Rice Genomics and Genetics 2024, Vol.15, No.2, 69-79 http://cropscipublisher.com/index.php/rgg 75 improves the transmission of disease resistance genes efficiency, and also avoids the carriage of undesirable genes unrelated to the target traits. This provides a more precise genetic improvement method for breeding more resistant rice varieties. 4 Molecular Breeding Strategies 4.1 Selection based on molecular markers Molecular marker -assisted selection (MAS) is an indirect selection using molecular markers that are closely linked to the target trait gene. It is a selection of the target trait at the DNA level and is not affected by the environment or alleles. The interference of dominant and recessive relationships makes the selection results reliable, and at the same time it can avoid the interference of dominant and recessive relationships between alleles, thereby achieving efficient improvement of comprehensive traits such as crop yield, quality and resistance; Marker-assisted breeding with marker genotype identification can be carried out in low generations and at any stage of plant growth. Co-dominant molecular markers allow the identification of recessive genes at the hybrid stage. The selection of target genes has the advantage of not being affected by gene expression and environmental conditions. Bergman et al. (2000) of the State Key Laboratory of Crop Genetic Improvement of Huazhong Agricultural University used MAS technology to successfully introduce the broad-spectrum bacterial blight resistance gene Xar21 into the excellent restorer line Minghui 63, and selected the genes to be less than 1.0 cM on both sides of Xcr21 Through a round of backcrossing and selfing, single plants homozygous for Minghui 63 alleles at most loci except Xar21 were obtained. It can be seen that as long as there are appropriate molecular markers, MAS can indeed improve the efficiency of resistance selection and speed up the process of disease resistance breeding. In addition to MAS technology, haploid selection (DH) breeding has made important progress in both theoretical and applied research. Through rice haploid breeding, we can speed up the breeding process, improve breeding efficiency, and provide new ways for rice breeding. At the same time, it can also reveal the genetic patterns and gene functions of rice, providing a basis for genetic improvement and functional gene research in rice. Some results have also been achieved in the practical application of rice blast. For example, haploid hybridization and cytology techniques are used to develop new rice varieties with disease resistance, stress tolerance and high yield. Haploid selection (DH) is a breeding method that accelerates gene fixation by halving the number of chromosomes in a plant's cells to obtain homozygous haploid plants. In disease resistance breeding, DH technology can accelerate the fixation of genotypes carrying disease resistance genes, thereby faster obtaining new varieties with stable disease resistance traits (Peng et al., 2021). Taking the fixation of rice blast resistance genes as an example, breeders first identify and select hybrid plants carrying rice blast resistance QTL through methods such as marker-assisted selection. Then, haploid selection technology is used to produce haploid offspring from these plants. Because a haploid plant is homozygous, all its cells carry the same genotype, including the target disease resistance gene. Through haploid selection, breeders can more quickly fix rice blast resistance genes in the entire plant population to form haploid germplasm with disease resistance traits. The combined application of these two technologies also shows great potential. By combining marker-assisted selection with haploid selection, breeders can accurately select plants carrying target disease-resistant genes while accelerating the fixation of disease-resistant genes. This not only improves breeding efficiency, but also ensures the stable transmission of disease-resistant genes, providing a more reliable way to cultivate crop varieties with stronger disease resistance. 4.2 Application of gene editing technology The basic principle of CRISPR-Cas9 technology involves CRISPR (Clustered regularly interspaced short palindromic repeats) sequences and Cas9 proteins. CRISPR sequences are a naturally occurring immune
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