*2.1.2.3 Lectins*

Lectins are a diverse group of carbohydrate-binding proteins, that are toxic to insects of the orders Homoptera, Coleoptera, Lepidoptera, and Diptera. The very first demonstration of enhanced resistance of transgenic plants expressing a foreign lectin used is the gene encoding the glucose/mannose-binding lectin from pea (*Pisum sativum*) [38]. The mode of action of lectins against insects is unknown, but it has been shown that some bind to midgut-epithelial cells [39], and some insecticidal lectins also show significant mammalian toxicity, including lectins from *P. vulgaris* (phaeton haemagluttinin, PHA), and the greater insecticidal activity is shown by chitin-binding lectins from wheatgerm (WGA) expression in transgenic maize [40].

Recent interest has primarily focused on the mannose specific lectin from snowdrop (GNA), which has shown activity against peach potato aphids, potato tuber moths [41] and the rice brown planthopper (*Nilaparvata lugens*) [42]. GN expressed in potato and tomato significantly reduced fecundity and enhanced resistance, respectively, in laboratory experiments [42]. When overexpressed in different species of tobacco, tomato, and sweet gum, the tobacco anionic peroxidase, which is involved in crosslinking and polymerisation, inhibition of digestive enzymes, and the generation of highly reactive, toxic species, led to significant levels of resistance to several lepidopteran, coleopteran, and peach potato aphid [43]. The expression of tryptamine and tryptamine-derived alkaloids in plants may serve as anti-oviposition and antifeedant agents, or as inhibitors of larval and pupal development, and when expressed in tobacco, inhibition of reproduction of the whitefly *Bemisia tabaci* by observed up to 97 percent [44].

#### *2.1.3 Resistance genes from animals*

Based on in vitro testing of proteolysis inhibition by several lepidopteran larvae midgut extracts, as potential insect-resistance proteins like bovine pancreatic trypsin inhibitor (BPTI), a1-antitrypsin (a1AT), and spleen inhibitor (SI) were identified and introduced into a variety of crop. Proteinase inhibitors derived from *M. sexta* and expressed in cotton and tobacco were reported to impede *B. tabaci* reproduction [45, 46]. Despite the introduction of chitinase (from the tobacco hornworm) into tobacco, these plants only exhibited a limited level of resistance to lepidopteran larvae and peach potato aphids [47].

#### *2.1.4 Microbial proteins*

The bulk screening of microbial culture supernatants against specific pests has been one strategy to the discovery of novel insecticidal proteins. Two proteins, Vip1 and Vip2, were isolated from vegetative *Bacillus cereus* culture supernatants and have been shown to be acutely poisonous to maize rootworms [47]. Some *B. thuringiensis* vegetative culture supernatants include a protein (Vip3A) that is acutely poisonous to Agrotis and Spodoptera caterpillars [48]. These proteins activity is extremely similar to that of Bt-endotoxins, yet they are distinctly separate from them.

#### *2.1.5 Predator toxins*

Genes producing neurotoxins from predatory mites [49] and scorpions [50] have been inserted into recombinant baculoviruses, where they effectively boost the rate of killing.

### **3. Disease resistance**

Since the identification of the chemicals and genes involved in disease resistance in plants, attempts have been made to develop permanent disease resistance in commercially significant crop plants. Unfortunately, many of these efforts have failed because to the complexities of disease-resistance signalling and the wide range of infection routes employed by various pathogens. Although disease-resistant transgenic plants

#### *Genetically Modified Crops and Their Impact on New Era of Agriculture DOI: http://dx.doi.org/10.5772/intechopen.105937*

or seeds are not currently commercially accessible, future product development looks to be feasible as our understanding of pathophysiology and plant defence deepens.

In general, plants are protected by structural defence (plant cell wall, thick and waxy epidermis, trichomes, thorns) and chemical defence (production of secondary metabolites, proteins, and digestive enzymes) [51]. Plant defence response genes are classified into susceptibility genes (S), resistance genes (R), and non-host resistance genes (NHR). Pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) is broad-spectrum resistance, effective against non-host pathogens that fail to establish virulence [52, 53], and ETI is regarded to be the basis of R-gene resistance (host resistance). NHR is regarded as a quantitative trait encompassing numerous genes and varied pathways, whereas R gene resistance is regarded as a qualitative trait that adheres to Flor's gene-for-gene model and is dependent on a complex regulatory mechanism for pathogen detection and defence response [54, 55].

