Metallothioneins, Metal Binding Complexes and Metal Sequestration in Plants

Abstract

A heavy metal (HM) contaminated atmosphere, geosphere and hydrosphere pose a serious threat to plants. Plants growing in these environments acquire a wide range of adaptive strategies, the most prominent mechanisms being the synthesis of phytochelatins (PCs) and metallothioneins (MTs; Rauser 1990b, 1995; Reddy and Prasad 1990; Steffens 1990; Prasad 1997; Rengel 1997; Sanità Di Toppi et al. 2002). The HM deposition pattern has been correlated with forest decline and the concentration of PCs (Gawel et al. 1996). It has also been reported that certain plants function as hyperaccumulators of specific heavy metals owing to their efficient metal complexation processes (Reeves et al. 1995; Krämer et al. 1996; see Chaps. 1, 8, 13 and 14, this Vol.). Thus, metal biomolecular complexes are of considerable interest not only in the context of ecotoxicology, but also from a social point of view (Lobinski and Potin-Gautier 1998; Prasad 1998). The main mechanism of heavy metal tolerance in plant cells is chelatin through the induction of metal-binding peptides and the formation of metal complexes. The introduction or overexpression of metal-binding proteins has been used to increase the metal-binding capacity, tolerance or accumulation ability of bacteria and plants. Modification in the biosynthesis of PCs in plants has recently been utilized to enhance the metal accumulation in bacteria. In addition, various peptides consisting of metal-binding amino acids have been studied for increased heavy metal accumulation by bacteria. MTs and phytochelatins in plants contain a high percentage of cysteine sulfhydryl groups, which bind and sequester heavy metal ions in very stable complexes (Figs. 3.1--3.5)