Molecular Plant Breeding 2024, Vol.15, No.4, 198-208 http://genbreedpublisher.com/index.php/mpb 202 the number of breeding cycles required. An example of MAS in action is the introgression of the opaque2 (o2) allele into elite maize hybrids to enhance protein quality. By using gene-based simple sequence repeat (SSR) markers, researchers successfully developed quality protein maize (QPM) inbreds with improved lysine and tryptophan content, while maintaining high phenotypic resemblance to the original hybrids (Hossain et al., 2018). 4.3 Genomic selection (GS) Genomic selection (GS) is a powerful tool that uses genome-wide marker data to predict the breeding values of individuals. This method has been shown to increase genetic gains with fewer breeding cycles compared to traditional selection methods. For example, GS was used to improve the kernel dehydration rate (KDR) in an exotic×adapted maize population, resulting in significant genetic gains for KDR and other important traits (Yong et al., 2021). Additionally, GS has been applied to improve shelling percentage and other traits in maize, demonstrating its potential to enhance selection efficiency and accelerate breeding progress (Sun et al., 2018). 4.4 Hybrid breeding strategies Hybrid breeding strategies involve the development of hybrids by crossing two or more inbred lines. This approach can combine the desirable traits of different parental lines, resulting in superior hybrid performance. For instance, the introgression of temperate donor inbred lines into tropical elite maize lines led to the development of experimental single-cross hybrids with high grain yield potential and good ear prolificacy. These hybrids outperformed commercial check hybrids, highlighting the effectiveness of hybrid breeding strategies in improving maize performance (Musundire et al., 2021; Chen et al., 2024). In conclusion, the incorporation of exotic varieties into maize breeding programs through techniques such as introgression and backcrossing, MAS, GS, and hybrid breeding strategies has proven to be highly effective in enhancing genetic diversity and improving various agronomic traits. These methods offer significant potential for the continued improvement of maize cultivars to meet the demands of different environments and market needs. 5 Contributions to Agronomic Traits 5.1 Yield improvement Exotic maize varieties have significantly contributed to yield improvement through various genetic and agronomic advancements. For instance, the identification of quantitative trait loci (QTL) for grain yield under different nitrogen environments has enabled the development of low-nitrogen tolerant genotypes, which are crucial for improving food security in developing countries (Ribeiro et al., 2021). Additionally, the integration of doubled haploidy, high-throughput phenotyping, and genomics-assisted breeding has led to the development of elite stress-tolerant maize cultivars, further enhancing yield potential (Prasanna et al., 2021). Moreover, the discovery of gene leads that improve yield in field-grown elite maize breeding germplasm has provided valuable insights for future crop improvement (Simmons et al., 2021). 5.2 Abiotic stress tolerance (drought, heat, soil salinity) Exotic maize varieties have also played a pivotal role in enhancing tolerance to abiotic stresses such as drought, heat, and soil salinity. Intensive breeding efforts have led to the development of climate-resilient maize cultivars that can withstand various climate-induced stresses, including drought and heat (Prasanna et al., 2021). The genetic dissection of drought and heat-responsive agronomic traits has identified loci associated with stress tolerance, providing valuable targets for breeding programs (Li et al., 2019). Furthermore, metabolomics-driven gene mining has identified candidate genes that improve tolerance to salt-induced osmotic stress, demonstrating the potential for genetic improvement of maize under saline conditions (Figure 3) (Liang et al., 2021). 5.3 Biotic stress resistance (disease, pests) The development of exotic maize varieties has also contributed to biotic stress resistance, including resistance to diseases and pests. The breeding of elite tropical maize germplasm with tolerance to key biotic stresses has been a focus of multi-institutional efforts, resulting in the deployment of stress-tolerant maize cultivars across various regions (Prasanna et al., 2021). These efforts have been crucial in ensuring the resilience of maize crops against biotic stressors, thereby safeguarding yield and productivity.
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