Tree Genetics and Molecular Breeding 2024, Vol.14, No.3, 144-154 http://genbreedpublisher.com/index.php/tgmb 149 5.3 Comparative analysis with other tree species exhibiting salt tolerance Comparative analysis with other tree species reveals that similar mechanisms are employed across different species to confer salt tolerance. For instance, the glycine-rich RNA-binding proteins (GRPs) identified in the halophyte Sporobolus virginicus have been shown to enhance salt tolerance in transgenic Arabidopsis, suggesting a conserved role of GRPs in salt stress response across different plant species (Tada et al., 2019). Additionally, the downregulation of stress-associated protein 1 (PagSAP1) in Populus alba × P. glandulosa increases salt tolerance by maintaining cellular ionic homeostasis, a mechanism also observed in other salt-tolerant species (Yoon et al., 2018). The WRKY transcription factor family, including PagWRKY75 and PalWRKY77, has been shown to play significant roles in salt and osmotic stress responses in poplar, with similar regulatory functions observed in other plant species (Zhao et al., 2019; Jiang et al., 2020). These findings indicate that while specific genes and their regulatory networks may vary, the underlying mechanisms of salt tolerance, such as ion homeostasis, ROS scavenging, and stress-responsive gene regulation, are conserved across different tree species. 6 Technological Advances in Genetic Engineering 6.1 Role of CRISPR-Cas9 and other genome editing tools in enhancing salt tolerance CRISPR-Cas9 has revolutionized the field of genetic engineering by providing a precise and efficient method for genome editing. This technology allows for the targeted modification of specific genes, which is crucial for enhancing salt tolerance in plants. For instance, CRISPR-Cas9 has been successfully used to edit genes in rice, leading to improved salt stress tolerance by targeting key molecular pathways involved in salt stress response (Farhat et al., 2019; Tran et al., 2020; Nazir et al., 2022). In poplar, CRISPR-Cas9 has been applied to create targeted mutations, demonstrating its potential in woody plants as well (Wang et al., 2021). The ability to perform multiplexed editing, as shown in tomato, further highlights the versatility of CRISPR-Cas9 in addressing complex traits like salt tolerance (Nazir et al., 2022). 6.2 Advances in transgenic approaches and their application in poplar Transgenic approaches have also made significant strides in enhancing salt tolerance in poplar. Overexpression of specific genes, such as the H+-pyrophosphatase gene PtVP1.1, has been shown to confer salt tolerance by improving ion homeostasis and reactive oxygen species scavenging (Kerek et al., 2021). Similarly, the overexpression of the HD-Zip transcription factor PsnHDZ63 in Populus simonii × P. nigra has resulted in better morphological and physiological responses under salt stress (Chan et al., 2022). These advances demonstrate the potential of transgenic approaches in developing salt-tolerant poplar varieties by manipulating key genes involved in stress response pathways. 6.3 Challenges and future prospects for genetic engineering in poplar Despite the promising advances, several challenges remain in the genetic engineering of poplar for salt tolerance. One major challenge is the complexity of the poplar genome, which can complicate the identification and manipulation of target genes. Additionally, the long generation time of woody plants poses a significant hurdle for rapid genetic improvements. However, the development of high-throughput functional genomics tools, such as CRISPR-Cas9, offers new opportunities to overcome these challenges (Zafar et al., 2020). Future prospects include the integration of genome-wide association studies (GWAS) with genome editing to identify and target novel genes associated with salt tolerance. Moreover, the combination of CRISPR-Cas9 with other emerging technologies, such as RNA interference (RNAi) and transcriptional modulation, could further enhance the precision and efficiency of genetic engineering in poplar (Tran et al., 2020; Han et al., 2022). 7 Implications for Breeding and Conservation 7.1 Using genetic insights to develop salt-tolerant poplar varieties The identification and functional verification of salt tolerance genes in poplar species provide a robust foundation for breeding programs aimed at developing salt-tolerant varieties. For instance, the overexpression of the PeERF1 gene in Populus alba × Populus glandulosa has demonstrated significant improvements in salt tolerance, highlighting its potential as a candidate gene for breeding (Ge et al., 2022). Similarly, the PtGSTF1 gene has been
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