Bt Research 2025, Vol.16, No.4, 168-181 http://microbescipublisher.com/index.php/bt 171 3 Development History of Bt Genetically Modified Crops 3.1 Early research and commercialization process (Bt cotton, Bt maize) The research and development of Bt genetically modified crops began in the mid-to-late 1980s. Monsanto, the United States, introduced the Bt Cry1Ac gene into tobacco for the first time to obtain insect-resistant plants, proving the feasibility of the Bt gene function in plants. In the mid-1990s, Bt-gene cotton and corn were successively tested in North America. In 1996, the United States took the lead in commercially planting Bt cotton (resistant to cotton bollworms) and Bt corn (resistant to corn borers), creating a new era of industrialization of genetically modified crops. Immediately following, Canada, Australia and other countries also approved the cultivation of Bt insect-resistant crops. In China, the research teams of the Chinese Academy of Sciences and the Academy of Agricultural Sciences have carried out the research and development of Bt cotton since 1991, and in 1993, a new insect-resistant cotton product containing the Cry1Ab gene was cultivated (Zhang et al., 2000). After regional experiments and safety evaluation, China approved the commercial planting of the first Bt cotton variety in 1997, with an initial planting area of about 50 000 hectares. Since then, Bt cotton has been rapidly promoted in China's cotton area, and by 2008 the planting area reached 3.8 million hectares, accounting for more than 95% of the country's cotton fields. In terms of corn, the Bt corn commercialized in the United States in 1996 contains the Cry1Ab gene, which mainly prevents and controls European corn borers. It has spread more than 20% of the corn planting area in the United States in a few years. China's insect-resistant corn research and development has also made breakthroughs, and has successively cultivated anti-corn borer transformants containing Cry1Ie and Cry1Ah genes. In 2009, China issued production and application safety certificates for two Bt corn lines (series BVLA430101 and IE09S034), marking an important step in the industrialization of Bt corn (Wang et al., 2022). 3.2 Development of new generation Bt crops (multigene polymerization, bivalent and trivalent genes) With the widespread cultivation of single Bt gene crops, signs of target pest resistance have begun to appear in some areas. In order to delay resistance and expand the prevention and control spectrum, the new generation of Bt crops focus on multigene polymerization and cultivate bivalent or multivalent insect-resistant crops. In terms of insect-resistant cotton, the United States and Australia launched double-price Bt cotton as early as 2003, and Bt cotton in India and other countries has also gradually upgraded from unit price to double-price. Bivalent Bt cotton can express two Bt proteins simultaneously, which is more lethal to targets such as cotton bollworms and significantly delays the development of resistance. In addition to "stacking" multiple Bt genes on the same crop, the artificial design of the new Bt gene is also a hot topic of research and development. For example, directed evolution or structural modification of Cry toxic proteins, create new insecticidal variant genes (such as mCry1Ac, mCry2Ab, etc.) and apply in combination with existing genes (Muralimohan et al., 2020). Chinese researchers successfully polymerized Cry2Ab, Cry1Ab and EPSPS genes on sugarcane, cultivated transgenic sugarcane strains that have both resistance to borer and glyphosate resistance, and predicted that their resistance to sugarcane borer can be maintained for at least 100 generations through simulation experiments (Zafar et al., 2020). These results show that multigene polymerization has greatly expanded the insect resistance spectrum and sustainability of Bt crops, and is the main direction of current Bt genetically modified crop development. 3.3 Research and promotion policies in major countries worldwide The global promotion process of Bt genetically modified crops is significantly affected by the policies and regulations of various countries. As a leading country in agricultural biotechnology, the United States has a relatively complete and relaxed regulatory system and has approved Bt crops more quickly. The United States has approved the commercial cultivation of dozens of insect-resistant genetically modified crops, including multi-generation Bt cotton, Bt corn and root insect-resistant corn. Since Brazil approved Bt cotton in 2005 and Bt corn in 2008, its output has increased significantly and has become a major exporter of cotton and corn (Valicente, 2014). These countries have passed legislation to ensure the intellectual property rights and biosecurity of genetically modified varieties, while providing popular science education to increase public acceptance.
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