Keywords

antioxidants; bioremediation; Cyt P450; genetic engineering; herbicides; metabolites; rhizosphere; resistance; pesticides; tolerance

17.1 Introduction

Pesticides are man-made and naturally occurring chemicals that control insects (Khan et al., 2010; Xiao et al., 2010), weeds, fungi, and other pests that destroy crops. Pesticide is a general term that includes a large number of biocidal compounds like fungicides, nematicides, insecticides, molluscicides, rodenticides, herbicides, and plant growth hormones. The most common pesticides like organochlorines (OC) have been widely used to overcome many diseases like malaria and typhus, but due to their recalcitrant nature, their use has been banned or at least reduced during last 50 years in the majority of developed nations. The application of various organophosphates (OP) during the 1960s, carbamates during the 1970s, and pyrethroids during the 1980s, along with the synthesis of other pesticides (1970s–1980s), have effectively reduced the pest hazards and resulted in increased productivity (Aktar et al., 2009).

Pesticides are the chemical species that cause death and avoid or reduce growth of plants or animals that are considered as pests. Herbicides are a class of pesticides that are used to kill weeds and other undesirable life forms in agricultural crops, including insects, while fungicides are employed to restrict the growth of molds and mildew. The use of disinfectants prevents the outbreak of bacteria and is also used to control mice and rats. Due to excessive use of these chemicals in agriculture, humans may be exposed to low concentrations of these biocidal residues through the food chain. The health effects of these pesticide residues are currently being investigated. Surveys have revealed that more prevalent cases of insomnia, dizziness, headaches, hand tremors, fatigue, and other neurological symptoms in children among the farming community are due the use of agricultural insecticides and pesticides. Hepel et al. (2012) established that catechol-containing compounds in the presence of copper(II) or iron(II) ions may induce a Fenton cascade leading to reactive oxygen species (ROS) generation, which are potent enough to cause damage to DNA. Currently, more than half of the world production of catechol is used by the pesticide and herbicide industry (Nurzhanova et al., 2013).

Restrained use of pesticides is critical to meet the food supply for an ever-increasing human population, and to avoid pest attack by ensuring the safety of human health. Crop protection and agricultural production are important factors of food safety, which can be ensured through judicious application of pesticides (Bolognesi, 2003). According to the United Nations Food and Agricultural Organization, the world’s potential human food losses are about 55%, which include pre-harvest (35%) and post-harvest (20%) losses. In advanced countries, about 10–30% of crop loss occurs due to pests; however, these losses are 75% in developing nations (Ohaya-Mitoko, 1997). Pesticides are crucial to enhance agricultural production, but at the same time they are toxic recalcitrant substances. Almost 90% of food is exposed to different herbicides, which are used in crops (Rosa et al., 2008). The herbicide residues in food are a consequence of their being directly sprayed onto crop plants and less likely due to residues persisting in the soil (Businelli et al., 1992). The presence of pesticide residues in food plants, especially vegetables, represents the most critical threat to human health. Best agricultural management practices can greatly reduce the risk associated with pesticide food chain contamination.

Aktar et al. (2009) have discussed the benefits and hazards of the application of pesticides in agriculture. Overwhelmingly large benefits result from the application of pesticides on agricultural crops and these xenobiotics present an ideal opportunity to study risk assessment. It has been estimated that developing countries need $8 billion on an annual basis. An in-depth comparison of financial gains from the use of pesticides and their subsequent impact on human health is required. The total cost and associated benefits vary among developed and developing countries. To ensure food security, pesticide use in developing nations is more intensive and the risk to human health is sometimes more demanding. However, the approach to pesticide use should be more prudent and rational, and practiced on scientific principles rather than merely for profit. Numerous factors, i.e., sex, race, age, state of health, socioeconomic status, and diet, have to be taken into account to make a proper scientific judgment on the effects of pesticide residues on human health. The long-term exposure to minute quantities of pesticides is greatly influenced by their interaction with other pollutants found in air, water, and food (Aktar et al., 2009).

Pesticide contamination is a greater risk to the sustainability of the environment and non-target organisms like soil flora and fauna, birds, and fishes. However, the cost of these pesticides is ever-increasing and their environmental toxicity is a consequence of their increased use. In reality, the persistent use of herbicides has caused greater damage to the environment. Weedicides are more toxic in elevated concentration. Aktar et al. (2009) recommended the standard procedure of minimizing xenobiotic residues and their harmful impact on the environment through their safer use and utilization of non-chemical pest control strategies. Focus should be centered on the avoidance of serious health consequences and endorsement of health as a lucrative venture for workers to achieve sustainable financial development. Health education should be based on information, capacity, and performance, and communities should be educated on the risks to health of pesticides and how their exposure can be reduced (Aktar et al., 2009).

