Oil Composition Variations - Genetics, Genomics and Breeding of Crop Plants - page 261

“linolenic” acid (Mongrand et al. 1998). Membrane lipid composition is
modulated by the environmental temperature, where “palmitic” membranes
are typical of tropical species and “linolenic” membranes are more northern.
This means that deep constraints limit the variation range for lipid
membranes to enable biological functions.
Desaturation of fatty acids occurs sequentially from C18:0, C18:1, C18:2,
to C18:3 or from C16:0 to C16:1 and so on. For seed oils these steps occur in
the developing embryo. Elongases add the acetate motif to fatty acids to
lengthen them to C:20, C:22, and higher ( Table 8-1 ). All these steps that are
detailed in many reviews enhance the diversity of storage lipids (Roscoe et
al. 2001).
Storage lipids are much more variable in composition and in quantity
than structural lipids. Humans have consumed many plant oils that are not
in use now, but some are still utilized for health or body care (e.g., Camellina ,
Oenothera , flax). Fruit oils (olive) and seed oils (oil crops) are not stored in
the same structure. In the seed, oil droplets are surrounded by a membrane
made up of a phospholipid monolayer embedded with oleosin proteins
(Cummins et al. 1993). In fruit oil, this membrane is absent and the oil
droplet floats in the tissue.
8.1.3 The Role of Mutations to Unravel Fatty Acid Biosynthesis
Pathways
A mutation interrupts one biochemical pathway and causes the
accumulation of the compound before the block. Thus, mutations have been
found very useful to enhance knowledge about biosynthetic pathways.
Practically, a mutant releases a specific oil composition that could be of
value in the market. However, biochemical pathways are complex networks
and one single mutation may not have the expected effect on one compound
only. Consequently, to unravel a mutation mechanism and to determine
which enzyme is involved may be more or less difficult. Regulation of a
biochemical pathway is generally due to a feedback effect by the end product
on one of the initial enzymes. Thus, a mutation may lead to deregulation of
the pathway with complex effects on the accumulated products. From a
genetic point of view, this is called as epistasis, but it does not suggest any
mechanism. The biochemical mechanism of several mutations have now
been unravelled in model and cultivated plants. Sobrino et al. (2003) reviewed
several experiments performed in Cordoba to modify sunflower oil
composition that will be detailed further.