Plant Adaptation to Salinity Stress: Significance of Major Metabolites

Abstract

These genes increase the plant’s tolerance to salt stress by producing proteins and metabolites that protect the cell against stress. More secondary metabolites including anthocyanins phenols, saponins, flavonoids, carotenoids, and lignins, etc., are produced by plants in salty conditions, but previous studies have only looked at a small portion of these compounds. Antioxidant activity and phenolic compound accumulation under salt stress have been linked in several studies. Proline accumulates in the cytoplasm and the vacuole, where it functions as an osmolyte and protects macromolecules against denaturation. Polyamines play a role in salt tolerance by regulating gene expression and ion flux. This means that metabolites are crucial for plant response to salt stress and maintaining agricultural productivity in salt-affected environments.

1. Introduction

The salinization of soil has emerged as one of the most significant environmental and socioeconomic problems on a global scale, and it is anticipated that this problem will become much more severe as a result of forecasted changes in the climate. It affects food security, water availability, and human health. Salinization of soil occurs when salt accumulates in the soil, making it unsuitable for crop growth. This can be caused by the accumulation of naturally occurring salts or by the introduction of salt-laden irrigation water. Salinization can also be caused by inadequate drainage, where water accumulates in the soil and dissolves and accumulates salts from the soil. Salinization can reduce crop yields and cause environmental damage such as increasing the salinity of nearby rivers and streams. To prevent salinization, farmers can use soil amendments to reduce the salt content of the soil and use irrigation practices that avoid the accumulation of salt in the soil. This can be caused by over-irrigation, poor drainage, and high levels of evaporation. The economic effects of salinization of soil can be seen in the form of reduced crop yields, increased. It is a process where the salt content in soil increases, which can endanger plant life and lead to soil erosion. Salinization is a result of increased precipitation, flooding, or irrigation that saturates the soil with salt. Salinization can also be the result of the chemical precipitation of salt from the atmosphere [1]. High levels of salt can cause soil structure degradation. It can lead to the formation of a hardpan layer, which can make it difficult for roots to penetrate the soil. This can further reduce the crop yields significantly. Soil salinity can have a significant adverse effect on crop yield. High levels of salinity can reduce crop yields by reducing germination rates, reducing plant growth, and increasing the susceptibility of plants to disease. Salinity can also reduce the availability of essential nutrients, resulting in stunted growth and reduced yields. Additionally, high salinity levels can result in increased water stress, which can further reduce crop yields. Recent estimates show that salt affects about 1125 million hectares of land around the world and that every year, 1.5 million hectares of land cannot be used for farming because the soil is too salty [2, 3]. It is hard to figure out how much agricultural production is lost, but it is thought that between 25% and 50% of all irrigated land is affected by salt accumulation, leading to decreased yields and the abandonment of agricultural land. It is estimated that up to 1 billion people in the world are affected by salinization and that the problem is growing. The World Bank has estimated that up to $20 billion a year is lost due to salinization [4]. Due to the extremely complex natural processes, abiotic stress inhibits plant development, reduces agricultural production, and further contributes to excessive soil degradation. Farmers’ incomes and other local economies are solely slowed [1].

Plants make substances called secondary metabolites that help them do well in their environment. A variety of responses occur in plants and other species as a result of these tiny chemicals. They signal the continuation of perennial growth or the onset of dormancy, and they are responsible for triggering blooming, fruit set, and abscission. Depending on the circumstances, they can attract or repel microbes. There are over 50,000 distinct secondary metabolites present in plants. Secondary plant metabolites are responsible for the activities of therapeutic plants and a variety of modern pharmaceuticals. Secondary metabolites are produced by plants in response to environmental and biotic stimuli. These include defensive chemicals, attractants, and toxins. They are also responsible for plant growth, development, and health. Universities and drug companies are always looking for new secondary products in plants in the hopes of finding new products or, even better, new ways to treat diseases. As secondary metabolites, once-held hopes that better appreciating natural product distribution might aid in plant taxonomy were also entertained. This supplementary rationale is no longer relevant because plant classification is increasingly done by comparing DNA sequences [5, 6]. In this chapter, we talk in-depth about the fundamental contribution of secondary metabolites that enable plants to sustain soil salinity, increase agricultural production, and have no negative effects on the economy or people’s health.

