Extensive contamination of the environment caused by heavy metals is one of the many consequences of fast industrial development and civilizational progress which started in the previous century and is still underway. Traditional methods of removing heavy metals from the environment are very expensive and invasive, therefore new and cheap methods are needed. High hopes are put on the possibility of using plants in the process called phytoremediation. Some plants have naturally adapted to life on soils with high heavy metal content. The use of modern methods of molecular biology can be very helpful in obtaining transgenic plants that would be able to take up and accumulate high amounts of heavy metals.
The studies on the effect of heavy metals on plants have shown that they cause intensification of two types of unfavorable processes: ? inactivation of macromolecules and cellular structures, ? induction oxidative stress. All molecular, structural and metabolic changes on the level of molecules, tissues and organs lead to changes of plant morphology. One of the change is inhibition of plant growth, reflected as a reduction of its size, mass of either the whole plant or its part, organs or tissues. In response the plant activates processes restoring its homeostasis. In removing reversible changes a particular role is played by the processes of heavy metal detoxification as well as removal of active forms of oxygen.
Heavy metals have been increasing in the environment as a result of either natural processes or human industrial activities. Many of the heavy metals affect and damage various developmental and biochemical processes causing reduction in growth, inhibition of photosynthesis and respiration and degeneration of main cell organelles. It is mostly due to the promoting effect of heavy metals on the formation of harmful reactive oxygen species (ROS) which disturb the whole cellular machinery. There is a requirement for a balance between the uptake of essential metal ions and the ability of plants to protect sensitive cellular structures and activities from excessive level of metals. The resistance of plants to heavy metals depends on the reduction of uptake and translocation from the root to the shoot, binding by appropriate ligands and, finally, transferring to the vacuole. The phytotoxic effect of heavy metals is effectively counteracted by the metal-binding proteins and peptides like metalothioneins, chaperones and phytochelatins as well as some organic acids. Another very important aspect of the heavy metal detoxication is the presence of an efficient ROS scavenging system consisting of low molecular antioxidants and antioxidant enzymes. Some plants can hyperaccumulate metal ions that are toxic for other species. Such plants can serve as donors of traits that could be used to clean up the environment. Several methods can be applied to create plants able to remove the xenobiotics from the environment: sexual or somatic hybridization, mutagenesis, in vitro selection of metal-resistant cell lines and engineering of metal-accumulating transgenic plants. The use of specially selected and engineered metal-accumulating plants for environmental clean-up is a novel technology called phytoremediation. This rapidly emerging biotechnology consists of some branches suitable to toxic metals remediation: 1) phytoextraction ? the use of plants to remove heavy metals from the soil, 2) phytostabilization ? the use of plants to complex and eliminate the availability of toxic metals in soils, 3) rhizofiltration ? the use of plant roots to remove heavy metals from polluted waters. Some new approaches concerning the use of transgenic plants as sensitive bioindicators of toxic heavy metals and soils contaminated with radionuclides are presented.
This review summarises the mechanisms of heavy metal tolerance in plants. Plants have many endogenous genetic, biochemical, and physiological properties that make them ideal agents for soil and water remediation. Phytoremediation is widely viewed as an ecological alternative to the environmentally destructive physical remediation methods currently in use.
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