Genes and Enzymes for In-Planta Phytoremediation of Air, Water and Soil

Abstract

Plants harbour highly versatile enzymatic machineries to attack and detoxify pollutants. Similarities to the mammalian detoxification led to the coining of the term “green liver” for plant xenobiotic metabolism. Important enzyme classes such as cytochrome P450 monoxygenases, glutathione S-transferases, glycosyltransferases and transporters are involved in both kingdoms. The availability of the first whole plant genome sequence of Arabidopsis thaliana revealed an unforeseen complexity ofthese enzyme classes. Genetic and biochemical diversity, by far exceeding at least single microorganisms, seems to exist in plants. In agreement with previous investigations at the enzymatic level both terrestrial and aquatic plants possess an enormous potential for phytoremediation of soil, water and air iflimitations due to insufficient uptake into plants can be overcome. This is exemplified by the detoxification of herbicides, halogenated phenols and anilines, and formaldehyde by the action of plant enzymes. Two examples are discussed at the biochemical and genetic level. Plants can detoxify airborne formaldehyde by a glutathione-dependent formaldehyde dehydrogenase. Recombinant expression of an Arabidopsis UDP-glucose dependent glucosyltransferase showed activity towards both endogenous and xenobiotic substrates by a single enzyme. Plants frequently do not completely degrade xenobiotics, but rather form conjugated metabolites and “bound” residues. However, these potential contaminants can be easily removed by harvesting. In order to exploit this enormous potential of plants, promising approaches extending their endogenous capacity have been initiated. Transgenic organisms that express heterologous enzymes in order to specifically degrade compounds or to increase the mobility and uptake ofrecalcitrant xenobiotics are being pursued to make phytoremediation procedures useful in practice.