**5. Heavy metal stress**

Plants required a small amount of HM for the proper functioning of their physiological processes. When the concentration exceeds the threshold value, these HM become toxic for plants. Excess of arsenic (As) causes photosynthesis inhibition, and decreases biomass and yield; cadmium (Cd) toxicity causes chlorosis, reduced water, and nutrient uptake, browning of root tips, and ultimate death; chromium (Cr) and lead (Pb) stress cause reduced nutrient uptake and disturbance in metabolic pathways, respectively. Mercury (Hg) and zinc (Zn) toxicity cause reduced photosynthesis due to the inhibition of photosystems I & II. Excess of nickel (Ni) causes retarded seed germination, reduced plant height, reduced root length, and reduced chlorophyll content [42].

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

### *5.1.1 Induction of the organic acid biosynthetic pathway*

TaALMT1 gene isolated from *Triticum aestivum* was inserted into tobacco and barley crops. Transgenic tobacco and barley showed increased tolerance to HM stress because TaALMT1 induces the expression of the malate biosynthetic pathway. Malate acts as a metal chelator and causes metal efflux. SbMATE gene isolated from sorghum was inserted into *A. thaliana.* Transgenic *A. thaliana* showed increased HM-stress tolerance. SbMATE induces the expression of citrate transporter for metal efflux [43].

#### *5.1.2 Genetic engineering for Cd toxicity tolerance*

Cd is a highly toxic metal due to its fast mobility and persistency. A very small concentration of Cd is lethal to plants. Different genetic engineering approaches have been implied to develop transgenic plants that can withstand Cd toxicity. *gsh1* isolated from *E. coli* was inserted into *Brassica juncia,* which showed increased Cd tolerance. gsh1 gene encodes for γ-glutamylcysteine synthetase for the synthesis of glutathione (GSH) and phytochelatins (PCs). GSH plays role in HM-induced ROS scavenging by initiating the ascorbate-glutathione cycle. PCs form a complex with HM (HM-PC), which is transported to the vacuole for detoxification. *N. tabacum* modified with RCS1 gene of *O. sativa* showed higher cysteine synthase activity [44]. CDna-LTC1 a nonspecific transporter of Cd was introduced in tobacco that showed increased Cd tolerance due to less storage of Cd in roots [45].

### *5.1.3 Induction of the expression of metallothioneins (MT)*

MTs act as chelator that binds with free metals and releases them slowly. MT1 gene isolated from chickpea was inserted into *A.thaliana, which* showed increased HM-stress *Molecular Mechanisms and Strategies Contributing toward Abiotic Stress Tolerance in Plants DOI: http://dx.doi.org/10.5772/intechopen.109838*


#### **Table 4.**

*Molecular responses of transgenic plants to HM stress.*

tolerance by upregulation of antioxidative enzymes (APX, GPX, GSH, GR) and reduced electrolyte leakage [46]. OsMT1e-P, a MT gene of *Oryza sativa* was inserted into tobacco that showed improved HM-stress tolerance (Cu and Zn) by metal ions compartmentalization and vacuolar sequestration [47]. Human MT2 was inserted into tobacco and oil seed crops. Transference of various MT genes (human MTIA, human MTII, yeast CUPI, pea PsMTA, and TaMT*)* into *A. thaliana, Brassica compestris,* and *N. tabacum* showed increased HM-stress tolerance due to overexpression of GSH-S- transferase activity. BcMT1 and BcMT2 genes from *Brassica compestris* were inserted into *A. thaliana and* showed improved tolerance to HM stress by upregulation of the activity of anti-oxidative enzymes [48].