1.1 Soil salinity causing factors

Soil salinization can be caused by a number of factors, but most commonly it is due to improper irrigation practices that allow salts to accumulate in the soil. Other causes of salinization include soil type, water table fluctuations, and high evaporation rates. In some cases, poor agricultural practices, such as excessive fertilization, can also lead to soil salinization. In order to prevent soil salinization, it is important to practice proper irrigation techniques, ensure that the soil has adequate drainage, and make sure to use the appropriate amount of fertilizer for the crops being grown. Additionally, certain soil amendments, such as gypsum, can help reduce the number of salts in the soil [2, 7]. One of the most common and bad effects of soil flooding is that it makes the soil saltier. Salinity is based on how much salt is dissolved in the soil, and it can be changed by a number of things. Poor drainage can also make it easier for certain types of weeds to grow, which can lead to soil erosion and other problems. It can also cause the soil to get harder, which can make it harder for plants to get enough oxygen and nutrients. Soil salinization occurs when seawater or other salt water floods a field or other piece of land. When water evaporates, salt and other minerals are left behind. These can build up in the soil and make it less fertile. This can cause long-term problems with soil salinity in places that flood often [7]. To mitigate the effects of soil salinity caused by flooding, farmers and landowners should improve drainage, plant salt-tolerant crops, and restrict the amount of irrigation water permitted to pool on the land. When soil is inundated, water from rivers, streams, and other sources can carry enormous quantities of dissolved salts. High concentrations of nutrients, such as nitrogen and phosphorus, can also contribute to an increase in salinity. Inadequate drainage can also raise the water table, leading to flooding and other water-related problems [8, 9]. Salt can build up in the soil as a result of over-irrigation. This can cause a variety of issues, including decreased crop yields, soil structure damage, and plant toxicity. Salt buildup can also cause water logging, which reduces soil oxygen levels and damages plant roots. Farmers should employ water-saving irrigation techniques, such as drip irrigation, to mitigate the impacts of soil salinity caused by over-irrigation. Farmers should also utilize soil tests to detect the salinity of the soil and alter their irrigation operations accordingly. Finally, the use of soil supplements, such as gypsum, can aid in the reduction of salt accumulation in the soil [10]. Climate change and natural disasters (such as a tsunami) that make the soil saltier are still problems for agriculture. When the Earth’s temperature rises, the water that falls as rain or snow becomes increasingly saline. An increase in saline water can dissolve salts in the soil, making plant growth more challenging. Also, when the Earth’s seas warm, more salt is released into the atmosphere. This rise in salinity can kill marine life, making it impossible for them to live. Finally, as the Earth’s surface dries up, salts collect in the soil more easily [11]. Drought, excessive salinity, and cold temperatures are all climatic factors that have a negative impact on plant growth and crop yield. The growth of a plant and the production of secondary metabolites are both affected by environmental conditions such as temperature, humidity, and the intensity of the light, as well as the availability of water, minerals, and carbon dioxide. Cellular dehydration brought on by exposure to salt generates osmotic stress, which in turn leads to the loss of cytoplasmic water and a consequent shrinking of the cytosol and vacuoles. Depending on the severity of the salt stress, plants may either accumulate or decrease a number of different secondary metabolites [12]. Generally, salt-stressed plants are known to accumulate a wide range of secondary metabolites, including polyamines, polyphenols, and flavonoids, which act as antioxidants and chelators of metal ions, thereby helping to protect the plants from oxidative damage caused by the salt stress. In addition, the accumulation of these compounds can also help to increase osmotic pressure, allowing plants to cope with the osmotic stress caused by salt. On the other hand, plants under salt stress may also decrease the synthesis of certain secondary metabolites, such as terpenes and alkaloids, which can be toxic to plants when present in high concentrations. Therefore, the accumulation or decrease of secondary metabolites in salt-stressed plants is largely dependent on the severity of the salt stress.

1.2 Plant defenses against salt stress and metabolic alertness

There are several different ways in which plants react when they are subjected to salt stress. The physiological and the biochemical are the two categories that can be used to classify these responses (Figure 1). Alterations in the growth rate of the plant are one type of physiological reaction. Other types of physiological reactions include changes in photosynthesis, respiration, transpiration, water uptake, nutrient uptake, and hormone production. Plants respond to salt stress by increasing the concentration of compatible solutes in their cells, which helps them maintain their turgor pressure and prevent dehydration [13]. Compatible solutes are small molecules that can be accumulated in the cell without disrupting its osmotic balance. Examples of compatible solutes include sugars, amino acids, and polyols. Plants also respond to salt stress by activating various stress-response pathways, such as the MAPK, calcium, and ABA pathways, which help them cope with the stress and protect them from damage [14]. Stomatal movement also gets affected under this salt stress. Plants close their stomata to reduce the amount of water lost through transpiration. This helps them conserve water and reduce the amount of salt that enters the plant. Stomata are small pores on the surface of leaves that open and close to regulate the exchange of gases and water vapor. When the stomata are closed, the plant reduces the amount of water vapor that is released into the atmosphere. This helps the plant conserve water and reduce the amount of salt that enters the plant [15, 16]. In some cases, plants may shed their leaves and can cause root growth inhibition, which reduces the amount of water and nutrients taken up by the plant to cut the amount of salt uptake. Salt stress led to antioxidant enzymes (such as superoxide dismutase, catalase, and peroxidase), osmoprotectants (such as proline, glycine betaine, and polyols to help them maintain their turgor pressure and prevent dehydration), stress hormones (abscisic acid and jasmonic acid), and phytohormones (gibberellins and cytokinins) production [17, 18]. Other types of physiological responses include changes in the structure of the plant such as an increase in the production of root hairs and thicker cell walls. Changes in the plant’s metabolism, such as the creation of enzymes and other proteins that assist the plant in coping with salt stress, are examples of biochemical responses that take place as a result of salt stress. In addition, plants may create compounds that assist them in absorbing and storing salt, or they may develop compounds that assist them in excreting excess salt as a defense mechanism against the effects of salt stress.

Figure 1.

Salt stress and metabolic reactions are physiological and biochemical: salt stress intensity affects secondary metabolite accumulation in salt-stressed plants. Salt stress increases osmotic potential, decreases water potential, increases photosynthetic rate, and decreases stomatal conductance. Salt stress increases antioxidant enzyme activity, stress-related gene expression, proline buildup, and soluble sugar accumulation. Plants need these responses to stay healthy and avoid salt stress.

