**4. Biochemical and molecular mechanisms of drought tolerance**

Plants via morphological adaptation avoid a range of environmental stressors. The drought tolerance mechanism is connected with several biochemical, morph physiological and molecular processes. The hormonal interaction inside the plant body controls these activities intensely.

*Enhancing Water Use Efficiency by Using Potassium-Efficient Cotton Cultivars Based… DOI: http://dx.doi.org/10.5772/intechopen.112606*

#### **4.1 Abscisic acid (ABA)**

ABA is a natural plant stress hormone; stress response, growth, and reproductive behavior in cultivated plants. Osmotic stress in plants is linked to drought and the low available water induced by the synthesis of ABA and adaptation mechanisms [22]. The abscisic acid production is triggered after the reception of stress signals by the plasma membrane, excluding the xanthoxin transition into ABA. This usually happens in the cytoplasm [120]. ABA is typically rooted and transmitted by vascular tissues to higher regions of the plant [111]. In cotton, ABA is detected and transmitted using ABAdependent or ABA-independent transmission, whereas the former are essential actors for stress-responsive gene expression under many stressors, including osmotic pressures. Many receptors in the plasma membrane, cytosol, the envelope of chloroplast, and nucleus have been identified. The plants display a low ABA concentration in the nonstress environment; sugar nonfermenting protein kinase 2 (SnRK2) is suppressed by protein phosphorylation of 2C (PP2C). ABA enhances drought tolerance in cotton plants through the modulation of stress-related genes. ABA-induced overexpression of cotton genes GhCBF3 in Arabidopsis led to drought tolerance in transgenic lines, with greater levels of relative water, chlorophyll, and proline than wild type [121]. The AREB1 and AREB2 are more expression-level compared to the wild transgenic line, while the stomata aperture is lower when ABA is treated. Suggesting that, through the ABA signaling route, GhCBF3 may increase drought resistance.

#### **4.2 Jasmonic acid (JA)**

Plant phytohormone and its active by-products known as jasmonate are considered Jasmonic acid (JA). In fighting many biotic and abiotic stressors, it plays a crucial function. In addition, JA is related to improving root structures, tendril coiling, pollen generation, and fruit maturation [122]. Exogenous application of jasmonate has been found to enhance plant performance in drought environments [18, 123]. The mechanism and production of jasmonic acid signaling were intensively researched. The repressor protein jasmonate-zim (JAZ) plays an important function in the JA signalization pathway as the JA signaling switch. Jasmonate/Jasmonate-Zim (JAI3/JAZ), proteins that are not under stress or lack of JA, are associated with and eliminate various elements of transcription, including myelocytomatosis (MYC2). Nonetheless, the degradation of JAZ proteins, as illustrated above, occurs under deficiency water, producing the effect of active transcription factors, that is, MYC2, which regulates stress tolerance-related genes [32]. Plant hormones usually do not work in one path, but depend partly on each other in different periods to control the environment and development paths. In plants, signal transduction occurs, and many changes may be organized to adapt in a challenging way to harsh conditions [123].

#### **4.3 Reactive oxygen species (ROS)**

The fractional reduction of ambient O2 is responsible for the formation of reactive oxygen species (ROS). The cell ROS consists of four categories: radical hydroxyl (HO·), radical anion superoxide (O2−), peroxide hydrogen (H2O2), and singlet oxygen (1O2). HO· and 1O2 are very reactive and may oxidize DNA and eventually causes cell death with RNA, lipids, and proteins [75]. The generation of ROS induces subcellular locations, namely the cell wall, chloroplast, nucleus, mitochondria, and plasma membrane [124]. Drought production increases such as a reduction in CO2 fixation

leading to a decrease in NADP+ reconstruction during the Calvin cycle. This lowers the activity of the photosynthesis chain. Moreover, the Mehler reaction in electrodes may potentially increase the generation of ROS during photosynthesis by too many electrons leaking to O2 [125]. By donating an electron in photosystem-I, the Mehler reaction decreases O2 to O2. O2− via superoxide dismutase, which can be converted into water by ascorbate peroxidase, may be changed into hydrogen peroxide. It is difficult, however, to evaluate ROS levels of those created by photorespiration during the Mehler reaction. The photo-respiratory route also increases moisture stress, especially if RUBP oxygenation is strong because of the partial fixing of CO2. Some 70 percent of overall humidity stress generation H2O2 takes occurs by photorespiration [125].

Plants have sophisticated systems to check the ROS redox homeostasis, to avoid extra ROS in cells. Changes in the metabolism of antioxidant enzymes may impact drought resistance in cotton plants. Plants have created antioxidant mechanisms to keep growing. This system consists of enzyme and nonenzyme additives. These enzymes include dismutase of superoxides, ascorbate peroxidase, and peroxides of guaiacol, reductase monodehydroascorbate, catala, reductase, and glutathione reductase of dehydroascorbate. Nonenzymatic components include reduced ascorbic acid (AA), flavonoids, carotenoids, proline, glutathione, and (GSH). Both components operate together to break ROS [126, 127]. The Halliwell Asada route detoxifies the H2O2 with the ascorbate peroxidase along with NADH MDAR, and GR [127]. In MDHAR, Ascorbate decreases MDHA. However, 2 MDHA molecules can be converted into MDHA and dehydroascorbate without being enzymatically reduced to ascorbate via the NADH and GR cycles [128]. The presence of NADPH reduces glutathione (GSH) via GR oxidation. Glutathione reductase activity rises with humidity stress, retaining oxidized and decreased levels of glutathione [129]. If oxidative signals and/or losses occur, the balancing of antioxidant activity and ROS generation determines whether [26]. The antioxidant capacity of various cotton cultivars affects the potential resilience to dry conditions. The moisture stress in cotton causes ROS formation; however, the ROS scraping process may also be improved and maintained by APX and GR activities [130].

The use of nutrients (Zn) was shown to reduce oxidative damage to cotton caused by polyethylene glycol (PEG). This raises the amount of CAT, APX, SOD, and enzyme-free antioxidants [52]. Increasing GR activities and better lump-sum levels have been reported in [130] drought-tolerant (CCRI-60). The CCRI-60 was capable of scavenging free radicals and protecting the plants against severe circumstances, in comparison to the sensitive (CCRI-27). This indicates better development and increased drought stress resistance. GbMYB5 down-control in Gossypium barbadense has resulted in decreased activity with antioxidants such as CTA, peroxidase (POD), SOD, and Glutathione S-transferase (GST) [30]. Further research is, however, necessary to discover genes in drought-resistant cotton cultivar pathways associated with the antioxidant enzyme. The use of Zn and K supplies can also help strengthen the cotton plant's antioxidant system [131].