#### *5.1.4 Induction of the expression of metal transporter genes*

Metal transporters are important for the transportation and compartmentalization of free metal ions. Genetic engineering mainly focuses on the expression of metal transporter genes in plants. Induction of AtPHT1/AtPHT7 genes isolated from *A. thaliana* along with YCF1 gene of *Saccharomyces cerevisiae* in tobacco showed much more As tolerance and accumulation. TgMTP1 gene from *N. Goesingense* was genetically engineered into *A. thaliana* that showed improved Zn tolerance [49]. *Znta* gene isolated from *E.coli* was inserted into *A. thaliana* that showed increased resistance to Pb and Cd. Znta gene encodes for V-type ATPase metal transporter that transports free metal ions from cytoplasm to vacuole for sequestration [50]. PvACR3 transporter gene isolated from *Pteris vittata* was introduced in *A. thaliana,* which showed increased tolerance to As [49]. A brief review of the molecular responses of transgenic plants to HM stress has been summarized in **Table 4**.

### **6. Salinity stress**

Increased concentration of soluble salts in the soil causes salinity stress. Salinity stress causes high-level accumulation of Na + and Cl- ions within the cytoplasm, which disturb enzyme activities and photosynthetic processes. It also causes ROS-induced oxidative damage to lipids, proteins, and nucleic acids. Other adverse effects include decreased nutritional value of plants, salinity-induced osmotic stress, decreased rate of seed germination, and decreased plant growth and productivity.

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

#### *6.1.1 Induction of the expression of Na+/H+ antiporter*

Genetic engineering approaches focus on the identification of various genes that encode ion transporters, antiporters, cationic channels, compatible solutes, osmoprotectants, etc. Ion transporters play important role in the selective transport of ions and maintain the optimal level of these ions. Vacuolar Na+/ H+ antiporter catalyzes the exchange of Na + from the cytoplasm to vacuole for sequestration. It helps in maintaining cellular homeostasis, pH, and cell turgidity. *B. napus* modified with AtNHX gene from *A. thaliana.* AtNHX gene encodes Na+/H+ antiporter. The transgenic plant showed increased salt tolerance, growth, and photosynthetic rate. Similar results were observed when *Brassica juncia* was transformed with pgNHX1 gene [54]. *T. aestivum* was modified by vacuolar Na+/ H+ antiport gene AtNHX1 from *A. thaliana*. The transgenic wheat plant showed a lower accumulated level of Na + in leaves. Transformed *A. thaliana* with a high expression level of AtNHX1 gene showed high salt tolerance. AtNHX1 gene is responsible for the compartmentalization and sequestration of Na + into the vacuole [55]. *O. sativa* was modified by Na+/H+ antiport gene nhaA from *E.coli*. Transgenic rice showed better salt tolerance, seed germination rates, growth, and productivity [56].

#### *6.1.2 Induction of the expression of SOS gene*

The high salt level is detected by receptors, which increases the cytosolic level of calcium. SOS3 binds to free Ca and activates the expression of SOS3 protein kinase. SOS3-SOS2 complex induces the expression of SOS1 gene, which encodes Na+/H+ antiporter. *ThSOS1*-*ThSOS5* genes were isolated from *T. hispidia* and inserted into *A. thaliana.* The transformed plant showed increased salt tolerance due to increased ROS scavenging activity, and lower MDA and H202 levels [57]. *SOS1* and *AHA* genes were isolated from *Sesuvium portulacastrum* and coexpressed in *A. thaliana*. Transgenic *A. thaliana* showed increased salt tolerance due to rapid Na + extrusion and regulated cellular homeostasis [58].

#### *6.1.3 Induction of the expression of HKT1-type transporters*

HKT1 transporters are responsible for regulating Na homeostasis by keeping a balance between Na and K in the cytoplasm. PpHKT1 gene isolated from almond rootstock was inserted into *A. thaliana*. The transgenic plant showed reduced electrolyte leakage, longer lateral roots, and increased salt tolerance [59]. McHKT2 gene isolated from *Mesembryanthemum crystallinum* was inserted into *A. thaliana*. Transgenic *A. thaliana* showed increased salt tolerance due to lower root Na uptake and lower Na concentration in xylem sap. A short review of molecular responses of transgenic plants to salt stress has been summarized in **Table 5**.

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


**Table 5.**

*Molecular responses of transgenic plants to salt stress.*
