A large proportion of the fatty acids in plant membrane lipids are polyunsaturated, which helps to maintain membrane fluidity and cell integrity, particularly at low temperatures (Wolter et al. 1992). The critical importance of ER-synthesised polyunsaturated fatty acids to the plant's ability to resist exposure to low temperatures is shown by the fad2 mutant of Arabidopsis, which lacks the capacity to make polyunsaturated fatty acids in the ER. When grown at 22 °C fad2 mutant plants are almost indistinguishable from wild type. If transferred to 6 °C, however, fad2 plants show signs of stress through accumulation of anthocyanins, leaf necrosis and death (Miquel et al. 1993) whereas wild type are unaffected. The appearance of symptoms in the fad2 mutant is relatively slow compared to plant species that are known to be chill-sensitive plants where rapid membrane disruption is evident. Evidence suggests that the decreased supply of polyunsaturated fatty acids from the ER to the mitochondrion leads to changes in lipid:protein ratios and micro-viscosity of the mitochondrial membranes and modifies mitochondrial function in the mutant (Caiveau et al. 2001). Linolenic acid, which contains three double bonds, is also used as a substrate for the production of oxylipin signalling molecules such as jasmonic acid (Feussner and Wasternack 2002). Triple fad3, fad7, fad8 mutant Arabidopsis plants are unable to synthesise any 18 : 3 and are subsequently unable to synthesise jasmonic acid and are very sensitive to attack by pests (McConn et al. 1997). The pathways of fatty-acid desaturation were identified by a classical biochemical genetic approach using Arabidopsis mutants (Browse and Somerville 1991; Miquel and Browse 1998; Wallis and Browse 2002). As mentioned in the introduction, a significant proportion of polyunsaturated fatty-acid synthesis is catalysed by the fatty-acid desaturases of the endoplasmic reticulum (Browse and Somerville 1991), and in many species of higher plants this proportion is very high (Mongrand et al. 1998).
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