Role of Plant Secondary Metabolites and Phytohormones in Drought Tolerance: A Review

Human activity has contributed to global warming both in the past and in the present, resulting in drought stress that have an impact on both plants and animals. Because plants are less able to withstand drought stress, plant growth and productivity are reduced. In order to lessen the impact of drought stress on plants, it is essential to develop a plant feedback mechanism for drought resistance. Drought is also one of the major abiotic stresses resulting from moisture deficit. It adversely affects plant growth and is one of the major causes of reduced crop yield under field conditions. In a nutshell, enhancing the root system, leaf structure, osmotic balance, comparative water contents, and stomatal adjustment are thought to be the most important characteristics for crop plants to resist drought. In response to drought stress, plants possess acclimation mechanisms that involve secondary metabolites (SMs) and phytohormones (PHs) to alter physiological, biochemical and molecular responses. Plant natural compounds such as flavonoid, polyphenol, isoprene (volatile) and isoprenoid (non-volatile) are the most commonly studied SMs under drought stress conditions. The plant accumulates SMs as an adaptive response that performs an essential role as a powerful antioxidant, synthesizing and transporting metabolites and enzymes, stabilizing cellular components, signaling and regulating genes. Usually, SMs interact with PHs to overcome constraints of drought stress. Synthesis and accumulation of SMs can be manipulated using the in-vitro culture technique to a preparedly enhanced level for drought tolerance in plants. The PHs such as abscisic acid (ABA), ethylene, auxins, cytokinins, gibberellic acid, salicylic acid, jasmonic acid, brassinosteroids and strigolactones play a vital role in drought stress signaling pathways of plants. The ABA is a major hormone that plays a key role in drought stress tolerance. The PHs, reduce transpiration losses through closing stomata, promoting leaf senescence and reducing leaf area. In addition, ABA enhances root growth, increases cytosolic Ca²⁺, depolarises the membrane and activates the gene responsible for avoidance of water stress. The present review briefly highlights and critically assesses the roles of SMs and PHs in drought tolerance, as evidenced by recent successes in crop plants.