids and simple soluble
peptides to combat the osmolytic stress conditions.
11.2.1.2 Stress Incited Generation of ROS and Activation
of Anti-Oxidant Systems: Consorted Damage and Repair
Mechanisms
Plants perceive cold stress via numerous cell membrane receptors which in succes-
sion modulate the expression of several tolerance genes to achieve cold acclimation.
In addition to the osmotic stress the plants also experience immediate oxidative
stress in retaliation to cold injury. It is characterized by the surplus generation of
Reactive Oxygen Species (ROS), including singlet oxygen (
1
O
2
), superoxide anion
radicals (O
2
-
), malondialdehyde (MDA), hydroxyl ions (HO), and hydrogen
peroxide (H
2
O
2
), as a result of interrupted respiration (over-reduction of the respira-
tory ETC cycle) and photosynthesis (inhibition of Calvin-Benson cycle)
mechanisms. The assembled ROS fails to metabolize and further acts as signaling
molecules that exhibit dual roles during acclimation. Undoubtedly, they first target
the DNA and damage the proteins and lipids by uncontrolled oxidation and peroxi-
dation within the cells respectively. Also, they are known to upregulate the genes
encoding both non-enz ymatic and enzymatic radical scavenging machinery in the
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plant organelles like chloroplast, mitochondria, and peroxisomes (Neill et al. 2002;
Gechev et al. 2003; Suzuki et al. 2012). Therefore, the latter provides the plant with a
natural defense mechanism to protect itself from necrosis caused due to the former
secondary oxidative stress and maintain cellular redox homeostasis.
11 Signaling Molecules in Medicinal Plants Response to Cold Stress 177
Free radical scaveng
ing enzymes include NADPH oxidase, superoxide dismutase
(SOD), oxalate and amine oxidase, reduced glutathione (GPX), ascorbate (APX)
peroxidases, glutathione S-transferase (GST), glutathione reductase, catalase,
monodehydroascorbate reductase (MDHAR), phenylalanine ammonia-lyase
(PAL), dehydroascorbate reductase (DHAR), and few others (Gill and Tuteja
2010; Mir et al. 2015; Abid et al. 2019). On the other hand, The non-enzymatic
mechanisms activated by ROS include the augmented expression of senescence-
related proteins, stress-responsive proteins, protein kinases, escalated Cytosolic
calcium influx and associated downstream signal transduction pathways, DNA
damage repair proteins, associated interaction of ABA and H
2
O
2
in stomatal closure,
elicitors like hormones, osmolytes, tocopherols, carotenoids, secondary metabolites
like flavonoids, phenolics and others (McAinsh et al.
1996). The collegial effect of
both enzymatic and non-enzymatic radical scavenging machinery plays a predomi-
nant role in detoxifying the pooled ROS and altogether protects the plant from cold-
induced injury and promotes its acclimation to cold stress (Chen and Raji
2020).
One of the classical examples delineating the role of antioxidant systems during
cold stress is better understood in Withania somnifera L. Dunal (aka Ashwagandha,
Indian Ginseng, and Winter Cherry) from Solanaceae family, the plant is widely
known for its classical therapeutic/curative roles in neurodegeneration, cancer
therapies, worm infections, CVDs, diabetes, and others (Sangwan et al. 2004;
Mukherjee et al. 2021). Withanolides, secondary metabolites released in the roots
and leaves act as an important catalyst in exhibiting pharmacological effects (Mir
et al.
2015). However, previous studies have proved that LT stress-induced ROS and
antioxidant mechanisms play an important role in the biogenesis and accumulation
of Withanolides and that their concentrations vary in diverse species and contrasting
genotypes (Kumar et al. 2012). Lipid peroxidation and a significant increase in the
superoxide ions is the immediate response engineered within Ashwagandha during
cold stress. Bilal Ahmad Mir et. al documented that, the two genotypes AGB002
and AGB025 have shown increase in the superoxide ions starting from first day till
seventh day with 102% and 140% respectively. Much effective coordinated antioxi-
dant machinery involving CAT and SOD in the former genotype is presumed to
convert superoxide and peroxide ions to water and oxygen much more readily
compared to the latter. These antioxidant enzymes reduce lipid peroxidation and
improve cell viability. Further glutathione reductase maintains the GSH/GSSG ratio
that facilitates errorless protein folding and upregulation of defense genes that
protects the roots and leaves from necrosis and achieve cold acclimation.
