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Chapter 6 Impact of salinity stress on medicinal and aromatic plant biotechnology 243
cal and biochemical processes, boosting antioxidant defenses, improving water relations, and maintaining ionic equilibrium under saline conditions [123].
Genetic approaches, such as identifying and overexpressing salt-tolerance genes
and utilizing advanced gene editing techniques, provide long-term solutions for developing MAP varieties with enhanced salt tolerance. These methods not only improve
plant resilience but also increase the production of valuable secondary metabolites
[124]. Practical agronomic methods, including efficient irrigation management, mulching, and organic amendments, offer easily implementable solutions to manage soil salinity in MAP cultivation [125]. These practices contribute to improved growth and
sustainable soil management.
The emerging field of nano-fertilizers and nanoparticles offers promising outcomes in enhancing nutrient efficiency and plant stress tolerance. However, further
studies are required to fully understand their long-term implications and optimize
their use across different MAP species [126, 127].
As soil salinity continues to pose challenges, integrating these diverse strategies
will be vital for the sustainable cultivation of MAPs [128]. Future research should aim
to explore the combined effects of different approaches, tailoring holistic solutions to
specific plant species and growing conditions. Moreover, it is essential to examine the
impact of these salt tolerance strategies on the quality and quantity of medicinal compounds, ensuring no compromise on their therapeutic value [129, 130]. By adopting
these strategies, it is possible to enhance the growth, yield, and medicinal properties
of MAPs under saline conditions, addressing environmental challenges sustainably
[131, 132].
6.9 Physiological and biochemical responses
to salinity stress
Medicinal plants utilize various physiological and biochemical mechanisms to adapt
to and endure salinity stress. This section focuses on the primary adaptive strategies
these plants employ to thrive in saline conditions [133].
6.9.1 Water relations and osmotic adjustment
Salinity stress profoundly affects plant water relations by lowering the osmotic potential of the soil, thereby restricting water uptake and creating conditions akin to
drought, even when soil water is available. To overcome this challenge, medicinal
plants deploy adaptive mechanisms aimed at maintaining water balance [134]. One
critical strategy involves the accumulation of compatible solutes, such as proline, glycine betaine, and soluble sugars. These molecules act as osmoprotectants, helping to
244 Fatemeh Ahmadi
regulate osmotic potential within cells and sustain cellular functions under stress. For
instance, in Moringa oleifera, exposure to salinity stress resulted in a significant increase in proline and soluble sugar levels. This accumulation was closely associated
with a reduction in osmotic potential in both roots and leaves, highlighting their role
in osmoregulation and the plant’s ability to adapt to saline conditions [135]. These
mechanisms not only enable plants to maintain water uptake but also support overall
stress resilience, ensuring survival and productivity in saline environments. Relative
water content (RWC) typically declines under salinity stress. In a study involving five
Coleus species, RWC was observed to decrease significantly with rising salinity levels.
For instance, in Coleus aromaticus, RWC dropped from 103% to 77.6% under high salinity. Similarly, leaf water potential (LWP) also declined. The same study reported a
reduction in LWP from −0.02 MPa to −1.2 MPa in Coleus aromaticus and from
−0.05 MPa to −2.14 MPa in Coleus forskohlii under high salinity conditions [136].
6.9.2 Ion homeostasis and nutrient balance
Salinity stress disrupts ion balance in medicinal plants, leading to ionic toxicity and
nutrient deficiencies that hinder growth and development [137]. This stress typically
results in excessive accumulation of sodium (Na
interfere with cellular functions and exacerbate nutrient imbalances. For example, in
Moringa oleifera, salinity causes a significant rise in Na
ing potassium (K
+
) concentrations [138]. A balanced K+/Na+ ratio is crucial for salt tolerance, as it supports essential enzymatic functions; however, salinity often lowers
this ratio due to increased Na
+
Na
disrupt the uptake of other vital nutrients like K+, calcium (Ca2+), and magnesium
2+
(Mg
) through competitive interactions, leading to further nutrient shortages and
+
uptake and limited K+ absorption. The high levels of
metabolic disturbances that impair growth [139]. To counter these effects, maintaining
ion homeostasis is essential. Strategies such as selective ion transport, compartmental-
+
izing Na
into vacuoles, and enhancing the uptake of K+ are vital for improving salt
tolerance in medicinal plants. These mechanisms highlight the importance of managing salinity stress for sustainable cultivation of medicinal plants in saline environments.
