**3. Temperature stress (chilling/freezing and heat stress)**

Temperature variations across the world have a direct effect on plant productivity. There is a prominent change in the growth and survival patterns of plants under temperature stress. Temperature stress is divided into two categories, that is, low-temperature stress (chilling injury) and high-temperature stress (heat stress).

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

## **3.1 Chilling injury**

Morphologically, chilling injured leaves become purple or red in color, and wilting of leaves is also observed. Growth is retarded and foliage of leaves appears soggy. At the cellular level, changes in membrane structure and composition due to decreased fluidity and permeability of plasma membrane were observed. Decreased photosynthesis activity under low temperature is mainly due to distorted and swollen thylakoids, reduction in size and number of starch granules, unstacking of grana, and disappearance of the chloroplast envelope. Condensation of chromatin, alternation in the appearance of nucleolus, Golgi apparatus, and endoplasmic reticulum has also been reported under low temperatures.

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

### *3.2.1 Induction of dehydration-responsive element (DRE)*

DREB1A is a transcription factor that interacts with DRE to induce the expression of cold-responsive genes (COR). At low-temperature, ICE transcription factor turn on the expression of CBF/DREB genes, which induces freezing tolerance [20]. Transgenic *A. thaliana* over-expressing ICE-1 showed increased tolerance to chilling stress by regulating CBF and other cold-responsive regulons [21]. In wheat, ICE-1 homologs TaICE141 and TaICE187 are overexpressed to activate the wheat CBF family. Transgenic *A. thaliana* modified with these homologs showed increased freezing tolerance by transcriptional and post-transcriptional changes [22].

### *3.2.2 Induction of cold-responsive LEA proteins*

LEA proteins act as antifreeze proteins that prevent ice nucleation and the formation of ice crystals. They slowed the growth and recrystallization of ice. PmLEAS is a cold-responsive gene, which is expressed in *Prunus mume* under chilling stress. Transgenic tobacco modified with PmLEAS showed increased freezing tolerance by modifying the composition of the lipid bilayer to increase the proportion of unsaturated fatty acid. It also increases the activity of the desaturase enzyme [23].

#### *3.2.3 Expression of LcFIN1 gene in transgenic A. thaliana*

LcFIN1 gene is overexpressed in sheep grass to provide adaptation to cold stress. Transgenic *A. thaliana* modified with LcFIN1 gene showed high germination rates and long survival time period due to the accumulation of compatible solutes, membrane stabilization, reduced ROS generation, and expression of COR genes [24].

#### *3.2.4 Induction of dehydrins*

Dehydrins are thermostable, hydrophilic, and cryoprotective protein molecules. They are molecular chaperone. Chilling stress results in the formation of secondary structures of RNA. Dehydrins prevent the formation of secondary structures by acting as molecular chaperons. They also protect other proteins and enzymes from denaturation. ABA treatment induces the expression of dehydrins. PmLEA


#### **Table 2.**

*Molecular responses of transgenic plants to chilling stress.*

is a cold-responsive gene of *P. mume,* which is overexpressed under chilling stress. Transgenic tobacco modified with PmLEA showed increased chilling tolerance due to reduced lipid peroxidation and electrolyte leakage [25]. Similarly, the maize ZmDHN2B gene inserted in tobacco provides cold adaptations by preventing the destabilization of membranes. It also increases the unsaturated to saturated fatty acid ratio to prevent ice crystals formation [26].

### *3.2.5 Induction of compatible solutes*

Compatible solutes, such as amino acids, proline, polyamines, and sugars, provide molecular adaptations under chilling stress. Glycine betaine is very important for osmotic adjustments and subcellular functions. CodA (choline oxidase) isolated from *Arthrobacter globiformis* is inserted in transgenic *A. thaliana* provides cold acclimation [27]. Zinc finger protein gene OSISAP1 from rice is inserted into tobacco plants that showed increased growth and survival rates under chilling stress. OSISAP1 encodes for proline biosynthesis enzyme [28]. A short review of molecular responses of some transgenic plants has been summarized in **Table 2**.

### **4. Heat stress**

Heat stress means temperature above a threshold level, which causes irreversible damage to plant growth and development. Scorching of leaves, leaf senescence, abscission, fruit discoloration, reflective leaf hair, leaf curling, and vertical leaf orientation are the main morphological effects of higher temperature. Heat stress induces the production of NH3 within plant tissues. It leads to ammonia toxicity. CAM pathway is responsible for the high production of organic acids, such as pyruvate, citrate, malate, PEP, and oxaloacetic acid. These organic acids prevent ammonia toxicity within the cytoplasm under heat stress.

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

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

#### *4.1.1 Induction of heat-shock proteins (HSPs)*

Expression of genes for the synthesis of various hormones, such as ABA, ethylene, salicylic acid, and brassinosteroids, are very important for thermotolerance. These hormones stabilize the heat-shock transcription factors and help them to bind with heat shock-related genes. DcHSP17.7 is a heat shock-related gene of carrots. It was inserted into the potato under the control of 35S promoter. Transgenic tomatoes showed increased tolerance to heat stress by stabilization of the tertiary structure of proteins and enzymes [34]. Fad8 is a cytosolic protein of *Brassica napus,* which is overexpressed in the tobacco plant. Transgenic tobacco showed much more heat sensitivity, which shows that silencing of fad8 is important for heat stress tolerance because *fad8* encodes for desaturase enzyme [35]. OsHSFA2e was isolated from *oryza sativa* and was introduced in *A. thaliana.* Resultant transgenic *A. thaliana* showed increased thermotolerance by upregulation of HSF-related genes [35]. Slhsp gene isolated from *Solanum lycopersicon* was introduced in *N. tabacum*. hsp101 gene was isolated from *A. thaliana* and was introduced in *O. sativa.* Both of the transgenic plants showed increased thermotolerance due to prevented protein aggregation [36]. *A. thaliana* AtPLC9 gene is responsible for heat tolerance as it induces the expression of HSPs and HSFAs. AtPLC9 gene was inserted into *O. sativa.* Transformed rice plants showed increased heat stress tolerance due to the over-expression of OsHSFAs, calcium ions, and calmodulin-related genes [37].

#### *4.1.2 Induction of membrane associated lipid metabolism*

Heat stress increases membrane fluidity, which causes disruption of cellular functions and membrane permeability. Plants achieve adaptation to heat stress by


#### **Table 3.**

*Molecular responses of transgenic plants to heat stress.*

increasing saturated fatty acids in membrane composition. Fad7 gene isolated from *A. thaliana* was introduced in *N. tabacum*. This gene encodes for the desaturase enzyme. Transformants that showed silencing of fad7 gene were able to adapt to heat stress more effectively. Similarly, fad8 isolated from *B. napus* was introduced in *N. tabacum.* Transformants with silenced fad8 gene showed better growth, chlorophyll content, and photochemical efficiency [38]. A short review of molecular responses of transgenic plants to heat stress has been summarized (**Table 3**).
