**8. GM crops**

**6. Glutathione S-transferases and evolved herbicide resistance**

both a direct role and an indirect role in evolved herbicide resistance [17].

**resistant crops**

114 Herbicide Resistance in Weeds and Crops

**7. Taxonomic effects in herbicide-resistant weeds and deployment of** 

Evolved herbicide resistance (EHR) has become a threat to agriculture around the world [12, 160, 161]. Evolved herbicide resistance in weeds was initially reported in 1970 and generally considered during the 1970s throughout the 1990s [80, 162]. The rate of instances has precipitated significantly during those decades. Up to now, the advancement of imperviousness to various herbicides with various mode of action has additionally been detected inside various weed species [51]. The detection of resistance to glyphosate, and the introduction of transgenic glyphosate-resistant crops in the 1990s, also the recent expansion of cases of evolved resistance to glyphosate in weeds, likely because to greater glyphosate usage, have inspired a renewal of interest and resurgence of research into this phenomenon [113, 163].

Glutathione S-transferases (GSTs) which catalyze the conjugation of glutathione to variety of hydrophobic, electrophilic substrates, are multifunctional enzymes. Glutathione S-transferases (GSTs) have a special role in protecting the plant from oxidative stress (e.g., from reactive oxygen species), thus functioning as protective mechanism [16]. Glutathione S-transferases (GSTs) detoxify several herbicides in some crop and weed species. These enzymes play a role in stress response [148–150]. Glutathione-conjugated herbicides can be sequestered in the vacuole or exuded via root tips [149, 151]. Herbicide-metabolizing GSTs have been purified and characterized from several crops [150, 152]. Some studies such as molecular modeling, mutagenesis studies, and also the resolution of the 3D structure of plant GST (including herbicide-induced GST) provide an understanding of the molecular basis of GST-catalyzed herbicide binding and how single amino acid substitution(s) can improve GST catalytic efficiency and affect substrate specificity for herbicides and xenobiotics [153–155]. Because the Glutathione S-transferases (GSTs) catalyze the conjugation of triazines to glutathione through their high activity, these herbicides are selective for corn. This feature cause to widespread utilize of triazines can elect some weeds with glutathione S-transferases capable to eliminate them. Actually, in some weed species such as *Abutilon theophrasti*, developed GST-intervened triazine herbicide resistance has been observed [49, 156]. More researches demonstrated that enhanced activity of glutathione S-transferases is owing to higher catalytic susceptibility compared with overexpression enzyme or presence of a novel glutathione S-transferases [157]. This shows a conceivable transformation (mutation) in the gene of glutathione S-transferase gene which could better herbicide binding and so glutathione S-transferase catalytic performance. Resistance to atrazine as a singular nuclear gene with sectional predomination is inherited in this biotype [5]. It was demonstrated that in a resistant *Echinochloa phyllopogon* biotype, fenoxaprop-*p*-methyl resistance can be due to glutathione-herbicide conjugation [80]. Investigates with multiple resistant *A. myosuroides* biotypes with increased P450-catalyzed herbicide metabolism also show that they have higher GST activity [149, 158, 159]. Generally, GST enzymes can play

> Herbicide-resistant crops (HRCs), sometimes called herbicide-tolerant crops, are crops made resistant to herbicides either by transgene technology or by selection in cell or tissue culture for mutations that confer resistance [172, 173]. Two techniques have been for all intents and purposes connected to generation of HR plants by hereditary building monetarily are: the presentation of the quality encoding the herbicide-inactivating catalyst, the presentation of the mutant, or outside quality. The advance in quality cloning and quality exchange, particularly utilizing *Agrobacterium tumefaciens*, has made hereditary building the most well known at present [174].

#### **8.1. The gene encoding the herbicide-inactivating enzyme**

This strategy has been most widely applied for the production of HR crops. The key step is to clone the gene encoding a herbicide-inactivating or detoxifying enzyme with high specificity and efficiency. Glufosinate-resistant crops sold as Liberty Link were produced by the introduction of the bar gene encoding the glufosinate-inactivating enzyme. The *bar* gene was cloned from Streptomyces hygroscopicus which produced bialaphos, the precursor of glufosinate (phosphinothricin). The *bar* gene encodes phosphinothricin-N-acetyl-transferase (PAT) which acetylates bialaphos to an inactivated form and prevents autotoxicity of bialaphos in the bacterium [174].

