Tree Genetics and Molecular Breeding 2024, Vol.14, No.2, 57-68 http://genbreedpublisher.com/index.php/tgmb 58 include osmotic adjustment, antioxidative defense, and hormonal regulation, which collectively enhance the tree's ability to withstand adverse environmental conditions (Bhusal et al., 2021; Bano et al., 2022). For instance, the abscisic acid (ABA) signaling pathway plays a crucial role in stress sensing and response, activating downstream transcription factors that regulate stress-responsive genes (Polle et al., 2019; Gao et al., 2019). Additionally, the integration of transcriptomic and genomic data has provided insights into the conserved and unique genetic responses to different stresses acROSs various tree species (Benny et al., 2020; Bano et al., 2022). 2.2 Key genes involved in drought, salt, and cold resistance Several key genes have been identified that contribute to drought, salt, and cold resistance in trees. For drought resistance, genes involved in osmotic adjustment, such as those regulating proline and other osmolytes, are crucial. In salt stress, genes associated with ion transport and sequestration, such as those encoding for sodium transporters and vacuolar proton pumps, play significant roles (Zhang et al., 2020). Cold resistance is often mediated by genes involved in the synthesis of cryoprotective proteins and antifreeze proteins (Chen et al., 2013; Jin et al., 2017). Notable examples include the WRKY transcription factors, which are involved in both drought and salt stress responses (Gao et al., 2019), and the NACtranscription factors, which play roles in cold and drought tolerance by interacting with other stress-responsive genes. Additionally, calcium-dependent protein kinases (CDPKs) and mitogen-activated protein kinase kinase kinases (MAPKKKs) have been shown to modulate stress responses by regulating reactive oxygen species (ROS) and other signaling molecules (Chen et al., 2013). 2.3 Methods for identifying and characterizing resistance genes The identification and characterization of resistance genes in trees have been greatly facilitated by advances in molecular biology and bioinformatics. Transcriptome meta-analysis is a powerful approach that integrates data from multiple studies to identify differentially expressed genes under stress conditions (Benny et al., 2020). This method allows for the identification of conserved stress-responsive genes acROSs different species and conditions. Functional genomics approaches, such as gene overexpression and silencing, have been employed to validate the roles of candidate genes in stress tolerance (Chen et al., 2013; Gao et al., 2019). Additionally, techniques like yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays are used to study protein-protein interactions and elucidate the regulatory networks involved in stress responses (Jin et al., 2017). Chromosome mapping and the identification of molecular markers are also crucial for breeding programs aimed at developing stress-resistant tree cultivars. 3 Drought Resistance Mechanisms in Trees 3.1 Physiological and biochemical pathways influenced by drought resistance genes Drought resistance in trees involves a complex interplay of physiological and biochemical pathways. Key regulatory genes and transcription factors play crucial roles in modulating these pathways to enhance drought tolerance (Figure 1). For instance, the Dehydration-responsive element D (DRE1D) and ethylene response factor (ERF61) genes have been identified as significant contributors to drought stress resistance in cotton, influencing various stress-responsive molecular networks (Bano et al., 2022). Similarly, in Arabidopsis thaliana, transcription factors such as HSF, AP2/ERF, and C2H2 have been shown to play critical roles in drought stress response mechanisms, highlighting the importance of transcriptional regulation in drought tolerance. Additionally, the WRKY transcription factor WRKY8 in Solanum lycopersicum has been found to enhance drought tolerance by regulating stress-responsive genes and maintaining higher water content in leaves (Gao et al., 2019). Figure 1 shows the network analysis of differentially expressed transcription factor genes under salt and drought stress. Each color of the node in the figure represents a specific transcription factor family, such as AP2/ERF,
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