Legume Genomics and Genetics 2024, Vol.15, No.5, 257-269 http://cropscipublisher.com/index.php/lgg 258 analysis will contribute to a better understanding of the mechanisms underlying drought tolerance in soybeans. Ultimately, it will provide a foundation for developing more resilient soybean varieties through breeding and biotechnological interventions. 2 Physiological Bases of Drought Tolerance 2.1 Water uptake and retention mechanisms Root morphology plays a critical role in water uptake and retention in soybeans under drought conditions. Enhanced root system architecture, including increased root length, surface area, and number of root tips, has been shown to improve water uptake efficiency. For instance, gene-editing of the GmHdz4 transcription factor in soybeans resulted in a significant increase in root system architecture parameters, which contributed to better drought tolerance (Figure 1) (Zhong et al., 2022). Additionally, the identification of genes such as GmACX1, GmMS, and GmPEPCK, which regulate root traits, underscores the importance of root morphology in drought resilience (Xiong et al., 2020). Aquaporins are integral membrane proteins that facilitate water transport across cell membranes, playing a vital role in maintaining cellular water homeostasis under drought conditions. The soybean plasma membrane intrinsic protein GmPIP2;9, for example, has been shown to enhance water uptake and transport, leading to improved drought tolerance. Overexpression of GmPIP2;9 in transgenic soybean plants resulted in increased net CO2 assimilation, stomatal conductance, and transpiration rates, indicating less stress and better water management (Lu et al., 2018). Furthermore, aquaporins modulate hydraulic conductance and root water transport properties, which are crucial for drought adaptation (Sousa et al., 2020; Patel and Mishra, 2021; Han, 2024). 2.2 Osmotic adjustment Osmotic adjustment through the accumulation of osmolytes such as proline and soluble sugars is a key mechanism for maintaining cell turgor under drought stress. Studies have shown that drought-stressed soybean plants accumulate higher levels of soluble sugars, proteins, and proline, which help in osmotic balance and stress mitigation. The role of melatonin in enhancing osmolyte content and antioxidant enzyme activities further supports the importance of osmotic adjustment in drought tolerance (Cao et al., 2019). Maintaining cell turgor is essential for plant survival under drought conditions. The accumulation of osmolytes helps in retaining water within cells, thereby maintaining turgor pressure. Gene-editing of GmHdz4 in soybeans has demonstrated better maintenance of turgor pressure through osmolyte accumulation, which contributes to higher drought tolerance (Zhong et al., 2022). Additionally, the role of aquaporins in regulating cellular water homeostasis further aids in maintaining cell turgor during water deficit (Sousa et al., 2020; Patel and Mishra, 2021). 2.3 Stomatal regulation and transpiration control Stomatal regulation is crucial for controlling water loss through transpiration. Reducing stomatal density and adjusting stomatal aperture can significantly improve water-use efficiency. For instance, drought-stressed soybean leaves showed reduced mRNA levels of stomatal development genes, leading to a decrease in stomatal density and index, which helps in conserving water (Tripathi et al., 2016). High-yielding soybean varieties also exhibit physiological adjustments such as enhanced photoprotective defenses and higher intrinsic water-use efficiency under drought conditions (Buezo et al., 2018). Abscisic acid (ABA) plays a pivotal role in stomatal regulation under drought stress. The transcription factor GmWRKY54, for example, enhances stomatal closure by activating genes in the ABA signaling pathway, thereby reducing water loss and conferring drought tolerance (Wei et al., 2019). The involvement of nitric oxide (NO) in altering stomatal characteristics and hydraulic conductivity further highlights the complex hormonal regulation mechanisms that contribute to drought resilience in soybeans (Sousa et al., 2020).
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