|
|
|
Altered fatty acid composition in Brassica and soybean seed oil
Oils and fats are triglyceride esters of glycerol involving aliphatic straight chain fatty acids, which may be either (i) saturated, with no double bonds, or (ii) unsaturated, with 1, 2 or more double bonds. The properties of oil or fat depends on the number of carbon atoms and the number of double bonds that are present in the constituent fatty acid(s).
The melting point of oil or fat decreases with the decrease in the number of carbon atoms and Increase in the number of double bonds, so that the oils and fats with fewer carbon atoms and more double bonds will have relatively low melting point, which in its turn determines the oil quality.
|
Therefore, fatty acids are characterized by the number of carbon atoms and the double bonds, which are generally given in parentheses after the name of a fatty acid. For instance, saturated fatty acids like stearic acid (18:0), palmitic acid (16:0) and lauric acid (12:0) respectively have 18, 16 and 12 carbon atoms with no double bonds.
Similarly, the unsaturated fatty acids like oleic acid (18:1), linoleic acid (18:2) and linolenic acid (18:3), each has 18 carbon atoms, but differ in having 1, 2 and 3 double bonds respectively. Erucic acid (22:1) that is common in Brassica seeds and .is often responsible for heart disease, has 22 carbon atoms and a single double bond.
|
While oils and fats, with high proportion of saturated fatty acids, and derived from palm, coconut and cacao, are important for industrial uses, those with high proportion of unsaturated fatty acids (derived from oilseed crops (soybean, sunflower, groundnut, safflower, sesame, cotton, etc.) are used as cooking oil, margarine, salad oil, etc.
In edible oils, relative proportion of saturated fatty acids and unsaturated fatty acids can be altered using transgenic approach. in the past, specialized fatty acid composition in the seed oil, desired for edible and industrial purposes, has been achieved by conventional plant breeding or by mutagenesis programmes.
|
|
Following are some examples: (i) removal of erucic acid from rapeseed oil to create ‘canola’; (ii) reduction in linolenic acid content in flax; (iii) increase in stearate content (six times that in wild type) in safflower (upto 12% stearate) and soybean (upto 30% stearate). Tissue specific antisense RNA expression has recently been used for reducing ‘stearoyl-ACP desaturase’ (ACP = acyl carrier protein) activity in seeds, leading to alteration in the ratio of saturated to unsaturated fatty acids.
Due to tissue specific expression of anti-sense RNA, integrity of membrane lipids in leaf remained unaffected. ‘Stearoyl-ACP desaturase’ catalyzes the first desaturation step in seed oil biosynthesis, converting 'stearoyl-ACP' to 'oleoyl- ACP.' Seed specific anti-sense gene constructs of B. rapa's (syn-B. campestris) stearoyl-ACP desaturase were used for production of transgenic B. rapa and B. napus plants.
|
|
|
This led to reduction in desaturase activity, resulting into a dramatic increase in the level of stearate in the seeds. High stearate B. napus, though unsuitable as a cooking oil, was developed to provide for an alternative source of saturated fatty acids, which are generally obtained from cacao butter.
This high level of saturated fatty acids, due to high melting point of their corresponding fats, are useful for getting semisolid products like margarine and shortenings (fats for pastries, etc.).
|
High laurate transgenic canola (rich in lauric acid, which is a saturated fatty acid with 12:0 constitution) has also been produced and commercialized as a variety named 'Laurical' . Oil processed from this transgenic canola is similar to coconut and palm oils, and is being sold to industry for use in chocolate candy coatings, coffee whitener, icings, frostings, and whipped topplings. This oil is also used in cosmetic industry.
In contrast to the above example, where the proportion of saturated fatty acids (stearate, laurate) was increased through genetic engineering, there are other examples, where the proportion of unsaturated fatty acids was increased, because according to health experts, for cooking oil, monounsaturated fats should be preferred over saturated fats found in beef, pork, cheese and other dairy products.
In soybean, while the oil processed from the conventional soybeans have an oleic acid (a monounsaturated fatty acid) content of 24%, that processed from the new transgenic soybeans has been shown to have oleic acid content, as high as 80%. Oil produced from these varieties is similar to that of peanut and olive oils. High oleic acid was obtained through anti-Sense technology by blocking the activity of olyl desaturase enzyme, which converts oleic acid into linoleic acid.
|
|
