Purcell and Perlak (2004) investigated the global impact of Bt cotton. It was reported that since its introduction in 1996, this insect resistant transgenic from of cotton has been widely accepted and grown by farmers. Bt cotton has been cultivated on both, commercial and small-scale farms, and farmers with small holdings have benefited majorly. The adoption of Bt cotton has facilitated growers in terms of increased productivity and various economic and social advantages.
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Wang et al., (2008) reviewed the impact of Bt cotton on the economic and social systems of 169 Chinese famers between 2002 and 2003. Results of the survey showed that the adoption of Bt cotton significantly increased agricultural productivity, in turn increasing the farmers’ income. These economic gains had led to better education, activities of leisure and health care facilities for the farmers. However, there were certain factors found to limit productivity. These included the high prices of Bt cotton, along with lack of technical knowledge and skills required to cultivate and maintain the crop. Thus, to further promote and integrate the growth of Bt cotton on smallholdings, continuous research and knowledge about the characteristics of this transgenic crop is required.
Qaim and de Janvry (2005) conducted a survey in Argentina to analyze the changes in agricultural productivity and the use of pesticides because of Bt cotton cultivation. Data showed that productivity had increased significantly, and the application of chemical toxins was reduced by 50%. The efficiency of the insect resistant crop versus chemical pesticides was also evaluated, by applying a damage control framework on various farm types. Smallholdings grossed the major benefits, and the durability of this pest-resistant feature was further analyzed using biological models. Hence, the adoption of Bt cotton on a wider scale would result in enhanced efficiency and environmental benefits. However, long-term studies regarding farmers’ attitudes and the prolonged use of Bt cotton are required to conclusively establish these findings.
Abdullah (2010) established that Bt cotton had been widely accepted and was being exponentially cultivated in Punjab, Pakistan. Further investigation into pest resistance revealed that the transgenic cotton was not susceptible to SBW (Bollworm Complex). However, the incidence of SBW was consistent throughout the growing season. Moreover, Bt cotton was susceptible to attack by sucking pests, such as the MB pest. Growth regulators would be effective against such disease-causing agents.
Rao (2009) conducted a study in 2005-2006, which analyzed the impact of Bt cotton varieties on 8000 acres of land, namely IR-NIBGE-2, IR-FH-901, IR-CIM-443 and IR-CIM-448. The findings of this study revealed that farmers from Bahawalpur, Karor Pakka, Muzaffergarh and Multan had investigated and observed various traits attributed to the transgenic crop, including its resistance to different pests, abiotic stress and productivity. When compared with non-Bt cotton crops, the above-mentioned traits were found to be superior in the Bt varieties.
Chen et al., (2000) reported that the concentrations of Bt toxin varied in different segments of the plant. Protein concentrations were significantly higher in fully expanded leaves as compared to germinating petioles, stems and roots. Ovaries at anthesis also expressed noticeably higher concentrations of the toxin proteins when compared to stamens and pistils at the flowering phase. It was also established that in a seven- to nine-leaf stage plant, the toxins were expressed in higher quantities in fully expanded leaves on the central stem, whereas younger leaves near the terminal stem expressed the lowest quantities of the protein toxins.
Producing Bt cotton varieties which express dual-gene endotoxin proteins have been reported to improve the efficacy of larval control and reduce the risk of developing resistance Greenplate et al., 2003; Penn et al., 2001). Bakhsh et al. (2009) investigated the spatio-temporal expression of the two insecticidal genes present in Bt cotton, namely Cry1Ac and Cry2A. Results showed that the endotoxin expression was inconsistent, and varied among different cotton varieties as well as among different parts of the plant. Moreover, the expression of these genes in different plant segments gradually declined as the plant developed. Maximum gene expression was recorded in the leaves of Bt cotton, followed by squares, bolls and anthers, with the least amount of endotoxins in the petals. Despite using the promoter 35S CaMV, the level of toxins was the least in the fruiting part of the plant.
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Kranthi et al., (2005) conducted experiments to confirm the integration of Cry1Ac and Cry2A. Isolation of genomic DNA from the transformed plants was done as described by Dellaporta . PCR was performed to detect the integration of the two genes. PCR was also used for the amplification of internal DNA fragments of 565 bp and 600 bp respectively, with the help of gene specific primers, as illustrated by Saiki et al. Results revealed that there was an almost twofold variation in the expression of Cry1Ac in the growing season of transgenic cotton cultivars. This expression declined with plant growth, and fell below its critical lethal level (1.9µg /g) after 110 days of plantation, leaving the crop susceptible to attack by bollworm.
Katageri et al., (2007) used a synthetic Cry1Ac δ-endotoxin gene to transform a superior genotype of cotton, Gossypium hirsutum L., using Agrobacterium-mediated genetic transformation. Apical shoot meristems were used as explants and were integrated with Agrobacterium tumefaciens (strain EHA 105). Regeneration of the explants was done using a selection medium containing kanamycin. Transgenic progeny was obtained by selfing the transformed explants and grown in a greenhouse. PCR and southern hybridization was used to screen for the presence of neomycin phosphotransferase (nptII) and Cry1Ac genes. The expression of Cry1Ac was measured using Quan-T ELISA kits. Results of bioassays of T2 and T3 progeny showed promising results in terms of resistance against bollworm.
Rashid et al., (2008) used sonication assisted Agrobacterium-mediated transformation to express Cry1Ac and Cry2A genes in a local variety of cotton, CIM-482. The presumed transgenic progeny was confirmed using various bioassays, namely southern hybridization, PCR and western blotting. This method was found to delay targeted insect resistance in cotton.
Stewart et al., (1998) stated that the currently grown varieties of Bt cotton had generated greater profits when compared with conventional cotton varieties. Edge et al. (2001) reported the benefits of including Bt cotton as an integral component of Insect Pest Management (IPM). These include the decreased use of broad-spectrum insecticides and pesticides, enhanced and targeted pest-control, various economic advantages, increased productivity and ultimately, superior biological control.
Various studies that determined the quantities of Cry1Ac and Cry2Ab proteins in different parts of the cotton plant Bollgard II® have been conducted. Sivasupramaniam et al., (2008) reported that Cry1Ac levels were lower in the calyx, flower bracts and large leaves as compared to terminal petals, leaves, ovules and pollen. Cry2Ab protein was found to be the lowest in calyx and the highest in ovlues. Adamczyk et al., (2001b) stated that the highest levels of Cry1Ac were present in squares and flowers, and significantly low in ovules and pollen (Greenplate et al., 1998). Gore et al., (2001) reported the highest expression of Cry1Ac protein in petals and bracts, and lowest in anthers. Wan et al., (2005) performed studies on Bollgard® and showed that Cry1Ac toxins were much higher in leaves, petals and stamens as compared to ovules and bolls.
The above studies conclude that an understanding of the susceptibility of how larva grow on Bollgard II® cotton is essential. Techniques such as the sandwich enzyme-linked immunosorbent assay (ELISA) may be used to quantify the expressions of both Bt endotoxin genes, Cry1Ac and Cry2Ab, in plant extracts (Adamczyk et al., 2000; Greenplate, 1999). Furthermore, the variations in results suggest that differences in methodologies, growing conditions (field or greenhouse) and environmental factors (temperature, water-stress, application of fertilizer, etc.) may play a role in influencing the quantities of endotoxins found in different plant parts, plant types and plant varieties at particular stages of growth and development.
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