Tree Genetics and Molecular Breeding 2025, Vol.15, No.2, 70-79 http://genbreedpublisher.com/index.php/tgmb 72 species offer stability. Through hybridization, they complement each other's strengths. Hu et al. (2022) indicated that the core resource bank of the Actinidia chinensis complex is to lay a solid foundation for future genetic improvement and resource conservation. 4 Hybridization Techniques 4.1 Controlled pollination methods Controlled pollination involves artificially transferring the pollen of selected male plants onto the stigma of the target female plants to ensure that the offspring have the required genetic combination. It is a fundamental step in cultivating new varieties with specific traits. Iliescu et al. (2022) specifically tested the pollen germination rates of different male hybrids and their compatibility with female plants in a kiwifruit breeding project in Romania, which could ensure successful pollination and the smooth development of seeds. Marcellán et al. (2022) found that male parents can affect the success of pollination, as well as the quantity and germination of seeds, indicating that the choice of pollination control is crucial to the overall hybridization outcome. 4.2 In vitro fertilization and embryo rescue In vitro fertilization and embryo rescue are suitable for addressing problems such as long seed dormancy time and low germination rate. They are carried out in a controlled laboratory environment, where embryos are taken out for individual cultivation to ensure their smooth development into healthy seedlings. Marcellán et al. (2022) demonstrated that in vitro embryo culture methods have been successfully employed in kiwifruit breeding to cultivate interspecific hybrids, which are suitable for selecting rootstocks with ideal traits. 4.3 Genetic markers and molecular tools in hybridization Genetic markers, molecular tools and other technologies enable breeders to more accurately identify the parental origin of hybrid plants and screen out individuals with ideal traits more quickly. Zhao et al. (2022) found that fluorescence in situ hybridization (FISH) and genomic in situ hybridization (GISH) have been optimized for application in kiwifruit, which can clearly distinguish chromosomes and determine which genes come from which parent. Chłosta et al. (2021) demonstrated that sex-linked molecular markers can identify female sources in callus or regenerated plants and determine at an early stage which plants are worth preserving, thereby enhancing breeding efficiency. 4.4 CRISPR/Cas9 and gene editing CRISPR/Cas9 can modify the genes of kiwifruit in a targeted manner, which is beneficial for breeders to obtain the desired traits more quickly. In 2018, Varkonyi-Gasic et al. modified the CENTRORADIALIS-like gene through CRISPR/Cas9 technology, successfully transforming kiwifruit into a compact plant and enabling it to flower earlier, accelerating the breeding process. It can also create new traits that traditional breeding cannot achieve. CRISPR/Cas9 is expected to help cultivate new kiwifruit varieties that are more disease-resistant, have better fruit quality and can adapt to different environments (Varkonyi-Gasic et al., 2018; 2021) 5 Strategies for Hybrid Breeding 5.1 Objectives: disease resistance, yield improvement, fruit quality Improving disease resistance can reduce the losses caused by diseases and make planting more stable. When the output increases, the income of fruit farmers will also be higher and the economic benefits will be better. The improvement of fruit quality such as good taste, delicate texture and rich nutrition can meet consumers’ preferences and make it easier to stand out in the market. They are the most core directions in kiwifruit breeding. 5.2 Breeding for abiotic stress tolerance Zhang et al. (2019) found that the transcription factor AcMYB3R of type R1R2R3-MYB could enhance the drought and salt tolerance of Arabidopsis thaliana, indicating that it might have a similar effect in kiwifruit and could be used to cultivate more drought-resistant and salt-tolerant varieties (Figure 1). Jin et al. (2021) demonstrated that the bZIP type transcription factor AchnABF1 is crucial for plants to cope with osmotic stress and low temperatures, providing theoretical support for the development of cold-resistant kiwifruit varieties.
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