Plants can not get away from potentially dangerous situations, so they have evolved to have different defense mechanisms to protect themselves. These include physical barriers such as thorns, chemical defenses such as toxins and poisons, and camouflage to blend in with their environment. Some plants also produce chemicals that attract predators of the herbivores that would otherwise feed on them. Some plants, such as certain species of grasses, are able to tolerate high levels of salinity. Other plants may be able to survive in the short term but suffer long-term damage. In extreme cases, the plants may die. There are several strategies that can be employed to help plants cope with high levels of salinity such as using salt-tolerant varieties of plants, avoiding over-irrigation, and using soil amendments to improve the soil’s structure and drainage [19]. Metabolites are used for a variety of purposes in plants. Plants can use these metabolites to help regulate their internal water balance, allowing them to cope with salt stress [19, 20]. Such compounds help plants detoxify and eliminate excess salt from their cells. They can be used to protect the plant’s cells from the damaging effects of salt. They can be used as a source of energy, to help regulate growth and development, to produce hormones, to protect against environmental stress, to produce pigments, to aid in defense against pathogens, and to help in the synthesis of other molecules [21]. Some plants have evolved mechanisms to cope with soil salinity. These mechanisms involve the production of metabolites, such as proline, glycine betaine, and trehalose, which help the plant to maintain its water balance and protect its cells from the damaging effects of salt. Additionally, some plants produce special root structures that help to reduce the uptake of salt from the soil.

The prospects of metabolic alertness in crop production against salinity stress are very promising. By understanding the role of metabolic alertness in the response to salt stress, researchers can develop new strategies to improve crop performance under salinity. It refers to the ability of a crop to detect and respond to salt stress. It could improve the drought tolerance of crops by reducing the amount of water lost through transpiration. It helps crops adjust their metabolic processes to adapt to the salinity environment. Various studies have shown that crops with higher metabolic alertness respond better to salinity stress than crops with lower alertness. This is done by increasing the production of certain proteins that are involved in salt tolerance. It also involves the upregulation of certain genes that are involved in salt tolerance. Finally, metabolic alertness can be used to improve the nutrient content of crops by increasing the uptake of minerals and other nutrients.

2. Secondary metabolic profile of plants under soil salinity

The secondary metabolic profile of plants in soils under saline conditions shows an increase in the activity of enzymes involved in the synthesis of secondary metabolites. These enzymes are typically more active when the concentration of salts is high. In addition, plants may upregulate the expression of genes encoding other enzymes involved in the synthesis of secondary metabolites in response to salt stress. Several stressors are imposed on plants, such as the presence of certain elicitors or signal molecules, resulting in the accumulation of secondary metabolites [21, 22]. It is possible for the secondary metabolites to be created in reaction to a wide variety of stimuli such as being injured, experiencing extreme cold or heat, dehydration, or exposure to salt or light. These compounds are also capable of being created as a result of the metabolic response that the plant has developed in reaction to being under stress. It is common for secondary metabolites to play a role in a plant’s defensive mechanism, whether it is against herbivores or pathogens. They are also able to contribute to the communication between plants, both with one another and with their surrounding environment.

In general, soils that are dry and infertile are more likely to become salinized since they are less able to resist the accumulation of salts. Salinity can also increase as a result of climate change as warmer temperatures allow more water to evaporate, leaving behind more salts. The inability of plants to absorb water due to an overabundance of salts is one of the many reasons why salinization is a resource concern that threatens agricultural output. This soil salinity includes sodium (Na⁺), potassium (K⁺), chloride (Cl⁻), and sulfate (SO₄²⁻). Ions, such as Na⁺ and Cl⁻,are not utilized by plants for nourishment; however, ions, such as K⁺ and SO₄²⁻, are. As a consequence of this, sodium ions and chloride ions are regularly investigated about soil salinity. The term “sodicity” refers to the degree to which the concentration of Na⁺ in the soil solution rises in comparison to the concentration of other exchangeable cations [23]. Indirectly, through the worsening of soil physical conditions, salinity and sodicity affect plant growth through their impacts on the water intake by plants, the availability of nutrients for plants, and by imposing plant toxicity. These effects can be attributed to salinity and sodicity’s respective impacts on water intake by plants, nutrient availability for plants, and plant toxicity. Both natural processes and human-caused interventions can contribute to an increase in salinity [24, 25]. The situation might become severe when salt contributions to the soil surface become too high. It exclusively hampers seed germination and plant growth. Alternatively, sodium (Na⁺) may cause the soil to become more dispersed. The electrical conductivity (EC) of the ground is used to assess the dangers posed by salt, whereas the exchangeable sodium percentage (ESP) is used to assess the dangers posed by sodium. By making EC more concentrated, salt dispersion may affect the possible damage [24]. Salts will nearly usually first occur at lower elevations on a landscape, and they will gradually ascend to higher heights through time. This phenomenon is occurring on millions of hectares worldwide, including in Australia, Canada, Montana, Minnesota, and North Dakota and South Dakota. It also occurs in other nations, including New Zealand and Australia. There is a correlation between a decline in agricultural yields and profitability and an increase in soil electrical conductivity (EC) in locations where salt has impacted the soil [26].

2.1 Plant metabolites and its significance in environment

Plants store many compounds or “specialized metabolites.” These tiny compounds affect plants and other living things. They blossom, fruit, then abscise or retain everlasting growth. Beside fighting bacteria, these compounds can also draw in or ward off pests. These substances are referred to as “secondary metabolites” [27]. These chemicals help a creature adapt to environmental changes and interact with other organisms. They protect against viruses, pests, and herbivores, respond to environmental stress and connect organisms. So far, about 50,000 secondary metabolites that come from plants have been studied. Plant secondary metabolites are the main therapeutic agents in ancient and modern medicines. It is the goal of many academic and pharmaceutical institutions to discover new goods or, better yet, new treatment approaches for a wide range of disorders by doing extensive research into the plant’s secondary compounds. Once upon a time, it was thought that understanding the spread of natural products would help classify plants [26, 28]. During a plant’s life, as it interacts with its complicated multi-kingdom microbiome, which is made up of both good and bad microbes, these specialized metabolites have been shown to play one of the most important and noticeable roles. Plant microbiomes are also in charge of controlling how a plant’s metabolism works. Because of this, plant microbiomes are involved in a lot of the things listed above. A lot of plant secondary metabolites are important to the economy because they are good for human health [29, 30] and help increase the amount of food that can be grown. Even though several protein-substrate and protein-metabolite complexes have been identified, the majority of their biological functions remain unconfirmed [31]. Scientists have identified functioning secondary metabolites and metabolic pathways in plants using metabolomics. These findings apply to both fundamental and applied research. Nuclear magnetic resonance spectroscopy, Fourier transform near-infrared spectroscopy, capillary electrophoresis mass spectrometry, gas chromatography-MS, liquid chromatography-MS, MS imaging, and live single-cell MS are some of the most prevalent methods (LSC-MS). These approaches generally work together since they examine distinct metabolites. These techniques aid researchers in gaining a more comprehensive understanding of how metabolic networks in plants are regulated under different biotic and abiotic circumstances.