17.2 Global pesticide use

A dramatic rise in pesticide usage throughout the globe is evident along with the ever-increasing human population and crop production (Zhang et al., 2011). The increasing grave misuse of pesticides is causing critical damage to the environment. The status of pesticide pollution was analyzed on a global scale (especially in China) to protect human health and endangered plant and animal species. During this course, pesticide development and usage were also considered and reviewed. The global pesticide consumption pattern has changed significantly over the last 50 years. Compared to pesticides, herbicide consumption has rapidly escalated, while the proportionate consumption of insecticides and fungicides/bactericides has declined. China is the major herbicide manufacturer and exporter worldwide. Pesticide contamination of the environment and resultant deaths in China remain a serious issue. An alternate option of bio-pesticides has been suggested by Zhang et al. (2011).

Historically, the use of pesticides has been divided into three periods (Zhang et al., 2006): (1) the main period covers the time prior to the 1870s when naturally occurring chemicals like sulfur were used as pesticides; sulfur was used in ancient Greece for pest management; (2) the next period, from the 1870s to 1945, employed inorganic synthetic compounds; the natural compounds found in various plant extracts along with some synthetic inorganics were the main focus of this period; (3) the last period, since 1945, comprises the time when organic synthetic pesticides were used. Over the last 60 years, synthetic pesticides like DDT, 2,4-D, and later HCH and dieldrin replaced inorganic and natural pesticides. Also, a number of synthetic pesticides were prepared called chemical pesticides. The use of synthetic organic herbicides has remained a hallmark for human society, which has greatly facilitated pest control and enhanced agricultural productivity. During the initial years of last period, the main types of organic insecticides, i.e., carbamate, organophosphate, and organochloride, were synthesized. Subsequently, herbicides and fungicides were used as the main pest control strategy. However, the use of insecticides is seen to be gradually declining and herbicides will gain popularity for the future (Zhang et al., 2011).

The United States Environmental Protection Agency reported the consumption of 3 billion kg of herbicides during 2001, which corresponded to 2 kg per capita in the USA (Toxipedia.org 2011). Roughly 400 million kg of active components and 600 different chemicals were listed as herbicides. Agriculture production used around 300 million kg of herbicides and 45 million kg (11.5%) were sprayed on lawns, gardens, and other areas. Besides this, approximately 1 billion kg were sprayed as disinfectants, and 0.3 billion kg were utilized as wood preservatives (Pesticide Use Statistics, 2011). Globally, around 2 billion kg of active pesticide ingredients were sprayed in agriculture production during 2001. Table 17.1 presents use of pesticides in the USA. Pesticide utilization is gradually decreasing in the USA in view of environmental protection. During 1972, the use of DDT and other organo-chlorinated pesticides was banned in the USA. Since 1975, the use of these chemicals decreased by 35% without affecting agricultural productivity (SDNX, 2005). The total cost of pesticides in the USA during 2001 was $11.09 billion of which $7.4 billion was used in the agricultural sector. Globally, the price of pesticides to enhance agriculture productivity during 2001 was US$31.8 billion (Toxipedia.org 2011).

Future pesticides will be extremely proficient with high biological reactivity, which will result in the decline of pesticide usage and will reduce environmental toxicity. These advanced pesticides will be less toxic, pollution free, and thus eco-friendly. Another recent concept is the development of bio-pesticides that employ direct use of either various life forms or their biochemical intermediates under field conditions and synthetic products from genetically modified organisms (GMOs), pathogenic insects, wild plants, or pathogenic microbes (Zhang and Zhang, 1998; Zhu et al., 2002).

The following benefits are associated with bio-pesticides:

Hundreds of bio-pesticides currently exist, of which around 30% are synthesized on a commercial basis (Xu, 2008). Forty-four percent of these bio-pesticides are being used in the USA, Canada, and Mexico, while consumption in the rest of the world is 56% (Qin and Kong, 2006).

