Maize Genomics and Genetics 2024, Vol.15, No.2, 49-59 http://cropscipublisher.com/index.php/mgg 52 Another limitation is the environmental dependency of trait expression. Genotype by environment interactions can affect the performance of bred varieties, making it challenging to predict how new hybrids will perform under different growing conditions. This variability necessitates extensive field trials across multiple environments, further extending the breeding timeline (Burger et al., 2008). Moreover, conventional breeding is often resource-intensive, requiring significant investments in labor, land, and facilities for field trials and seed production. These costs can be prohibitive, especially for public breeding programs and smaller seed companies with limited budgets (Jumbo et al., 2011). In conclusion, while conventional breeding has been instrumental in the development and improvement of maize, it faces challenges that limit its efficiency and scope. Addressing these challenges through technological advancements and complementary breeding techniques will be essential to meet the future demands of global agriculture. 3 Genetic Engineering in Maize 3.1 Historical development of genetic engineering The history of genetic engineering in maize dates back to the late 20th century when scientists began to explore the potential of recombinant DNA technology. The first genetically modified (GM) maize plants were developed in the early 1980s using methods such as Agrobacterium-mediated transformation and particle bombardment. These techniques allowed for the introduction of foreign genes into the maize genome, enabling the development of transgenic maize with improved traits (Dunder et al., 1995). One of the earliest successes in genetic engineering was the development of Bt maize, which expresses a gene from the bacterium Bacillus thuringiensis. This gene produces a protein toxic to certain insect pests, significantly reducing the need for chemical pesticides and leading to widespread adoption of Bt maize in the mid-1990s (Wisniewski et al., 2002). Since then, genetic engineering has expanded to include traits such as herbicide resistance, drought tolerance, and enhanced nutritional content. 3.2 Techniques and methods 3.2.1 Recombinant DNA technology Recombinant DNA technology involves the manipulation of DNA to create new genetic combinations that are not found in nature. In maize, this technology typically involves the insertion of genes that confer desirable traits, such as resistance to pests or tolerance to herbicides. The process begins with the identification and isolation of the gene of interest, which is then inserted into a plasmid vector. This vector is introduced into maize cells using methods like Agrobacterium-mediated transformation or particle bombardment (Yadava et al., 2017). Agrobacterium-mediated transformation utilizes the natural ability of Agrobacterium tumefaciens to transfer DNA into plant cells. The bacterium carries a plasmid with the gene of interest, which integrates into the plant genome, allowing for stable genetic modification. Particle bombardment, or biolistics, involves shooting microscopic particles coated with DNA into plant cells. This method is particularly useful for maize, which can be challenging to transform using Agrobacterium due to its recalcitrant nature (Hong et al., 2019). 3.2.2 CRISPR-Cas9 and other genome editing tools CRISPR-Cas9 has revolutionized genetic engineering by enabling precise, targeted modifications to the genome. This system uses a guide RNA to direct the Cas9 enzyme to a specific DNA sequence, where it creates a double-strand break. The cell's natural repair mechanisms then fix the break, allowing for the introduction or deletion of specific genes. This technology offers several advantages over traditional recombinant DNA methods, including higher precision, fewer off-target effects, and the ability to make multiple simultaneous edits (Hue et al., 2018). Other genome editing tools, such as zinc finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs), also allow for precise genetic modifications. These technologies have been used to develop maize varieties with enhanced traits, such as improved drought tolerance and resistance to various pests and diseases (Yang and Yan, 2021).
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