Biochemical and Molecular Effects of Lead Heavy Metal Toxicity in Plants: A Phytoremediation Approach

Lead is one of the most abundant, ubiquitous toxic elements posing a critical concern to human and environmental health. It causes deleterious effects at the subcellular and molecular levels in plant cells. The effect of lead depends on the concentration, type of salt, soil properties and plant species. Visible symptoms include chlorotic spots, necrotic lesions etc. in leaf surface, senescence of leaf and stunted growth. One of the detrimental effects of lead stress is the generation of reactive oxygen species (ROS) and free radicals which are highly reactive and toxic and cause damage to proteins, lipids, carbohydrates and DNA which ultimately result in oxidative stress. Plants have a range of potential mechanisms at the cellular and molecular levels to accumulate/ tolerate lead without showing any toxicity. Biochemical mechanisms of Pb tolerance are antioxidant defense system which works in concert to control the cascades of uncontrolled oxidation, detoxification mechanisms that include the sequestration of Pb in the vacuole, phytochelatin synthesis and binding to glutathione and amino acids. Molecular mechanisms involve the expression of genes against heavy metal stress for hyperaccumulation of metals in plants. Phytoremediation, the use of plants to extract, sequester and / or detoxify heavy metals including lead is regarded as an effective, non-intrusive and socially accepted technology to remediate contaminated sites. The recent progress of molecular techniques has helped to improve the performance of phytoremediation technology as well as plant adaptation to extreme metallic environment. This review summarizes lead heavy metal toxicity induced biochemical and molecular changes in plants, lead removal by phytoremediation and genetic engineering approaches for improving lead-tolerance in plants.