17.3 Why pesticide-/herbicide-tolerant plants?

The toxicological and environmental effects of synthetic herbicides are an increasing concern for human health and environmental protection agencies (Sunohara et al., 2010). The production of highly specific herbicides at very low concentrations is a prerequisite to controlling the target plants without harming other non-target organisms. The fine toxicological and environmental performances are still highly preferred in spite of the registered 400 herbicides (Sunohara et al., 2010). Due to its frequent use, pesticide resistance assists the ability of plants and other organisms. Such resistance follows the rules of evolution, resulting in the survival of the fittest and induces a heritable alteration. Multiple herbicide selections may be disregarded if a weed has acquired resistance to numerous herbicides (i.e., cross-resistance). Apparently, reducing herbicide choices may result in important economic and environmental consequences to agriculture. The evaluation of herbicide-resistant weeds involves complex and costly procedures. Because of cross-tolerance, consistent efforts should be put forth to overcome herbicide resistance. The herbicide resistance concern has resolutions and it is best to consider them as a resource. Later, plans for resistance prevention may be devised (Gunsolus, 2008).

Besides certain disadvantages, herbicide resistance in plants may be beneficial for certain plant species. The discovery of herbicide-resistant (HR) weeds during the 1970s activated an interest in imitating this involuntary development into crop breeding (Madsen and Streibig, 2013). The associated growth in biochemistry realized the incorporation of genes responsible for tolerance in vulnerable plants. The conventional route for producing herbicide-resistant crops (HRCs) was initially carried out by traditional breeding methods. However, genetic engineering emerged as the major contributor to producing HRCs, but the technology has been under scrutiny for its benefits and other ethical issues. HRCs had been produced on a commercial scale during last 30 years when the OAC Triton HR biotype was developed and released in Canada. This cultivar was the product of breeding between the HR Brassica rapa L. oilseed rape plant (Hall et al., 1996). Genetically modified (GM) HRCs encompassed the bulk of cultivated areas where GM crops were grown (James, 2001). GM HRCs are normally considered as “first generation crops” and their efficacy has been queried due to hypothetical threats to users and the environment. In the case of a few HRCs, herbicides were substituted with a less promising ecological profile. Additionally, costs of weed control programs in glyphosate-tolerant soybean were reduced in conventional and HRCs due to the low cost of herbicides (Madsen and Streibig, 2013).

Glyphosate-tolerant soybean crops offer farmers a vital tool for fighting weeds and are compatible with no-till methods, which help preserve topsoil. As glyphosate has a strong affinity to become adsorbed to the soil particles, there was little chance of having residues on subsequent crops. The number of chemical sprays on soybeans was decreased to 12% for the season 1995–1999. However, an increase was observed in terms of total concentration of active constituents sprayed (Carpenter and Gianessi, 2001). The implementation effects of GM crops cannot be separated from other factors affecting insect killer applications (Heimlich et al., 2000). According to the American Soybean Association, environment protection by HR soybean involves changes in tillage procedures, pesticide application, and enhanced weed control (Anderson, 2001). The commercially available HR rice varieties are other examples. Agronomically, there are two arguments in favor of development of HR rice: (1) improvements of management in the weed related with rice, particularly red rice and other taxa (Gealy and Dilday, 1997; Olofdotter et al., 2000) and (2) provision of an alternative method to control weeds which formerly attained tolerance to specific pesticides, particularly monocots like Echinochloa spp. (Wilcut et al., 1996; Olofdotter et al., 2000). HR rice authorizes the usage of substitutes to presently practiced herbicides (Olofdotter et al., 2000).

Soil loss due to farming procedures is troublesome in many parts of the world. Generally, HRCs will be encouraging for environment protection through pest control in relation to conservative measures. Such practice will allow crop growers to use conventional farming methods in order to decrease soil erosion, especially through non-tillage (Duke, 2001). In the case of HRCs, vulnerabilities may be considered as qualitative estimates including the probability and harshness of both instant and outstanding severe impacts on human health, the environment, and the economies of farmers. Such undesirable effects may be caused by many traits such as crop specificity, particularly resistant features, weeds, atmospheric conditions, and farming practices (Madsen et al., 2002).