The plant microbiome helps the plant fight disease [32], get nutrients [33], and protect itself from living and nonliving environmental threats [34, 35]. Large-scale parallel sequencing enhanced plant microbiome research 15 years ago. These studies discovered plant microbiomes and interactions. Seed, core, synthetic community, defensive, and epiphytic microbiomes are some of the examples. Plant microbiomes respond differently to biotic and abiotic stimuli. More evidence reveals that complex feedback loops between plants, microorganisms, and their physical and chemical environments shape plant microbiomes. Genomic and molecular biology developments make studying plant specialized metabolites’ biosynthesis pathways’ structural and regulatory components easier. These methods can also be used to develop and test synthesizing-deficient mutant strains [6, 36]. Apart from these environmental elements such as light, temperature, soil water, soil quality, and salinity all have a significant role in the secondary metabolites’ ability to accumulate. Changes in a single environmental element, even when all others are held constant, can affect the levels of secondary metabolites in most plant species. Plants are sensitive to the ionic or osmotic pressure induced by salinity, which can either promote or decrease the accumulation of particular secondary metabolites. Their secondary metabolites provide protection of the plant cells from the oxidative damage produced by ion accumulation at the cellular and subcellular levels; salt stress may operate as an elicitor of secondary metabolites, which mitigate the harmful effects of salinity [37].

2.2 Secondary metabolic profiles decrease with salinity stress

As salinity increases, secondary metabolic profiles decrease in a variety of species. This can be seen in the decreased levels of metabolites such as carbohydrates, lipids, and proteins. This decrease in metabolic activity can be due to a number of factors, including a decrease in the number of cells and a decrease in the activity of metabolic enzymes. An increase in soil saltiness and ion accumulation represents one of the important abiotic factors that adversely affect the growth and production of cultivated plants. High NaCl concentrations impede plant growth due to a decrease in hydraulic conductivity (hyperosmotic stress) and the accumulation of ions to harmful levels for their proliferation (hypertonic stress). Plants change their biochemical and physiological processes in response to these stresses. Observing gene regulation, production of functional proteins, and accumulation of tiny molecules (i.e., metabolites) has allowed researchers to concentrate on plant signal perception and adaptability to an unfavorable environment [38, 39]. Salinity stress can have various effects on plants, including changes in their secondary metabolism. Secondary metabolites are compounds produced by plants that are not directly involved in growth and development but instead play roles in various functions such as defense against herbivores, attraction of pollinators, and communication with other organisms [26, 40].

Several studies have reported that salinity stress can lead to a decrease in the production of secondary metabolites in plants. For example, a study was conducted on exposure of basil plants to salt stress resulted in a decrease in the levels of certain secondary metabolites, including β-carotene, cryptoxanthin, lutein flavonoids, and phenolic acids in the leaves and flowers. Salinity stress has been shown to decrease the levels of certain secondary metabolites in basil plants [41]. Another study reported that salinity stress reduced the levels of secondary metabolites in grapevine leaves, including stilbenoids and flavonoids [42]. However, it is important to note that the effect of salinity stress on secondary metabolism can vary depending on the plant species and the specific metabolites involved. Some studies have also reported an increase in the production of certain secondary metabolites in response to salinity stress, suggesting that the relationship between salinity stress and secondary metabolism is complex and not fully understood. As a result of salinity stress, plants produce secondary metabolites that help them to adapt to the new environment. These metabolites can help plants reduce water loss, increase their resistance to pests and pathogens, and increase their salt tolerance.

Several studies have shown that sugars, amino acids, and organic compounds, which are primary metabolites, play a role in osmotic regulation. In contrast, secondary metabolites, which are the final products of primary metabolites, are more species-specific and are connected with plant protection due to their numerous roles (e.g., serving as antioxidant activity, superoxide radical (ROS) scavengers, and regulatory molecules) [43]. Even though the production of secondary metabolites such as phenols, saponins, flavonoids, carotenoids, and lignins, etc. usually goes up in salt-stressed plant, only a few target compounds have been looked at in detail in previous studies [44, 45]. Several studies have shown that this link between antioxidant activity and the buildup of phenolic compounds under salt stress is true for most plants, and research on the topic is constantly expanding. However, there are specific plant species that have been found to be more resistant to the oxidative damage caused by salt stress, and these are typically plants that have high levels of antioxidant activity. One such plant is rosemary, which has been shown to have high levels of antioxidant activity and resistance to salt stress. This is likely due to the presence of high levels of polyphenols in rosemary, which are powerful antioxidants that can protect cells from damage caused by oxidative stress [46]. Other plants that are known to be resistant to salt stress and have high levels of antioxidant activity are grapefruit, black tea, and green tea.