+
) and chloride (Cl–) ions, which can
+
and Cl– levels while decreas-
6.10 Molecular mechanisms of salt tolerance
Salinity represents a major environmental challenge, severely impacting plant growth,
physiological processes, and overall productivity [140]. To cope with salinity stress, plants
have evolved intricate molecular mechanisms that enable them to maintain ion homeostasis, regulate osmotic balance, and mitigate oxidative damage. These adaptations in-
Figure 6.2: Effect of soil salinity on the ion homeostasis and nutrient balance.
Chapter 6 Impact of salinity stress on medicinal and aromatic plant biotechnology 245
clude the activation of ion transport systems to control Na+ and K+ distribution, the accumulation of osmoprotectants such as proline and glycine betaine to adjust cellular osmotic potential, and the enhancement of antioxidant defenses to counteract ROS. Together, these mechanisms allow plants to survive and adapt to high-salt environments,
ensuring their functionality under adverse conditions [141].
A key mechanism for salt tolerance is the Salt Overly Sensitive (SOS) pathway,
which is activated when plants detect high salinity. Calcium sensors such as SOS3 and
SCaBP8 recognize salt stress and activate SOS2, a protein kinase. This, in turn, phosphorylates SOS1, a Na
+/H+
antiporter, which facilitates the removal of excess Na+ from
cells [142]. SOS1, composed of an N-terminal transmembrane region and a cytosolic
regulatory C-terminal domain, is activated through phosphorylation by SOS2. This
prevents the interaction of SOS1 with inhibitory proteins like 14-3-3, thereby improving the plant’s salt tolerance. Beyond cellular ion regulation, SOS1 also plays a role in
controlling Na
+
transport between roots and shoots, reducing Na+ buildup in leaves
and improving overall salt tolerance [143, 144].
The ability to sustain a proper K
+
/Na+ balance is a critical determinant of salt tolerance, ensuring the stability of cellular processes and overall plant health. A key
player in this balance is the HIGH-AFFINITY K
+
TRANSPORTER 1 (HKT1) family, which
246 Fatemeh Ahmadi
actively minimizes Na+ accumulation in shoots by retrieving Na+ ions from the xylem
sap and redirecting them to the roots or vacuoles. This mechanism helps reduce toxic
+
levels of Na
in above-ground tissues, thereby protecting photosynthetic machinery
and other essential cellular functions. In Arabidopsis thaliana, mutations in the
AtHKT1 gene have demonstrated enhanced salt tolerance, particularly in plants deficient in components of the SOS pathway. This indicates a synergistic interaction between HKT1-mediated Na
+
regulation and the SOS pathway, which is involved in Na
extrusion and homeostasis. Together, these systems coordinate to maintain ion balance, ensuring sufficient K
+
levels while preventing excessive Na+ accumulation. Such
findings underscore the complexity and importance of integrated molecular mechanisms in salt tolerance [145, 146].
The role of HKT1 transporters extends beyond basic ion regulation, as they represent a potential target for genetic and biotechnological approaches aimed at improving salt tolerance in MAPs. By enhancing the efficiency of HKT1 transporters or their
interaction with pathways like SOS, it may be possible to develop MAPs capable of
thriving in saline environments, while maintaining optimal growth and secondary
metabolite production.
