Legume Genomics and Genetics 2024, Vol.15, No.6, 291-302 http://cropscipublisher.com/index.php/lgg 294 3.3 Advantages of MAS over traditional breeding methods MAS offers several advantages over traditional breeding methods. Efficiency: MAS allows for the selection of desirable traits at the seedling stage, reducing the time and resources needed for field trials (Ribaut and Ragot, 2006). Precision: By targeting specific genetic markers, MAS increases the accuracy of selecting plants with the desired traits, leading to more consistent and reliable outcomes (Ouyang et al., 2022; Eltaher et al., 2023). Speed: MAS accelerates the breeding process by enabling the early identification of desirable traits, thus shortening the breeding cycle (Patil et al., 2016). Cost-Effectiveness: Although the initial setup for MAS can be expensive, it ultimately reduces the costs associated with extensive field trials and phenotypic evaluations (He et al., 2014; Hassan et al., 2023). 3.4 Examples of MAS applications in other crops MAS has been successfully applied in various crops to improve traits such as drought tolerance, disease resistance, and yield. Common bean (Phaseolus vulgaris L.): MAS has been used to identify RAPD markers associated with drought resistance, leading to improved performance under stress conditions. Maize (Zea mays L.): Marker-Assisted Backcross (MABC) selection has been employed to improve grain yield under drought conditions, resulting in hybrids that perform significantly better under water stress (Ribaut and Ragot, 2006). Faba bean (Vicia faba L.): MAS has been used to develop resistance to diseases such as ascochyta blight and rust, as well as to improve traits like growth habit and nutritional value (Torres et al., 2010). Rice (Oryza sativa L.):MAS has been utilized to introgress QTLs associated with drought tolerance into elite rice lines, enhancing their ability to withstand adverse conditions (Hassan et al., 2023). 4 Genetic Basis of Drought Tolerance in Soybean 4.1 Key genetic loci and quantitative trait loci (QTLs) associated with drought tolerance Drought tolerance in soybean is a complex trait controlled by multiple genetic loci. Several studies have identified key QTLs associated with drought tolerance. For instance, a study using a Recombinant Inbred Line (RIL) population from a cross between drought-tolerant and drought-sensitive cultivars identified five QTLs on five chromosomes, with one QTL on chromosome 16 accounting for 17.177% of the phenotypic variation (Ouyang et al., 2022). Another study identified 10 QTLs on seven chromosomes, with significant loci on chromosomes 1, 2, 7, 10, 14, 19, and 20, explaining up to 12.9% of the phenotypic variance (Figure 2) (Dhungana et al., 2021). Additionally, a comprehensive study on a Chinese cultivated soybean population detected 75 and 64 QTLs for drought tolerance indicators, explaining 54.7% and 47.1% of phenotypic variance, respectively (Wang et al., 2020). 4.2 Major genes involved in drought tolerance mechanisms Several genes have been implicated in the mechanisms of drought tolerance in soybean. For example, nine candidate genes were identified within a QTL on chromosome 16, including Glyma.16G036700, Glyma.16G036400, and Glyma.16G036600, which are annotated as NAC transport factor, GATA transport factor, and BTB/POZ-MATH proteins, respectively (Ouyang et al., 2022). Another study identified 177 candidate genes grouped into nine categories, with ABA and stress responders being major components (Wang et al., 2020). Furthermore, RNA-seq analysis of wild soybean identified differentially expressed genes (DEGs) associated with water and auxin transport, cell wall/membrane integrity, antioxidant activity, and transcription factor activities, highlighting the complexity of the genetic response to drought stress (Aleem et al., 2020). 4.3 Importance of identifying QTLs for MAS Identifying QTLs is crucial for Marker-Assisted Selection (MAS) as it allows for the precise incorporation of drought tolerance traits into new soybean varieties. The identification of QTLs linked to drought tolerance traits enables breeders to select for these traits more efficiently, thereby accelerating the development of drought-tolerant cultivars. For instance, the QTLs identified in various studies can be used to develop molecular markers that facilitate the selection of drought-tolerant genotypes in breeding programs (Ren et al., 2020; Dhungana et al., 2021; Ouyang et al., 2022). The establishment of QTL-allele matrices, as demonstrated in the Chinese cultivated soybean population, provides a robust framework for predicting optimal crosses and enhancing drought tolerance through MAS (Wang et al., 2020).
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