Flavonoids, polyphenols, tannins, and anthocyanins are some other secondary metabolites that were found in large amounts and may help plants tolerate salt by making their antioxidants work better [47]. Flavonoids are a type of secondary metabolite that is found in large amounts in plants. These compounds can help plants tolerate salt. Polyphenols are another type of secondary metabolite that can help plants tolerates salt. Tannins are types of secondary metabolite that can help plants resist damage from salt. Anthocyanins are another type of secondary metabolite that can help plants resists damage from salt. Yang et al. used saponin as a priming agent to help quinoa plants grow from seeds in salty environments. This is because saponins can get rid of ROS [48]. In this way, by analyzing the changes in metabolites at the whole-metabolome scale and using these metabolic profile changes along with other “omic” analyses such as genome, transcriptome, and proteome analysis, one can figure out the regulatory networks and find biomarkers that control stress responses and can be used to improve plants [39, 49].

2.3 Plant reactions against the salt stress factors

Plants can not get away from potentially dangerous situations, such as those caused by biotic and abiotic stresses, which affect them at every stage of their lives. Stress does not hurt some desert plants, but it can kill others [50]. Some plants have a physiological or biochemical defense against salt stress factors. These plants can accumulate substances that scavenge or chelate ions or that suppress the activity of salt-sensitive enzymes. For example, several plant species accumulate compounds such as salicin or protocatechuic acid that bind to and suppress the activity of ion channels in the cell membrane. Other plants produce compounds such as phenolic acids that inhibit the activity of salt-sensitive enzymes. Plants protect themselves from a wide range of stresses with systems that are complex and well-balanced. There are three main types of systems in plants: photosynthesis, respiration, and homeostasis. Photosynthesis is the process that plants use to create energy from the sun [51]. Respiration is the process that plants use to release energy from the food they eat. Homeostasis is the system that plants use to maintain the correct level of water, minerals, and energy in their cells.

When plants are exposed to high levels of salt, it can disrupt their ability to take up water and nutrients, which can have a negative impact on their metabolism and overall growth. To cope with this stress, plants have evolved a variety of strategies to adjust their metabolism and maintain their physiological functions. Plants adjust their rates of photosynthetic activity, stomatal conductance, transpiration, cell wall architecture, membrane remodeling, cell cycle and division rates, and a variety of other physiological and metabolic activities in response to environmental stresses [52]. This can be achieved by altering the expression of genes that are involved in ion transport and osmotic adjustment. For example, some plants will increase the production of compatible solutes, such as proline and glycine betaine, which help to maintain cellular osmotic balance and protect cellular structures from damage. Other plants may increase the expression of genes involved in ion transport, such as the Na+/H+ antiporter, which helps to remove excess sodium from the cell and maintain cellular pH. In addition to these gene expression changes, plants may also alter their metabolic pathways in response to salt stress. For example, some plants may increase their production of antioxidants, which can help to protect against oxidative stress and cell damage. Others may alter their carbohydrate metabolism such as increasing the breakdown of starch to provide energy for growth and maintenance. Stress signals turn on the plant’s main metabolism, which makes biosynthetic intermediates for the secondary metabolism. The stress response system and the inducible defense system root stress signals turn on the plant’s main metabolism in soil salinity. This increases the rate of uptake of salt ions from the soil, which can lead to increased plant growth and survival in saline soils [53] depend on the ability to turn on or off a number of genes and a number of molecular and cellular processes that have to do with defense. To deal with harsh conditions, plants make SMs from primary metabolites in their cells.

Salt stress causes a reduction in plant growth and development; it also has an effect on carbon combustion, ion uptake, nutritional requirements, and energy metabolism, and it alters the amounts of secondary metabolites, which are crucial physiological markers in salt stress tolerance. Recent advancements have been made in the identification and characterization of the systems that enable plants to resist high salt concentrations and drought stress. These processes allow plants to survive in harsh environments. In plants that are subjected to stressors, such as the presence of a variety of elicitors or signal molecules, the deposition of secondary metabolites frequently takes place [10, 54].

3. Research methods and analysis of metabolic alertness during salt stress

Metabolic alertness during salt stress can be studied using a range of research methods and analyses. These can include in vitro studies, animal studies, and human studies. In vitro studies involve culturing cells and introducing salt stress to observe the metabolic alertness response. This can be done using a range of techniques such as fluorescence microscopy, flow cytometry, and metabolite assays. Animal studies involve exposing animals to the plants grown under salt stress and observing their metabolic alertness response. This can be done using behavioral tests, metabolic assays, and gene expression analyses. Human studies involve exposing individuals to salt stress and observing their metabolic alertness response. This can be done using various methods such as dietary interventions, metabolic assessments, and physical activity tests. The analysis of metabolic alertness during salt stress will depend on the research method used. For in vitro studies, the data can be analyzed using a variety of techniques such as statistical tests, machine learning algorithms, and network analyses. For animal studies, the data can be analyzed using techniques such as behavioral tests, metabolomic profiling, and gene expression analyses. For human studies, the data can be analyzed using techniques such as dietary interventions, metabolic assessments, and physical activity tests [55]. In case of salt stress in plants [56, 57], metabolic alertness during salt stress in plants can be studied by a variety of methods and techniques have been mentioned below:

3.1 Molecular analysis

1. RNA sequencing: RNA sequencing is a powerful tool for studying gene expression in plants under salt stress. It can be used to identify and quantify changes in gene expression in response to salt stress.

2. qPCR: qPCR is a sensitive and quantitative method for measuring gene expression in plants. It can be used to measure the expression of genes involved in metabolic pathways and to identify changes in gene expression in response to salt stress.

3. Proteomics: proteomics is a powerful tool for studying protein expression in plants under salt stress. It can be used to identify and quantify changes in protein expression in response to salt stress.

3.2 Physiological analysis

1. Photosynthesis: photosynthesis is a key metabolic process in plants. It can be used to measure the effects of salt stress on photosynthetic efficiency, as well as to identify changes in photosynthetic rate in response to salt stress.