Protein kinases and phosphatases are vital regulators of ion transport and salt
stress adaptation. For example, the GSK3-like kinase BIN2 inhibits SOS2 activity
through phosphorylation, acting as a negative regulator of salt tolerance. During
post-stress recovery, SOS3 and SCaBP8 facilitate BIN2’s movement to the membrane,
where it represses SOS2 activity by phosphorylating it at a specific residue (T172).
This process enables growth-related signaling proteins like BES1 and BZR1 to promote recovery, showcasing BIN2’s role in balancing stress tolerance and growth recovery [147, 148].
Calcium signaling is central to plant responses to salinity, involving various calcium-dependent proteins and transporters that mediate stress signaling. For example,
AtANN1, a calcium-binding membrane protein, regulates Na
root cells. Plants lacking AtANN1 exhibit excessive Na
+
+
influx and K+ efflux in
uptake and K+ loss, leading to
compromised root growth under salinity stress. Similarly, AtANN4, another calciumbinding protein and putative transporter, plays a crucial role in maintaining cytosolic
calcium levels and activating the SOS pathway during salt stress [149, 150].
Osmotic adjustment represents one of the most critical strategies that plants employ to adapt to salinity stress. This adaptation involves the accumulation of compatible solutes, such as proline, glycine betaine, and soluble sugars, which function as osmoprotectants. These solutes play multiple roles: they reduce the osmotic potential
within cells, maintain water uptake, stabilize cellular structures, and protect enzymes
and proteins from denaturation caused by ionic and osmotic stress [151]. The biosynthesis of these solutes is tightly regulated by specific genes and enzymes, such as P5CS
(Δ1-pyrroline-5-carboxylate synthetase) in proline synthesis, which shows heightened
expression under salinity stress. Similarly, BADH (betaine aldehyde dehydrogenase),
+
Chapter 6 Impact of salinity stress on medicinal and aromatic plant biotechnology 247
crucial for glycine betaine synthesis, is upregulated in response to saline environments, enabling plants to better manage osmotic and ionic imbalances [152].
Salinity stress also intensifies the generation of ROS, such as superoxide anions,
hydrogen peroxide, and hydroxyl radicals. These ROS can severely damage cellular
components, including membranes, proteins, and nucleic acids, leading to impaired
growth and metabolic dysfunction [153]. To mitigate this, plants activate a duallayered antioxidant defense system comprising enzymatic and nonenzymatic components.
The enzymatic defense includes superoxide dismutase (SOD), which converts superoxide radicals into hydrogen peroxide, catalase (CAT), which decomposes hydrogen peroxide into water and oxygen, and ascorbate peroxidase (APX), which uses ascorbic acid as a substrate to neutralize hydrogen peroxide. Glutathione peroxidase
(GPX) also contributes to detoxifying peroxides. Under salinity stress, the activity and
gene expression of these enzymes are significantly upregulated, enhancing the plant’s
ability to neutralize ROS and prevent oxidative damage.
Complementing enzymatic antioxidants, nonenzymatic antioxidants such as ascorbic acid, glutathione, flavonoids, and phenolic compounds provide additional protection. These molecules scavenge free radicals and reinforce cellular defenses. The
synthesis and regulation of these antioxidants involve complex transcriptional, posttranscriptional, and signaling pathways, ensuring a rapid and effective response to
oxidative stress [154].
By integrating osmotic adjustment and robust antioxidant defenses, plants demonstrate a multifaceted response to salinity stress. These mechanisms not only ensure
cellular homeostasis but also contribute to overall plant resilience, offering valuable
insights for breeding and biotechnological strategies aimed at enhancing the salt tolerance of crops and medicinal plants. These approaches hold promise for sustainable
agriculture and the cultivation of salt-tolerant plant species in challenging environments.
Plant hormones coordinate various salt stress responses. ABA is a key hormone
that is upregulated under salinity, triggering adaptive responses such as stomatal closure, the induction of stress-responsive genes, and the accumulation of osmolytes
[155]. The ABA signaling pathway involves receptors, protein phosphatases, and transcription factors (TFs) working in a complex network. Other hormones, including jasmonic acid (JA), SA, and ethylene, also contribute to salt tolerance, often interacting
with ABA signaling to fine-tune responses.