The best possible weed eradication often needs chronological applications of glyphosate in glyphosate-resistant (GR) crops. The proper schedule for such application is very crucial and the spray application in relation to weed appearance is important (Swanton et al., 2000). A high selection pressure on weed plants is caused by high annual doses of glyphosate. In a span of around 8 years, a shift in GR weed composition is usually observed (Shaner, 2000; Benbrook, 2001), in which case discrete pesticides are required to eradicate these GR weeds (Shaner, 2000). The classical, post-emergence pesticides were anticipated for effective weed control of GR soybean to support the elimination of GR weeds like Sesbania exaltata (Raf.) Cory, Ipomoea spp., or Amaranthus rudis Sauer (Payne and Oliver, 2000). Growing GR corn and soybean in their rotation will not result in the control of native corn by glyphosate (Shaner, 2000). Flow of genes within similar crop species can be an alternate method to develop weed-resistant crop populations. Steady spray of pesticides with a similar mode of action can lead to selection of the single gene responsible for resistance if detected, which can be further implemented for similar weed control. Such practice will result in conveying pesticide resistance in plants and natives.

Amplified use of pesticides can be a risk in a few parts of the world, while the toxicity of these pesticides to humans or environmental health is not fully characterized. The toxic effects caused by the extensive use of pesticides on ground water and the pesticide residues in different food plants are critical. Extensive application of herbicides for HT crops may be due to increased pest tolerance to respective pesticides, which proved a compelling factor for farmers to employ high concentration pesticide sprays. Farmers considered that pesticides would not exert harmful effects on plants and they would realize greater yields by controlling weeds. Moreover, resistance of weeds or native plants further compels farmers to enhance pesticide usage for effective weed control and increase crop yield.

The use of herbicides affects the biodiversity of a field when pesticides are used in greater quantities to control weeds or wild species. In addition, weeds show differential response to various herbicides and other methods to eradicate undesirable vegetation; this may result in vegetation shifting of a particular field. According to FAO (2001), consequent reduction in the diversity of local field species will be hazardous for growth of HR crops of genetic origin. Further, HR is not predicted to cause genetic diversity fluctuations among wild plant species because herbicide application is also prevalent outside cultivated lands. Moreover, HR traits cannot result in selective advantage excluding plants exposed to herbicides (Poulsen, 1995; Madsen et al., 1998). Thus, a meager loss of genetic diversity among wild plant species is anticipated in natural habitats. Despite major ethical and ecological threats to genetic diversity of native species, few GM crop plants have been considered safe under discreet scientific norms, e.g., HR sugar beet (Madsen and Sandøe, 2001). However, the final release of HR varieties has to surpass several strict scientific assessments (Madsen et al., 2002).

17.4 Mechanisms of Herbicide/pesticide tolerance in plants

Herbicide/pesticide tolerance is a complicated process involving numerous components of a plant. These include phytochromes, plant antioxidant machinery, glycoproteins, and interaction of various metabolic systems (Figure 17.1). Phytoremediation is one potential method for reducing risk from these pesticides. Genetic heterogeneity of wild populations and weedy species growing on pesticide-contaminated soil provides a source of plant species tolerant to these conditions (Nurzhanova et al., 2013). In this section, we will deal with various aspects of herbicide/pesticide resistance mechanisms found in plants.

17.4.2 Bioremediation and phytoremediation

The understanding of the biochemical pathways of pesticides in various plants and microbes is crucial for efficient clean-up, safe use of these chemicals, and their bioremediation in soil and water. Metabolism or co-metabolism may be involved in the pesticide metabolism, which is a multi-step process. The majority of pesticides undergo widespread breakdown in plants and the environment. The breakdown of these pesticides involves a number of diverse physicochemical rearrangements like redox reactions, hydrolysis, and conjugation (Hoagland et al., 2000). The mechanisms of bioremediation and/or phytoremediation are presented in Figure 17.2. Two major mechanisms of phytoremediation for organic pesticides are rhizodegradation and phytoextraction (Pascal-Lorber and Lauren, 2011). Co-metabolism is a term that describes the biological transformation of various organics but does not act as a source of energy and this process also aids in the biotransformation of the pesticides (Alexander, 1994). During early co-metabolic stages, the chemical reaction decreases the toxicity of the pesticides for both target and non-target plants and increases the susceptibility of pesticides to various biological, biochemical, or physicochemical degradation. Various enzymes like hydrolytic enzymes (esterases, amidases, nitrilases, etc.), transferases (glutathione S-transferase, glucosyl transferases, etc.), oxidases (cytochrome P-450 s, peroxidases, etc.), and reductases (nitroreductases, reductive dehalogenases, etc.) take part in the initial stages of co-metabolism (Hatzios, 1991; Mandelbaum et al., 1995). Plants and microbes have many common enzymes for the detoxification of various synthetic organic toxins. However, there are many differences in the details of the metabolic processes of these pesticides (Hoagland et al., 2000).