2. Enzymatic activity: enzymes are involved in a variety of metabolic pathways in plants. Measurement of enzymatic activity can be used to identify changes in metabolic activity in response to salt stress.

3. ion transport: ion transport is an important process in plants. Measurement of ion transport can be used to identify changes in ion transport in response to salt stress.

3.3 Field experiments

Field experiments can be used to study the effects of salt stress on plant growth and productivity. These experiments can be used to measure changes in plant growth and productivity in response to salt stress, as well as to identify changes in metabolic processes in response to salt stress.

As a whole, the systematic research methodology and its analysis to control the salt stress in the crop plants has resulted in the development of a variety of techniques to maximize crop yields and minimize salt stress (Table 1). Firstly, the use of salt tolerance genes and the development of transgenic plants have been found to be effective in controlling salt stress. Secondly, the use of crop rotation, soil amendments, and the application of fertilizers and micronutrients can be helpful in controlling salt stress. Thirdly, the selection of salt-tolerant crop varieties and the use of drip irrigation techniques also play a major role in controlling salt stress. Finally, the use of bioremediation techniques, such as the use of bacteria and fungi to absorb the salt from the soil, can also help in controlling salt stress.

Research method	Analysis
Observational studies: observing plants in their natural environment to determine their response to salt stress and metabolic alertness.	Statistical analysis: analyzing data collected from observational studies and experiments to identify trends in the plant’s response to salt stress and metabolic alertness.
Experiments: creating controlled environments to study the effects of different levels of salt stress and metabolic alertness on plants.	Molecular analysis: analyzing gene expression, proteome, and metabolome data to identify key genes and pathways involved in the plant’s response to salt stress and metabolic alertness.
Molecular techniques: using techniques such as gene expression, proteomics, and metabolomics to study the plant’s response to salt stress and metabolic alertness.	Comparative analysis: comparing the plants’ response to salt stress and metabolic alertness with other species to identify differences and similarities.

Table 1.

The prospective approach to control the salt stress in the crop plants.

4. Behavior of anthocyanins in soil salinity

Anthocyanins are a type of phenolic pigment that dissolves in water. Pigments are found in glycosylated forms. The red, purple, and blue pigments found in fruits and vegetables are called anthocyanins. The anthocyanin content of berries, currants, grapes, and even some tropical fruits is quite high. Edible vegetables rich in anthocyanins have a red to purple hue and include leafy greens, cereals, roots, and tubers. There are several different types of anthocyanin pigments, but the most common one is cyanidin-3-glucoside, which is present in a wide variety of plant species [58]. They are glycosides and acylated, while anthocyanidins are 3-hydroxy, 3-deoxy, and O-methylated. Cyanidin, delphinidin, pelargonidin, peonidin, petunidin, and malvidin are some of the most common types of anthocyanidins. Aside from the usual anthocyanins, plants have also been found to have acylated anthocyanins. Acylated anthocyanin has four subtypes: acrylated, coumaroylated, caffeoylated, and malonylated. These compounds become active under the saline stress. Some of them get induced in response to salt stress and hike the amount of synthesis of anthocyanins in the concerned plant. While anthocyanins appear red in acidic conditions, they change to a more typical blue color when the pH level is raised. Extraction, separation, and measurement of anthocyanins have all greatly benefited from the use of chromatography [59]. There have been several studies that link the production of anthocyanins to the induction of various types of stress in plants; however, very few of these studies have examined the rise in anthocyanin levels that occurs in response to salt stress [60].

Anthocyanins in soil salinity can help promote beneficial microbial activity in the soil. Anthocyanins are plant pigments that can help protect the plant against environmental stress such as soil salinity. They can also act as an antioxidant, scavenging for free radicals and reducing oxidative stress. Furthermore, they can induce the production of plant growth regulators and plant hormones, which can stimulate beneficial microbial activity in the soil. This includes bacteria and fungi that can improve soil fertility and nutrient availability. Additionally, anthocyanins can also stimulate the production of enzymes and metabolites that can help to increase the microbial activity in the soil. When plants are subjected to salinity stress, their metabolic processes undergo alterations. These changes manifest themselves first in the vegetative part of the plant (i.e., the leaves) and later in the reproductive organs of the plant (i.e., the flowers). Thus, leaves with intact polyphenols and antioxidants may indicate that the plant’s response to salt stress occurred before the 10th day. Salinity may have decreased anthocyanins in flowers because metabolic activities were inhibited, other pigments accumulated, or the plant’s antioxidative mechanisms had exhausted their supply [61]. Salinity affects anthocyanin accumulation differently, thus more research is needed. In salinity-exposed tomato genotypes, For instance, Borghesi et al. [62] demonstrated that the accumulation of anthocyanins in two different tomato genotypes reacted in opposing ways when they were exposed to saline. The study suggests that the production and localization of anthocyanins may help the plant acquire resistance to a variety of environmental challenges and that the adaptive advantages of anthocyanins are considerably less muddled in nonreproductive tissues [63].