TFs are essential for regulating salt-responsive gene expression. TF families such
as AP2/ERF, bZIP, MYB, NAC, and WRKY are particularly important. For example,
DREB (Dehydration-Responsive Element Binding) TFs from the AP2/ERF family regulate genes associated with stress responses. Overexpression of certain DREB genes has
improved salt tolerance in various species. Similarly, NAC family members, such as
ANAC092, are involved in processes like senescence and salt stress adaptation [156,
157]. Posttranscriptional and posttranslational modifications fine-tune plant responses
248 Fatemeh Ahmadi
to salinity. MicroRNAs (miRNAs) regulate gene expression by targeting transcripts related to ion transport, osmolyte production, and antioxidant defenses [158]. At the protein level, modifications like phosphorylation, ubiquitination, and sumoylation adjust
the activity and stability of critical proteins involved in salt stress responses.
Epigenetic mechanisms, including DNA methylation and histone modifications,
play significant roles in salt tolerance. Salt stress often alters methylation patterns,
leading to changes in gene expression. Histone acetylation and methylation also regulate the activity of salt-responsive genes. For instance, histone deacetylase HDA6 has
been shown to modulate stress-related gene expression in Arabidopsis [159, 160].
Membrane lipid composition is dynamically adjusted under salinity to maintain
fluidity and support the function of membrane-bound proteins. Similarly, modifications in the cell wall, such as changes in pectin content and composition, contribute to
maintaining integrity and regulating ion transport under salt stress [161]. Omics technologies have provided new insights into salt tolerance mechanisms. Transcriptomics
has identified salt-responsive genes, proteomics has uncovered protein abundance
and modifications, and metabolomics has highlighted shifts in metabolic pathways
under salinity, including changes in primary and secondary metabolite profiles [162].
Salt tolerance in plants involves a coordinated interplay of physiological, biochemical,
and genetic processes [163]. These include ion homeostasis, osmotic adjustment, antioxidant defenses, hormonal signaling, transcriptional regulation, and cellular adaptations. Understanding these mechanisms is crucial for improving crop resilience and
productivity under salinity stress. Future research should focus on integrating these
mechanisms into cohesive strategies to enhance salt tolerance in plants, ensuring
global food security [164].
6.11 Biotechnological approaches for improving
salt tolerance
Biotechnology offers innovative solutions to address the challenges posed by soil salinization, enabling the development of salt-tolerant crop varieties [165]. These approaches range from advanced genetic engineering techniques to improved plant
breeding methods supported by genomics (Figure 6.3). The goal is to create crops that
maintain productivity under saline conditions, thereby addressing global food security concerns [166]. Plant cell cultures provide a valuable tool for identifying salttolerant traits. Screening large populations of cells under controlled conditions, using
pure salts, salt mixtures, or seawater, has been effective in selecting salt-tolerant lines
[167, 168]. Traits identified in cell cultures are often expressed in regenerated plants
and inherited through subsequent generations, demonstrating the potential of this
technique for developing salt-tolerant crops [169]. Cell culture-based selection offers
the advantage of consistent selection pressure and rapid evaluation of large popula-
Chapter 6 Impact of salinity stress on medicinal and aromatic plant biotechnology 249
tions, which can be challenging in field-based methods [170]. Utilizing wild germplasm
is a valuable strategy for introducing salt tolerance traits into economically important
crop species. Wild relatives often possess resilience traits that have been lost during
domestication and breeding for higher yields [171]. Advanced hybridization techniques, such as embryo rescue and protoplast fusion, enable breeders to overcome
crossing barriers between distantly related species. These techniques allow for the
transfer of genetic diversity from wild species to cultivated crops, introducing novel
salt-tolerance genes. For instance, salt tolerance traits from wild relatives have been
successfully incorporated into wheat and rice, improving their ability to grow in saline soils [172, 173].
