Heavy-Metal-Induced Reactive Oxygen Species and Methylglyoxal Formation and Detoxification in Crop Plants: Modulation of Tolerance by Exogenous Chemical Compounds

Heavy metals (HMs) include both essential micronutrients displaying toxicity when present in excessive amounts and the elements that do not play any physiological role in living organisms. HMs occur naturally in soil and water, but due to the anthropogenic emissions their concentrations in the environment have increased. Contamination by HMs leads to the decrease in crop productivity and poses a threat to human health as a result of biomagnification of HMs in the food chains. The toxicity of HMs is a complex phenomenon due to their pleiotropic effects including the disturbance of metabolism and signaling, damage of macromolecules and organelles, enhanced formation of toxic metabolites such as reactive oxygen species (ROS), and methylglyoxal (MG) leading to oxidative stress. In the present chapter, we focus on the participation of HMs in ROS and MG formation in plant cells and the protective mechanisms that evolved to counteract it. The role of exogenously applied chemical compounds, such as signaling molecules (nitric oxide, hydrogen sulfide, and hydrogen peroxide), phytohormones, antioxidants, osmoprotectants, chelators and nutrients in providing tolerance to HMs in crop plants will also be discussed.