**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 (SO4 2−). Ions, such as Na+ and Cl− ,are not utilized by plants for nourishment; however, ions, such as K+ and SO4 2−, 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

#### *Plant Adaptation to Salinity Stress: Significance of Major Metabolites DOI: http://dx.doi.org/10.5772/intechopen.111600*

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 proteinmetabolite 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 saltstressed 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].
