Plant Gene and Trait 2024, Vol.15, No.5, 265-274 http://genbreedpublisher.com/index.php/pgt 269 transcription factors, such as those from the MYB and bZIP families, adds another layer of complexity to breeding programs aimed at enhancing stress tolerance (Zhang et al., 2019; Jin et al., 2023). 4.3 Environmental and regulatory challenges Environmental factors and regulatory constraints pose additional challenges to breeding stress-tolerant kiwifruit. The impact of climate change, with increasing instances of extreme weather conditions, necessitates the development of cultivars that can withstand a range of stresses, including drought, salinity, and high temperatures (Zhang et al., 2019; Tu et al., 2023). Furthermore, regulatory hurdles related to the use of genetic modification and genome editing technologies, such as CRISPR/Cas9, can slow down the development and commercialization of new stress-tolerant varieties (Varkonyi-Gasic et al., 2021). The need for extensive field trials and compliance with biosafety regulations adds to the time and cost involved in bringing new cultivars to market. 4.4 Consumer acceptance and market dynamics Consumer acceptance and market dynamics also play a crucial role in the success of breeding programs. While developing stress-tolerant kiwifruit is essential for ensuring crop resilience and productivity, it is equally important to consider consumer preferences for fruit quality, taste, and nutritional value. For instance, the introduction of new cultivars with enhanced stress tolerance must not compromise the desirable traits that consumers expect, such as sweetness, texture, and shelf life (Commisso et al., 2019; Abid et al., 2020). Additionally, market dynamics, including competition from other fruit crops and changing consumer trends, can influence the adoption and success of new kiwifruit varieties. Breeding programs must therefore strike a balance between improving stress tolerance and maintaining or enhancing fruit quality to meet market demands. 5 Case Studies 5.1 Successful breeding programs or studies that have developed stress-tolerant kiwifruit varieties Several breeding programs and studies have successfully developed stress-tolerant kiwifruit varieties by focusing on different types of environmental stresses such as drought, salinity, heat, cold, and waterlogging. One notable study identified the R1R2R3-MYB transcription factor AcMYB3R in kiwifruit, which enhances drought and salinity tolerance when overexpressed in Arabidopsis thaliana. This gene upregulates stress-responsive genes, providing a valuable resource for improving stress tolerance in kiwifruit through molecular breeding (Figure 2) (Zhang et al., 2019). Another study focused on the heat shock transcription factor (Hsf) gene family in two kiwifruit species, Actinidia chinensis and Actinidia eriantha. The study identified 36 Hsf genes in A. chinensis and 41 in A. eriantha, which were found to be crucial for high-temperature tolerance. The findings offer a theoretical foundation for developing heat-stress-resistant kiwifruit varieties (Tu et al., 2023). Salt stress tolerance has also been a significant focus. A study on four different kiwifruit genotypes under varying salt concentrations revealed that the ZM-H genotype fromA. valvata exhibited the highest salt tolerance. This genotype could serve as an important germplasm resource for breeding salt-tolerant kiwifruit rootstocks (Abid et al., 2020). Cold stress tolerance was addressed by identifying the bZIP transcription factor AchnABF1 in kiwifruit, which enhances cold tolerance by upregulating key genes associated with ABA-dependent and ABA-independent pathways. This gene also improves ROS-scavenging ability, making it a valuable target for breeding cold-tolerant kiwifruit varieties. Waterlogging tolerance was studied in Actinidia valvata, where transcriptome analysis revealed that the roots of the KR5 genotype responded to waterlogging stress through carbohydrate and free amino acids metabolism and ROS scavenging pathways. This study provides insights into the factors contributing to waterlogging tolerance, which can be used for breeding waterlogging-tolerant kiwifruit varieties.
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