Role of phytohormones in plant response to drought and salinity stresses

Plants being sessile are exposed to a plethora of abiotic stresses, such as drought, cold, heat, and salinity, as well as biotic stresses inflicted by fungi, bacteria, viruses, and nematodes. Their survival depends on a complex network of proteins and regulatory molecules that can help to perceive the stress signal and relay a stress response. Phytohormones form a critical part of this network and play a crucial role in the growth of plants regulating the processes of photosynthesis, stomatal conductance, ion homeostasis, cell division, senescence, antioxidant enzyme activity, and reproduction. Phytohormones, such as abscisic acid, jasmonic acid, salicylic acid, and ethylene, are also implicated in stress response pathways, and their interaction with cytokinin, auxin, gibberellin, brassinosteroids, and strigolactones is vital for mitigating drought and salinity stress. Drought and salinity trigger a water deficiency in the cell leading to osmotic stress, causing similar physiological and biochemical responses. The signaling mechanism activated to counter the stress involves the activation of these phytohormones that in turn activate transcription factors and stress response genes thus conferring stress tolerance. This chapter focuses on the role of different phytohormones under abiotic stresses such as drought and salinity and how they signal the stress response.