Hormonal Regulation of Heavy Metal Toxicity and Tolerance in Crop Plants

In crop ecosystems, the accumulation of heavy metals (HMs), such as iron (Fe), cadmium (Cd), zinc (Zn), copper (Cu), chromium (Cr), arsenic (As), lead (Pb), silver (Ag) and nickel (Ni), often derived from industrial and agricultural activities, compromises food security and human health. Plants, as sessile organisms, are exposed to HMs in soil, air, and water; and even excess metallic micronutrients essential for plant metabolism (Fe, Zn, Cu, Mn) can cause toxicities and physiological disorders. The HM stress overproducing reactive oxygen species (ROS) increases oxidative stress damaging membranes and macromolecules. The HM-induced adverse effects interfere with plant growth and development (PG&D), decreasing crop yield and quality. Phytohormones (PHYs) are chemical messengers regulating PG&D, even under HM stress, triggering hormonal signaling pathways with activation of responsive genes to alleviate the HM stress. This chapter summarizes the main findings on hormonal signaling pathways and responses of plants exposed to HM stress, presenting strategies that could be explored in plant breeding programs improving the phytoremediation of HM-contaminated soil.