Tree Genetics and Molecular Breeding 2024, Vol.14, No.3, 144-154 http://genbreedpublisher.com/index.php/tgmb 145 physiological processes. For instance, salt stress can disrupt ion homeostasis by causing an excessive accumulation of sodium (Na+) and chloride (Cl-) ions in plant tissues, which in turn inhibits the uptake of essential nutrients like potassium (K+) and calcium (Ca2+) (Yoon et al., 2018; Zhang et al., 2019b). This ionic imbalance can lead to reduced photosynthetic efficiency, stunted growth, and even cell death. Additionally, salt stress induces the production of reactive oxygen species (ROS), which can damage cellular components such as lipids, proteins, and nucleic acids (Guo et al., 2019; Ge et al., 2022). To mitigate these effects, poplar trees activate antioxidant enzymes like superoxide dismutase (SOD) and peroxidase (POD) to scavenge ROS and protect cellular integrity (Guo et al., 2019; Gao et al., 2022). 2.2 Overview of salt tolerance mechanisms in plants Plants have evolved a variety of mechanisms to cope with salt stress, which can be broadly categorized into ion homeostasis, osmotic adjustment, and ROS scavenging. Ion homeostasis involves the regulation of ion transporters and channels to maintain a balance between Na+, K+, and Ca2+ ions. Key players in this process include the salt overly sensitive (SOS) pathway, which helps in the extrusion of Na+ from the cytoplasm and its sequestration into vacuoles (Yoon et al., 2018; Zhang et al., 2019b). Osmotic adjustment is achieved through the accumulation of compatible solutes such as proline, glycine betaine, and sugars, which help to maintain cell turgor and protect cellular structures (Guo et al., 2019; Ge et al., 2022). ROS scavenging mechanisms involve the activation of antioxidant enzymes and the synthesis of non-enzymatic antioxidants to neutralize ROS and prevent oxidative damage (Guo et al., 2019; Gao et al., 2022). Additionally, transcription factors like DREB and NAC play crucial roles in regulating the expression of stress-responsive genes, thereby enhancing the plant's ability to withstand salt stress (Guo et al., 2019; Zhang et al., 2019a). 2.3 Historical approaches to studying salt tolerance in poplar Research on salt tolerance in poplar has a rich history, with early studies focusing on the physiological responses of different poplar species to salinity. Over time, advancements in molecular biology and genomics have enabled more detailed investigations into the genetic and molecular bases of salt tolerance. Transcriptome analyses have been instrumental in identifying key regulatory genes and pathways involved in salt stress responses (Zhang et al., 2019b; Ge et al., 2022). For example, the PeERF1 gene fromPopulus euphratica has been shown to enhance salt tolerance when overexpressed in transgenic poplar lines (Ge et al., 2022). Similarly, downregulation of the PagSAP1 gene in Populus alba × P. glandulosa has been found to increase salt tolerance by improving ionic homeostasis and stress-responsive gene expression (Yoon et al., 2018). Recent studies have also explored the role of specific genes like NAC13 and PtGSTF1 in conferring salt tolerance through mechanisms such as ROS scavenging and cell wall modification (Zhang et al., 2019a; Gao et al., 2022). These historical approaches have laid the foundation for current and future research aimed at developing salt-tolerant poplar varieties through genetic engineering and breeding programs. 3 Gene Screening Techniques 3.1 Advanced genomic tools used in salt tolerance gene screening Advanced genomic tools have significantly enhanced the efficiency and accuracy of identifying salt tolerance genes in poplar. One such tool is the Weighted Gene Co-expression Network Analysis (WGCNA), which was used in combination with Genome-Wide Association Studies (GWAS) to identify key regulatory factors associated with salt resistance in Populus euphratica. This approach led to the discovery of the PeERF1 gene, which showed significant differences in expression levels under salt stress conditions (Ge et al., 2022). Another powerful tool is RNA sequencing (RNA-seq), which has been employed to profile the expression patterns of various genes under salt stress. For instance, RNA-seq analysis identified 63 HD-Zip transcription factors in poplar, with PsnHDZ63 being significantly up-regulated under salt stress (Guo et al., 2021). Additionally, transcriptomic and metabolomic analyses have been used to understand the molecular mechanisms underlying salt tolerance. For example, the transcriptomic analysis of transgenic Arabidopsis expressing glycine-rich RNA-binding proteins from Sporobolus virginicus revealed upregulation of stress-related pathways, providing insights into the roles of these proteins in salt tolerance (Tada et al., 2019).
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