Plant Secondary Metabolites and Abiotic Stress Tolerance: Overview and Implications

*Attiqa Rahman, Ghadeer M. Albadrani, Ejaz Ahmad Waraich, Tahir Hussain Awan, İlkay Yavaş and Saddam Hussain*

## **Abstract**

Plant secondary metabolites (PSM) are one of the major sources of industrially important products such as food additives due to their distinctive tastes, smells, and flavors. Unlike primary metabolites such as carbohydrates, lipids, and proteins, these secondary chemicals are not involved in plant growth, development, and reproduction but play a significant role in ecosystem functioning. These secondary biochemicals also play a key role in plant communication and defense, particularly under different environmental stresses. Plants may exhibit a defense response to combat these abiotic environmental stressors by generating a variety of PSMs to minimize cell and tissue damage. Secondary metabolites are very diverse (almost more than 200,000) in nature, majorly classified into terpenoids, phenolic compounds, nitrogen, and sulfur-containing secondary metabolites, separated based on biosynthetic pathways (shikimate pathway, mevalonic pathway, and tricarboxylic acid cycle pathway). This chapter summarizes the stimulating effects of different abiotic stressors (heavy metals, cold and high temperature, light, salinity, and drought) on secondary metabolite production. A major focus is given on the synthesis of secondary metabolite and accumulation in plants under stressful conditions, and their role in the regulation of plant defense.

**Keywords:** secondary metabolites, ecosystem regulator, abiotic stresses, phenolics, plant defense, terpenoids

## **1. Introduction**

Primary and secondary metabolites are two categories of substances synthesized by plants. Primary metabolites that directly contribute to plant growth and development include lipids, proteins, and carbohydrates. On the other hand, secondary metabolites (SMs) are multipurpose metabolites that frequently participate in plant defense and environmental communications [1]. Interspecies communication, controlling the activity of enzymes, signaling, and defense are just a few of the tasks performed by SMs [2].

The SMs play a major role in the plant's capacity to endure abiotic stressors. The plant defense system has developed under a variety of stress detection systems, including transmembrane recognition, and the creation of SMs, which ultimately help plants to withstand these harsh or stressful conditions [3, 4].

Secondary metabolism also permits ecological interactions between plants and some other species and contributes to the capacity of plants to adjust and survive in response to environmental conditions throughout their lifecycle [5]. Plants are often at risk from a variety of abiotic stresses, such as oxidative and toxic metals, extreme cold, flooding, salinity, and drought [6]. Plants create a variety of low molecular weight chemicals known as secondary metabolites, such as anthocyanins, phenolic compounds, alkaloids, flavonoids, steroids, and terpenes. These phytochemicals are vital for a plant's protection, adaptation, and environmental adjustment [7].

Plant secondary metabolites are distinctive sources of flavorings, food additives, and medications with significant industrial use [8]. A transition from a rapid growth period to a stationary(inactive) phase in plants is typically due to the highest amounts of SMs production. Secondary metabolism is a crucial component of organic metabolism and biological processes; it mainly relies on primary metabolism for the contribution of the necessary enzymes, ATP, and cellular machinery and has a role to make a significant contribution to the producer organism's long-term survival. However, their manufactured organisms can grow and develop even without these secondary compounds, recommending that secondary metabolism is not necessary, at least for short-term or temporary survival [9].

Regulating plant tolerance at the cellular level is crucial for controlling stress responses and signal transduction systems [10]. For instance, the capacity of cotton to synthesize flavonoids and derivatives of cinnamic acid under drought stress tolerance indicated that they are highly effective at scavenging ROS [11], whereas the ability of reed plants to produce isoprene during heat-induced stress suggested that they have an effective antioxidant capacity that can quench oxygen [12]. There are more than 200,000 SMs known in plants [13]. Some SMs are unique to certain plant taxa, and their concentrations might differ between populations and individual plants depending on tissue type and specific plant development stage [14]. Although such SMs variations may be a result of genetic variability, their concentrations are also influenced by abiotic factors (growth circumstances) that are anticipated to get worse with climate change (such as heat stress, drought, and UV radiation) [15]. It has been believed that increased synthesis of the majority of SMs by a plant' s mechanism for chemical defense response is linked to their resistance to stress and that this may be one explanation for the existence of variation in the type and quantity of SMs produced by different plant' s taxonomic groups [16, 17].

