**2. Drought stress**

Decreased water availability in soil or excessive loss of water from plants due to high transpiration rate causes drought stress. At the vegetative stage, it affects growth, development, turgidity, and stomatal conductance. At the cellular level, drought stress causes damage to cell division, expansion, nutrient uptake, chlorophyll content, and CO2 assimilation [6].

#### **2.1 Molecular responses of genetically engineered plants**

Genetic engineering basically focuses on the identification of key genes involved in drought stress resistance mechanisms and their potential transfer to different crops

#### *Molecular Mechanisms and Strategies Contributing toward Abiotic Stress Tolerance in Plants DOI: http://dx.doi.org/10.5772/intechopen.109838*

through recombinant DNA technology. These candidate genes are involved in osmotic adjustments, induction of dehydrins, synthesis of abscisic acid (ABA), indole-3-acetic acid (IAA), polyamines for maintaining turgidity, tissue water potential, net photosynthesis, stomatal conductance, growth of plants, and development of deeper and prolific root system [7].

#### *2.1.1 Induction of trehalose biosynthesis*

Trehalose is a compatible osmoprotectant that plays its role in improving root architecture, maintaining the integrity of thylakoid membrane and sugar signaling pathway, and increasing photosynthesis, stomatal conductance, and drought resistance [8]. The bacterial OtsB gene is introduced in tobacco for better growth and development under drought stress by promoting a deeper and more prolific root system. Such transgenic tobacco plants show more photosynthetic and water-retaining capacity [9]. The yeast TPS1 gene is involved in developing drought tolerance in transgenic plants by regulating carbohydrate levels through SnRK1and ABA signaling pathways to regulate stomatal conductance [10].

#### *2.1.2 Induction of LEA protein*

LEA (late embryogenic abundant protein) plays a role in the sequestration and compartmentalization of ions. They also protect other proteins from degradation during cellular dehydration. HVA1 gene isolated from barley is introduced in rice for higher growth rates, hydraulic conductivity, and water permeability under drought stress [11]. Transgenic wheat containing barley HVA1 gene showed overexpression of aquaporins (PIP1, PIP2,NIP, TIP3, XIP) for cell proliferation, ions transport, germination, and morphogenesis under drought stress [9].

#### *2.1.3 Induction of proline and polyamines biosynthetic pathways*

Proline and polyamines are compatible solutes, whose expression is regulated under drought stress. They are of low molecular weight and are highly soluble in the cytosol. P5CS gene isolated from moth bean is inserted in tobacco plants for detoxification of ROS and stabilizing structures of membranes, enzymes, and proteins [10]. Transgenic soybean plants modified with P5CR gene showed increased proline accumulation for plant growth and development under drought stress. It also protects the lipid bilayer from damage during cellular dehydration [11]. Genetic engineering of the induction of polyamine biosynthetic pathway mainly focuses on two species rice and tobacco. Transgenic rice modified with oats ADC (arginine decarboxylase) shows increased biomass due to regulated plant growth, antioxidant defense, and metabolism [12].

#### *2.1.4 Induction of transcriptional factors (TFs)*

Transcription factors are proteins that bind to the promoter of the respective gene to regulate its expression. Under drought stress, different transcription factors are activated, such as DREB (DREB1A, DREB1B, DREB1C) and CBF (CBF1, CBF2, CBF3). Transgenic wheat modified with DREB1A gene from *Arabidopsis thaliana* showed increased drought resistance by overexpressing drought-tolerant genes [13].


**Table 1.**

*Molecular responses of transgenic plants to drought stress.*

#### *2.1.5 Induction of MAPK pathway*

MAPK is involved in phosphorylating other protein molecules that initiate a downstream oxidative signaling cascade. NPK1 gene isolated from tobacco is inserted in maize to increase drought tolerance of transgenic maize by protecting photosynthetic machinery under drought stress [14].

#### *2.1.6 Induction of ABA signaling pathway*

Under drought stress, the level of ABA increases to cause the closure of stomata and to prevent water loss through transpiration. Genetically engineered plants modified with ERA1gene isolated from *A. thaliana* showed better growth, development, and net photosynthesis under drought stress as compared to control. Identification and transference of candidate genes from donor to transgenic plant and their molecular responses have been summarized in **Table 1**.
