Silene cucubalus

In contrast to MT induction, the available evidence suggests that PC synthesis is not stimulated by direct transcriptional activation. PCs can be synthesised very rapidly (within 5 min) in response to metals entering the cells, even when protein synthesis is totally inhibited (Scheller et al., 1987 Robinson etal., 1988). Phytochelatin synthase thus appears to be a constitutive enzyme. Loeffler et al. (1989) have shown that, for the enzyme purified from Silene cucubalus, metal ion induction is a post-... 

An important harmful effect of metals at the cellular level is the alteration of the plasma membrane permeability, leading to leakage of ions like potassium and other solutes (Passow and Rothstein, 1960 Wainwright and Woolhouse, 1978 De Filippis, 1979 De Vos et al., 1988, 1991). After supply of copper ions Ohsumi et al. (1988) demonstrated for yeast cells and De Vos et al. (1989) for root cells of Silene cucubalus that the permeability barrier (controlled by means of potassium leakage) of the plasma membrane was almost immediately lost. Oshumi et al. (1988) also reported a quick release of amino acids, especially glutamate and aspartate. After McBrien and Hassall (1965) and Overnell (1975), who studied potassium release from algal cells, the increased permeability of the plasma membrane may be considered to constitute the primary toxic effect of copper. 

De Vos et al. (1989) suggest that the copper-induced damage to the permeability barrier in roots of Silene cucubalus is caused by a direct metal action on both membrane lipids and thiols. They propose that the first damaging effects of copper ions is the oxidation and cross-linking of membrane protein sulphydryls. However, they also adjudge an important role to the copper induced membrane lipid peroxidation, possibly due to direct free radical formation in the membrane this effect could be enhanced by a depletion of thiols such as glutathione which results in a concomitant decrease of the cellular defence system against free radicals. 

Zinc Zinc-sensitive Silene cucubalus Leaves Mathys (1977)... 

A transient increase of catalase capacity was found after application of a toxic lead concentration to Zea mays seedlings (Hoxha et al., 1985). Evidence was presented for a protective role of catalase after intoxication with heavy metals. De Vos (1991) reported a copper-induced increase of catalase capacity in copper-sensitive Silene cucubalus. In Lemna minor fronds and Allium cepa roots, the same enzyme induction was observed with mercury (Subhadra et al., 1991). 

Table 6-2. Enzymes of the intermediary metabolism induced after application of toxic concentrations of metals. Most of the experiments were performed on Phaseolus vulgaris cultivars, except those who are specially marked, which were performed on Glycine max (1) and on a zinc resistant (2) and a zinc sensitive (3) clone of Silene cucubalus (after Van Assche and Clijsters, 1990b, adapted).

Enzyme induction is an indirect effect of metal toxicity. In consequence, it only appears after in-vivo metal application. Increase in capacity in the presence of toxic concentrations of metals implies that the enzyme involved is insensitive to or well masked from direct metal action. In Silene cucubalus POD and to a lesser extent ICDH were shown to be very tolerant to zinc, copper and cadmium applied in-vitro (Mathys, 1975). MDH extracted from roots of both copper tolerant and non-tolerant clones of Agrostis stolonifera was found to be insensitive in-vitro to copper concentrations up to 180 im. However, in the literature, conflicting results are reported about the effects of in-vitro and in-vivo application of metals on enzymes. In Phaseolus vulgaris, GDH was inhibited by cadmium in-vitro, while an induction was found after in-vivo application of a toxic dose of the same metal (Weigel and Jager, 1980b). 

De Vos ChHR, Vonk MJ, Vooijs R and Schat H (1992) Glutathione depletion due to copper-induced phytochelatin synthesis causes oxidative stress in Silene cucubalus. Plant Physiol 98 853-858. 

A similar cadmium-binding complex of peptides is produced by the alga Chlorella fusca as well as other Phyto-phyta (Gekeler et al., 1988). Phytochelatins have been identified in the roots of heavy-metal-sensitive Acer pseudoplatanus and resistant Silene cucubalus plants grown in zinc-rich soil, whereas plants grown in the absence of this metal lacked these peptides. Metal-binding phytochelatins appear to be specifically induced in plants in heavy-metal-enriched ecosystems (Grill et al., 1988). 

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