Many soil microbes like Arthrobacter, Burkholderia, Pseudomonas, and Sphingomonas have the ability to use a variety of xenobiotics. Bumpus (1993) reported that white rot fungi possess various enzymes employed in lignin and other xenobiotic degradation (lignin peroxidase and manganese peroxidase). The pesticide degradation by various microbial species was reported by many workers; however, bacterial species are not capable of pesticide degradation but rare cases of plants are found (Carr et al., 1985; Ramanand et al., 1988; Ryan and Bumpas, 1989; Krueger et al., 1989; Nagasawa et al., 1993; Mandelbaum et al., 1995; Leung et al., 1997). Previously, the 2,4-dichlorophenoxyacetic acid and 2,4,5-trichlorophenoxyacetic acid were reported to be biodegraded by various bacterial cultures (Alcaligines, Arthrobacter, Flavobacterium, and Pseudomonas) (Häggbloom, 1992). Certain herbicides were completely biodegraded while serving as a nitrogen source such as atrazine degradation, accomplished by Pseudomonas sp. ADP (Mandelbaum et al., 1995); while paraquat was mineralized by the yeast Lipomyces starkeyi (Carr et al., 1985).

Dubey and Fulekar (2013) reported rhizoremediation—the biodegradation catalyzed by plant–microbe interaction to mineralize various organic pollutants. Rhizodegradation of pesticides like pyrethroid presents a potentially cheap and promising method of reclamation of polluted sites. Hydrolysis and photolysis are the most effective in degradation of cypermethrin in soil. The degradation route involving the hydrolysis of the ester bonds produces 3-phenoxybenzoic acid (PBA) and cyclopropane carboxylic acid derivatives, especially 3-(2,2-dichlorovinyl)-2,2-dimethyl cyclopropane carboxylic acid (DCVA) (Kaufman et al., 1981). Aerobic microbes accomplish the degradation of cypermethrin under oxic conditions. Factors like the presence of heavy organic soils and reduced microbial metabolism were associated with high prevalence of this pesticide in the soil. The end result of rhizodegradation is always formation of non-toxic end products. The roots of plants directly affect the structure and density of the soil microbes; this is called rhizosphere effect. The structure of root and its secretions have significant bearing on the biochemical reactions and biodegradation occurring in the root zone. Several workers reported pesticide rhizodegradation (Yu et al., 2003; Singh et al., 2004; Sun et al., 2004), metal uptake (Gaur and Adholeya, 2004), and the mineralization of a few other organic compounds through rhizodegradation (Jordahl et al., 1997; Nakamura et al., 2004; Chaudhry et al., 2005; Biryukova et al., 2007). It was also reported that such mineralization was greater in the rhizosphere compared to bulk soils. Dubey and Fulekar (2013) reported the ability of Pennisetum pedicellatum to mineralize cypermethrin. The microbes capable of pesticide degradation were found higher in the rhizosphere. The effective rhizodegradation of cypermethrin was reported by Stenotrophomonas spp. from the rhizosphere of P. pedicellatum. It was suggested that rhizodegradation can serve as an effective tool for the reclamation of natural habitats (Dubey and Fulekar, 2013).