Recombinant DNA technology has revolutionized the development of salttolerant crops. This approach involves identifying, cloning, and transferring salttolerance genes into target species. Genomic tools like forward and reverse genetics
in model plants such as Arabidopsis thaliana are often used to identify candidate
genes [174]. Once isolated, these genes can be introduced into crops through transformation techniques like Agrobacterium-mediated transformation or particle bombardment. Despite its success in creating transgenic crops with enhanced salt tolerance,
commercialization often faces challenges such as regulatory hurdles and public skepticism [175].
Improving salt tolerance in plants heavily relies on effective ion transport mechanisms. The SOS pathway, initially studied in Arabidopsis, has been a focal point for
genetic modifications aimed at maintaining ion balance by promoting the exclusion
of sodium (Na
+/H+
a Na
Additionally, manipulating genes from the HIGH-AFFINITY K
(HKT1) family has helped improve Na
+
) from cells [176]. Overexpressing components like SOS1, which encodes
antiporter, have been shown to enhance salt tolerance in various species.
+
+
exclusion and maintain K+/Na+ ratios under
TRANSPORTER 1
saline conditions [177]. Osmolytes such as proline, glycine betaine, and soluble sugars
are crucial for osmotic adjustment and cellular protection during salinity stress. Genetic engineering efforts have focused on increasing the production of these compounds [114]. For example, overexpressing the enzyme P5CS, which is involved in proline synthesis, has significantly enhanced salt tolerance in multiple plant species.
Similarly, modifying non-glycine betaine-producing plants to synthesize this osmolyte
has proven effective in improving resilience to salinity. Salt stress also triggers the
production of ROS, which can damage plant cells. Strengthening antioxidant defenses
is vital for mitigating this damage, and genetic engineering has targeted both enzymatic antioxidants like superoxide dismutase (SOD) and catalase (CAT), as well as
nonenzymatic antioxidants such as flavonoids [121, 132]. Overexpressing genes responsible for these antioxidants has led to notable improvements in salt tolerance
among plants.
High-throughput genomics technologies have accelerated the discovery of salttolerance genes. Tools like genome-wide association studies (GWAS) and quantitative
trait loci (QTL) mapping have identified key genes and alleles associated with salinity
Biotechnological Approaches
Plant Cell Cultures
Rapid Screening
Antioxidant Systems
Genetic Engineering
Genomics & CRISPR
QTL IdentificationNa+ Transport Manipulation
Figure 6.3: Biotechnological approaches for improving salt tolerance.
250 Fatemeh Ahmadi
Chapter 6 Impact of salinity stress on medicinal and aromatic plant biotechnology 251
tolerance in crops like rice, providing valuable markers for breeding programs [84,
105]. Transcriptomics has further revealed complex gene expression networks, offering new targets for genetic improvement [16].
The advent of CRISPR/Cas9 has enabled precise gene editing to improve salt tolerance. This technique allows for targeted modifications of genes involved in salinity
+
responses, such as those regulating Na
transport. For instance, CRISPR/Cas9-mediated
modifications in rice improved salt tolerance by enhancing ion homeostasis. As it
avoids the introduction of foreign DNA, this method may address regulatory and public acceptance concerns surrounding genetically modified crops [114, 122]. Despite
these advances, challenges remain. Salt tolerance is a complex trait involving multiple
genes and pathways, making single-gene modifications often insufficient for achieving
robust and stable improvements. Future efforts will likely focus on multigene engineering and the manipulation of regulatory networks for comprehensive tolerance.
Additionally, it is crucial to ensure that increased salt tolerance does not compromise
yield or quality under nonstress conditions [75, 85]. Biotechnological approaches offer
transformative solutions for developing salt-tolerant crops. From the utilization of
cell cultures for screening to advanced genetic engineering techniques, these strategies have yielded promising results in enhancing salinity resilience [23]. As research
continues to uncover the molecular basis of salt tolerance and refine genomic tools,
the potential for creating more resilient crops will only grow. However, biotechnology
should complement traditional breeding and agronomic practices, forming part of a
holistic approach to address soil salinization and ensure global food security [117].
