Maize Genomics and Genetics 2024, Vol.15, No.3, 136-146 http://cropscipublisher.com/index.php/mgg 139 genotypic data into GS models can further enhance the accuracy of predictions and accelerate the breeding process (Zhang et al., 2017; Rice and Lipka, 2021). 3.6 CRISPR/Cas9 and gene editing CRISPR/Cas9 and other gene-editing technologies offer precise tools for modifying specific genes associated with desirable traits. These technologies can be used to introduce or knock out genes to enhance traits such as yield, stress tolerance, and disease resistance. While the provided data does not include specific examples of CRISPR/Cas9 applications in maize, the potential for these technologies to revolutionize maize breeding is immense. By enabling targeted modifications, gene editing can complement traditional breeding strategies and accelerate the development of improved maize varieties. The utilization of genetic diversity in maize improvement involves a combination of traditional and modern breeding strategies. Introgression breeding, heterosis and hybrid breeding, marker-assisted breeding, GWAS, genomic selection, and gene editing each play a crucial role in enhancing the genetic potential of maize and addressing the challenges posed by changing environmental conditions and increasing food demand. 4 Case Studies of Successful Maize Improvement 4.1 Disease resistance 4.1.1 Northern corn leaf blight Northern Corn Leaf Blight (NCLB), caused by the fungal pathogen Setosphaeria turcica, is a major threat to global maize production. Resistance to NCLB has been a key focus in maize breeding programs. Genome-wide association studies (GWAS) have identified multiple significant SNPs and haplotypes associated with NCLB resistance, which can be targeted in breeding programs (Rashid et al., 2020). The research highlights the polygenic nature of NCLB resistance, and in tropical environments, quantitative resistance is preferred due to its robustness against evolving pathogens (Figure 2). This study provides valuable insights into utilizing genetic markers to enhance crop disease resistance, laying the groundwork for future breeding efforts. By integrating quantitative resistance strategies controlled by multiple minor-effect genes, it is possible to achieve durable NCLB resistance across diverse environments (Galiano-Carneiro and Miedaner, 2017). Additionally, large-scale genomic prediction models can improve the accuracy of disease resistance breeding by using genotyped and phenotyped training sets (Technow et al., 2013). 4.1.2 Maize lethal necrosis Maize Lethal Necrosis (MLN) is a devastating disease caused by the synergistic interaction of Maize chlorotic mottle virus (MCMV) and any of the cereal-infecting potyviruses. Breeding for MLN resistance involves evaluating maize lines under artificial inoculation and identifying genomic regions associated with resistance traits. Recent studies have identified several SNPs linked to MLN resistance, providing valuable markers for marker-assisted selection and genomic prediction (Technow et al., 2013). These findings are crucial for developing MLN-resistant maize varieties, particularly in sub-Saharan Africa where the disease is prevalent. 4.2 Abiotic stress tolerance 4.2.1 Drought tolerance Drought stress is a major constraint to maize production, and improving drought tolerance has been a key objective in maize breeding. Advances in genetic dissection through linkage mapping and association mapping have identified key genetic components underlying drought tolerance(Liu and Qin, 2021). The use of transgenic approaches and genome editing technologies has further enhanced the efficiency of breeding for drought tolerance (Liu and Qin, 2021). Additionally, the identification of genetic variants in genes such as ZmVPP1, which contribute to drought tolerance, has provided direct targets for genetic engineering and selection (Wang et al., 2016). 4.2.2 Heat tolerance Heat stress, often occurring in conjunction with drought, poses a significant challenge to maize production. Studies have shown that certain maize landraces exhibit combined tolerance to drought and heat stresses, making them valuable genetic resources for breeding climate-resilient maize varieties (Nelimor et al., 2019). The
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