The successful transfer of R genes that express specific effector proteins in a gene-for-gene manner among plant species has resulted in long-term resistance and crop protection against a variety of pests and diseases. Polymorphisms in the coiledcoil (CC) and nucleotide-binding (NB) domains have been shown to be critical for recognition specificity, hence R genes could be edited [56]. In wheat, a two-amino acid mutation in the NB domain of the R gene (PM3F) protein was known to improve the resistance spectrum and HR response. S gene mutations can give both broad-spectrum and pathogen-specific resistance. Most S genes play functional roles in the plant, and mutations in them can cause serious pleiotropic defects. For example, CRISPR–Cas9-mediated disruption of the *OsSEC3A gene* in rice improved defence response against *Magnaporthe oryzae*, but the resistant plant also had dwarf stature and a lesion-mimic phenotype [57]. Similarly, leaf chlorosis was observed in wheat plants that had been MLO gene-edited for resistance to powdery mildew [58–60]. Rice *OsSWEET14* and 11 mutations resulted in shorter height and pollen abortion [61, 62].

Understanding the processes of NHR, which has a long lifespan and a broad spectrum, is essential for breeding disease-resistant cultivars. Type I NHR is asymptomatic [63, 64], whereas Type II NHR is similar to host resistance ("gene-for-gene") and is associated with the hypersensitive response triggered by pathogen penetration into plant tissue and activation of the resistance gene [65–68]. For conferring and developing disease resistance technologies are RNAi, TALENS, CRISPR/Cas, ZFN used transfer gene in various crops (**Table 2**) [76].

### **4. Herbicide resistance**

Herbicides are essential components of today's integrated weed management strategy. To address the issue of repeated use of the same herbicide resulting in the evolution of resistant weeds, complicating their control, the development of diverse herbicide-tolerant crops is critical.

#### **4.1 Photosystem based herbicides**

Herbicides of various types disrupt various organelles and parts of plants. For example, now recognised by numerous names, including QB (Quinone), D1, encoded by the psbA gene,the 32-kDa weighted protein found in the thylakoid membrane acts


#### **Table 2.**

*RNAi, CRISPR/Cas 9 and TALEN mediated gene transfer for various diseases in different crops.*


#### **Table 3.**

*Amino acid exchanges in wild and mutant plant proteins.*

as a herbicide binding protein for a variety of proteins was identified using a photoaffinity marker (azidoatrazine) [73, 75]. This is the first gene found for herbicide tolerance research. Atrazine, the most commercially important PS II herbicide, is the most extensively used herbicide in maize and sorghum as these crops can extensively degrade atrazine by glutathione conjugation. Herbicide-tolerant weeds (*Senecio vulgaris*) discovered in maize for atrazine, simazine, and other s-traizine category herbicides, which were previously susceptible for these herbicides, demonstrated maternal inheritance and conferred 100 times less binding affinity for herbicide in thylakoid membrane [77–79]. There have been six recorded amino acid exchanges (**Table 3**) [80–83]. The 3D arrangement of the amino acids revealed that different herbicide classes had different binding sites on the QB protein.

#### *4.1.1 Glyphosate*

Glyphosate N-(phosphonomethyl) glycine is a post emergence herbicide and is a potent competitive inhibitor of the enzyme 5-enol-pyruvyl shikimic acid 3-phosphate (EPSP) synthase and it is rate limiting enzyme for aromatic amino acids synthesis pathway [84]. A gene (*aroA*) from gram negative bacteria *Salmonella typhinutrium* (now the same found in *Aerobacter aerogenes* [85], possessing mutation on 101th

#### *Genetically Modified Crops and Their Impact on New Era of Agriculture DOI: http://dx.doi.org/10.5772/intechopen.105937*

position of amino acid wherein proline is changed to serine a gene (*aroA*) encoding resistant EPSP enzyme against glyphosate was transformed to tobacco with octopine synthase promoter through agrobacterium transformation which led to insensitive response to herbicide [86]. The mutated gene was fused to an octopine synthetase promotor to enable expression in plants leading to regeneration of transgenic plants with glyphosate tolerance and impaired growth due to compartmentation of aromatic amino acid biosynthesis and disruption of EPSP synthase and other shikimic acid pathway enzymes located in chloroplasts. The precursor protein is directed into the chloroplast by amino-terminal regions known as transit peptides, which are eliminated during the absorption process. EPSP synthase is also a precursor protein that is enzymatically active and binds glyphosate. A fusion gene encoding the sequence of a plastidic transit peptide before the aroA sequence, which exhibits a 40-fold increase in EPSP-synthase activity in petunia plants, is likely to provide a greater level of resistance [87, 88].