Ocimum sanctum and Ocimum basilicum, two prerogative species of the genus
Ocimum are well known for their SMs terpenoids and phenylpropanoids including
linalool, 1,8 cineole, methyl chavicol (estragole), and eugenol (Poonkodi 2016). The
essential oil extracts of these plants containing the vital SMs have proven to have an
ameliorating effect on laryngeal cancer, type-2-diabetes and co-morbid active
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tuberculosis, cough, headache, diarrhea, warts, constipation, worms, and kidney
malfunctions (El-Beshbishy and Bahashwan 2012; Joshi 2014). Methyl eugenol
and methyl chavicol are prominent phenylpropanoids, produced as by-products of
the shikimate pathway. Phenylalanine ammonia-lyase (PAL), Cinnamate-4-hydrox-
ylase (C4H), eugenol synthase 1 (EGS1), eugenol O-methyl transferase (EOMT),
and chavicol O-methyl transferase (CVOMT) are the key enzymes that play critical
roles in the genesis of these phenylpropanoids. Expression of genes encoding these
enzymes is highly affected by cold stress due
to the release of abundant ROS and
successive DNA damage. A report by Ramin Rezaie et al. shown a significant
increase in the levels of superoxide radicals in the two species and so the activity
of SOD has greatly been upregulated (Rezaie et al.
2020). Severe cold stress/freeze
injury downregulates the antioxidant enzymes like APX, CAT, and PAL leading to
oxidative damage. Whereas, it has been observed that, the non-freezing low
temperatures trigger the cold-responsive genes present in the promotors of EGS1,
EOMT, and C4H resulting in the upregulation of their expression. Therefore, PAL,
C4H, EGS,1, and EOMT collectively promote the release of abundant Methyl
chavicol and Methyl eugenol subduing the cold stre
ss and enhancing the availability
of pharmaceutically important SMs.
178 G. K. Dugganaboyana et al.
11.2.1.3 Intracellular Calcium Responses: Key Mediators
of Cross-Tolerance
Calcium, a secondary messenger plays an important role in sensing environmental
stress and transducing the stress signals intracellularly. Intracellular calcium influx is
therefore considered as a stimulus to cold stress that elicits various downstream
signal transduction pathways leading to numerous physiological, metabolomic, and
molecular alterations in response to cold injury (Xu et al. 2022). Cytoskeletal
reorganization and disruption of membrane integrity as a result of oxidative stress
are key stimuli that drive the activation of intracellular Calciu m permeable channels.
Phosphatidylinositol 4,5-bisphosphate (PIP2) mediates the incre ase in intracellular
calcium influx from the sub-cellular/organellar calcium reserves, such as extracellu-
lar compartments, vacuole, mitochondria, and others (Ahmad et al.
2010). The
influxes of calcium within the cell are sensed by the C-terminal calmodulin-like
domain within Calcium-dependent protein Kinases (CDPKs/CPKs),
CBL-interacting protein kinases (CIPKs), calcineurin B-like proteins (CBLs), Pro-
tein phosphatases, and MAPKs including AtPK6, AtPK19, AtMPK3, and
AtMPKK1 leading to the phosphorylation of several stress-responsive proteins,
such as late embryogenesis abundant proteins (LEA), antifreeze proteins, cold
shock proteins, and others (Mahajan and Tuteja
2005).
Late Embryogenesis Abundant Proteins
Late Embryogenesis Abundant proteins have been classified into 8 subgroups
(LEA1, LEA2, LEA3, LEA4, LEA5, LEA6, dehydrin, and SMP), are small, hydro-
philic, heat-stable proteins that accumulate in the leaves in response to the upsurged
intracellular calcium, antioxidant mechanisms, and osmolytes as a consequence of
cold injury and desiccation (Garay-Arroyo et al.
2000; Zeng et al. 2018). It is
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intended to stabilize membrane integrity, prevent protein denaturation, promote
renaturation of unfolded and partially folded proteins, sequester metal ion binding,
and fortify the structural integrity of macromolecules including enzymes, nucleic
acids, lipids, and others during cold stress (Zeng et al.
2018; Yamaguchi-Shinozaki
and Shinozaki 1993; Campos Alvarez et al. 2002).
11 Signaling Molecules in Medicinal Plants Response to Cold Stress 179
Gastrodia elata (Aka. Tianm
a) is a
saprophytic perennial herb of the
Orchidaceae family well known for its usage in traditional Chinese Pharmacopoeia
for the treatment of headache, hypertension, dizziness, epilepsy and neurodegenera-
tive diseases mainly Parkinson’s Disease and Alzheimer’s disease (Manavalan et al.
2012). Phytochemical analysis of G. elata showed that under low temperatures, the
plant produce s all 8 subtypes of LEA proteins referred to as GeLEAs in various
subcellular compartments like cytosol, chloroplast, mitochondria, nucleus, and few
others, protects the cells from both primary and secondary stress injuries preceding
cold acclimation.
