Molecular Soil Biology 2024, Vol.15, No.5, 205-215 http://bioscipublisher.com/index.php/msb 208 3.3 Hormonal regulation (e.g., ABA) in water stress Hormonal regulation plays a crucial role in plant responses to drought stress, with ABA being a key hormone involved in this process. ABA can regulate various physiological processes by its negative and positive crosstalk with phytohormones in response to drought conditions. In pennyroyal (Mentha pulegium L.), ABA content is highest in the plants, inoculated with plant growth-promoting rhizobacteria (PGPR) under severe drought stress (Asghari et al., 2020). Additionally, ABA, interacts with other hormones like jasmonates (JA), salicylic acid (SA), and ethylene (ET), modulates developmental processes and signaling networks that contribute to plant defense against water stress (Wahab et al., 2022). In Quercus species, hormonal changes were observed under water deficit conditions, with Q. cerris showing a higher antioxidant capacity and hormonal modulation compared to Q. robur (Kebert et al., 2022; Sobrino-Plataen et al., 2024). By understanding these biochemical regulation mechanisms, researchers can formulate the strategies to improve plant resilience and in water deficit stress response, ensure sustainable agricultural productivity under changing climatic conditions. 4 Molecular Response Mechanisms 4.1 Gene expression and regulatory networks related to water deficit Plants respond to water deficit through complex gene expression and regulatory networks. Key transcription factors (TFs) and genes are activated in drought stress response. For instance, overexpression of 9-cis-epoxycarotenoid dioxygenase (NCED) and acetaldehyde dehydrogenase (ALDH) enhances drought resistance by increasing ABA biosynthesis, which are crucial TFs for stress response. Additionally, genes such as EgrNCED3, EgrPYR1, and EgrDREB2.5 are upregulated in drought-tolerant clones of Eucalyptus, indicating their role in ABA-dependent and independent pathways (Martins et al., 2018). The regulatory networks also involve TFs such as DREBs and AREBs, which mediate stress-responsive gene expression to enhance drought resistance (Takahashi et al., 2020). 4.2 Activation of signaling pathways (e.g., ABA signaling) ABA signaling is a central pathway in plant response to water deficit. ABA is synthesized mainly in leaves upon receiving drought signals from roots and regulates various physiological and molecular responses, including stomatal closure to reduce water loss (Takahashi et al., 2018; 2020). The ABA signaling pathway involves several protein kinases such as SnRK2s and MAPKs, which detect ABA influx in guard cells and mediate stomatal closure (Takahashi et al., 2018). Furthermore, the SAPK9-OsMADS23-OsAOC pathway in rice modulates ABA and JA biosynthesis, influence drought tolerance (Lv et al., 2022). The circadian clock system also plays important role in regulating ABA production and signaling, thereby influences stomatal responses and water-use efficiency (Yari Kamrani et al., 2022). 4.3 Roles of proteomics and metabolomics in water deficit response Proteomics and metabolomics provide advanced insights into the biochemical changes in plants under water deficit. Xiong et al. (2019) identification and analysis differential metabolites and differentially expressed proteins functions of rice spikes indicate that the drought mainly promoted carbohydrate metabolic process, carbon fixation in photosynthetic organism pathway, the energy metabolism pathway, and ROS metabolic process functions. The dynamic regulatory networks in wheat roots, characterized through transcriptomics and proteomics, highlight the involvement of various biosynthetic pathways and protein-protein interactions in drought tolerance (Rahimi et al., 2021). These studies underscore the importance of integrating proteomic and metabolomic data to understand the comprehensive response mechanisms of plants to water deficit. 5 Ecological Adaptation and Evolution 5.1 Changes in plant community structure and species diversity Water deficit significantly impacts plant community structure and species diversity. Studies have shown that drought conditions lead to a shift in species composition, favoring drought-tolerant species over those less adapted to water scarcity. For instance, a meta-analysis revealed that water stress inhibits plant growth and photosynthesis
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