For many millions of years, plants have adapted to the presence of various metals in varying amounts in soils. Some metals, such as zinc, nickel, cobalt, and copper, function as nutrients when eaten by humans in small amounts, but are toxic when consumed in excess. Heavy metals that are toxic even in trace amounts include mercury, lead, cadmium, silver, gold and chromium. Human activities such as mining, municipal waste disposal, and manufacturing have increased heavy metal pollution to dangerous levels in some areas. These chemicals cause oxidative damage, which destroys lipids, DNA, and proteins.
Certain plants, called hyperaccumulators, cope with excess heavy metals in the environment by taking them in and sequestering them in vacuoles, which are bubble-like structures in their cells. Sometimes the plant combines a pollutant with another molecule, a process called chelation. Organic acids often serve this role. Citric acid, for example, surrounds and thereby detoxifies cadmium, and malic acid does the same for zinc. A class of polypeptides called phytochelatins can also bind metals and escort them to vacuoles. Yet a third strategy that plants use to control metal accumulation is to employ a class of small, metal-binding proteins called metal-lothioneins. The intentional use of plants that use any of these ways to take heavy metals from soil is termed phytoremediation. It is a form of bioremediation.
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