Rice Genomics and Genetics 2024, Vol.15, No.4, 164-177 http://cropscipublisher.com/index.php/rgg 172 6.2 Drought and salinity tolerance Understanding the genomic basis of stress tolerance has been critical for developing drought- and salinity-tolerant rice varieties. Studies have identified QTLs associated with these traits, such as those found in the Oryza sativa IR64×Oryza glaberrima interspecific backcross populations, which revealed significant genetic variability and QTLs linked to yield-enhancing traits under stress conditions (Bharamappanavara et al., 2023). Breeding programs have leveraged genomic insights to develop rice varieties with enhanced stress tolerance. For instance, the introduction of genes from wild rice species through genetic transformation and genome editing has led to varieties with improved drought and salinity tolerance. The Agrobacterium-mediated transformation system has been successfully used to introduce stress-tolerance traits from wild rice into cultivated varieties, demonstrating the potential for neodomestication (Xiang et al., 2022). Figure 4 CRISPR/Cas9-targeted mutagenesis of Os8N3 in rice to confer resistance to Xanthomonas oryzae pv. oryzae (Adopted from Kim et al., 2019) Image caption: a: Bacterial blight resistance phenotypes of the xa13 mutant rice lines (T1). Rice plants 12 DAI with Xoo. From left to right: Kitaake (Kit), transgenic line (XA21, 7A-8) carrying Xa21 driven by the ubiquitin promoter, and transgenic lines (OsU6a xa13m/Kit, T1) carrying the OsU6a::xa13m-sgRNA/pHAtC construct. Arrowheads indicated the end of the lesion. WT: wild type: Ho: homozygous; Bi: bi-allelic: He; Heterozygous. b: Lesion lengths measured 12 DAI in Kitaake, XA21, and OsU6a xa13m/Kit T1. Error bars in the graph represent standard error of at least three leaves from each plant. Letters indicate a significant difference at P < 0.050 by Tukey’s HSD test (Adopted from Kim et al., 2019) The study by Xiang et al. (2022) successfully established an efficient gene transformation and editing system for Chaling Common Wild Rice (CLCWR). Using thin-layer tissue of mature seed endosperm and Agrobacterium-mediated transformation, the researchers achieved genetic modification of this wild rice with an AA genome (Figure 5). The study showed that CLCWR callus tissue is easily inducible and regenerable, with transformation efficiencies ranging from 87% to 94%. The efficiency for single-gene and multi-gene editing was 60%~70% and 20%~40%, respectively. Additionally, compared to Nipponbare (Nip), CLCWR exhibited a higher frequency of hygromycin-resistant callus and transformation efficiency. Through CRISPR/Cas9-mediated gene editing, the system successfully introduced specific gene variations, demonstrating the potential for efficient gene function studies and molecular breeding applications. This research not only showcased the application of the Agrobacterium-mediated transformation system in wild rice but also provided a valuable foundation for future crop domestication using gene editing technologies. 6.3 Biofortification and quality improvement Biofortification aims to enhance the nutritional value of rice by increasing the content of essential nutrients. Genetic approaches have identified key genes involved in nutrient biosynthesis pathways. For example, the
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