The hereditary potential to generate hydroxyatrazine was formerly credited to a 1.9-kb AvaI DNA section isolated from Pseudomonas sp. ADP (de Souza et al., 1996). The study confirmed open reading frame atzA responsible for coding an enzyme that converts atrazine to hydroxyatrazine. The enzyme AtzA was isolated, homogenized, and its structure revealed that it was chlorohydrolase instead of oxygenase. The selection of plant species to accomplish the rhizodegradation is very important. Thus, the intensity of pesticide-degrading bacteria is always high in the rhizosphere of plants inhabiting polluted soils. The physical parameters of root are the most critical in rhizoremediation; these parameters include width of root, depth, surface-to-volume ratio, root biomass, and surface area, and these parameters are variable among various plant species, which results in variable herbicide resistance (Dubey and Fulekar, 2013). Due to the extensive shallow root system in grasses, they are very promising candidates for rhizoremediation. As roots absorb plenty of water from the soil, they help in the movement of pollutants to the rhizosphere (Erickson, 1997). Pennisetum pedicellatum resisted large amounts of pesticide cypermethrin (Dubey and Fulekar, 2013). Yu et al. (2003) also reported that diversity and richness of soil microbes were important for pesticide metabolism. An interesting report stated that the excessive pesticide that does not reach a target organism is absorbed by plants like many vegetables and fruits, and also processed foods (González-Rodríguez et al., 2011). Kreuz and Martinoia (1999) concluded that the principle metabolic step in the detoxification of a pesticide is hydrolysis or oxidation, generating metabolites that could be processed further by secondary enzymatic conjugation to endogenous substrates like glutathione (GSH), carbohydrates, or organic acids. The final disposition of such substrates is carried out in vacuoles as conjugates, and/or extracellular disposal. An important constituent in the regulatory metabolism is the transfer of intermediates like GSH or glucosyl conjugates from the cytoplasm by specific ATPase conjugate pumps (Hoagland et al., 2000). It was reported that genetic control of enzymes in higher plants is different from that in prokaryotic organisms. In bacteria, operon clusters possess genes regulating the metabolism of pesticides. The enzymes involved in the metabolism of pesticides can be stimulated or suppressed by the same effect. The eukaryotes are different in this regulation where multiple effectors regulate these enzymes and a gene is regulated by a specific promoter. Such systems have innumerable proteins that influence expression, i.e., specific cytoplasm enzymes (WD proteins) controlling the transcription process, and they are merely restricted to the cytoplasm (Ma, 1994). A thorough understanding of the gene regulation of plant gene expression is yet to be accomplished (Hoagland et al., 2000). Nandula et al. (2008) established glyphosate resistance in a rye grass T1 population partially due to abridged glyphosate absorption and translocation, while in the T2 population it was caused by decreased transport of glyphosate.

17.5 The selection process of tolerant plants

The HR herbaceous plants are privileged via the process of natural selection in polluted habitats because they are better adapted to such conditions compared to non-tolerant plants. The selection pressure in numerous herbs is directed to the natural evolutionary progress of HR genotypes consequent to a broad variety of pesticides. The herbicide-sensitive plants display noticeable impacts on their growth and reproduction upon exposure to various herbicides. The seedlings of the majority of plants are the most susceptible to pesticide toxicity. However, the considerable toxic effects may be displayed during vegetative and reproductive growth periods. Many annual herbs exhibit the process of selection for herbicide resistance under herbicide exposure. The mechanism of tolerance under pesticide exposure in perennial plant species is different from annuals. The response to agrochemical pollution is similar to abiotic stress. However, the mechanism of tolerance in these herbaceous plants is supposed to be linked with their better genetic make-up, which enables them to survive under stressful conditions. How better genetic make-up originated is not clear (Gunsolus, 2008). Herbicides have not been thought to cause gene mutations allowing herbicide resistance. The HR species may be found in low numbers within large plant populations and survive till reproduction when herbicides are applied; while other susceptible species are wiped out. Upon continuous use of herbicides on HR species, the number of such HR plants increases with the passage of time. Selection pressure functions like a sieve, which sorts the HR species from intolerant ones and the end result is the survival of just HR species. Being herb killers, herbicides have the ability to exert selection pressure on the weed populations. The susceptibility of a weed species results in its elimination upon exposure to an herbicide; susceptibility has a linear relation with weed control. Consequently, the rate of selection for herbicide resistance may be quick if the same herbicide is repeatedly used on similar weeds. Thus, even the use of very effective herbicides may increase the number of HR weed species. HR plant species are detected only when present in about 30% of total weed populations. In many instances it was observed that use of the same pesticide resulted in a 1% HR population after many years of pesticide use. Prolonged use of the same herbicide will result in the growth and reproduction of HR weed species. Gunsolus (2008) listed the following features of herbicides that may result in HR species: (1) they may target solitary sites of weeds; (2) spraying the same herbicide for repeated periods of time in the same crop; (3) repeated use for several growing seasons having a single site of action; (4) herbicides employed exclusively for additional weed control programs and sometimes called “stand-alone” pesticides.

