Maize Genomics and Genetics 2024, Vol.15, No.2, 60-69 http://cropscipublisher.com/index.php/mgg 61 Recent advancements in genetic engineering have also focused on developing crops with multiple herbicide tolerance traits to address the issue of herbicide-resistant weeds. For example, maize varieties tolerant to both glyphosate and glufosinate have been developed to provide farmers with more flexible and effective weed management options (Fu et al., 2021; Bao et al., 2022). The primary objective of this study is to evaluate the impact of genetic engineering on herbicide tolerance in maize. Assess the effectiveness of genetically engineered herbicide-tolerant maize in improving weed control and crop yields. Examine the environmental and economic implications of adopting herbicide-tolerant maize varieties. Investigate the potential risks associated with the development of herbicide-resistant weeds and the increased use of herbicides. Explore the advancements in genetic engineering techniques that have contributed to the development of herbicide-tolerant maize. By synthesizing findings from multiple research studies, this study seeks to provide a comprehensive understanding of the benefits and challenges associated with genetically engineered herbicide-tolerant maize. The insights gained from this study will inform future research and policy decisions aimed at optimizing the use of genetic engineering in agriculture for sustainable crop production. 2 Genetic Engineering Techniques for Herbicide Tolerance 2.1 Genetic engineering techniques for herbicide tolerance Transgenic approaches involve the introduction of foreign genes into the maize genome to confer herbicide tolerance. One of the most common genes used in this context is the CP4-EPSPS gene, which provides resistance to glyphosate, a widely used broad-spectrum herbicide. For instance, the development of transgenic maize expressing both CP4-EPSPS and bar genes has shown significant tolerance to glyphosate and glufosinate, allowing for effective weed management and reducing the risk of herbicide resistance in weeds (Yu et al., 2023). Additionally, the use of codon-optimized synthetic CP4-EPSPS genes has been demonstrated to confer high levels of glyphosate tolerance in transgenic rice, suggesting similar potential applications in maize (Hummel et al., 2018). Genome editing techniques such as CRISPR/Cas9 and TALENs have revolutionized the field of genetic engineering by enabling precise modifications at specific genomic loci. CRISPR/Cas9, in particular, has been employed to introduce targeted mutations in the EPSPS gene, resulting in glyphosate-resistant crops. For example, CRISPR/Cas9-mediated homology-directed repair has been used to introduce amino acid substitutions in the EPSPS gene of rice, conferring glyphosate resistance and enhancing grain yield (Figure 1) (Sony et al., 2023). Similarly, the CRISPR/Cas9 system has been utilized to optimize glyphosate tolerance in rapeseed by precise gene replacement, demonstrating the potential for similar applications in maize (Wang et al., 2021). 2.2 Key genes involved in herbicide tolerance The EPSPS (5-enolpyruvylshikimate-3-phosphate synthase) gene is a critical target for glyphosate, which inhibits the shikimate pathway essential for the biosynthesis of aromatic amino acids. Mutations in the EPSPS gene can render the enzyme less sensitive to glyphosate, thereby conferring resistance. For instance, the introduction of mutant variants of the EPSPS gene with specific amino acid substitutions has been shown to provide glyphosate tolerance in rice (Achary et al., 2020). Additionally, the co-expression of EPSPS with other genes, such as glyphosate oxidase, has been demonstrated to enhance glyphosate tolerance and reduce herbicide residues in transgenic crops (Wen et al., 2021). Apart from the EPSPS gene, other genes have also been identified to confer herbicide tolerance. For example, the bar gene, which encodes phosphinothricin acetyltransferase, provides resistance to glufosinate, another commonly used herbicide. The co-expression of CP4-EPSPS and bar genes in transgenic maize has been shown to confer dual herbicide tolerance, allowing for more flexible and effective weed management strategies (Yu et al., 2023). Additionally, the igrAgene from Pseudomonas, which encodes a glyphosate detoxifying enzyme, has been used in combination with EPSPS to enhance glyphosate tolerance and reduce herbicide residues in transgenic rice (Fartyal et al., 2018).
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