Mohammed Hamdan Edan Al-Issawi

Unlike some of the heavy metals (e.g. Mo, Cu, Cr, Co, V, Ni, Zn, Mn, Fe), there are other metals (e.g. Ag, Hg, As, U, Pb, Cd……etc) that are not proven to have a role in plant life. On the contrary, their presence in the environment hinders plant growth. Thus, may enter the food chain, which in turn forms a great danger to human health because they are not soluble and can be accumulated in organism’s cells, besides the life age of heavy metals is relatively long and this causes their accumulation in organisms’ bodies. Chemical weapons used in wars are considered one of the most important sources of heavy metals found in the environment followed by the accumulated waste in cities, sludge places, dust, the engines smoke, and wastes and gases from factories.  In addition, soil rock weathering is considered one of the heavy metals resources especially cadmium.   The over use of phosphate fertilizers can also be source of heavy metals. On the other hand, it has to be noted that sewerage, which is discarded in rivers is seen as one of the most dangerous sources of heavy metals.

 There are no apparent symptoms of heavy metals once they enter the biological system; therefore, their impact is hidden and appears after many years due to their accumulation ability. Cadmium (Cd) is one of the widespread heavy metals in the environmental systems, and it comes third after mercury and lead. It had not been proven that Cd has a positive role in plant life. Plant functions start to be declined when Cd concentrations get higher than 3 mg kg^-1 TDM. Cd has a strong ability to bind with Sulphur causing protein damaging. It also has a great ability to bind with oxygen and nitrogen compounds disturbing the oxidation balance in plant cells.  Cd competes with Ca⁺² which is important to cell signaling through kinase enzymes; therefore, it disturbs cell signaling. As for Cd transporters, there is no evidence that Cd has specific transporter; however, some researchers proposed that Zn and Fe transporters can transport Cd with less efficiency. The Zn transporter ZNT1 had been found to transport Cd but with less efficiency. Besides, Cd decrease from cell expansion through accumulating H₂O₂ in the cell wall which in turn makes it solid, and this explains the weakness in root growth in Cd-affected environments. Upon exposure to high concentrations of Cd, Auxins and Gibberellins, which are important in cell signaling might be damaged. On the other hand, Cd increases the content of Abscisic acid (ABA) and Ethylene (ET) in root cells and leads to plant vegetative growth reduction. Ethylene reduces cell expansions, while ABA helps plants adapt against water stress.

The genotoxic is the most important damage that heavy metals can cause to the organisms as they may lead to genetic variations in the genome during replication and recombination of DNA.  Some researchers indicated that DNA damage is due to the oxidation action by free radicals (ROS), which might partition DNA and may lose some nitrogen bases. As for humans, Cd may enter the body by smoking, inhaling polluted air, or eating contaminated food. Cd may cause Osteoporosis, renal failure, lung cancer, and reproductive performance.

To remove or reduce cadmium effects, many methods were employed and they all focused on two main directions. The first direction is to clean the polluted areas through many physical and chemical protocols, which included thermal methods, physical separation, washing or stabilization, and solidification. Although those methods are effective, they are cost-effective and destroy soil structure and activity or soil microbes. Phytoremediation is one of the effective methods that can remove Cd from soils by using some of the forage crops. It is very effective, not cost-effective, and environment friendly as well. Those kind of plants are hyperaccumulators used and are grown in polluted areas then they can be discarded or treated. The second direction adopted to avoid Cd impacts is to hinder Cd from reaching edible plant parts. This can be achieved by using plant breeding programs and molecular techniques. This gives a clear image of how Cd is mobile inside plants at physiological and genetic levels. In addition, good plant nutrition reduces the toxicity of Cd and controls its accumulation in edible plant parts.