Herbicide resistance is more likely to be triggered due to application of a single site of action herbicide, and the mutation in merely one gene is sufficient to accomplish its binding to the target site. It is more likely to develop an HR weed population when dissimilarity in a single gene is needed. The use of herbicides having multiple sites of action will not result in the development of HR plant species. Such resistance is generally against those herbicides to which plants were exposed; there will be no resistance to other herbicides. The presence of many binding sites for a particular site of action may be the reason and such sites of action are specific to each herbicide. Thus, many herbicides may have the ability to bind with the same enzyme but at diverse sites of action. Consequently, the cross-resistance of a herbicide cannot be predicted; apart from the fact that a particular herbicide works at single or multiple sites, it may be transformed by target plants until it reaches its site of action. The speed of herbicide metabolism is crucial in shaping damage to plants and weed management. An example is the change of metabolic rate caused by a single gene mutation in biotypes of atrazine-resistant velvet leaf (Abutilon theophrasti). However, the majority of metabolic processes have polygenic control in nature and thus there is a little chance for weed species to be resistant against a particular pesticide based on its improved metabolic functions. As metabolic process influences the action of herbicides with different sites of action, metabolic resistance can be a challenge. The reduction in selection intensity may be the key factor to preventing herbicide resistance. Consequently, weeds also exhibit an ability to adapt various pesticide management programs. When an HR weed dominates a field, two important points should be focused on for their effective control: (1) ability of a weed to reproduce and (2) dispersal of HR weed seeds.

The ability of HR weed spread depends upon its reproductive success to become a dominant weed species. Based on increased viability of certain HR weeds after establishment in a field, it may be difficult to eradicate in spite of costly chemical application. Due to different seed dispersal means, farmers should consider interception of the seed dispersal mechanism as well as employing effective herbicide management strategies. A number of strategies were developed by the North Central Weed Science Society (NCWSS) and Herbicide Resistance Committee to either avoid HR weeds or eradicate them. These include (Gunsolus, 2008):

17.6 Conclusion and future prospects

Continuous endeavors for greater yields have prompted scientists to invent new herbicides/pesticides. Pesticide application is still the most effective and accepted mode of protection of plants from pests, and has contributed significantly to enhance agricultural productivity and crop yield. Thus, the future work on herbicide/pesticide resistance could be promising if it involves the identification of metabolites responsible for detoxification of these applied xenobiotics, various enzymes, their controlling genes, and the rhizosphere bacterial species conferring herbicide/pesticide tolerance to plants. The quest of biotechnology to identify resistance responsible genes could be promising, which in turn could be transferred to transgenic plants for enhancing resistance. Among major resistance mechanisms, the roles of enzymes like Cyt P450 in plants have a supreme function in the metabolism for the detoxification of herbicides. The detoxification and metabolism of herbicide-catalyzed enzymes of Cyt P450 present an outstanding resistance mechanism found in HR plants. Knowledge of the pesticide resistance mechanism in plants and other microorganisms is crucial for devising a safe plan for its use and biodegradation in contaminated environments.

Secondary plant metabolites also play a crucial role in developing the multitude of enzymes responsible for the breakdown of various organic pollutants. Further research work is required to provide a link between secondary plant metabolites and enzymatic diversity, which can be applied in fields for pest management, bioremediation, and fine chemical production (Singer et al., 2003). Pest management has been challenged by economic and ecological constraints globally. The characterization and synthesis of new and effective insecticides is crucial to combat increasing resistance. The plant extracts having active insecticidal properties seem to be promising to control some of these problems. Thus, continuous efforts are required to explore novel active molecules with innovative mechanisms of action via gene transformation methods.

Recent research on gene shuffling furnished glyphosate resistance in plants, moreover, has resulted in numerous genes being patented, and substantial efforts being put into developing herbicide-resistant transgenic crops. Currently, few commercialized herbicide-resistant crops are available, e.g., glyphosinate- and glyphosate-resistant herbicides. However, transgenes have significantly greater environmental risk than HR crops. Further, wild-type species can be screened for resistance genes and then transferred to susceptible plant species for increasing survival and suitability in the natural environment. However, transgenic plants pose a greater risk than HR crops. Ameliorating the allelopathic mechanism in plants can result in greater resilience and minimize herbicide/pesticide usage. However, this work could be painstaking and the practical outcome would be difficult to achieve. Transforming wild-type species as transgenes can increase their suitability in natural ecosystems; however, transgenes have significantly greater environmental risk than HR crops. Research on the development of allelopathic plants is in progress with the aim of reducing the use of herbicides/pesticides. Even if made successful and safe, this technology will not be available for at least 10 years (Duke et al., 2007).