Plant Gene and Trait 2024, Vol.15, No.5, 265-274 http://genbreedpublisher.com/index.php/pgt 268 development of a RAD-based linkage map has facilitated the identification of markers linked to important traits, including stress tolerance, thereby enhancing the efficiency of breeding programs (Scaglione et al., 2015). QTL mapping has been crucial in understanding the genetic basis of stress tolerance in kiwifruit. By mapping QTLs associated with traits like drought and salt tolerance, researchers can pinpoint specific genomic regions that contribute to these traits. This information is vital for developing new varieties with enhanced stress tolerance through targeted breeding strategies (Scaglione et al., 2015). 3.3 Genomic and transcriptomic approaches Genomic and transcriptomic approaches have provided deeper insights into the molecular mechanisms underlying stress tolerance in kiwifruit. Genome-wide association studies (GWAS) and transcriptome analyses have identified key genes and pathways involved in stress responses. For instance, transcriptome analysis of waterlogging-tolerant kiwifruit roots revealed significant changes in carbohydrate and amino acid metabolism, as well as ROS scavenging pathways, under stress conditions (Li et al., 2022). Several key genes have been identified that play crucial roles in stress tolerance. For example, the R1R2R3-MYB transcription factor AcMYB3R has been shown to enhance drought and salinity tolerance by upregulating stress-responsive genes in transgenic plants (Zhang et al., 2019). Similarly, the bZIP transcription factor AchnABF1 has been implicated in cold and osmotic stress tolerance through its role in ROS metabolism and ABA-dependent pathways (Jin et al., 2021). 3.4 Biotechnological interventions Biotechnological interventions, including genetic engineering and CRISPR/Cas9, offer promising avenues for developing stress-tolerant kiwifruit. For instance, overexpression of the NAC domain transcription factor AvNAC030 in transgenic plants has been shown to enhance salt stress tolerance by improving osmotic adjustment and antioxidant defense mechanisms. Additionally, CRISPR/Cas9-mediated editing of key genes involved in stress responses, such as AcePosF21, has demonstrated potential in enhancing cold tolerance by regulating ascorbic acid biosynthesis and reducing oxidative damage (Liu et al., 2023). While these biotechnological approaches hold great promise, they also raise potential ethical and regulatory considerations. The use of genetic engineering and genome editing technologies must be carefully evaluated for their long-term impacts on the environment and biodiversity. Moreover, public acceptance and regulatory frameworks will play crucial roles in the adoption of these technologies in commercial kiwifruit breeding programs. 4 Key Challenges in Breeding Stress-Tolerant Kiwifruit 4.1 Genetic diversity and breeding constraints Breeding stress-tolerant kiwifruit is significantly hampered by the limited genetic diversity within the species. The high heterozygosity and polyploidy of kiwifruit complicate the breeding process, making it challenging to select and stabilize desirable traits. For instance, the application of genome selection (GS) in kiwifruit breeding has shown promise but still faces hurdles due to the complexity of the genome and the need for improved genotyping approaches. Additionally, the dioecious nature of kiwifruit, where male and female plants are separate, further complicates breeding efforts as it necessitates the selection of appropriate male parents, which is both time-consuming and costly. 4.2 Complexity of stress tolerance traits Stress tolerance in kiwifruit involves a complex interplay of multiple physiological and biochemical pathways. For example, the response to cold stress in kiwifruit involves various metabolic pathways such as starch and sucrose metabolism, MAPK signaling, and plant hormone signal transduction (Sun et al., 2021). Similarly, salt stress tolerance is mediated through intricate networks involving glycine betaine, pyruvate metabolism, and glutathione biosynthesis (Abid et al., 2022). The multifaceted nature of these stress responses makes it difficult to pinpoint and breed for specific tolerance traits. Moreover, the expression of stress-responsive genes and
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