International Journal of Molecular Evolution and Biodiversity, 2025, Vol.15, No.2, 84-98 http://ecoevopublisher.com/index.php/ijmeb 88 4.2 Osmotic adjustment through proline and sugar accumulation Under drought stress conditions, plants must maintain normal water potential and metabolic activities of cells. Osmotic regulation becomes the core mechanism, among which proline and soluble sugars are the main regulatory factors. The accumulation of these two types of small molecules helps to maintain cell turgor pressure, regulate osmotic balance, and protect organelles and membrane structures. Studies have found that drought-tolerant rapeseed varieties have significantly higher levels of proline and sugar accumulation than susceptible varieties (Chaghakaboodi et al., 2021; Batool et al., 2022). Proline can not only stabilize protein structure in cells, but also effectively scavenge free radicals and alleviate oxidative damage caused by stress. At the same time, soluble sugars not only participate in osmotic regulation, but also provide cells with necessary energy support to maintain basic metabolic functions. The enrichment of these osmotic regulating substances is usually closely related to the increase in the activity of related enzymes, thereby further enhancing the plant's resistance to drought (Batool et al., 2022). Therefore, focusing on osmotic regulation pathways and screening materials with stronger proline and sugar accumulation capabilities is one of the important strategies to improve rapeseed drought tolerance. 4.3 Antioxidant defense system and regulation of reactive oxygen species (ROS) Under drought conditions, reactive oxygen species (ROS) often accumulate rapidly in plants. If these unstable molecules cannot be removed in time, they can easily trigger a series of oxidative stress reactions such as membrane lipid peroxidation, protein inactivation and DNA damage. To defend against this damage, plants rely on a sophisticated antioxidant enzyme system. Key enzymes including superoxide dismutase (SOD), catalase (CAT) and peroxidase are important barriers to remove ROS and maintain cellular redox balance (Batool et al., 2022). Studies have shown that drought-tolerant rapeseed can usually maintain higher levels of antioxidant enzyme activity under drought stress, while drought-sensitive varieties are more susceptible to damage by reactive oxygen species. The enhancement of antioxidant enzymes helps maintain cell metabolic balance and alleviate the adverse effects of oxidative stress. For this reason, strengthening antioxidant defense mechanisms has become one of the important directions of current drought-resistant breeding. By regulating key genes related to the antioxidant system, enzyme activity can be increased at the molecular level, providing a new path to improve the physiological stability and survival ability of rapeseed under drought conditions. 4.4 Regulatory role of hormone signaling pathways in drought response Plant hormones constitute a complex signal network to regulate the drought response process. ABA, as a core signal molecule, accumulates rapidly under water stress, triggering key physiological responses such as stomatal closure and osmotic regulation (Lin et al., 2020). Ethylene and cytokinin synergistically participate in this process. The former regulates the remodeling of the root system architecture, while the latter affects cell proliferation and expansion, jointly enhancing the drought resistance of plants. The discovery of the rapeseed BnA.JAZ5 gene reveals a new mechanism of hormone cross-regulation. This gene integrates the ABA and JA signaling pathways and affects water use efficiency by regulating the dynamics of stomatal development (Cao et al., 2022). This multi-hormone synergistic action mode provides a new perspective for understanding plant environmental adaptation strategies. Analyzing the interaction mechanism of the hormone signaling network has important application value. By targeting and regulating key node genes, precise intervention in drought response can be achieved, opening up new ways to cultivate high-yield and stable-yield rapeseed varieties. 5 Breeding Approaches for Drought Tolerance 5.1 Conventional breeding techniques and challenges Traditional breeding methods have always occupied a core position in the cultivation of drought-resistant rapeseed varieties. Technical means such as systematic selection, recurrent selection, backcrossing and mutagenesis all use existing natural genetic diversity to improve target traits (Ashraf, 2010; Begna, 2022). For example, by selecting
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