Due to greater public knowledge regarding the health benefits the demand for canola (Brassica napus L.) oil has significantly increased throughout the past few years as it is characterized by very low concentration of saturated fatty acids, a relatively low monounsaturated fatty acids and an intermediate contents of polyunsaturated fatty acids, with a good balance between the omega-6 and omega-3 fatty acids ( Cardoza and Stewart .2007 ).
Oilseed rape (Brassica napus and B.rapa) is very important in temperate climates, such as north Europe and Canada, and contributes about 12% to total world oil and fat production (Gunstone et al., 2007).Its production contributed 13.8 billion dollars to the Canadian economy in 2008 (Goodwin, 2008).
Among the storage compounds which are accumulated during seed development, seed oil is the major carbon and energy reserves for germination and seedling growth.
The process of oil synthesis involves a number of organelles and metabolic pathways within the cell. Oil synthesis occurs in the plastids in which acyl chains of varying lengths are produced from acetyl-Co A and malonyl-Co A. This initial step is the beginning of fatty acid biosynthesis and is considered to be the major rate limiting step in the entire fatty acid metabolic pathway (Baud et al., 2008).
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The conventional approach for the production of brassica oil has a major restriction of available genetic resources .The transgenic expression levels, site and insertion number in the genome and stability of the trail all influence the successfulness of this approach(Scarth and Tang 2006).
The biochemical pathway resulting in the accumulation of triacylglycerols in seeds requires genetic regulation. Genes involved in the regulation of embryo and seed development affects the production and accumulation of oil in seeds and coordinated expression of many genes involved in this pathway influence the production and accumulation of lipids in seeds (Sharma et. al., 2008.)
Very low levels of saturated fatty acids, high levels of short and medium chain fatty acids ,high lauric acid high caprylic acid and caprylic acid, high palmitic acid, high stearic acid, very high oleic acid and super high eruic acid have been developed using a transgenic approach (Scarth and Tang .2006).
The transgenes that are being used provoke the expression of genes encoding for enzymes involved in the synthesis of fatty acids as well as incorporating fatty acids into triacylglycerols (Scarth and Tang .2006).
In seed plants, the maturation phase of embryo development and totipotency through somatic embryogenesis are the two basic aspects. Seed development can be divided into embryo morphogenesis and seed maturation. In Arabidopsis thaliana (a close relative of canola) the main storage compounds accumulated in seed consist of oil stored as triacylglycerols (TAGs) and seed storage proteins (Baud et al.,2008).Triacylglycerols (TAGs) are the essential human nutritional and valuable feed stocks for chemical industry.(Cernac and Benning, 2004).
The process of embryogenesis requires the synthesis and accumulation of seed storage macromolecules which include proteins, starch, sugars and lipids, seed storage lipids are synthesized as triacylglycerols and stored in oil bodies (Baud et al., 2008).Oil bodies are specialized organelles consisting of a matrix of TAGs that are enclosed by a layer composed of phospholipids and structural proteins and originate from the endoplasmic reticulum (Baud et al., 2008).
Lec1 gene is responsible for developing a cellular environment that promotes embryonic growth which is essentially coordinated the two stages involved in embryo growth, morphogenesis and maturation (Stone et al., 2001).
Braybrook and Harada (2008) propose that LEC TFs activate the specific target genes underlies, their roles in the somatic embryogenesis and maturation phase and also suggest the effect of LEC transfer factor on the balance of different hormones that might connect their roles in totipotency and maturation phase.
Casson and Lindsey (2006) Suggest that the role of LEC1 in embryonic cell destiny control requires auxin and sucrose to embryonic differentiation and advance cell division.
Lotan et al., (1998) stated that LEC1 encodes a transcription factor subunit associated to the HAP3 subunit of the CCAAT binding factor family. While FUS3 and LEC2 encode B3 domain transcription factor (Stone et al., 2001).The LEAFY COTYLEDON class of genes (LEC1, LEC2 and FUSCA3 a ndFUS3) have been identified as main regulators of late embryogenesis (Stone et al.,2001).
LEC2 is required for the maintance of suspensor morphology, requirement of cotyledon identity, succession through the maturation phase and inhibition of premature germination. LEC 2 is a transcriptional regulator that establishes a cellular environment enough to initiate embryo growth (Stone et al., 2001).
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Lec 1 is a vital regulator of embryo development that activates the transcription of genes essential for both embryo morphogenesis and cellular differentiation (Lotan et. al 1998).
Arabidopsis leafy cotyledon1 (LEC 1) is a crucial regulator required for normal development during the early and late phases of embryogenesis that is enough to induce embryonic development in vegetative cells ( Kwong et al., 2003).
Shen et al., (2010) reported that over expression of maize (Zea mays) LEAFY COTYLEDON 1 (Zm LEC 1) increased seed oil by 48 percent, but reduce seed germination in maize. To separate oil increase from the unwanted agronomic characters, a LEC1 downstream transcription factor, maize WRINKLED 1(Zm WRI 1) was identified whose over expression results in oil increase as much as ZmLEC1 and it did not affect germination, seedling growth or grain yield. These results shows the importance of field testing for developing a commercial high-oil product and draw attention to ZmWRI1 as a hopeful target for increasing oil production in crops ( Shen et al., 2010).
Development of seed is not only controlled by a network of transcription factors but also by many of external factors as light and internal signals as verying levels of sugars and PGRs like ABA.(Finkelstein and Gibson,2002).WRI1 gene strongly effect the embryonic response as well as the sensitivity to the presence of abscisic acid. Without involving the ABI3 genes, the Wri1 gene influences the ability of the developing embryo to sense sugar (Cernac et al., 2006).
The molecular basis for the regulation of oil accumulation and to identify genes necessary for this process, Arabidopsis mutants producing wrinkled, partly filled seeds were isolated and analysed. The results of this analysis suggested that during seed filling wri1 is involved in the developmental factor of carbohydrate metabolism ( Focks and Benning 1998).The gene product of WRI1 is directly involved in the regulation and control of embryo maturation which results triacylglycerols accumulation within the seed .(Cernac et al., 2006).
LEC1 regulates the expression of LEC2 which controls the expression of the WRI1 gene.WRI1 is required to regulate the action of LEC2 in fatty acid metabolism (Baud et al., 2007).
Liu et al., (2009) studied the effect of B.napus WRI1 on oil content. Their results showed that over expression of B napus WRI1 resulted in 10-40 % increased seed oil content and enlarged seed size and mass. Detailed investigation on transgenic embryos indicates an increased cell size other than cell number.
Increasing demand for a sustainable agricultural output together with a need to use plants as source of raw materials for industrial products have led to recent initiatives to direct the quality and quantity of canola seed oil.
Keep in view all information, it is hypothesized that constitutive expression of LEC1, LEC2 and WRINKLED1 genes can influence the embryogenesis and are the hopeful target for the increasing oil production in canola.