Deciphering the Role of Phytohormones and Osmolytes in Plant Tolerance Against Salt Stress: Implications, Possible Cross-Talk, and Prospects

Salinity is one of the main abiotic stresses, which adversely affects the growth, and development and ultimately yield of crop plants. In recent years, phytohormones have been involved in reducing the adverse effects of salt stress. For instance, phytohormone auxin is important for stabilizing the cellular pH environment and activating Na+/ H+ antiporters under salt stress. The application of gibberellins helps increase the seedlings' water status and maintain RNA and protein levels to maintain growth under salinity stress. Some other types of phytohormones and osmolytes such as abscisic acid, salicylic acid, and ethylene have also been reported to reduce the Na+/ K+ ratio and Na+ and Cl- content, improve the antioxidant enzymes activities and regulate the expression of various genes, including NTHK1, AtMEKK1, AtRSH3, Cat1, thereby enhancing the plant resistance to salt stress. The individual effects of phytohormones have been discussed in various studies, but comprehensive studies on the application of various phytohormones under salinity stress are lacking. Hence the emphasis of this review is on current and former studies that identify the molecular mechanisms by which phytohormones/ osmolytes regulate plant tolerance to salinity stress in promoting growth and development. In this review, we summarized the role of various phytohormones, i.e., cytokinin, abscisic acid, gibberellins, jasmonic acid, auxins, brassinosteroids, and ethylene which directly or indirectly play a vital role in modulating the response of the crop to salinity stress. Furthermore, the role of various osmolytes, i.e., proline and glycine betaine, to enhance salinity tolerance was also highlighted. In addition, the interaction and crosstalk between phytohormones and osmolytes under salinity stress in the plant was adequately elaborated. In the end, future research needs about the application of phytohormones and osmolytes have also been discussed.