Plant-lead interactions: Transport, toxicity, tolerance, and detoxification mechanisms

Natural and human activities introduced an excess level of toxic lead (Pb) to the environment. Pb has no known biological significance and its interactions with plants lead to the production of reactive oxygen species (ROS). Pb and/or ROS have the potential to cause phytotoxicity by damaging the tissue ultrastructure, cellular components, and biomolecules. These damaging effects may possibly result in the inhibition of normal cellular functioning, physiological reactions, and overall plant performances. ROS play a dual role and act as a signaling molecule in plant defense system. This system encircles enzymatic and non-enzymatic antioxidative mechanisms. Catalase, superoxide dismutase, peroxidase, and enzymes from the ascorbate-glutathione cycle are the major enzymatic antioxidants, while non-enzymatic antioxidants include phenols, flavonoids, ascorbic acid, and glutathione. Pb removal from contaminated sites using plants depend on the plant's Pb accumulation capacity, Pb-induced phytotoxicity, and tolerance and detoxification mechanisms plants adopted to combat against this phytotoxicity. However, the consolidated information discussing Pb-plant interaction including Pb uptake and its translocation within tissues, Pb-mediated phytotoxic symptoms, antioxidative mechanisms, cellular, and protein metabolisms are rather limited. Thus, we aimed to present a consolidated information and critical discussions focusing on the recent studies related to the Pb-induced toxicity and oxidative stress situations in different plants. The important functions of different antioxidants in plants during Pb stress have been reviewed. Additionally, tolerance responses and detoxification mechanisms in the plant through the regulation of gene expression, and glutathione and protein metabolisms to compete against Pb-induced phytotoxicity are also briefly discussed herein.