4.2.2. Terpenoids

Plants have developed a complex resistance system that depends on the swift perception and instigation of secondary metabolites to adopt different environmental stress factors in an ecosystem [203]. Among all, terpenoids establish a broad and structurally diverse group of lipophilic secondary metabolites, which are produced in plants from isoprene units (C5H8) [204]. Physiologically, terpenoids play an important role as phytohormones, such as the sesquiterpenoid abscisic acid (ABA) and the diterpenoids gibberellic acid (GA), against biotic and abiotic stresses. Studies have shown that the phytohormone abscisic acid (ABA) triggers defense mechanisms, such as facilitating responses to drought and water stress by adapting the membrane properties [205]. Moreover, terpenoids show antioxidant and antibiotic activity that maintains lipid membranes and increases environmental stress tolerance against herbivores [206].

Terpenoids also function as phytoalexins (LMW antimicrobial compounds), prepared as part of the plant defense mechanism in response to abiotic and biotic factors. For example, many diterpene phytoalexins have been reported in *Oryza sativa* [207]. Similarly, in cotton plants, sesquiterpenoid phytoalexins, such as gossypol, hemigossypolone, and heliocides, as defensive metabolites accumulated both above and below the ground against pathogens and herbivores [208]. Moreover, in maize and rice leaves and roots, diterpene phytoalexins are produced, including zealexins, kauralexins, and oryzalexins that exhibit antimicrobial properties and respond against pathogenic fungal blast diseases, such as rice blast caused by *Magnaporthe grisea* [207,209]. Additionally, UVr and TM stress induced the accumulation of rice phytoalexins. According to Vaughan et al. [209], the accumulation of phytoalexins in response to drought is root-specific and does not affect the level of phytoalexins aboveground. However, the reduced content of the terpenoid compound was described in cotton species in drought conditions [191,210]. Yusuf et al. [211] noticed that the increased content of soluble alcohol tocopherol with antioxidant properties shows a significant role in the mitigation of stress by stabilizing the cell membranes induced by salinity, TM, and osmotic potential in *B. juncea*. Furthermore, the content of saponins in soybean plants was recognized as one of the crucial secondary metabolites related to the resistance of salt stress [212].

**Table 2.** The response of various types of metabolites against different abiotic stresses.



#### **Table 2.** *Cont.*

#### 4.2.3. Nitrogen-Containing Secondary Metabolites

Plants have developed several defense mechanisms against invading enemies, such as microbial pathogens and herbivorous animals, as well as abiotic factors, e.g., drought, waterlogging, and salinity, which are considered for the high loss of crop production worldwide [217]. However, plants have developed a complex defense system of secondary metabolism against these stressors, including the nitrogen-containing secondary metabolites, such as alkaloids, cyanogenic glycosides or glucosinolates, and non-protein amino acids (Figure 4) [158,217]. Previously, nitrogen-containing secondary metabolites were considered unwanted materials of plants and are known now for their resistivity towards different stress factors [119]. Among the phytochemicals, alkaloids are heterogeneous groups of secondary metabolites consisting of one or more nitrogen atoms produced under abiotic stress conditions. It has been found that alkaloids perform a significant role against microbial pathogens and herbivorous animals. Besides, more alkaloid contents and derivatives are produced in abiotic stress conditions. For example, poppy plants make more alkaloids when there is a drought period as well as under salinity stress [214]. In lupins (*Lupinus termis*) cultivars, the content of alkaloids was also influenced by the drought and activated yeast extract treatment [216].

**Figure 4.** Chemical structures of some plants derived primery and scndary metabolites with key importance in different era of lfe. Among all, some commonly known alkaloids (**A**), cyanogenic glycosides (**B**), and (**C**) non-protein amino acids along with their protein amino acids analogues.

Additionally, glucosinolates and cyanogenic glycosides are sulfur and nitrogen-containing secondary metabolites derived from glucose and amino acids. Similarly, Rodziewicz et al. [96] demonstrated that all these natural compounds play a significant role against different environmental factors (biotic and abiotic). Mewis et al. [215] showed that in *A. thaliana*, under drought and water logging conditions tend to increase aliphatic compounds of glucosinolate and flavonoids. Moreover, in *B. juncea,* the increased level of glucosinolate was observed during the vegetative stage under water deficit conditions. In plants, apart from the essential 20 amino acids, there are more than 200 free plant cell amino acids that

are not assimilated into proteins. These free amino acids are called non-protein amino acids. Their major function in plants is to respond to various environmental stresses [158,217].

#### **5. Conclusion and Future Perspectives**

The acceleration of climate change increases the severity of damage to crop productivity under environmental stress. Understanding the role that primary and secondary metabolites play during stress resistance mechanisms is important for developing crop species and improving their stress resistance, ensuring that the need for food security is met for a growing global population. However, less has been understood about the function of these metabolites against environmental stresses in plants, especially abiotic stresses. In the current review, we have provided an overview of the role of primary (amino acids, polyamines, carbohydrates, glycine betaine, and lipids) and secondary (phenolics, flavonoids, terpenoids, alkaloids, and glycosides) metabolites against several abiotic factors, such as drought, salinity, temperature, UVr, and TM. Analysis of more than 200 articles allowed us to describe the main responses of primary and secondary metabolite products of different plant species to abiotic stresses. Metabolomics has occupied a prominent place in plant stress physiology and biology research. Metabolic change due to abiotic stress is complex to describe the variability between different plant species. Nevertheless, metabolomics needs more extensive research in data annotation, assessment, processing, and evaluation. Progress in "omics" tools and bioinformatics and enhanced assimilation of the data from varying molecular levels is needed. Hence, to expose the full picture of sustaining mechanism, which will lead to new biomarkers of resistance towards biotic and abiotic stresses. Affirmation of the impact of environmental stresses on plants and their metabolite level responses recorded valued genes about the mechanism underlying such acclimation. However, the balancing mechanism between the gene expression and the subsequent metabolic phenotype is a big challenge nowadays. Therefore, comprehensive research of the dynamic behavior of metabolic systems is a great task for researchers in systematic biology. Furthermore, identifying the genetic background behind the diversity of primary and secondary metabolites produced by plants will help in improving and developing stress tolerance. Manipulating and overexpressing genes related to the biosynthetic pathway of secondary metabolites could be a solution for plant tolerance to environmental stress conditions.

**Author Contributions:** Conceptualization: S.A. and U.S.; Investigation: Z.-H.T. and S.A.; Resources: S.U., U.S., A.A.E. and Y.K.; Software: S.U. and A.A.E.; Supervision: Z.-H.T.; Writing—original draft: S.U.; Review and editing: A.A.E., J.K., and Y.K.; Equal contribution: S.U.; References: A.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** The key Research and Development project of Heilongjiang Province, China (JD22A008).

**Acknowledgments:** The authors are thankful to the China Scholarship Council (CSC) for support and to the Key Laboratory of Plant Ecology, Northeast Forestry University, Harbin 150040, China, for providing an excellent research environment. Moreover, Uzma Salam and Shakir Ullah are thankful to Zhong-Hua Tang for his continuous support and assistance during their studies.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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