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Figure 1. 

Schematic representation of ABA and MAPK signaling pathways integrated with CDPK, ROS, and GABA signaling pathways regulating transcriptional responses under salt and drought stress. Stress signaling pathways are divided into three main modules, indicated in the figure. (a) ABA perception and signal initiation (the cyan rectangle). Under drought and/or salinity stress, endogenous ABA levels increase^[13,31], which promotes the binding of ABA to PYR/PYL/RCAR receptors^[32]. This causes PYR/PYL/RCAR to recruit protein phosphatase 2C (PP2C^[17]) and to inhibit PP2C phosphatases, leading to activation of SNF1-related protein kinases (SnRK2s^[33,34]). In the presence of ABA, activation of SnRK2s leads to phosphorylation of downstream target proteins^[35], which subsequently regulate transcription. (b) MAPK cascade activation, ROS, SOS, Ca²⁺, and GABA signaling (the blue rectangle). Stress signals (e.g., ABA, Ca²⁺, H₂O₂, O₂⁻, GABA) activate a conserved MAPK cascade^[35] involving sequential phosphorylation of MAPKKKs, MAPKKs, and MAPKs^[11,13,14]. CDPK, MAPKs (e.g., MAPK1), and ion transporters such as SOS1 have been associated with drought and salt stress^[1,10,29]. Stress-induced cytosolic Ca²⁺ signals activate the SOS signaling pathway. ABA induces changes in chloroplast activity and increases ROS production that further contribute to MAPK cascade activation and antioxidant enzyme responses^[36]. Low cytosolic pH and high Ca⁺² concentration activate glutamate decarboxylase (GAD), which catalyzes GABA biosynthesis from L-glutamate^[36]. (c) Transcriptional regulation (the violet rectangle), activated SnRK2s, CDPKs, and MAPKs phosphorylate transcription factors (e.g., bZIP, NAC, AP2/ERF, MYB, DREBs, WRKY, GRAS, and AREBs/ABFs), which bind to cis-regulatory elements in promoters of stress-responsive genes. The target genes mediate protective responses, including osmolyte accumulation, ROS detoxification, and ion homeostasis^[37]. Arrows indicate directional signaling. Solid lines represent experimentally established interactions. Dashed lines indicate hypothetical interactions. Arrow connections represent positive regulation, and block connections indicate negative regulation. Different modules of the ABA signaling pathway are color-coded for clarity. Blue: channels and transporters. Pink: signaling molecules. Light green: ABA receptors. Dark green: phosphatases and kinases. Purple: Transcription factors. Orange: Ca²⁺ receptors. Key interactions and gene functions were compiled from the following studies: (AsCDPK, AsSOS1, MAPKs under salinity^{[1,21,29,38−40]}) (CDPK26 and MAPK1 in drought responses^{[10,12,13,17]}) (SnRK2s in ABA-dependent pathways^[33,41]), (PYL/RCAR5 and SnRK2s in ABA signaling under salt and drought stress^{[19,32,42,43]}). The hypothetical interaction between ROS and Ca²⁺ influx is supported by evidence from model species showing that ROS elevates cytosolic Ca²⁺ concentration in guard cells^[44]. The proposed hypothetical CDPKs-MAPKKKs interaction is inferred from their concurrent transcriptional induction under salt stress in P. nuttalliana and previously reported signaling cross-talk in Arabidopsis^[24,45]. The putative link between CDPK-ROS signaling and between SOS3-NHX is based on evidence from wheat^[46,47]. At the same time, the hypothetical interaction between ABA and NHX is supported by studies in Brachypodium^[48].

Figure 2. 

Integrated strategy for plant tolerance to drought and salinity stresses.