Mechanisms of Selected Plant Hormones under Heavy Metal Stress

The present review focuses on the role of auxin (IAA), cytokinins (CKs), ethylene (ET) and abscisic acid (ABA) under heavy metal stress. From the onset of heavy metal stress in plants, cellular levels of ABA tend to increase, leading to the establishment of signal communication and encoding of genes associated with the enzymes involved in antioxidant stress responses. The mechanisms of IAA phytohormones in terms of enhancing plant tolerance to stress mainly comprise developing a link between hydrogen peroxide (H2O2) and auxin homeostasis, which can result in forming a signaling interaction between them. The primary anti-phytotoxicity mechanism of ET in response to heavy metal stress is related to modulating the interaction of ethylene with reactive oxygen species (ROS) signaling, which is reflected in a synergy between induction of ROS and synthesis of 1-aminocyclopropane-1-carboxylic acid (ACC). CKs, particularly when applied exogenously, make a positive contribution to alleviating stress emanating from plant metal exposure. This is achieved by a reduction in ROS levels inside plant cells. Generally, CKs can ameliorate heavy metal stresses via a three-way interaction involving exogenous CK, CK metabolism, and CK signaling. Our aims in this work were to review the major hormonal responses of plants elicited by heavy metal stress.