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intracellular availability of essential metals within certain limits. They may as well serve to reduce the availability of nonessential metals. The capacity of these systems can be varied by de novo synthesis of the chelating compounds. The heavy metal-PC complexes may also be relevant for animal and human nutritional studies since these complexes reflect the state of heavy metal chelation within these plants (Gekeler et al.,1989). The "phytochelatin response" or synthesis of Cd-binding polypeptides, is one of the few examples in plant stress biology in which it can be readily demonstrated that the stress response (PC synthesis) is truly an adaptive stress response (Steffens, 1990). It has been shown that the energy necessary for PC production is considerable: since PCs derive from GSH, Cd-stressed cells have to restore the GSH used to form them, by activating the enzymes catalyzing GSH biosynthesis (Sanità di Toppi and Gabbrielli, 1999). However as discussed by Ow (1996), other factors can be more important than the PC level for efficient Cd detoxification, such as a PC reductase enzyme which seems to be essential to guarantee sufficient reducing power to prevent the oxidation of Cd-induced PCs and ineffective Cd-binding. Also in higher plants high levels of PCs may not be sufficient for complete Cd detoxification. The rapid formation of HMW complex, highly stabilized by S 2- groups, seems to particularly decisive in Cd detoxification (Speiser et al., 1992; Zenk, 1996). Because of their ability to bind heavy metal ions, PCs are considered to play a role in cellular metal homeostasis and metal detoxification (Grill et al., 1988; Rauser, 1990; Steffens, 1990). Moreover, it is suggested that PCs are involved in differential heavy metal tolerance, i.e. naturally or artificially selected intraspesific heritable differences in the ability to tolerate high levels of metal exposure. Increased metal tolerance could involve increased or accelerated production of PCs or the formation of more stable metal-PC complexes due to either an increase in the PC chain length or an increase in the incorporation of labile sulfide into the complex (Rauser, 1995). It is possible that the plants can cope effectively with Cd stress by means of the mechanisms of avoidance, detoxification and repair which are provided by PCs and vacuolar compartmentalizationi.e. amount of proteins, rapidity in HMW formation, number of γ-Glu-Cys units, high incorporation of Phytochelatin and cadmium 53 S 2- , level of reduction of PCs. If Cd levels are low in the soil or culture medium but the exposure time is long, a general cellular homeostatic process may be the plant management to chronic Cd stress (Sanità di Toppi and Gabbrielli, 1999). If Cd levels are high or very high and the exposure time is short, plant can manage this acute Cd stress by detoxification (constitutive response) and repair. Cd tolerance (adaptive response), which is mostly observed in high concentrations of Cd with a long exposure time, in higher plants should be defined as the natural or artificially given capacity regulated by interacting genetic and environmental factors, thus the development of tolerance should be a long-term process (Sanità di Toppi and Gabbrielli, 1999). The mechanisms required to adapt to highly contaminated environments may involve just one of these processes (Meharg, 1994). Molecular genetic approaches have brought important advances in our understanding of PC biosynthesis (Clemens, 2001). The identification of PC-synthase genes from plants and other organisms and also the possibility to utilize the plant genes in mammalian cells to enhance the resistance to Cd toxicity by PC activity, is a significant breakthrough that will lead to a better understanding of the regulation of a critical step in PC biosynthesis. Nonetheless, we must keep in mind the numerous other aspects of PC biosynthesis and function, and the ways in which they, too, are regulated at a cellular and physiological level in response to heavy-metal exposure. These include aspects of sulfur assimilation, GSH and sulphide biosynthesis, PC compartmentalization and the signal pathways through which metal toxicity leads to gene regulation (Cobbett, 2000, 2001). Increasing pollution of the environment caused by heavy metals is becoming a significant problem in the modern world. The use of plants and the concept of phytoremediation of contaminated soils has been increasingly supported by research in recent years (Salt et al., 1998; Cobbett, 2000; Rennenberg and Will, 2000). To investigate the heavy-metal detoxification processes will allow us to explore the mechanisms by which some species are capable of hyper-accumulation of metals such as Cd and how they may be best used for phytoremediation. Understanding the genetic and molecular basis of such mechanisms is an important goal in developing plants for the phytoremediation of contaminated environments.
54 Gülriz Bayçu References Abrahamson SL, Speiser DM and Ow DW. A gel electrophoresis assay for phytochelatins. Anal Biochem. 200: 239-243, 1992. Bayçu G. Cadmium tolerance and cadmium-binding polypeptides in Ailanthus altissima. In: Progress in Botanical Research. Tsekos I and Moustakas M (Eds). Kluwer Academic Publishers, London. 1998. Chardonnens AN, Tenbookum WM, Kuijper LDJ, Verkleij JAC and Ernst WHO. Distribution of cadmium in leaves of cadmium tolerant and sensitive ecotypes of Silene vulgaris. Physiol Plant. 104: 75-80, 1998. Chen J, Zhou J and Goldsbrough PB. Characterisation of phytochelatin synthase from tomato. Physiol Plantarum. 101 : 165-172, 1997. Clemens S. Molecular mechanisms of plant metal tolerance and homeostasis. Planta. 212: 475-486, 2001. Clemens S, Kim EJ, Neumann D and Schroeder JI. Tolerance to toxic metals by a gene family of phytochelatin synthases from plants and yeast. EMBO J. 18: 3325-3333, 1999. Cobbett CS, May MJ, Howden R and Rolls B. 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Page 21 and 22: Figure 3: Several sources of PAs ar
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Page 31 and 32: 74 Seyhan Altun et al. Altun (1996)
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