Molecular Plant Breeding 2025, Vol.16, No.1, 63-72 http://genbreedpublisher.com/index.php/mpb 65 2.3 Antioxidant defense system Enzymatic antioxidants, such as superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD), play a pivotal role in mitigating oxidative stress caused by drought. These enzymes help in scavenging reactive oxygen species (ROS), thereby protecting cellular components from oxidative damage. Studies have reported increased activities of SOD, CAT, and POD in drought-tolerant soybean varieties, which contribute to their enhanced drought tolerance by maintaining cellular redox homeostasis (Moloi and Merwe, 2021; Begum et al., 2022; Wang et al., 2022). Non-enzymatic antioxidants, such as ascorbate and glutathione, also play a crucial role in the antioxidant defense system during drought stress. These antioxidants work synergistically with enzymatic antioxidants to neutralize ROS and protect cells from oxidative damage. study indicates that drought stress leads to an increase in the levels of ascorbate and glutathione in soybean plants, which helps in maintaining cellular integrity and enhancing drought tolerance (Moloi and Merwe, 2021; Sheteiwy et al., 2021; Wang et al., 2022). In summary, the physiological mechanisms of drought tolerance in soybean seed germination involve a complex interplay of osmotic adjustment, water uptake and retention, and antioxidant defense systems. The accumulation of proline and soluble sugars, the properties of the seed coat, the role of aquaporins, and the activities of enzymatic and non-enzymatic antioxidants are all critical factors that contribute to the drought resilience of soybean seeds. These insights provide a comprehensive understanding of the physiological responses of soybean to drought stress, which can inform breeding programs aimed at developing drought-tolerant soybean varieties. 3 Molecular Mechanisms of Drought Tolerance in Soybean Seed Germination 3.1 Gene expression regulation Transcription factors (TFs) play a crucial role in regulating gene expression in response to drought stress. For instance, the WRKY transcription factor GmWRKY54 has been shown to enhance drought tolerance in soybean by activating genes involved in ABA and Ca2+ signaling pathways. GmWRKY54 directly binds to the promoter regions of genes such as PYL8, SRK2A, CIPK11, and CPK3, thereby promoting stomatal closure to reduce water loss (Wei et al., 2019). Another important TF, GmNFYB17, has been identified to regulate drought resistance by enhancing root growth and improving physiological traits such as relative water content (RWC) and superoxide dismutase (SOD) activity (Sun et al., 2022). Additionally, the GmHdz4 transcription factor, when edited via CRISPR/Cas9, has shown improved drought tolerance by promoting root system architecture and maintaining turgor pressure through osmolyte accumulation (Zhong et al., 2022). Stress-inducible promoters and genes are critical for the activation of drought-responsive pathways. The GmWRKY54 gene, driven by a drought-induced promoter (RD29a), has been shown to confer drought tolerance by activating a large number of stress-related genes (Wei et al., 2019). Similarly, the TGA transcription factor GmTGA17, which is strongly induced by drought stress, enhances tolerance by increasing chlorophyll and proline contents and reducing malondialdehyde (MDA) content in transgenic plants (Li et al., 2019). The GmNAC019 transcription factor also mediates drought tolerance in an ABA-dependent manner, demonstrating higher survival rates and stronger antioxidant defense under water-stressed conditions (Hoang et al., 2019). 3.2 Signal transduction pathways Abscisic acid (ABA) plays a pivotal role in drought response by regulating various physiological and molecular processes. The GmWRKY54 transcription factor activates ABA receptors and SnRK2 kinases, which are upstream components of the ABA signaling pathway, thereby enhancing drought tolerance (Wei et al., 2019). The GmNAC019 transcription factor also functions as a positive regulator of ABA-mediated responses, leading to lower water loss rates and improved drought tolerance (Hoang et al., 2019). Additionally, the overexpression of the Arabidopsis bZIP transcription factor AREB1 in soybean has been shown to activate cross-signaling responses under drought stress, further highlighting the importance of ABA-dependent pathways (Fuhrmann-Aoyagi et al., 2020).
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