## 2 Generic Pathways for Plant Response to Abiotic Stresses  
According to our current knowledge about stress signaling pathways, the generic signaling pathway for any given abiotic stress can be divided into the following major steps: signal perception, signal transduction, stress-responsive gene expression and the activation of physiological and metabolic responses [23]. When plants are exposed to drought or salinity stresses, they produce signals that are perceived at the membrane level by ion channels, membrane receptors, receptor-like kinases and many other proteins. Upon perception, complex intracellular signals are then initiated and cause the generation of many messengers such as Ca\({}^{2+}\), reactive oxygen species (ROS), phytohormones and other stress signaling molecules [24; 25; 26]. Those signals are then transduced further inside the cell and interact with other molecules that will ultimately lead to the induction of the stress-induced gene that encodes for proteins that will particularly enhance the plant's tolerance to abiotic stresses directly or indirectly. The plant response to abiotic stresses can be described as the coordination of expression of genes that encode for a variety of products that contribute to the plant's adaptation to abiotic stresses [27; 28]. Those products could carry out any function that supports the plant in the adaptation to the stress; they could be, for instance, secondary metabolites that carry out essential protective functions against abiotic stresses, transcription factors that facilitate the activation of second-level stress-inducible genes or any other functional or regulatory molecules [29]. Based on the protein products of the stress-inducible genes, they can be divided into two functional categories: (1) genes that encode for products that directly affect the plant's tolerance to salinity and water deficit stresses, such as heat stress proteins (HSPs) or chaperones, LEA proteins, osmoprotectants, antifreeze proteins, detoxification enzymes and free-radical scavengers [30], and (2) stress-responsive genes that encode for protein products that influence the abiotic stress resistance by regulating the expression of other downstream target genes and modulate the signal transduction cascades. The latter category includes genes that may produce phytohormones such as abscisic acid (ABA), jasmonic acid (JA), salicylic acid (SA) and ethylene [7]. It also includes genes that encode for other signaling molecule products that contribute to the signaling cascade regulation of gene expression, such as the mitogen-activated protein kinases (MAPKs), calcium-dependent protein kinases (CDPKs) and a great number of stress-inducible transcription factors (Figure 1). Figure 1: Generic pathway for plant response to salinity and drought stresses in Arabidopsis thaliana cell. Salinity and drought elicit Ca\({}^{2+}\) signals through the SOS pathway to achieve ionic homeostasis. When the ions are increased, it is first perceived by the ionic stress sensors, and then the Ca\({}^{2+}\) levels are increased. The Ca\({}^{2+}\) binds to the SOS3 protein and then interacts with the SOS2 protein kinase. The Ca\({}^{2+}\)-SOS3–SOS2 complex phosphorylates and activates the SOS1 membrane protein, which will cope with the ionic stress. This change in cytoplasmic Ca\({}^{2+}\) level is sensed by calcium sensors (CDPKs, CaMs, CMLs and CBLs) that interact with their downstream signaling components, which could be RBOHD or transcription factors. The activation of RBOHD leads to the generation of ROS molecules, which in turn aid in the signaling mechanism through the interaction with many molecules such as phytohormones or several kinases. Those phytohormones (ABA) or the protein kinases interact with the stress-inducible TFs. Those stress-inducible TFs carry specific DNA-binding domains that bind to specific regulatory sequences found upstream of the stress-inducible gene and initiate transcription.