6.11.1 Genetic engineering strategies
Genetic engineering has become a transformative tool in improving salt tolerance in
plants by enabling precise modifications to enhance their resilience to saline environments [152]. This approach focuses on isolating and transferring key salt-tolerant genes,
particularly those involved in pathways critical for managing salinity stress. One such
pathway is the SOS pathway, which regulates ion homeostasis and prevents sodium toxicity. Enhancing the expression of SOS1, a gene encoding a plasma membrane Na
porter, has shown significant results, with transgenic plants exhibiting better growth and
yield under saline conditions due to improved Na
+
efflux and ionic balance. Similarly, the
HKT1 family of transporters, which facilitates the removal of excess sodium from xylem
tissues, has been a key target, as modifications to HKT1 expression have effectively reduced sodium buildup in shoots and enhanced overall salt tolerance [164]. Another critical strategy involves targeting genes associated with osmotic adjustment, such as P5CS
(Δ1-pyrroline-5-carboxylate synthetase), which is vital for proline synthesis. Overexpression of P5CS increases proline levels, allowing plants to better manage osmotic stress.
Similarly, enhancing glycine betaine production through genetic modifications has
proven effective in enabling plants to maintain osmotic balance in saline conditions [127].
+/H+
anti-
252 Fatemeh Ahmadi
Efforts to enhance salt tolerance in plants have increasingly focused on strengthening antioxidant defense systems to combat oxidative damage caused by ROS generated during salinity stress [105]. Overexpressing genes for enzymes such as superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) have been
shown to improve a plant’s capacity to detoxify ROS, thereby reducing oxidative
stress. Recent advancements in genome editing technologies, particularly CRISPR/
Cas9, have provided new opportunities for enhancing salt tolerance [136]. This technique allows for precise modifications of specific genes or regulatory regions without
introducing foreign DNA. CRISPR/Cas9 has been successfully used to edit genes associated with salinity tolerance, leading to crops that can better withstand salt stress,
while addressing concerns related to traditional transgenic methods [36]. These developments highlight the potential of genetic engineering in tackling the challenges
posed by salinity, offering innovative solutions for cultivating salt-tolerant crops and
medicinal plants, which is essential for sustainable agricultural productivity in saline
environments [74].
6.11.2 Identification of salt-tolerant genes
The discovery of genes associated with salt tolerance is essential for developing crops
resilient to salinity. Researchers have employed various methods to identify and understand the genes involved in these mechanisms. GWAS have become a powerful approach to identifying QTLs linked to salinity tolerance [110]. For example, studies in
rice have revealed several QTLs that influence salt tolerance traits, providing a valuable resource for breeding programs. Transcriptomics, such as RNA sequencing, has
highlighted the complexity of gene networks activated during salinity stress, offering
new candidates for genetic improvement. Research in Melia azedarach identified pivotal genes, including CBL7, SAPK10, EDL3, and AKT1, through Gene Ontology (GO)
analysis, which are crucial for salinity responses. Similarly, in rice, genes like OsSTL1
and OsSTL2 have been validated for their roles in salt tolerance, with OsSTL1 found
to be homologous to the SRP1 gene in Arabidopsis [162].
TFs are also critical regulators of salinity tolerance. In rice, TFs such as OsWRKY53
and OsMKK10.2 have been identified as key contributors to salt resilience. Furthermore,
zinc finger proteins have been implicated in regulating salinity responses, with several
genes from this family associated with salt tolerance traits [42]. Additionally, wild relatives of cultivated crops offer a valuable reservoir of salt-tolerant genes that have often
been lost during domestication. Advanced hybridization techniques now enable the
transfer of these traits from wild species to domesticated crops, improving their ability
to withstand salinity [118].