BIOCHEMISTRY AND MOLECULAR-BIOLOGY OF HEA VY-METAL ACCUMULATION IN HIGHER-PLANTS

Heavy metals, such as Cd, Cu, Zn, Bi, Ni, Hg and Pb are ubiquitous com ponents of the biosphere. They are either essential trace nutrients, b ut may be toxic, when present at concentrations higher than required f or optimal growth conditions. Contamination of soils with toxic metal ions may have adverse effects on plants and soil biota and may impose a risk for humans along the food chain. Populations of a variety of hi gher plant species are able to colonize these environments. One of the best-known examples are metal-hyperaccumulating plants, which have ev olved on these sites through natural selection. Hyperaccumulation is a n ecophysiological adaptation to metalliferous soils. These plants oft en accumulate only a specific metal, they grow slowly and have a small biomass. Little is known about their agronomic characteristics, breed ing potential and physiology. Interest in these metal-hyperaccumulatin g mechanisms has come from the developments in phytoremediation, a new technology, using high biomass metal-accumulating plants to extract t oxic metals from contaminated soils. The mechanisms of metal accumulat ion, which involve extracellular and intracellular metal chelation, pr ecipitation, compartmentalization and translocation in the vascular sy stem are poorly understood. Well-known metal-binding proteins include metallothioneins, metalloenzymes and various metal storage, carrier an d channel/transport proteins. In addition phytochelatins, low molecula r weight gamma-Glu-Cys-peptides with high affinity for certain metals are assumed to be involved in accumulation, detoxification and metabol ism of metal ions such as Cd, Zn, Cu, Pb and Hg in plant cells. At pre sent, the wheat E-c-protein is the only plant protein, that can be une quivocally designated as a metallothionein. Many metallothionein-like plant genes have been isolated from different species and the availabl e data indicate, that these genes have a role in metal metabolism. Mos t of their translational products remain to be purified from plant mat erial and sequenced. Metal-induced gene expression and regulation is a broad field for further investigations. Increased metal tolerance has already been obtained by the introduction of different metallothionei ns into higher plants. Only by analysing the mechanisms of metal uptak e and metal storage at the molecular level, it will be possible in fut ure, to develop crop plants with hyperaccumulating tendencies and to e stablish phytoremediation as a modem technology of environmental clean up.