4.1.5. Lipids

Lipids are a fundamental component of biological membranes, particularly the plasma membrane, which serves as the contact between the cell and its surroundings [72]. Lipids can be grouped into eight major types based on the chemical structure in conjunction with distinctive hydrophobic and hydrophilic components, such as fatty acids, glycerides, phosphoglycerides, sphingolipids, steroids, isoprenoids, glycolipids, and polyketides [139]. Being sessile organisms, plants are subjected to a wide variety of biotic and abiotic factors, such as temperature, drought, heavy metals, salinity, and pathogen attack. However, lipid-mediated signaling occurs in response to all these stressors (Figure 2). The plasma membrane, which is typically the signaling source of lipids, is commonly used by plants to sense these stimuli and transform the signal into subsequent biochemical metabolism. Generally, these are acclimating enzymes that have all been proposed as signaling lipids, such as phospholipases, lipid kinases, and phosphatases [140]. Commonly, lysophospholipid, fatty acid, phosphatides, triacylglycerol, inositol phosphate, oxylipins, sphingolipids, and nacylethanolamine are considered the major contributing signaling lipids molecules [141]. The conformation and activity of cellular proteins and metabolites are influenced by signaling lipids because they have the ability to temporarily attract molecular markers to the membrane.

The enzyme phospholipase A (PLA) is very important in the formation of fatty acids and lysophospholipids. Usually, lysophospholipids are present in very limited amounts in plant tissues; however, in stressed conditions such as freezing their quantity increases [142]. Some reports revealed the physiological role of lysophospholipids against various environmental stresses. Similarly, the phospholipase A2 (PlA2) has been shown to increase the production of some elicitors in poppy plants [143], while lysophosphatidyl-choline and lysophosphatidyl-ethanolamine act as signals transducers in arbuscular symbiosis in potato [144].

Fatty acids have also been demonstrated as stress-responsive lipids in plants. Oleic acids modulate nitric oxide-related proteins, thereby regulating nitric oxide and mitigating tolerance in *Arabidopsis* [145]. Moreover, fatty acids also regulate drought, salt, and heavy metals tolerance, as well as the wound-induced responses of pathogens/herbivores in plants [146]. Likewise, the responsive role of phosphatidic acid (PA), inositol polyphosphates, oxylipins, sphingolipids, and some other lipids have been studied in various plant species [147–149]. Some of the environmental stress factors under which the plant lipid responses were reported to include chilling, freezing, and wounding [150], pathogens [151], low-temperature stress [152], salt stress [153], and water and drought [154] stress response.

#### *4.2. The Response of Secondary Metabolites to Abiotic Stresses*

Primary metabolites are compounds that are related to important physiological functions in organisms. Hence, they are generally found in all plant species and are directly involved in growth, development, and reproduction [155]. Compared to primary metabolites, secondary metabolites are very definite in their function, as they are not directly involved in plant growth, development, and reproduction of organisms. Generally, they are species-specific that could be redundant in different situations [156]. Usually, they

are made under particular conditions for a definite purpose, such as defense against pathogens infection, enhanced resistance to abiotic stresses, and protect the harmful effect of UVr [157]. Furthermore, secondary metabolites produce different compounds important for several biochemical and biophysical processes in plant cells and tissues (Table 2). However, they have no common familiar physiological functions in plants, such as photosynthesis, respiration, translocation, transportation of solute, acclimatization of nutrients, and differentiation [158].

In addition, the specified plant species produce these natural products, and their concentration level is controlled to some extent with the growing period, environments, and adjustment progress [159]. Substantially, they attracted insects and animals for fertilization and seed spreading. The accumulation of phenyl amides in beans to the impact of abiotic factor (heat) was described, proposing an antioxidant role of these secondary metabolites [160]. Modern research tries to identify the key roles the secondary metabolites play in plants as indicators, antioxidants, and for other purposes. Secondary metabolites are also important in plants used by humans [161]. Besides, the compounds of secondary plant metabolites are distinctive means of food essences, medicines, flavorings, and other industrial materials [162]. In plants, the accretion of certain metabolites frequently occurs exposed to different stress factors, such as several phytohormones, elicitors, TM, and signal transduction compounds [163–165].

Some famous examples of secondary plant metabolites with medicinal properties include the anesthetic and antipyretic compounds salicin taken from *Salix* sp., which is used to make aspirin [166]. Similarly, other pharmacological secondary metabolites, such as taxol (anticancer), sequestered from pacific yew (*Taxus brevifolia*), and the strong obsessive compound morphine removed from opium (*Papaver somniferum*). Secondary metabolites have the following three major groups: phenolics, terpenes, and S and N comprising compounds (Figure 3) [167,168].

**Figure 3.** Some eminent examples with medicinal properties of secondary plant metabolites are (**A**) salicin, (**B**) taxol (paclitaxel), and (**C**) morphine.

