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  Section: Molecular Biology of Plant Pathways » Metabolic Engineering of the Content and Fatty Acid Composition
  of Vegetable Oils
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Alteration of Seed Oil Content


Initial efforts to engineer oil content in crops emphasized the overexpression of enzymes responsible for incorporation of acetyl-CoA into TAG. Modest improvements have been achieved, notably by overexpression of ACCase, which catalyzes the initial step in fatty acid synthesis, and DGAT, which attaches the last fatty acid to the oil molecule (Jako et al., 2001; Madoka et al., 2002; Roesler et al., 1997). Nevertheless, the many trials conducted to date suggest that no single enzyme in the synthetic pathway is limiting for oil accumulation in seeds. Since genes of the synthetic pathway are often coordinately expressed (Cronan and Subrahmanyam, 1998; Lee et al., 2002; Ruuska et al., 2002; Slabas et al., 2002), it is hoped that identification of the transcription factors responsible will ultimately permit global overexpression of the pathway. In addition, it is likely that partitioning and flux of carbon into pathways such as glycolysis and the pentose phosphate cycle that generate precursors and reducing capacity for de novo fatty acid synthesis are major factors in determining oil production (Rangasamy and Ratledge, 2000; Rawsthorne, 2002; Schwender et al., 2003).

Our understanding of carbon partitioning and flux control in seeds is currently in an early stage. Studies with isolated seed plastids have been useful for identifying cytosolic precursors of acetyl-CoA for fatty acid synthesis (Rawsthorne, 2002). The recent use of stable isotope labeling techniques coupled with nuclear magnetic resonance (NMR) and mass spectrometry has also provided useful insights into carbon flux in seeds (Schwender and Ohlrogge, 2002; Schwender et al., 2003). In addition, transcriptional profiling has revealed a comprehensive view of the timing and levels of expression of genes associated with the synthesis of oil, carbohydrates, and proteins during the development of A. thaliana seeds (Beisson et al., 2003; Ruuska et al., 2002). Undoubtedly, proteomic and metabolomic analyses will yield still greater understanding of metabolic networks associated with the regulation of carbon partitioning in seeds. With these data, it should ultimately be possible to uncover the basis for differences in the relative amounts of storage compounds in seeds of different plant species. With such information, it should be possible, for example, to understand why seeds of soybean contain 18% oil and 38% protein, while seeds of peanut, which is also a legume, contain 45% oil and 23% protein.

The roles of transcription factors in the global control of seed metabolism also require extensive investigation. To date, several transcription factors associated with carbon partitioning and seed development have been identified. For example, the transcription factor WRI1 has been linked to regulation of carbohydrate metabolism in seeds (Cernac and Benning, 2004; Focks and Benning, 1998). Whether transcription factors that can be manipulated to raise oil content in oilseeds and fruits will be identified is not yet clear. However, the stunted pickle-shaped roots of the A. thaliana pkl
mutant continue depositing the oil and protein characteristic of embryos during seed development (Ogas et al., 1997). Its gene product, PICKLE, is now known to be a master regulator of embryogenic transcription factors such as LEAFYCOTYLEDON1 and 2 and FUSCA3 (Brocard-Gifford et al., 2003; Ogas et al., 1999; Rider et al., 2003). Thus, the possibility that transcription factors can confer oil production to organs other than classical oilseeds and fruits should also be considered. Possible applications might include increasing the caloric content of vegetative organs for human and livestock nutrition or for the production of novel oils in organs such as roots that are less prone to herbivory.

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