Rice Genomics and Genetics 2024, Vol.15, No.3, 106-120 http://cropscipublisher.com/index.php/rgg 114 and modern breeding techniques with the genetic resources from wild and cultivated Oryza species, significant advancements can be made in rice improvement, ensuring food security and sustainability for the growing global population. 7 Case Studies of Successful Rice Improvement 7.1 Disease resistance Disease resistance is a critical aspect of rice improvement, as diseases can significantly reduce yield and quality. Several successful case studies illustrate how genetic resources from wild and cultivated Oryza species have been used to develop disease-resistant rice varieties. Bacterial blight (BB), caused by Xanthomonas Oryzae pv. Oryzae (Xoo), poses a major threat to rice production (Kumar et al., 2020). To enhance rice resistance to BB, researchers have adopted various strategies such as utilizing wild rice genes, marker-assisted breeding, and genome-wide association studies. Angeles-Shim et al. (2020) identified a new locus from the wild rice species Oryza latifolia that confers race-specific resistance to PXO339 (Philippine Xoo race 9A). The study showed that this locus was transmitted through two introgression lines (WH12-2252 and WH12-2256) and exhibited resistance to the PXO339 strain. Genotypic analysis and phenotypic segregation ratios indicated that this resistance is controlled by a single recessive gene (Figure 3). Further genomic analysis narrowed down the candidate region to a 1 817 kb segment on chromosome 12 and identified potential candidate genes regulating this resistance. The findings underscore the importance of wild rice species as a valuable source of new resistance genes and suggest integrating these genes into rice breeding programs to improve crop disease resistance (Angeles-Shim et al., 2020). Figure 3 Phenotype distribution in segregating populations (Adopted from Angeles-Shim et al., 2020) Image caption: The figure shows the distribution of lesion lengths in F2 and F3 segregating populations after inoculation with PXO339. In the F2 population, the lesion length exhibits a bimodal distribution, separating into resistant and susceptible phenotypes. Resistant plants have lesion lengths less than 4 cm, while susceptible plants have lesion lengths greater than 6 cm. The F3 population shows a similar bimodal distribution, with resistant plants having lesion lengths less than 5 cm and susceptible plants having lesion lengths greater than 6 cm. The lesion length distribution in the F2 population indicates a clear segregation of resistant and susceptible plants, consistent with a Mendelian 3:1 segregation ratio, and the F3 population displays a similar pattern. These results collectively support the conclusion that PXO339 resistance is controlled by a single recessive gene (Adapted from Angeles-Shim et al., 2020) Rice blast, caused by the fungus Magnaporthe Oryzae, is another major biotic stress affecting rice. The use of genes from wild and traditional rice varieties has successfully achieved resistance to rice blast. For instance, the study used marker-assisted backcrossing to improve the Indian elite rice variety ‘Krishna Hamsa’, making it resistant to bacterial blight (BB) and blast disease (Badri et al., 2022). Additionally, researchers have developed an introgression library that includes agronomic traits from all AA genome Oryza species, which encompasses traits for rice blast resistance. This provides a valuable resource for future breeding programs (Zhang et al., 2022).
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