## 7 Drought Stress and Mineral Nutrition
### Molecular and Phytohormone-Mediated Signaling Mechanisms of Drought Tolerance  
Molecular responses to adverse stress conditions involve highly regulated genes and signal transduction processes that aid plants to confront the stress conditions. C-repeat/dehydration-responsive element binding factors (CBF/DREB), mitogen-activated protein (MYB), cup-shaped cotyledon CUC, no apical meristem NAC TFs and zinc-finger proteins (ZFPs) are recognized as significant moieties in conferring plant drought tolerance [144]. _GsZFP1_ gene improved _Medicago sativa_ drought tolerance, suggesting that the GsZFP1 is effective to promote drought tolerant plants in genetic engineering breeding practices [145]. The overexpression of _SNACT_ in _Gossypium hirsutum_ elevates its ability to cope with water deficit and also escalates its root growth, which shows that bigger roots are useful in drought resistance breeding [146]. _BdWRRKY36_ gene stimulated transcription of stress-related genes, reduced electrolyte leakage, decreased ROS level and elevated chlorophyll amount, plant water status and antioxidant enzyme activities to enhance the drought tolerance [147]. MpCYS4 boosted closing of stomata, triggered the transcription activity of abscisic acid (ABA) and water-deficit-associated genes to confer drought tolerance and was associated with ABA induced stress signal transduction [148]. _Late embryogenesis associate_ (_LEA_) gene expression declined photosynthetic activity and boosted the plant antioxidant defense system to improve drought stress tolerance in three Linderniaceae species differing in desiccation tolerance [149]. In drought-tolerant _Malus domestica_, the foremost stimulatory strategy for high water use efficiency involves maintenance of C\({}_{3}\) cycle activity by enhancing the function of photosynthetic enzymes, alleviating e\({}^{-}\) transfer, diminishing ROS amount by controlling the photosynthetic e\({}^{-}\) transport chain, C\({}_{2}\) cycle and ROS mitigation ability to inhibit photoinhibition and improving photosynthetic activity [150]. Against water deficit stress, resulting signal transduction induced the generation of different constituents including phytohormones to respond and adapt to drought stress. ABA is useful in plant drought tolerance by triggering diverse signaling mechanisms [151]. Beside stimulating stomatal movement, root architecture and regulating photosynthesis, ABA-induced genes encoding drought-related proteins such as dehydins, ROS-detoxifying enzymes, regulatory proteins and phospholipid signaling enzymes can improve drought stress tolerance [152]. Improved amount of ABA induced a signaling pathway in guard cells which results in outflow of guard cells K\({}^{+}\) and reduced turgor pressure, ultimately causing stomata closure [44; 153]. ABA mitigated drought stress and increased the wheat tolerance ability by improving stem lengths and plant biomass, declining the level of H\({}_{2}\)O\({}_{2}\) and malondialdehydedehyde (MDA) [154]. Increased level of cytokinin amount in xylem sap induced stomata opening by diminishing its sensitivity to ABA [155]. Jasmonic acid (JA) synthesis-related genes were stimulated in the overexpressing lines of _VaNAC26_ which increased ROS scavenging and stimulated stomata closure and root growth, thereby promoting higher drought tolerance [156]. JA enhances plants drought tolerance by stimulating root growth, decreasing level of ROS and promoting stomatal closure [157]. Auxin regulates root development, functioning of ABA related genes and ROS metabolism to improve drought-tolerance [158]. Ethylene mediates synthesis of guard cell antioxidant flavanols in an EIN2 dependent manner and adversely affects stomata closing by suppressing drought mediated ROS formation [159], thereby resulting in another possible target for genetically engineered plants tolerant to drought. In view of the above, obtaining transgenic plants is a promising approach to improving drought tolerance traits in a shorter time as compared to classical breeding programs.