Abstract
Phytohormones act as chemical messengers and, under a complex regulation, allow plants to sustain biotic and abiotic stresses. Thus, phytohormones are known for their regulatory role in plant growth and development. Heavy metals (HMs) play an important role in metabolism and have roles in plant growth and development as micronutrients. However, at a level above threshold, these HMs act as contaminants and pose a worldwide environmental threat. Thus, finding eco-friendly and economical deliverables to tackle this problem is a priority. In addition to physicochemical methods, exogenous application of phytohormones, i.e., auxins, cytokinins, and gibberellins, can positively influence the regulation of the ascorbate–glutathione cycle, transpiration rate, cell division, and the activities of nitrogen metabolism and assimilation, which improve plant growth activity. Brassinosteroids, ethylene and salicylic acid have been reported to enhance the level of the anti-oxidant system, decrease levels of ROS, lipid peroxidation and improve photosynthesis in plants, when applied exogenously under a HM effect. There is a crosstalk between phytohormones which is activated upon exogenous application. Research suggests that plants are primed by phytohormones for stress tolerance. Chemical priming has provided good results in plant physiology and stress adaptation, and phytohormone priming is underway. We have reviewed promising phytohormones, which can potentially confer enhanced tolerance when used exogenously. Exogenous application of phytohormones may increase plant performance under HM stress and can be used for agro-ecological benefits under environmental conditions with high HMs level.
Highlights
In terms of the degree of danger to the human population across the globe, heavy metals (HMs) are thought to be in second place among pollutants
It has been reported that 70% of all HMs and their compounds routed in the human body came from food (Jaishankar et al 2014)
As this review is devoted to the role of phytohormones under HMs contamination in plants, it should be noted that some trace metals are necessary for plant growth and metabolism, but within reasonable limits
Summary
In terms of the degree of danger to the human population across the globe, heavy metals (HMs) are thought to be in second place among pollutants. In response to environmental stresses, a rapid increase in endogenous ABA levels has been observed, which activates the specific signaling pathways and modulates gene expression levels in the plant (Brien and Benkova 2013). GAs support the development of plant adaptation and resistance to various abiotic stresses and have protective effects on the toxicity of HMs (Maggio et al 2010).
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