Bromoxynil-resistant crops sold as BXN were produced by the introduction of the *bxn* gene encoding the bromoxynil-inactivating enzyme [175]. However, the *gox* gene encoding the glyphosate-inactivating enzyme was also introduced into some plant species, possibly to enhance the level of resistance [176]. The gene encoding the 2,4-dichlorophenoxyacetic acid (2,4-D)-inactivating enzyme was also cloned from a soil bacterium because 2,4-D was readily inactivated in soil [177]. *Alcaligenes eutrophus*, thus selected, utilized 2,4-D as the sole source of carbon. The *tfd*A gene from the bacterium converts 2,4-dichlorophenoxyacetic acid (2,4-D) to 2,4-dichlorophenol. An option technique is to use the quality encoding the catalyst required in the imperviousness to more than one herbicide, for example, glutathione S-transferase and cytochrome P-450 [178].

Resistance types of EPSPS from petunia or Salmonella, conveying Gly96 to Ala or Pro101 to Ser changes, were assessed in early transgenic plants. In spite of the fact that these changes do give imperviousness to glyphosate, the reactant properties of the modified protein are hindered, which decreases the force of the plants without herbicide. The popularized glyphosate-safe harvests contain the *Agrobacterium* CP4 EPSPS quality [176]. Focusing on the CP4 quality to the chloroplast presents an abnormal state of imperviousness to glyphosate sans the negative impacts connected with the single transformation EPSPS qualities portrayed previously. In some transgenic edit cultivars, glyphosate resistance is presented by a mix of the CP4 quality and a bacterial oxidoreductase quality that detoxifies glyphosate. An option wellspring of glyphosate resistance for transgenic plants is an EPSPS quality conveying two separate changes in a similar district of the quality (positions 101, 102 as well as 106) [179]. This two-fold mutant quality has not been presented in any marketed safe yields. Glyphosate resistance can likewise be founded on intensification of the EPSPS quality, prompting to expanded levels of transcript creation and EPSPS action [180–182]. Essentially, adequate EPSPS is created to titrate out the glyphosate, leaving an overabundance of chemical that remaining parts practical. This has been appeared in plants chose in tissue culture or through repetitive choice. Now and again overexpression is lost when the choice weight is expelled or when plants are recovered; in others, the quality has all the earmarks of being steady in recovered plants and their descendants. Be that as it may, this component has not been utilized to create glyphosate-safe yields [15].

#### **8.2. Mutant or foreign gene encoding the target enzyme with low affinity to the herbicide**

This strategy is applicable to the production of any HR crops. But it was applied to commercial HR crops, which have been restricted to glyphosate-resistant crops sold as Roundup Ready. Generally, when an enzyme with a high herbicide binding constant was produced by a mutant gene, its enzymological characteristics were found to be unfavorable for the maximal enzyme activity leading to decreased growth and fitness of the plants transformed with this gene. It was known that EPSPSs from some bacteria were naturally resistant to glyphosate. EPSPS from *Agrobacterium* sp. strain CP4 was selected with high glyphosate-resistance and catalytic efficiency in the presence of glyphosate. The CP4 EPSPS gene was cloned from the bacterium and used for the production of Roundup Ready crops such as soybean, canola, cotton, maize, and sugar beet [51]. The target enzyme of sulfonylurea, imidazolinone, and triazolopyrimidine herbicides is acetolactate synthase (ALS). Various resistant ALS genes were cloned from tobacco [183] and *Arabidopsis thaliana* [184]. These gene products showed different levels of resistance to sulfonylureas, imidazolinones, and triazolopyrimidines [15]. These resistant ALS genes were introduced into plants individually, or in combination, and conferred resistance to these herbicides. Though some of these genes conferred resistance even at field trials, these genes have not been used for commercialization [185].

#### **8.3. Novel tools for development of herbicide-resistant crops**

cloned from Streptomyces hygroscopicus which produced bialaphos, the precursor of glufosinate (phosphinothricin). The *bar* gene encodes phosphinothricin-N-acetyl-transferase (PAT) which acetylates bialaphos to an inactivated form and prevents autotoxicity of bialaphos in

Bromoxynil-resistant crops sold as BXN were produced by the introduction of the *bxn* gene encoding the bromoxynil-inactivating enzyme [175]. However, the *gox* gene encoding the glyphosate-inactivating enzyme was also introduced into some plant species, possibly to enhance the level of resistance [176]. The gene encoding the 2,4-dichlorophenoxyacetic acid (2,4-D)-inactivating enzyme was also cloned from a soil bacterium because 2,4-D was readily inactivated in soil [177]. *Alcaligenes eutrophus*, thus selected, utilized 2,4-D as the sole source of carbon. The *tfd*A gene from the bacterium converts 2,4-dichlorophenoxyacetic acid (2,4-D) to 2,4-dichlorophenol. An option technique is to use the quality encoding the catalyst required in the imperviousness to more than one herbicide, for example, glutathione S-transferase and