#### 4.2.1. Phenolic Compounds

In plants, phenolic compounds are recognized as the largest and essential group of secondary metabolites changing from simpler aromatic rings to more complicated ones, such as lignin, and play a significant physiological role in increasing the resistance and adaptableness suboptimal circumstances during the life cycle of plants [169,170]. Phenolics are produced in optimum and sub-optimum environments in plants and play a major role in various developmental mechanisms, such as cell division, balancing hormones, photosynthetic processes, and reproduction, as well as in the mineralization of nutrients [75]. These compounds constitute secondary metabolites, including lignins and tannins, flavonoids, isoflavonoids, anthocyanins, and coumarins [171]. Moreover, all these chemical compounds are produced in plants by the phenylpropanoid pathway, in that phenylalanine compound

is the main substratum that can do significant work in the resistance mechanism of plants against various stress factors in the environment [172].

The pathway of phenylpropanoid is regulated by biotic and abiotic factors, including drought, salt stress, TM, low or high temperature, wounding, pathogen attack, herbicide treatment, nutrient deficiencies, and UV radiations causing the accumulation of different phenolic compounds [75,173]. Consequently, the aggregations of phenolic compounds in plant materials are considered an important sustaining strategy of plants in harsh environmental situations. Hence, respond to these stresses and contribute to the removal of ROS, catalyzed-oxygenated reaction with the establishment of metabolic structures and obstructing the processes of oxidative enzymes, thus increasing evolutionary aptness [174,175]. Besides, phenolic accumulation is also considered a reliable feature and key defense mechanism under stress, leading to the enhanced creation of free radicals and other oxidative species in plants [176].

Moreover, to survive in oxidative stress conditions, plants had established two diverse biological ways, such as escaping ROS creation and eliminating it through enzymatic and non-enzymatic processes, such as the deposition of LMW antioxidants [177,178]. Further, studies revealed that the accumulation of LMW antioxidants results due to the activities of phenylalanine ammonia-lyase (PAL), chalcone synthase (CHS), and other essential enzymes [179]. Various physiological processes of plants related to growth and development in plants comprising seed germination, cell division, and synthesis of photosynthetic pigments, are influenced by phenolic acids and flavonoid accumulation for persistence and adaptation to environmental conflicts [180,181]. In particular, phenolic compounds consult greater tolerance in plants such as TM stress, which enhances the production of ROS and reduced growth [182], and phenolic compounds (flavonoids), in response, protect plants from oxidative stress damage through the chelation process [183,184]. Similarly, when plants are exposed to other abiotic factors can also affect their life cycle. Under drought conditions, the concentration of ferulic acid decreased, while the p-coumaric acid and caffeic acid increased in maize xylem sap, which could be supportive in stiffening and lignification of the cell wall [185]. Spatially confined fluctuations in cell wall phenolics were presented to be engaged in the advanced inhibition of wall extensibility and root growth, which can enable root acclimation to drought [186].

Various environmental stress factors mediated the synthesis of flavonoids, isoflavonoids, and anthocyanins. In plants, flavonoids play a defensive role due to their antioxidant properties when exposed to a water-deficit situation [187]. Moreover, Nakabayashi et al. [188] indicated that flavonoid significantly improves resistance in *A. thaliana* in water scare conditions. Similarly, phenolic acids and flavonoids as antioxidants and sunshades are involved in plants' response to a dry environment [189]. Other studies reported that different polyphenolic was associated with gene expression reforms in an account of potato plants under drought conditions, though the fluctuations were greatly specific to the cultivar [190]. Rodziewicz et al. [96] and Parida et al. [191] suggested that polyphenols are involved in conserving osmotic potential in cells and confiscating free radicals during drought stress. Besides, polyphenols affect the source and movement of organic and inorganic soil nutrients existing for plants and microbes and indicate a reply to nutrient insufficiency, therefore offering a way for identifying nutrient ailments earlier to the occurrence of evident symptoms [192]. Stress conditions of drought and waterlogging increased the flavonoids quercetin and rutin in the herbaceous pharmaceutical plant *Hypericum brasiliense*, whereas cold stress caused a different reaction [193]. Comparatively, a greater decrease in flavonoids was noticed in the sensitive genotypes; thus, they could show that the flavonoid content was imperative in sustaining the greater antioxidant activity in water-stressed conditions [96,194].

Furthermore, anthocyanins were identified to increase their content in plant tissues against drought and cold stress because of their antioxidant and ROS scavenging properties, which cause protection to plant cells [195,196]. In red-fleshed apple callus culture, low temperature (16 ◦C) tempted an increased level of anthocyanin [197].

Additionally, the assembly of phenolics rises into the cell wall either as suberin or lignin under low-temperature stress [198]. Though, suberin deposition and lignification increase the adaptability and resistance to cold stress [192]. Similarly, to respond to the negative effects of Uvr, endogenous phenolic compounds (flavonoids) accumulate in plant cells and make a shield under the epidermal layer, which protects the plant and the component of the cell from these harmful radiations [199]. Moreover, flavonolignan silymarin has been reported to accumulate in in-vitro cultures of *Silybum marianum* upon application of abiotic stress treatments, such as NaCl, polyethylene glycol, and gamma irradiation, because of the defensive mechanisms that cells perform to counteract the stress of these factors [200]. Therefore, stress elicitation successfully produces high-value phytoconstituents from medicinal crops [201,202].
