Molecular basis of salicylic acid–phytohormone crosstalk in regulating stress tolerance in plants

Due to their sessile nature, plants are often subjected to a plethora of abiotic (drought, heat, cold, salt and metal toxicity) and biotic (bacteria, virus, fungi and nematode) stress conditions, which constitute an immense danger to plant life and significantly impact crop growth, development, metabolism and production. It is well known that phytohormones are effective metabolites for reducing the negative impacts of abiotic and biotic stresses on agricultural plants over time. Salicylic acid is a powerful phenolic signalling biomolecule and a versatile plant growth regulator that has a key role in growth, metabolic and defence system of plants, thereby coordinating responses to pathogen attack and abiotic stress. This metabolite is crucial for the development of systemic acquired resistance (SAR) in plants because it causes the expression of genes associated with defence. Exogenous administration of SA promotes seed germination, development and blooming, up-regulates photosynthesis and boosts the activity of antioxidants that are enzymatic and non-enzymatic. Salicylic acid is an efficient signalling molecule that can modify physiological and metabolic processes in plants and can thus help in reducing environmental stress in plants over a long period of time. The less addressed issue, however, is the detailed investigation about the interaction of SA with major phytohormones as well as combined action of SA and other phytohormones that can support and influence the fundamental biochemical/physiological and molecular processes of plants against different forms of stress conditions (biotic and abiotic). The current review aims to document the detailed crosstalk of SA with other phytohormones (auxin, gibberellin, cytokinin, ethylene, abscisic acid, jasmonic acid, polyamines, melatonin, brassinosteroids and strigolactones) that confers protection and helps in the recovery of plants from different stress conditions.