The contamination of the environment with heavy metals has become a world-wide problem, leading to considerable losses in plant productivity and hazardous health effects, which contribute for the bioaccumulation and biomagnification in the food chain. Several activated oxygen species, designated reactive oxygen species (ROS), are continuously produced in plants as by-products of aerobic metabolism such as those reactions occurring in mitochondria, chloroplast and peroxisomes, which is accelerated under both biotic and abiotic, stress conditions, including exposure to heavy metals. Some of the ROS are highly toxic and must be rapidly detoxified by the various cellular mechanisms that plants and other organisms have evolved, to prevent or alleviate the damage that oxygen radicals can cause. The study of such a response may even allow the evaluation of pollution in the environment. However, research must be intensified in order to establish specific and general responses to metals by different crop plant species, as well as a maximum acceptable concentration in the soil and food. Information about some of the physiological and molecular mechanisms involved in antioxidant stress responses has begun to emerge, together with biological and engineering strategies designed to improve phytoremediation by reducing the amount of heavy metals in contaminated soils. In this review, we have presented a general survey of the information available in the literature related to the antioxidant responses by plants to heavy metals, including the current knowledge of the compartmentalization and transport of heavy metals.
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