Resistance types of EPSPS from petunia or Salmonella, conveying Gly96 to Ala or Pro101 to Ser changes, were assessed in early transgenic plants. In spite of the fact that these changes do give imperviousness to glyphosate, the reactant properties of the modified protein are hindered, which decreases the force of the plants without herbicide. The popularized glyphosate-safe harvests contain the *Agrobacterium* CP4 EPSPS quality [176]. Focusing on the CP4 quality to the chloroplast presents an abnormal state of imperviousness to glyphosate sans the negative impacts connected with the single transformation EPSPS qualities portrayed previously. In some transgenic edit cultivars, glyphosate resistance is presented by a mix of the CP4 quality and a bacterial oxidoreductase quality that detoxifies glyphosate. An option wellspring of glyphosate resistance for transgenic plants is an EPSPS quality conveying two separate changes in a similar district of the quality (positions 101, 102 as well as 106) [179]. This two-fold mutant quality has not been presented in any marketed safe yields. Glyphosate resistance can likewise be founded on intensification of the EPSPS quality, prompting to expanded levels of transcript creation and EPSPS action [180–182]. Essentially, adequate EPSPS is created to titrate out the glyphosate, leaving an overabundance of chemical that remaining parts practical. This has been appeared in plants chose in tissue culture or through repetitive choice. Now and again overexpression is lost when the choice weight is expelled or when plants are recovered; in others, the quality has all the earmarks of being steady in recovered plants and their descendants. Be that as it may, this component has not been utilized to create glyphosate-safe yields [15].

**8.2. Mutant or foreign gene encoding the target enzyme with low affinity to** 

This strategy is applicable to the production of any HR crops. But it was applied to commercial HR crops, which have been restricted to glyphosate-resistant crops sold as Roundup Ready. Generally, when an enzyme with a high herbicide binding constant was produced by a mutant gene, its enzymological characteristics were found to be unfavorable for the maximal enzyme activity leading to decreased growth and fitness of the plants transformed with

the bacterium [174].

116 Herbicide Resistance in Weeds and Crops

cytochrome P-450 [178].

**the herbicide**

Plant cells have three genomes and, in some plant seeds, two of these genomes are transformable: the nuclear genome and the genome of the plastids (chloroplasts). The plastid genome of photosynthetically active seed plants is a small circularly mapping genome of 120–220 kb, encoding 120–130 genes. It can be engineered by genetic transformation in a (still relatively small) number of plant species, and this possibility has stirred enormous interest among plant biotechnologists. There are considerable attractions associated with placing trans-genes into the plastid genome rather than the nuclear genome. First and foremost, the high number of plastids per cell and the high copy number of the plastid genome per plastid offer the possibility of expressing foreign genes to extraordinarily high levels, often one to two orders of magnitude higher than what is possible by expression from the nuclear genome [186, 187]. Second, transgene integration into the plastid genome occurs exclusively by homologous recombination, making plastid genome engineering a highly precise genetic engineering technique for plants. Third, as a prokaryotic system that is derived from a cyanobacterium acquired by endo-symbiosis, the plastid genetic system is devoid of gene silencing and other epigenetic mechanisms that interfere with stable transgene expression. Fourth, similar to bacterial genes, many plastid genes are arranged in operons offering the possibility to stack transgenes by arranging them in artificial operons. Finally, plastid transformation has received significant attention as a superb tool for transgene containment due to the maternal mode of plastid inheritance in most angiosperm species, which drastically reduces transgene transmission through pollen [188, 189]. Since the development of plastid transformation for the seed plant tobacco (*Nicotiana tabacum*) more than 20 years ago [64, 190], the community has assembled a large toolbox for plastid genetic engineering and also made some progress with developing plastid transformation protocols for additional species. Unfortunately, plastid transformation is still restricted to a relatively small number of species and not a single monocotyledonous species (including the cereals representing the world's most important staple foods) can be transformed. Thus, developing protocols for important crops continues to pose a formidable challenge in plastid biotechnology and significant strides forward are likely to require conscientious efforts and long-term investments in both the academic and the industrial sectors [26].
