Tree Genetics and Molecular Breeding 2025, Vol.15, No.2, 70-79 http://genbreedpublisher.com/index.php/tgmb 74 5.3 Breeding for biotic stress tolerance Breeders are striving to identify and utilize genes related to disease resistance and introduce them into new varieties to enhance the resistance of plants. Molecular markers and genomic tools have helped researchers identify many key genes. Zhang et al. (2019) discovered the role of AcMYB3R in abiotic stress and pointed out that it may have potential in enhancing the resistance of plants to phytopathogens. This genetic information can enable breeding work to be carried out more specifically, and kiwifruit varieties that are more resistant to diseases and pests can be selected and bred. 6 Recent Advances and Technologies 6.1 Application of CRISPR and gene editing The CRISPR/Cas9 technology can create directed mutations in the genes of kiwifruit, which is beneficial for breeders to obtain ideal new traits. Varkonyi-Gasic et al. (2018) modified the CENTRORADIALIS-like gene in kiwifruit using CRISPR/Cas9, transforming the perennial plant that originally grew in a vine-like pattern into a compact plant, enabling it to flower rapidly at the top and shortening the juvenility period. Wang et al. (2018) optimized the pairing method of sgRNA and Cas9, making CRISPR/Cas9 more efficient when editing multiple genes at one time and achieving a higher accuracy rate of mutations. Zhou et al. (2020) demonstrated that CRISPR/Cas9 can accelerate the breeding speed and improve the agronomic traits of kiwifruit, and it is a very promising new tool. 6.2 Use of genomics and bioinformatics in hybrid breeding Popowski et al. (2021) established a high-density genetic map using genotyping-by-sequencing (GBS) and identified QTL loci related to fruit quantity, weight, and storage hardness in hexaploid kiwifruit. Hanley (2018) demonstrated that advancements such as the completion of kiwifruit genome sequencing and the decreasing cost of genotyping have made breeding faster and more cost-effective. Cheng et al. (2019) and Jeon et al. (2023) both hold that GS predicts the breeding value of each plant by analyzing a large number of markers across the entire genome, and can also take into account both major and minor effector genes. Merrick et al. (2022) found that this method has begun to be applied in kiwifruit, which is expected to improve the efficiency and accuracy of hybrid breeding. 6.3 Marker-assisted selection (MAS) and genomic selection (GS) MAS is suitable for selecting traits controlled by one or a few genes and determining whether kiwifruit seedlings are male or female (Cheng et al., 2019). Merrick et al. (2022) hold that traits such as the size, taste, and disease resistance of fruits, which are determined by many genes together, need to be marked with GS, which takes all gene markers into account and is more comprehensive. Hasan et al. (2021) found that techniques such as molecular markers have improved the accuracy of screening and made the breeding process faster. The combination of MAS and GS can pick out plants with different advantages at different stages, making the breeding effect better. Hanley (2018) indicated that the kiwifruit varieties eventually bred would better meet the demands of growers and the market. 7 Case Studies of Successful Hybrids 7.1 Detailed examination of notable kiwifruit hybrids Marcellan et al. (2022) obtained varieties with changed rhizome structure by crossing Actinidia arguta with A. chinensis var. deliciosa, providing a new option for controlling plant size. In 2018, Varkonyi-Gasic et al. used CRISPR/Cas9 technology to conduct targeted mutations on the AcCEN4 and AcCEN genes, transforming the traditional perennial climbing kiwifruit into a compact plant, achieving rapid apical flowering and shortening the breeding cycle (Figure 2). Iliescu et al. (2022) screened out male hybrid varieties with a relatively high pollen germination rate, which is crucial for enhancing field pollination efficiency and fruit yield. 7.2 Breeding strategies used and outcomes achieved The hybridization of A. arguta and A. chinensis var. deliciosa was accomplished through conventional isploid hybridization, and new materials with ideal rhizome traits could be obtained without complex techniques such as embryo rescue (Marcellán et al., 2022). Another study was to mutate the AcCEN4 and AcCEN genes using
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