Abstract: Heavy metals impact on the cytoplasmic function in a number of different ways, principally by their binding to protein sulflhdryl groups, by producing a deficiency of essential ions and, eventually, by substituting the essemial ions. Other modes of toxicity are possible, including disruption of cell transport processes and oxidative damage by free radicals generated by metal redox cycling. Plants have developed a variety of biochemical defense strategies to prevent heavy metal poisoning. The possible defense mechanism in plant may involve: metal binding to cell walls, avoidance of uptake these toxic metal ions, reduction of heavy metal transport across the cell membrane, active efflux, compartmentalization and metal chelation. Phytochelatins that can tightly bind and sequester metals may play an important role in the accumulation of heavy metals and preventing them from entering the cell metabolic pathway, the rates of high molecular weight (HMW) metal phytochelatin complexes (Cd-Sa-complex) formation may be an important determinant of the plant tolerance. In addition, plants possess several antioxidant defense systems to protect themselves from the oxidative stress by heavy metals.

Key words: Heavy metal, Tolerance mechanism, Plant