Heavy metal stress and plant life: uptake mechanisms, toxicity, and alleviation

Heavy metal (HM) pollution in environment occurs due to their release from natural resources, such as rocks, ore minerals, volcanoes, weathering, and various anthropogenic activities like urban advancement, electricity generation, mining, and refinery industries. HMs are transition metals, which possess atomic mass more than 0.002 kg, weight about 5 N/m3, and density greater than 5 g/cm3. These metals are categorized as essential, such as copper (Cu), zinc (Zn), manganese (Mn), nickel (Ni), iron (Fe), cobalt (Co), molybdenum (Mo), selenium (Se), and nonessential metals. Essential HMs play vital regulatory roles in several cellular reactions, including electron transfer, in enzyme activation, in redox reaction, as well as in the synthesis of pigments, whereas nonessential metals, such as chromium (Cr), cadmium (Cd), lead (Pb), silver (Ag), mercury (Hg), and arsenic (As), have no role in any biological reaction, thus cause toxic impacts even at low concentrations by competing with crucial elements at protein-binding sites. Although when the amount of these metals increases beyond the optimum point, they cause toxicity in plants, by decreasing growth, causing soil quality deterioration and by affecting the yield. Toxicity of HMs depends on the concentration, reactivity, as well as on their oxidation capacity. Crops are more susceptible to these HMs, as they transport HMs to organisms through the food chain. HM-stressed plant shows alteration in cellular mechanisms and gene regulation. These HMs generate free radicals in cells, which further cause toxicity in plants. Nonessential metals deliberate various physiological reactions, through the alteration in biomolecules and in regulatory proteins or by replacement of crucial metals, therefore disturb the integrity of biomolecules and affects antioxidant defense system by generating reactive oxygen species. Plant acquires several defense approaches to safeguard against metal toxicity; such processes include sequestration, compartmentalization, exclusion, and inactivation by the secretion of organic ligands. Beside this, plants also induce antioxidant system as well as maintain the metal homeostasis by restricting the metal bioavailability.