The TAGs in edible oil crops contain the same fatty acids as those present in cytoplasmic membrane lipids since both storage and membrane lipids are produced by the same pathway. However, many plant species synthesise storage lipids that contain unusual fatty acids (Voelker and Kinney 2001). The chemical reactivity and physical properties of these diverse, unusual fatty acids are incompatible with the structural integrity and physiological functioning of the lipid bilayer of membranes. In developing seeds, storage and membrane lipid biosynthesis occur simultaneously in the endo-plasmic reticulum and share common enzymes and intermediates. Nevertheless, unusual fatty acids are excluded from the membrane lipids during seed development, which indicates an efficient segregation between triacyl-glycerol and membrane lipids (Millar et al. 2000). This discrimination is remarkable since, as described above, in many cases the modification reaction occurs when the fatty acyl groups are esterified to the membrane lipid PtdCho. A fundamental problem of plant lipid biology therefore relates to how plants are able to control the fatty-acid composition of membrane and storage lipids.
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