4.1.2. Polyamines

Plants are tested by different stress factors and adversely affect their growth, yield, and geographical circulation [107]. To survive the combative environmental stress circumstances, plants have developed many adaptive strategies, amongst which the accumulation of metabolites plays an important defensive role [108]. Metabolites strongly involved in stress resistance are the low-molecular-weight (LMW) acyclic polyamines [109]. Polyamines are the LMW nitrogen-containing organic compounds with more than two amino groups with a positive charge at the cellular pH, allowing them to link with negatively charged molecules, such as nucleic acids, phospholipids, and proteins [110]. Usually, polyamines are polycations essential for plant growth and development and play an important role in abiotic stress resistance in higher plants. Triamine spermidine, tetraamine spermine, and their diamine predecessor, putrescine, are the general polyamines [111]. Because of their cationic nature, these compounds have often been correlated to environmental stresses, such as drought, chilling, heat, TM, and salinity [112].

The results of Khan et al. [95] and Capell et al. [113] showed that the accumulation of spermidine with the up-regulation of spermidine synthase of *Cucurbita ficifolia* augmented several stress responses in a recombinant Arabidopsis plant, such as waterlogging and salinity stresses. It was shown that spermidine acts as a signaling molecule and controls the assertion of intricate genes in drought resistance. Furthermore, it has been demonstrated that polyamines are attributed to being involved in maintaining membranes shielding from damage under stressful environments [114] and controlling the formation of nucleic acid as well as enzyme activity [115]. Additionally, different findings revealed that polyamines play a significant role in oxidative stress by mitigating the balance state of ROS through their direct contact or indirectly regulating the antioxidant system and suppressing ROS production. Moreover, some authors hypothesized that polyamines could act as a cellular signal in plants throughout the stress responses [116].

#### 4.1.3. Carbohydrates

Carbohydrates produced during photosynthesis are the main building units that provide energy and support to the plant biomass [117]. Extensive studies revealed that non-living factors lead to the assemblage of non-structural saccharides, such as sucrose and lactose, simple sugars, or polyhydric compounds (alcohols and phenols), amongst various species of plants [118]. Particularly, there is a robust association between carbohydrate accretion and osmotic stress resistance, including oxidative stress (ROS) conditions, salt stress, and the scarcity of water [95]. As a source of carbon and energy in a cell, soluble carbohydrates may take a significant part in the metabolic processes of plants. Several stress factors may impact the level of these soluble carbohydrates because the accumulation of carbohydrates is associated with photosynthesis [119]. Rosa et al. [120] demonstrated that certain soluble sugars, such as sucrose and hexoses, improved stress tolerance by down-regulating the stress-related genes and up-regulating growth-related genes. Though, the contents of certain carbohydrates, such as raffinose, glucose, fructose, and maltose, are highly sensitive to environmental stresses and increase. However, the contents of myoinositol were reduced in barley roots during water-scarce conditions [121]. The findings of Sperdouli and Moustakas [122] revealed an increase and contact of augmented soluble carbohydrates, sustaining a great antioxidant defense in the leaves of *Arabidopsis thaliana* under dry environmental stress conditions. Studies showed renovation of carbon metabolism under salt-related stress (paraquat) in *A. thaliana* tissues and inferred by the researchers as a substitute approach to staying alive [122].

In water-deficit conditions, soluble sugars function as osmoprotectants, decreasing the harmful impact of osmotic stress and helps in sustaining the turgidity of cell and cell membrane stability by keeping plants from humiliation [123]. Under stress conditions, the increase in sugar quantity is generally the result of carbohydrate hydrolysis that needs enzymes with hydrolytic usage [124]. Moreover, carbohydrates that are soluble, such as disaccharides (sucrose and trehalose), oligosaccharides (raffinose and stachyose), and polymer of fructose molecules (fructans) next to their linked metabolic enzymes are essential compatible osmolytes associated with the scavenging of unstable molecules (ROS) during their assortment in plant tissues [125]. In low-temperature stress, sugar alcohols, such as polyols, function as osmoprotectants and shield cell membranes against ice adhesion [77]. Moreover, carbohydrates may act as signaling molecules [126]. The demonstrated data advocate a specific response of carbohydrates in plants. However, it should be noted that the accumulation of carbohydrates depends on the kind of stress to which it bared [127].

#### 4.1.4. Glycine Betaine

Glycine betaine (GB) is a widely studied quat compound, which is active in retaining the water balance between the plant cell and the environment during drought conditions. Moreover, GB playing a significant role in stabilizing the macromolecules, shielding photosynthesis, detoxification of reactive oxygen radicals, and as an osmoprotectant [128,129]. Several studies indicated their importance in improving plant tolerance under various abiotic factors. It has been shown that plants are distinguished according to the formation of GB, such as barley, spinach, maize, and wheat, produce and accumulate a higher quantity of GB in their chloroplast. However, some plant species cannot obtain substantial amounts of GB during stress, such as *A. thaliana*, rice, and tobacco [130]. Furthermore, it has been shown that transgenic plants could mitigate the impact of abiotic stresses. Therefore, efforts have been made to improve tolerance through glycine betaine biosynthesis to achieve transgenic plants. In transgenic plants, such as *Arabidopsis*, the *cyanobacteria* genes, such as glycine sarcosine methyltransferase, and in transgenic maize, a greater amount of GB accumulates. As a result, in transgenic *Arabidopsis*, resistance to drought and salt is greater; nevertheless, a recombinant plant of maize retained well in cold-related to non-transgenic cultivars [131,132].

Moreover, through genetic engineering, other transgenic plants with a GB-producing capacity have been achieved, including *Brassica juncea* and tobacco with greater tolerance

to salt and chilling, indicating a progressive ability to propagate and grow well related to wild-type in abiotic environmental conditions [133,134]. Besides, transgenic tomatoes with GB synthesis were more resistant to cold stress and produced fruit at a rate from 10 to 30% higher than the wild type. [135]. Though, the meditations of GB produced in every transgenic plant were scarce to control the osmotic stress to which plants were exposed. Similarly, previous studies showed that GB could enhance root growth and reduce oxidative stress. Additionally, the exogenous application of GB improves the stress tolerance of Cr in chickpea plants [136] and salinity stress in wheat [137]. Consequently, further protecting approaches of GB, such as defense against ROS and heavy metals stress, should be considered, which may enhance the tolerance level [138].
