Evolution of metal hyperaccumulation and phytoremediation hype

Cadmium accumulation and tolerance are discussed in a New Phytologist article by Krämer (2000), which comments on a paper by Lombi et al . (2000). In this context, a number of additional points should be made, putting the role of humans in the evolution of metal resistance into context and emphasizing what is the 'hype' of phytoremediation.
It is important that sites created by humans should not be overemphasized in considering the evolution of metal resistance. Plants resistant to heavy metals have their primary sites not on these, but on soils where ores are outcropping, the so-called metalliferous or orogenic soils (Ernst, 1974). Over thousands of years, natural exposure to a surplus of various metals, depending on the mineralization process, has driven the evolution of metal resistance in many plant species under the local environmental conditions. Many publications have shown that Thlaspi caerulescens can hyperaccumulate Zn (e.g. Vázquez et al ., 1992), and accumulate other heavy metals such as Cu and Pb depending on soil chemistry (e.g. Baker et al ., 1994). It has been known for more than 30 years that T. caerulescens gives a good response to experimentally supplied high Zn levels (Ernst, 1968). One of the ecological effects of hyperaccumulation of heavy metals is a defence against herbivorous insects (Boyd & Martens, 1994). This effect is enhanced by a preferential accumulation of heavy metals in the epidermal leaf layer (Heath et al ., 1997).