|Content of Metabolic Engineering of the Content and Fatty Acid Composition of Vegetable Oils
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.