Current scenario of Pb toxicity in plants: Unraveling plethora of physiological responses

Abstract

Lead (Pb) is an extremely toxic metal for all living forms including plants. It enters plants through roots from soil or soil solution. It is considered as one of the most eminent examples of anthropogenic environmental pollutant added in environment through mining and smelting of lead ores, coal burning, waste from battery industries, leaded paints, metal plating, and automobile exhaust. Uptake of Pb in plants is a nonselective process and is driven by H+/ATPases. Translocation of Pb metal ions occurs by apoplastic movement resulting in deposition of metal ions in the endodermis and is further transported by symplastic movement. Plants exposed to high concentration of Pb show toxic symptoms due to the overproduction of reactive oxygen species (ROS) through Fenton-Haber-Weiss reaction. ROS include superoxide anion, hydroxyl radical, and hydrogen peroxide, which reach to macro- and micro-cellular levels in the plant cells and cause oxidative damage. Plant growth and plethora of biochemical and physiological attributes including plant growth, water status, photosynthetic efficiency, antioxidative defense system, phenolic compounds, metal chelators, osmolytes, and redox status are adversely influenced by Pb toxicity. Plants respond to toxic levels of Pb in varied ways such as restricted uptake of metal, chelation of metal ions to the root endodermis, enhancement in activity of antioxidative defense, alteration in metal transporters expression, and involvement of plant growth regulators.