#### *4.1.2 Sulfonylurea and Imidazolinone herbicides*

The herbicides imidazolinones and sulfonylureas operate by limiting acetolactate synthase (ALS), the first enzyme in the biosynthetic chain that results in the production of branched chain amino acids [89–91]. Thus, chlorsulfuron is readily detoxified by wheat, barley, or oats by phenyl ring hydroxylation and consequent conjugation with glucose. Sulfonylurea-resistant mutant strains have been isolated from species as varied as bacteria, fungi, and plant cell cultures. Using cell biology techniques, maize plants resistant to imidazolinone and tobacco plants resistant to sulfonylurea herbicides have been successfully established [90, 91].

#### *4.1.3 Phosphinothricin*

L-Phosphinothricinis (an analogue of glutamate), a component of the tripeptide "bialaphos" produced by several *Streptomyces viridochromogenes*, is a potent irreversible inhibitor of glutamine synthetase with herbicidal activity causing a rapid increase of ammonia concentration in plants which leads to the inhibition in photosynthesis, which derives the plants to death of the plant cell [92, 93]. Resistance to L-phosphinothricin in alfalfa was established via overproduction of a glutamine synthetase gene connected to the promoter of the 35S transcript of cauliflower mosaic virus, and the construct was incorporated into the genome of N. Tobaccum var. W38 by *A. tumefaciens* [94, 95]. Transgenic plants demonstrated superior resistance to high doses of commercial formulations of phosphinothricin and bialophos after integrating the resistance gene from *Streptomyces hygroscopicus* to tobacco and regulating it with the 35S promotor of the cauliflower mosaic virus.The treated plants showed no elevation in ammonia concentration, signifying the total shielding of the plant glutamine synthetase from the herbicide's activity [94]. Herbicide resistance is acquired via TALENs-mediated gene mutation of OsALS in rice and ALS (SurA and SurB) in tobacco [96, 97] (**Table 4**).

### **5. Stress**

Abiotic stress is a highly complicated phenomena that involves biochemical and physiological changes in plant cells causing increased amounts of ROS (reactive oxygen species), that are extremely reactive, toxic and impact chlorophyll


#### **Table 4.**

*Genes from various sources responsible for mutation in herbicide tolerance.*

production, photosynthetic capability, and carbohydrate, protein, lipid, and antioxidant enzyme activities.

Genes that code for synthesis of osmolytes (proline, betain, etc.), water uptake and transport (ion transporters and aquaporin), transcriptional regulation and signal transduction mechanisms (MAPK) are identified from a variety of organisms and transformed into sensitive genotypes for generation of stress-protecting chemicals [98, 99]. The most efficient candidates for genetic transformation are transcription factors (TFs) that regulate the expression of a number of genes involved in conferring abiotic stress tolerance in plants [100].

#### **5.1 Drought tolerance**

Stress-related ring finger protein 1 (*OsSRFP1*), drought-induced SINA protein 1 (*OsDIS1*), and dry- and salt-tolerant protein 1 (*OsDST*) are negative regulators of drought tolerance, whose silencing increased antioxidant enzyme levels, reduced H2O2 concentrations, and enhanced drought tolerance in rice.

The CRISPR–Cas9 system was used to introduce novel alleles in Arabidopsis *OPENSTOMATA 2 (OST2*)-encoding gene—a key plasma membrane H+ ATPase causing two significant mutations at the *ost2* locus that led to constitutive functioning of proton pumps, induction of necrotic lesions and exhibiting a substantially higher rate of stomatal closure coupled with a lower rate of transcriptional water loss which resulted in enhanced drought tolerance (**Table 5**).