Another classical example of the release and accumulation of LEA proteins can
be deduced by frost tolerance developed in the genus Triticeae. Triticum aestivum
Linn. (Aka Wheatgrass), is a critical deep-rooted traditional herb belonging to this
genus used in the medicinal systems of both humans and animals. It is also known as
the powerhouse of micro and macromolecules, the antioxidants, enzymes, and
assemblage of SMs like terpenoids, phenolics, lignins, tannins, flavonoids, quinines,
alkaloids, and others engender a clique of diseases including anemia, lung and
CVDs, diabetes, anti-carcinogenic therapies, allergic responses, dermatosis,
nephrolithiasis/renal colics, and other conditions (Kosová et al. 2021; Asif Khan
et al. 2022). Amongst various subtypes of LEA released by the grass under stress
conditions, LEA-II/dehydrins cold regulated (COR) protein plays a significant role
in T. aestivum to overcome cold-induced dehydration with its distinctive zinc-finger
motifs. In response to cold stress, these intrinsically unstructured proteins (IUPs)
reach up to 1–2% of the total protein concentration and bind to copious water
molecules and promoting the formation of the protein secondary structures thereby
and achieve cryoprotection. Also, upregulated expressions of the genes WCOR14a/
b, and WCOR15 are known to release LEA-III chloroplast targeted proteins that play
an crucial role in preventing ice crystal formation, scavenging activity to overcome
the oxidative stress, boosted enzymatic activities, etc. all of which facilitates cold
acclimation/tolerance in plants (Kosová et al. 2021).
Antifreeze Proteins
Antifreeze proteins infer to the glycoproteins (AFGPs) or peptides (AFPs) capable of
protecting the plants from sub-zero temperatures. It is a key marker found in
psychrophiles and other temperate plants to stabilize the ice crystal formation and
inhibit their reformation during extremely low temperatures. A phenomenon called
thermal hysteresis (TH) is adopted by the AFGPs/AFPs where their ice-binding
domains adsorb irreversibly to the generated ice flakes and inhibit further crystalli-
zation (Eskandari et al. 2020). Hippophae rhamnoides L. (Aka. Sea buckthorn), the
Himalayan Wonder plant is a classic example of the release and cryoprotective
functions of AFPs. Hippophae rhamnoides L. are widely used in traditional
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Ayurvedic medicine for stress management, anti-atherogenic, anti-microbial, liver
dysfunctions, and other conditions (Suryakumar and Gupta 2011). Chitinase and
thaumatin-like proteins are putative antifreeze proteins found in the LT-induced
secretome analysis of H. rhamnoides. They acquire the anti-freeze property during
low temperatures and act as cryoprotectants against frost injury with a 60-fold
effective inhibition of ice crystallization and ice recrystallization inhibition (IRI)
and also serve to combat severe dehydration (Gupta and Deswal
2012)
.
180 G. K. Dugganaboyana et al.
Cold Shock Domain Proteins
Both freezing
and chilling temperatures primarily trigger the activation and expres-
sion of several Cold Responsive genes (COR) at the molecular level. The secondary
oxidative, osmotic, hormonal, and calcium-dependent signal transduction pathways
additionally prompt the upregulated expression of COR. C-repeat binding factors
(CBR) are the transcriptional ambassadors of COR with highly conserved cold-
signaling pathways that possess an indispensable role in the accretion of cold
tolerance (Thomashow
1999). One of the prominently expressed proteins by COR
is cold shock domain (CSD) proteins, rich in glycine and interspersed CCHC Zinc
finger motifs. These proteins are widely characterized into four subtypes namely,
Early Dehydration-Inducible (ERD), Low temperature induced (LTI), Cold-
inducible (KIN), and Responsive to Desiccation (RD), each of which is plant
species-specific and released at different stages of cold injury. CSDs of Pinus
tabuliformis, Pinus massoniana, and Pinus sylvestris, traditional Chinese herbs
employed mainly for anti-aging and anti-telomerase activity are RNA-chaperones
that prevent the secondary RNA structure formation and retaliate the degradation of
RNA by regulating various transcription and translation activities of vital proteins
involved in metabolism, cell structure, photosynthesis, and transcription (Barria
et al.
2013).
11.2.1.4 Dynamic Transcriptional Cascades and Alterations in Gene
Expressions Are a Cue for Cold Acclimation
CBFs /DREBs (cold-binding transcription factors/dehydration responsive element
binding factors) is the most sustained cold regulating mechanism supplemented with
activators like ICE1, CAMTA3, and BZR1/BES1, and successive repressors, such as
PIFs, MYB15, and EIN3 as its major components. The CBF/DREB-dependent
downstream transcriptional signaling is distinct for different plant species and
triggers the release of several cryoprotective agents that facilitates the plants to
develop resistance to low temperature. On the other hand, phytohormones, such as
auxin, ABA, ethylene (ET), GA, jasmonic acid (JA), cytokinins (CK),
brassinosteroids (BR), and SA display CBF-independent signaling pathways to
upregulate the COR genes due to cold stress (Ke et al. 2020).
11.2.1.5 Phytohormones: Chemical Messengers Transmitting Cold
Signals from Source to Sink
Hormones are signaling molecules that amalgamate the external stress stimuli with
the internal activation of the congenital traits to fight cold stress. Synergistic or
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