5. Behavior of flavonoid in soil salinity

In the kingdom of plantae, flavonoids are the secondary metabolites that are present in the widest variety of plant species. These chemicals have a wide variety of physiological and molecular functions in plants, including acting as signaling molecules, contributing to plant defense, influencing the transport of auxin, exhibiting antioxidant activity, and scavenging free radicals [64]. According to Sirin and Aslam [65], among the nonenzymatic antioxidants, phenols and flavonoids make a substantial contribution to removing free radicals in plants, which allows plants to tolerate salt stress by storing the antioxidants in a variety of tissues. Flavonoids are a group of compounds that are found in many plants. These compounds have been studied for their potential health benefits, and many are now known to be beneficial for human health. Flavonoids are a type of polyphenol. They play essential roles in soil salinity regulation [30, 41, 66]. They can reduce the number of ions, such as sodium and chloride, present in the soil solution, which can help maintain soil health and fertility [66]. They have also been shown to promote beneficial microbial activity in the soil, including improving the growth and health of beneficial bacteria and fungi. They also have antioxidant properties, which can help protect plants from environmental stressors such as high levels of salinity. They can also act as signaling molecules, helping plants regulate their response to salt stress. In addition, flavonoids can help promote beneficial microbial activity in the soil, which can improve the soil’s fertility and reduce its salt content. This can help boost the soil’s fertility, improve water retention and nutrient cycling, and reduce the risk of disease-causing pathogens. They can also help protect plants from environmental stressors such as drought and extreme temperatures [67, 68].

In plant materials, polyphenols can be found in both their free and bound forms. Phenolic acids make up the bulk of the polyphenols that are found in grains and baked goods derived from cereal, and approximately 75% of these are accessible in bound form. Plants’ development, reproduction, and eventual grain yield are almost entirely dependent on the leaf proteins [68]. It is not surprising that salt stress causes a decrease in protein content in plant leaves, given that proteins are known to be one of the first targets of reactive oxygen species (ROS) in living organisms. One of the principal sites of ROS damage is the chloroplast, where it leads to the degradation and inactivation of Rubisco, as well as many other changes in the thylakoid and stromal proteins [69].

Because of this, leaf protein concentration is an important indicator of salt stress [70]. One of the most debated topics in the literature is the extent to which genetic diversity affects the protein content of plant leaves when subjected to salt stress. Few studies have looked into how genetics interact with salt stress to affect protein levels. In addition, there is a lot of debate in the scientific literature concerning the way salinity has an effect. For instance, Birhanie et al. [71] found that salt stress similarly reduced total protein concentration in the shoots of two cultivars of wheat (tolerant and sensitive). In contrast, it was discovered that during salt stress, leaf protein concentration varied by genotype and increased [72, 73].

Many studies have been proposed that increasing the accumulation of suitable solutes, such as proline, can improve salt tolerance. Proline, such as other osmolytes, can eventually regulate redox potential through its effects on osmotic adjustment, membrane protection, and enzyme stability in the face of abiotic and biotic stresses [74]. Arabbeigi et al. [75] reported greater proline biosynthesis gene (P5CS) expression in Ae. cylindrica may be linked with salt tolerance. It is a bacterial species that is commonly found in salt marshes and coastal habitats. The P5CS gene is known to play a role in proline biosynthesis, which is a process that helps cells to build proteins. The researchers found that Ae. cylindrica cells that expressed high levels of the P5CS gene were more resistant to salt stress. Additionally, the study found that deleting the P5CS gene had no impact on the salt tolerance of Ae. cylindrica cells. The authors of the study say that the findings suggest that the P5CS gene may play a role in salt tolerance in Ae. cylindrica. They say that the findings could help to identify new strategies for preserving salt marsh habitats. In wheat, this finding agrees with that of Kumar et al. [76]. This is congruent to a certain extent with the findings of Ebrahim et al. [77] in barley, who also found that salt stress led to a huge buildup of proline but also found that salt-sensitive genotypes accumulated more proline in their leaves than salt-tolerant ones. Since the osmo-adaptive response includes proline buildup, whether or not it plays a special function in the resistance to abiotic and biotic stresses (such as salinity and drought) is debatable. Several explanations come to mind for this discrepancy. Variations in genotype or species, stress level or duration, and physiological maturity or development are particularly important to note when comparing research.

6. Behavior of phenolic in soil salinity

Increased salinity affects primary carbon metabolism, plant growth and development by osmotic stress, mineral deficiencies, ion toxicity, and physiological and biochemical disturbances. In general, the effect of salinity on plant growth and development is greatly reduced by the presence of phenolic compounds. Phenolic compounds have been shown to reduce the uptake of sodium and other cations, thereby reducing the amount of cations in the soil. Phenolic compounds can also increase the availability of phosphorus, potassium, magnesium, and calcium, which can help to reduce soil salinity. Phenolic compounds can also reduce the amount of bicarbonate and chloride ions in the soil, thereby reducing soil salinity [78].

Phenolic compounds are known to have a strong ability to absorb and react to soil salinity. They are capable of binding and immobilizing ions, including sodium, which helps to reduce the salinity of the soil. These compounds can also reduce the amount of cations present in the soil, which helps to reduce soil salinity [79]. These compounds can also form complexes with other compounds in the soil, preventing them from taking up and dissolving in the soil water, thereby reducing the salinity of the soil.

7. Discussion

Excessive salts can be deposited on the soil surface due to rainfall or irrigation, or salt can be injected into the soil due to oil and gas exploration or mining, both of which lead to salinization. Using salt-based insecticides and fertilizers can also contribute to salinization. Soil salinization is now a major issue in environmental and socioeconomic sustainability around the world. Soil salinization is the process of the soil becoming excessively salty. This can be caused by a number of factors, including overpumping of groundwater, the use of salty irrigation water, and the addition of salt to the soil through fertilizer and other means. The effects of soil salinization are wide-ranging and can have serious consequences for both the environment and human health [9, 80]. Salinization can lead to the formation of salt lakes and ponds, which can damage infrastructure and disrupt water supplies. It can also damage plants and animals and can increase the risk of waterborne diseases. Soil salinization is a growing problem around the world, and it is becoming increasingly important to address it. There are a number of ways to combat soil salinization, and it is important to stay up-to-date on the latest research and developments [81]. Some of the most commonly used methods include (Table 2).

