Molecular Plant Breeding 2025, Vol.16, No.1, 73-81 http://genbreedpublisher.com/index.php/mpb 75 3 Methodology in Molecular Breeding for Drought Resistance 3.1 Marker-assisted selection (MAS) for drought tolerance Marker-Assisted Selection (MAS) is a powerful tool in molecular breeding that leverages DNA markers to select plants with desirable traits, such as drought tolerance, at early stages of development. This method significantly reduces the time and resources required for breeding compared to traditional methods. MAS has been successfully applied in various crops to enhance traits like frost tolerance and disease resistance, demonstrating its potential for drought tolerance as well (Hasan et al., 2021; Tu et al., 2023; Wang and Zhang, 2024). By identifying and utilizing specific markers linked to drought tolerance, breeders can efficiently screen and select superior sweet potato varieties that are more resilient to water scarcity. 3.2 Genomic selection and quantitative trait loci (QTL) mapping QTL mapping has been instrumental in identifying genetic loci associated with drought tolerance in various crops. For sweet potato, QTL-seq, a modified bulked segregant analysis using next-generation sequencing, has been employed to pinpoint clusters of SNPs linked to important traits (Yamakawa et al., 2021). This method allows for the rapid development of tightly linked DNA markers, facilitating the selection of drought-tolerant varieties. The identification of stable QTLs across different environments and genetic backgrounds is crucial for the success of breeding programs aimed at improving drought resilience (Selamat and Nadarajah, 2021; Wang et al., 2024). Genomic selection (GS) involves predicting the genetic value of plants using genome-wide markers, which is particularly useful for complex traits like drought tolerance that are controlled by multiple genes. High-throughput genotyping (HTG) and phenotyping (HTP) platforms enhance the accuracy and efficiency of GS by providing comprehensive data on genetic and phenotypic variations (Bhat et al., 2020). These tools enable breeders to estimate genomic estimated breeding values (GEBVs) with high precision, accelerating the development of drought-tolerant sweet potato varieties (Halladakeri et al., 2023). 3.3 Genetic transformation and CRISPR applications Genetic transformation and CRISPR/Cas9 technology offer innovative approaches to introduce or modify genes associated with drought tolerance in sweet potato. CRISPR/Cas9, a precise genome-editing tool, allows for targeted modifications in the plant genome, enabling the introduction of drought-resistance genes or the knockout of genes that negatively affect drought tolerance (Rosero et al., 2020; Raj and Nadarajah, 2022). This technology, combined with traditional breeding methods, can significantly enhance the development of drought-resistant sweet potato varieties by directly manipulating specific genetic pathways involved in drought response. 4 Case Study Overview: Breeding Drought-Resistant Sweet Potato 4.1 Selection of parental lines and breeding strategy The selection of parental lines is a critical step in breeding drought-resistant sweet potato varieties. Parental lines are chosen based on their demonstrated drought tolerance and high yield potential. For instance, the study by Sapakhova et al. (2023) highlights the importance of selecting genotypes that exhibit physiological and biochemical traits conducive to drought resistance, such as antioxidant activity and stress protein production. Additionally, transcriptomic analyses, as discussed in Lau et al. (2018), can identify candidate genes associated with drought tolerance, which can be used to select and crossbreed suitable parental lines. 4.2 Development and testing of new varieties Once the parental lines are selected, the breeding strategy involves crossing these lines and developing new varieties through traditional and molecular breeding methods. The use of marker-assisted selection (MAS) is particularly effective in this phase, as it allows for the identification of drought-tolerant traits at the genetic level, thereby accelerating the breeding process (Sprenger et al., 2015; 2017). The development of new varieties is followed by rigorous testing under controlled drought conditions to evaluate their performance. For example, Lau et al. (2018) utilized polyethylene glycol (PEG) to simulate drought conditions and assess the differential gene expression in sweet potato cultivars, providing insights into the genetic mechanisms underlying drought tolerance (Figure 2).
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