Molecular Plant Breeding 2025, Vol.16, No.2, 146-155 http://genbreedpublisher.com/index.php/mpb 149 Another notable example is the use of wild maize relative Zea diploperennis to develop inbred lines with improved resistance to Striga hermonthica and drought. Hybrids derived from these inbred lines have shown superior performance and stability across different environments, highlighting the practical benefits of incorporating wild relatives into breeding programs (Shaibu et al., 2021). Additionally, the integration of exotic germplasm into sub-tropical breeding programs has led to the development of high-yielding and stable hybrids, further demonstrating the success of germplasm innovation in enhancing disease resistance (Nyoni et al., 2023). 4 Breeding Strategies Integrating DH Breeding and Germplasm Innovation in Maize 4.1 Strategies for integrating germplasm innovation with DH breeding Integrating germplasm innovation with doubled haploid (DH) breeding involves combining the genetic diversity and desirable traits from various germplasm sources with the efficiency of DH technology to produce homozygous lines rapidly. One effective strategy is the introgression of resistance loci from diverse landraces into elite maize lines. For instance, European flint landraces have been successfully used to improve resistance to Gibberella ear rot (GER) in elite maize lines through DH breeding (Akohoue et al., 2023). Additionally, the development of haploid-inducer lines carrying CRISPR/Cas9 cassettes allows for targeted genome editing, further enhancing the integration of desirable traits into elite lines (Meng et al., 2022). This approach can significantly shorten the breeding cycle by bypassing the need for repeated backcrossing. 4.2 Process for rapid selection of disease-resistant genotypes The DH breeding process accelerates the selection of superior genotypes by producing homozygous lines in a single generation. After germplasm innovation, such as the introduction of disease resistance genes, DH technology can be employed to rapidly fix these traits. For example, DH lines derived from biparental populations have been evaluated for resistance to multiple diseases, including maize lethal necrosis (MLN), under various environmental conditions. The use of genome-wide association studies (GWAS) and genomic predictions further aids in the rapid identification and selection of disease-resistant genotypes (Sadessa et al., 2022). This process ensures that only the most resistant lines are advanced in the breeding program. 4.3 Doubling and propagation of disease-resistant germplasm Ensuring stable inheritance and large-scale propagation of disease-resistant traits involves the efficient doubling of haploid chromosomes and the subsequent propagation of DH lines. Traditional methods of chromosome doubling often involve the use of toxic chemicals, but recent advances have explored the natural fertility of haploids to reduce reliance on artificial treatments (Kleiber et al., 2012). Additionally, the overexpression of modified CENH3 in haploid inducer lines has been shown to improve maternal haploid induction rates, facilitating the production of DH lines with higher efficiency (Meng et al., 2022). These advancements ensure that disease-resistant traits are stably inherited and can be propagated on a large scale. 4.4 Successful cases of integrating disease-resistant germplasm with DH breeding in maize Several successful cases highlight the integration of disease-resistant germplasm with DH breeding in maize. For instance, the introgression of resistance loci from European flint landraces into elite maize lines resulted in DH populations with significantly lower GER severity compared to the original elite lines (Akohoue et al., 2023). Another example is the use of DH technology to develop lines resistant to multiple diseases, such as gray leaf spot and turcicum leaf blight, through the identification of associated SNPs and the application of marker-assisted selection (Sadessa et al., 2022). These cases demonstrate the effectiveness of combining germplasm innovation with DH breeding to enhance disease resistance in maize. 5 Case Study: Application of Integrated DH Breeding and Germplasm Innovation in Specific Maize Diseases 5.1 Case study on resistance breeding for southern rust Southern corn rust (SCR), caused by Puccinia polysora, is a major threat to maize production, leading to significant yield losses. The integration of doubled haploid (DH) breeding and germplasm innovation has shown promising results in developing resistant lines. For example, a study utilizing four DH populations and their testcross hybrids identified five loci associated with SCR resistance, with an important locus on chromosome 10
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