Irrigation management	This includes adjusting the timing and amount of irrigation, using soil moisture sensors, using different types of water, and improving drainage systems.
Leaching	Leaching is a process by which salts are removed from the soil by flushing them away with water.
Drainage	Proper drainage helps to reduce the number of salts in the soil and can be achieved by installing tile drains or subsurface drains.
Amendments	Adding organic matter to the soil can help to reduce soil salinity and can also improve soil structure and fertility.
Cover crops	Planting cover crops, such as legumes and grasses, can help reduce soil salinity by taking up some of the salts from the soil.
Crop rotation	Rotating different crops in a field can help to reduce soil salinity as different plants have different salt tolerances.
Desalinization	Desalination is a process by which salts are removed from the water and can be used to reduce soil salinity in areas with high levels of water salinity.

Table 2.

Proper management of soil from salinity.

Salt lakes are a byproduct of salinization, which can also contribute to the formation of other environmental dangers such as toxic dust storms and the contamination of water supplies and agricultural land. The development of salt-sensitive habitats is one way in which salinization affects human health. Various efforts have been made to address the global problem of salinization. The United Nations Development Programme (UNDP) has launched a number of initiatives to help address the issue, including the Salinization of Soil Information and Capacity Building Centre (SalinSICB), which provides training and resources to help farmers and communities protect their soil from salinization, and the Salinization [80]. The Salinization of Soil Information and Capacity Building Centre (SalinSICB) was launched in March 2008 to support the development of effective soil salinization management practices in countries across the world. The centre provides access to soil salinization information and training to government officials, scientists, farmers, and other practitioners. It also works to build the capacity of countries to manage salinization and address its impacts [76, 82]. Salt stress is a major environmental stressor for plants. It has a negative effect on the environment and is a major contributor to reducing crop production. Plants are unable to grow and develop normally under salt stress, and their levels of secondary metabolites, which serve as important physiological markers of salt stress resistance, are typically altered as a result. Recent successes in identifying and characterizing salt-resistant systems in plants have paved the way for the creation of salt-resistant crops. Here, the chapter discussed the salt stress affects secondary metabolites and several vital plant medicines. During stress, increased cytosolic synthesis of secondary metabolites (anthocyanins, flavonoids, phenolics, and unambiguous phenolic acids) may protect cells from ion-induced oxidative damage by binding the ions and lowering cytoplasmic structural toxicity. Anthocyanins, flavones, phenols, and phenolic acids cause this. Plants adapt to salt stress by changing their metabolism. Understanding these systems may help researchers enhance agricultural output in salt-affected areas.

8. Conclusion and prospects

Over exploitation of groundwater, the use of irrigation water with high salt content, and the application of salt-based pesticides and fertilizers have all contributed to soil salinization, a major problem for environmental and socioeconomic sustainability in many parts of the world. Salinization occurs when salt concentrations in the soil become elevated, leading to a decrease in soil fertility and crop yields. This is often caused by the accumulation of salts in the soil, either due to a lack of leaching or due to the application of salt-based fertilizers and pesticides. The over-extraction of groundwater can also lead to the accumulation of salts in the soil as the water may contain a higher concentration of salts than the soil. In addition, irrigation with water that has a high salt content can lead to the accumulation of salts in the soil. To prevent soil salinization, proper irrigation management, the use of low-salt fertilizers and pesticides, and the proper management of groundwater extraction are all essential. Soil salinization is a problem for both terrestrial and aquatic ecosystems, causing problems for plants, animals, and people. Plants are able to adapt their metabolic processes and physiological activities in order to survive in environments with high levels of salt. This can inhibit plant growth, but plants are able to overcome this obstacle by changing the way they assimilate and use nutrients. When exposed to high levels of salt, plants have evolved mechanisms to adapt their metabolism and preserve physiological processes such as changing the expression of genes involved in ion transport and osmotic adjustment. In addition, plants can upregulate the expression of genes involved in antioxidant enzymes, which help protect the plant from oxidative damage caused by high salinity. Plants also increase the expression of genes involved in the synthesis of compatible solutes, which are osmotically active molecules that increase the osmotic potential of the cell and allow it to retain water. Lastly, plants can reduce their transpiration rates and close their stomata to reduce water loss. It is possible to do this by regulating the expression of genes involved in ion transport and osmotic balance. The expression of genes involved in ion transport and osmotic balance under soil salinity is to introduce a salt stress response gene. A salt stress response gene encodes a transcription factor that responds to salinity stress by upregulating the expression of ion transport and osmotic balance-related genes. This upregulation helps the plant to better cope with stress and improve its overall growth and survival.

There are a number of prospects to control soil salinity using secondary metabolites. Some of these include using antimicrobial agents to control bacterial populations, using plant-derived secondary metabolites to regulate salt uptake, and using plant-derived inhibitors of salinization enzymes. There are several secondary metabolites that have been shown to have the ability to mitigate the effects of soil salinity on plant growth. For example, polyamines such as spermidine and spermine have been found to reduce the toxic effects of salt on plant cells by maintaining ion homeostasis and reducing oxidative stress. Another group of secondary metabolites that have been studied for their potential to control soil salinity are plant growth regulators such as auxins, cytokinins, and gibberellins. These compounds have been found to improve plant growth and productivity under saline conditions by regulating the uptake and distribution of ions, maintaining water balance, and enhancing antioxidant defense mechanisms. In addition to polyamines and plant growth regulators, other secondary metabolites, such as flavonoids, alkaloids, and terpenoids, have also been shown to have potential for controlling soil salinity. These compounds have been found to improve plant growth and productivity by reducing oxidative stress, enhancing nutrient uptake, and regulating ion balance. Soil salinity can have a significant impact on plant metabolism and growth, leading to stunted growth and decreased yields that need to be minimized in the future forecast.