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How Plants Adapt Their Physiology to an Excess
of Metals
Martine Bertrand and Jean-Claude Guary
National Institute for Marine Sciences and Techniques, Conservatoire National des Arts et Métiers, Cherbourg,
France
Benoît Schoefs
University of South Bohemia, Budejovice, Czech Republic*
751I. INTRODUCTION
It is well established that trace metals are indispensable for physiological and biochemical processes in
plants. For instance, in plants growing in a Zn-deficient medium the stem fails to elongate and there is a
marked reduction of shoot fresh weight [1,2]. Resupply with Zn stimulates growth [1]. At the biochemi-
cal level, various metals are involved in the structure and the biological activity of many proteins (Table
1). Other molecules such as chlorophylls require Mg for their structure and function.
However, when metal levels increase in the environment, they reach concentrations that plants can
no longer tolerate [3]; their ions rapidly become highly toxic. This is especially true for heavy metals such
as Cd, Ag, Hg, and Pb,† which are not known to play a physiological role in organisms. Plants have de-
veloped defense mechanisms against metal pollution. These include the control of metal influx, active
metal efflux as well as intracellular, extracellular metal ion sequestration, and exclusion [4]. In this chap-
ter, we first describe adverse effects of metals on plant physiology and then focus more specifically on
the plant’s defense mechanisms. Both aquatic and terrestrial plants are considered.
The following abbreviations are used in this chapter: HM, heavy metal; MT, metallothionein; PC,
phytochelatin.
II. ADVERSE EFFECTS OF METAL IONS ON PLANT PHYSIOLOGY
AND BIOCHEMISTRYThe benefit that a plant can usually obtain from metal assimilation can turn to disaster when the metallic
concentration increases or when nonessential metals such as Cd, Hg, or Pb are absorbed. One can tenta-
tively define metal toxicity by stating that a metal becomes toxic when there is no free specific site for it.
Then it binds to any molecule that cannot chemically refuse it, modifying significantly the functional
properties of the host molecule. Consequently, the general metabolism is modified. In the worse case,
metal binding to functional proteins is irreversible, causing death of the cell.
*Current affiliation:Université Joseph Fourier, Grenoble, France.
† When no indication of the oxidation state is given, the symbol refers to the element and not to a particular chemical form
of it.