GENETIC ENGINEERING IN THE IMPROVEMENT OF PLANTS FOR

PHYTOREMEDIATION OF METAL POLLUTED SOILS

Sirpa Kärenlampi¹, Arja Tervahauta¹ and Päivi Kopponen²

¹Department of Biochemistry and Biotechnology, ²Department of Ecology and

Environmental Science, University of Kuopio, P.O.Box 1627, 70211 Kuopio, Finland

Heavy metals are natural elements cycling at low levels in bio-, geo-, atmos- and hydrospheric systems. At optimal levels, metals like Cu, Zn, Mn, Fe, Ni and Mo are essential to biological systems, being cofactors or parts of active enzymes. At increased levels, both essential and non-essential metals (Cd, Pb, Hg) are toxic. Local metal increases are caused by human activities: smelting, mining, processing, agricultural and waste disposal technologies. Due to technological advancements, metal concentrations in the air have significantly decreased. Metal concentrations in soils are increasing, leading to potential increases in leaching to water, uptake by plants and intake by human population. This is an increasing risks for human health and the enviromnent Contamination affects the growth and survival of plants and microbes. Some species have adapted to increased metal concentrations, and have developed a heritable tolerance to heavy metals. The mechanisms of metal tolerance have been explored mainly in microbes. Micro-organisms have adopted several ways to cope with heavy metals. The uptake of metals may be prevented by extracellular metal-binding compounds. Some microbes produce metallothioneins or phytochelatins. Resistance to non-essential heavy metals (Cd ete.) is based on selective and energy-dependent efflux system.

At high concentrations, heavy metals are phytotoxic. Some plants have developed mechanisms to cope with metals. Binding to the cell wall may prevent excess metal to reach the sensitive areas in the root. In the cytoplasm, metals are usually bound to proteins, polypeptides, organic acids or inorganic compounds. When plants are exposed to metals they often produce phytochelatins. Metaflothioneins are found in some plants. Transport of metals from cytoplasm to vacuole may be one control point for metal tolerance. Metals are taken into tonoplasts as complexes by an active transport system. In Cu tolerance, plasma membrane seems to play a more important role than the tonoplast membrane. To produce a plant for remediation and revegetation of metal contaminated soils, the tolerance mechanisms should be better understood. Also some characteristics of metal hyperaccumulator plants growing in metal-rich soils, like the abilities to absorb heavy metals effectively into the roots, and to transfer metals to the above-ground parts, would be desirable for phytoremediation. Unfortunately, the accumulator plants now known have a small biomass and they grow too slowly to be useful for bioextraction. The genes responsible for metal hyperaccumulation, if known, might be successfully transferred into larger and faster growing plants.

In order to increase the knowledge of mechanisims of metal tolerance and also the mechanism of metal hyperaccumulation we are studying the metal inducible proteins and metal tolerance genes of birch, Silene vulgaris and metal hyperaccumulating plants: metal inducible proteins of tolerant birch ecotypes have been found by 2D electrophoresis, putative metal tolerance genes (Silene vulgaris and hyperaccumulators) will be studied using genetic complementation of metal sensitive yeasts, by homology screening of plant CDNA library and differential display. The metal tolerance genes of plants and microbes have been first transformed to tobacco and will be later transferred in to larger and stronger plants in order to increase metal tolerance and influence the uptake and distribution of metals, to produce plants optimised for purification of agricultural fields and remediation of soils around industrial emission sites and by that way improve the quality of the environment. Tolerant plants can be used fo revegetation of metal contaminated areas which also helps immobilising the remaining metals in the soil.