Legume Genomics and Genetics 2024, Vol.15, No.5, 257-269 http://cropscipublisher.com/index.php/lgg 260 3 Biochemical Bases of Drought Tolerance 3.1 Antioxidant defense systems Enzymatic antioxidants play a crucial role in mitigating oxidative stress induced by drought conditions in soybeans. Superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) are key enzymes that help in scavenging reactive oxygen species (ROS). Studies have shown that the activities of these enzymes increase under drought stress, which helps in maintaining cellular homeostasis and protecting plant tissues from oxidative damage (Wang et al., 2022; Fatema et al., 2023). For instance, the drought-tolerant soybean variety ‘Heinong 44’ exhibited higher activities of CAT and POD compared to the drought-sensitive variety ‘Heinong 65’, indicating a robust enzymatic antioxidant defense system. Non-enzymatic antioxidants such as ascorbate and glutathione also contribute significantly to the antioxidant defense system in soybeans under drought stress. These molecules act as ROS scavengers and help in maintaining the redox balance within the cells. The total antioxidant capacity (T-AOC) has been observed to increase significantly in drought-tolerant soybean varieties, suggesting an enhanced non-enzymatic antioxidant defense mechanism (Wang et al., 2022). This increase in T-AOC is crucial for mitigating the adverse effects of drought-induced oxidative stress. 3.2 Metabolic pathways and stress metabolites Primary metabolites such as carbohydrates and amino acids play essential roles in the plant's response to drought stress. Carbohydrates serve as energy sources and osmoprotectants, while amino acids like proline accumulate in response to drought, aiding in osmotic adjustment and protecting cellular structures (Wang et al., 2022; Fatema et al., 2023). The accumulation of soluble sugars and proline has been reported to increase under drought conditions, contributing to the osmotic balance and stress tolerance in soybeans. Secondary metabolites, including flavonoids and phenolics, are crucial for enhancing drought tolerance in soybeans. These compounds have antioxidant properties and help in protecting the plant cells from oxidative damage. The production of secondary metabolites is often upregulated in response to drought stress, contributing to the overall stress tolerance mechanism (Dubey et al., 2019; Aleem et al., 2020). For example, transcriptome analysis has revealed the upregulation of genes involved in secondary metabolism, indicating their role in drought response. 3.3 Membrane stability and lipid peroxidation The lipid composition and fluidity of cellular membranes are critical factors in maintaining membrane stability under drought stress. Changes in lipid composition can affect membrane fluidity, which in turn influences the plant's ability to withstand drought conditions. Drought-tolerant soybean varieties have been shown to maintain better membrane stability, which is essential for preserving cellular integrity and function during water deficit (Castro et al., 2019; Fatema et al., 2023). Lipid peroxidation is a common consequence of oxidative stress induced by drought, leading to membrane damage. However, drought-tolerant soybean varieties exhibit lower levels of lipid peroxidation, indicating effective protective mechanisms. The accumulation of malondialdehyde (MDA), a marker of lipid peroxidation, is significantly lower in drought-tolerant varieties, suggesting better protection against oxidative damage (Castro et al., 2019; Fatema et al., 2023). The overexpression of specific genes, such as GmNFYB17, has been shown to enhance drought resistance by reducing MDA content and improving membrane stability (Figure 2) (Sun et al., 2022). In summary, the biochemical bases of drought tolerance in soybeans involve a complex interplay of antioxidant defense systems, metabolic pathways, and mechanisms to maintain membrane stability. These biochemical responses are crucial for enhancing the plant’s ability to cope with drought stress and ensure better growth and productivity under water-limited conditions (Buezo et al., 2018).
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