Adverse environmental conditions such as high and low temperature (Tremolieres et al. 1982; Pleines et al. 1987), salinity (Elenkov et al. 1996; Allakhverdiev et al. 1999), and heavy metals (Fodor et al. 1995; Howlett and Avery 1997; Jemal et al. 2000) change the composition of fatty acids in plants. High temperature has been found to result in a significant increase of C ; 81 and C182 desaturation, resulting in a higher C183 (Tremolieres et al. 1982; Pleines et al. 1987). In a study carried out by Ouarti et al. (1997), cadmium stress has been found to increase the proportion of C160 and decrease in the C182 and C183, in 17 day old tomato seedlings. These results suggest that metal treatment has induced an alteration in the fatty acid desaturation processes.
Furthermore, the accumulation of C rather
than C ; 80 indicated an alteration in the ratio of products from the fatty acid synthase. Similarly, Krupa and Baszynski (1989) reported that thyla-koids from 4-week-old tomato seedling grown for 14 days in nutrient solutions containing Cd showed a decrease in the content of all individual glycol- and phospholipids to approximately 75% of control. The greatest decrease was in the phos-phatidylcholine content. The fatty acid composition of the acyl lipids extracted from the thylakoids was characterized by a significant decrease in the trans-8-3 hexadecanoic acid component of the phosphatidylglycerol and by a tendency for the linolenic acid content in all lipids to fall.
Kelly-John et al. (2003) reported that metal contamination showed reduction in fatty acids of actinomycetes fungi, etc. Moreover, environmental pollution such as "industrial pollution" also resulted in decrease in the amount of major and minor fatty acids (Patel-Davendra et al. 2004). Abiotic stress including heavy metals cause molecular damage to plant cells and rupture the cell membrane, leading to oxidative stress in plants, i.e., increase in oxidative enzymes (Zhang et al. 2005). This also resulted in production of H2O2, which can convert fatty acids to toxic peroxides, thereby destroying biological membrane through lipid peroxidation.
The changes in the composition or molecular arrangement of membranes might also play a role toward heavy metal resistance, either by modifying the permeability of membranes to ions or by altering the membrane-bound enzyme activities (Verkleij and Schat 1990). Cooke and Burden (1990) considered changes in lipid under metal stress as one of the mechanisms most likely involved in the regulation of plant plasma membrane ATPases, which produce the proton electrochemical gradient responsible for primary transport processes in higher plants. This enzyme has already been described as a key regulatory enzyme that controls many important functions including cell division and elongation (Serrano 1989). Zel et al. (1993a, b) reported that heavy metal decreased membrane fluidity in the Al-sensitive fungus Amanita muscaria, but an increase in membrane fluidity was observed in Al-resistant fungus Lactarius piperatus. Apparently cell decompartmentalization and modification of membrane functions represent the first target for metal toxicity. The changes in Cd can induce disturbance of membrane lipid turnover. Cadmium has been found to enhance lipoxygenase activity (Somashekaraiah et al. 1992) which is responsible for catalyzing lipid peroxidation by using membrane lipids as substrates, particularly unsaturated fatty acids. Likewise, the products of lipoxygenase reaction mainly peroxy, alkoxy, and hydroxyl radicals are themselves reactive and can result in further membrane lipid deterioration and also affect other macromolecules in cells.
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