Physiological and biochemical mechanisms of plant adaptation to the stress action of heavy metals: implications for the phytoremediation technology

Heavy metal exposure affects several morphological and physiological processes in plants, altering water balance, nitrogen metabolic activities, and nutrient uptake and distribution. In particular, heavy metals can interact directly or indirectly with the photosynthetic machinery causing a reducing metabolic capacity for CO2 fixation. Heavy metals can also induce the production of reactive oxygen species (ROS) that can damage cell components such as nucleic acids, proteins and polysaccharides modifying the cellular redox state. The excess of metal accumulation in plant tissues can result in severe stress symptoms such as chlorosis, necrosis, decrease in water potential and transpiration, inhibition of growth and death. To face the heavy metal presence in water and soil, plants have evolved a wide array of morphological, physiological and biochemical modifications allowing them to avoid or limit metal absorption or, in contrast, to accumulate and safely store the metals at cellular and tissue levels. Such processes represent the basic properties to be exploited for the utilisation of plants in the metal decontamination of soil and water through the bio-technology called phytoremediation. To be really effective and widely applied, phytoremediation needs the availability of plant species, clones and ecotypes able to grow in many different environmental conditions. In order to improve the ability of plants to survive and perform the metal remediation of soil and water, a genotype selection for superior performances in metal tolerance and accumulation is required. Thus, the characterisation of the morpho-physiological and bio-molecular traits involved in these processes represents a key point in order to assist the selection of plants for phytoremediation.