This chapter elaborates on the sources and classification of plant secondary metabolites, their synthesis under unfavorable conditions, and their role in modulating plant growth and abiotic stress tolerance.

### **2. Sources and classification of secondary metabolites**

A simple classification of plant secondary metabolites includes three main groups: terpenes (which include plant volatiles, cardiac glycosides, carotenoids, and sterols), phenolics (which include phenolic acids, coumarins, lignans, stilbenes, flavonoids, tannins, and lignin), and nitrogen-containing compounds (which include alkaloids and glucosinolates) [18].

*Plant Secondary Metabolites and Abiotic Stress Tolerance: Overview and Implications DOI: http://dx.doi.org/10.5772/intechopen.111696*

#### **2.1 Terpenoids**

Terpenoids are the largest and most diverse collection of natural products and contain thousands of isoprene units ranging from hemiterpenes to rubber, varying in structure from linear to polycyclic molecules. Each terpene is produced by the condensed isoprene units, and all of them are classified based on the number of carbon (5C) units that make up the basic structure [19].

A monoterpene is a compound composed of two isoprene units (C10) and is found in a wide range of plants. Terpenoids were first reported in their volatile forms, where they contributed to abiotic stress tolerance and chemical communications between species [20]. Terpenes have several uses for attracting pollinators, protecting plants from herbivores [21], and also acting as deadly insecticides and insect repellents [22].

#### **2.2 Phenolics**

Phenolics are a crucial subclass of aromatic PSMs, with one or many acidic hydroxyl groups linked to phenyl-ring [23, 24]. Phenolics are a family of heterogeneous molecules made up of about 10,000 chemicals that are both soluble in water and organic solvents [25]. Plants, fungi, and bacteria can make these substances [26].

Phenolic compounds are classified based on their structural characteristics (phenols (C6), phenolic acids (C6-C1 or C6-C3), flavonoids (C6-C3-C6), and tannins (C6-C3-C6)). They can occasionally be further divided according to the degree of polymerization: (I) High molecular weight phenolic compounds, such as tannins, are water-soluble molecules that are strongly polymerized and fall under two subfamilies with distinct properties (II). The majority of saprotrophic organisms readily use low molecular weight phenolic compounds, which are discovered in most plants in glycosylated and solubilized form; however, some of them have specific biological activities such as cinnamic acids, in dicot angiosperms, the bulk of hydrolyzable tannins are present. Pro-anthocyanidins is another name for condensed tannins, which are polymers of flavan-3-ols (flavonoids) [27].

They serve significant roles in giving fruits, flowers, and seeds color, smell, and flavor, and they also have powerful antioxidant qualities because of their physical and chemical makeup [28]. Moreover, these substances have an impact on interactions between plants, such as allelopathic inhibition of targeted species growth [29].

#### **2.3 Nitrogen-containing secondary metabolites**

Glucosinolates, alkaloids, and cyanogenic glycosides make up a significant class of nitrogen-containing SMs. Alkaloids are basically alkaline compounds, which have low molecular weight and make up a large group of secondary metabolites [30]. The majority of alkaloids occur in their free state as N-oxides or mix with acids to form salts that can easily dissolve in water [24]. Alkaloids are identified due to the presence of at least one nitrogen atom, and they have a noticeable physiological impact on animal behavior; for instance, many of them have poisonous or analgesic effects (such as cocaine or morphine) [21].

Cyanogenic glucosides (CNglcs), also known as α-hydroxy nitrile glucosides, are naturally occurring bioactive plant secondary metabolites generated from amino acids and primarily composed of the sugar D-glucose [31]. According to Picmanová et al. [32], CNglcs are thought to be crucial for plant development, growth, and tolerance against different abiotic stressors [33].

#### **2.4 Sulfur contains secondary metabolites**

Phytoalexins, defensins, glutathione (GSH), and allicin are just a few of these substances that have been linked either directly or indirectly to plants' protection against microbial diseases [34]. The GSH is one of the primary forms of organic sulfur, which is present in soluble parts of plants and is essential for controlling plant development and growth as well as working as an antioxidant in cells under stress [35].

