The Role of New Members of Phytohormones in Plant Amelioration under Abiotic Stress with an Emphasis on Heavy Metals

In addition to regulating plant growth and development, phytohormones play an essential role in the response to abiotic and biotic stress - especially heavy metal stress. In response to environmental stressors, phytohormones act as signaling molecules in both exogenous and endogenous signaling pathways. In parts of the rhizosphere, phytohormones have an exogenous role, and when regulating plant growth and development, they have an endogenous role. Phytohormones and their associated signaling pathway network are involved in plant responses to heavy metal stress. These molecules can improve the plant defense system by increasing antioxidant enzyme activity and degrading lipoperoxidation and H2O2. Phytohormones are divided into classical phytohormones such as auxins, ethylene, gibberellins, and cytokinins, that have been investigated for many years and the new members, such as jasmonates, brassinosteroids, and strigolactones, which have been little studied. In this review article, we investigated the main mechanisms involved in the amelioration of heavy metals by the three new phytohormones to provide more knowledge about their detoxification mechanisms.