Higher plants synthesize glucosylates (GSL), a type of low molecular weight plant glucosides containing nitrogen and sulfur to boost their resistance against predators, competitors, and parasites. These compounds degrade into toxic or repellent volatile defense chemicals, for example, allylcys sulfoxides in allium and mustard oil and glucosides in *Cruciferae* [36]. A typical structural characteristic of Brassica phytoalexins is an indole or closely similar ring structure. These secondary metabolites are different compared to other well-known GSLs and appear to exclusively be produced by the plant family Cruciferae. Due to their high value, multiple research teams have looked at cruciferous phytoalexins as well as their biological activities over the past few decades [37].

### **3. Production sites and biosynthesis of secondary metabolites**

Within plant cells, many secondary compounds are biosynthesized from primary metabolites, and their production can be stimulated under many abiotic elicitors and signaling molecules [38]. Under stressful environments, specific SMs conditionally accumulate in different plant sections [39]. The Krebs cycle and the Shikimate pathway are run where the precursors of metabolites are formed. The important starting materials for SMs are primary metabolites. Based on their chemical makeup, intended use, and distribution in plants, primary and SMs can be separated from one another. In most plants, the basic biosynthesis routes of metabolites remain conserved and most of the primary metabolites are present in all tissue types. Several basic metabolic frameworks have emerged because of the preservation of this metabolic core. Many modifications in fundamental structures result from regular glycosylation, methylation, hydroxylation, acylation, oxidation, phosphorylation, and prenylation, as well as from fewer chemical modifications brought on by the specialized enzymes.

The three main categories of SMs can be separated based on biosynthetic pathways. Phenolic substances are synthesized through the shikimate pathway. Terpenes are synthesized through the mevalonic pathway, while the N-containing substances are synthesized through tricarboxylic acid cycle pathway [40]. The precursor of the shikimate pathway (from the pentose phosphate pathway) is shikimic acid, which is produced when erythrose 4-phosphate and phosphoenolpyruvate from the glycolytic pathway combine. Phenylalanine, tyrosine, and tryptophan are the precursors for PSMs like phenolics, and N-containing compounds are produced *via* the shikimate pathway and also serve as the building blocks for protein synthesis [39]. In contrast to tryptophan (which is the precursor of alkaloids, phytoalexins, indole glucosinolates, and plant hormones like auxin), tyrosine further produces isoquinoline alkaloids, pigment betalains, and quinones (such as tocochromanols and plastoquinone) [41].

*Plant Secondary Metabolites and Abiotic Stress Tolerance: Overview and Implications DOI: http://dx.doi.org/10.5772/intechopen.111696*

### **4. Metabolomic approaches for quantifying secondary metabolites**

Understanding the kinds and quantities of SMs generated by the kingdom of plants is beneficial for plant research since this production reveals how plants have evolved in nature to meet various challenges [42]. We need to use a wide range of various analysis tools to get better handling of metabolites for plant metabolomics due to the wide variety of chemical classes and characteristics as well as their enormous dynamic range of metabolite concentrations in plants.

After the organic substrate has been extracted using a non-polar solvent, the only approach for terpene analysis is through GCMS (gas chromatography-mass spectroscopy). The principle behind the technique is the specific absorption of near-infrared electromagnetic radiation by different OCs (organic compounds), which is known as near-infrared spectrometry (NIRS). This method creates spectra reflecting the material's organic composition (nitrogen, lignin, cellulose, hemicelluloses, etc.) [43]. Another recently created comprehensive method is called "metabolomics," which enables quantitative determination and all metabolites' identifications in different tissue by using a variety of analytical methods such as gas/liquid chromatography (GC/LC) combined with mass spectrometry (MS) or nuclear magnetic resonance (NMR) spectroscopy. Metabolomics provides a full snapshot of the chemical makeup of a tissue (such as a leaf) at a specific moment, enabling somebody to focus on the least understood chemicals and evaluate or recognize metabolites that have insufficient information [44].

Folin-Ciocalteu method, used to calculate the concentration of OH− phenolic compounds that are bound to the benzene ring is the method most frequently employed to examine the phenolic fraction of plants [45]. The Folin-Ciocalteu reagent reacts with phenolic groups to produce a blue complex, which is the basis for this procedure [46].

The STME (Silicone tubing microextraction) is a technique created by Mohney et al. [47] and is one of the newer techniques that open up new opportunities for studying secondary metabolites in soils. Direct placement of sorbent microtubes in the soil allows for *in situ* monitoring of allelochemicals released from roots [48].
