ABSTRACT
Since the first report on the successful transformation of plants (Hernalsteens et al., 1980) molecular tools (e.g. suitable transformation systems, regulatory sequences and genes) have been developed allowing the genetic manipulation of metabolic pathways in transgenic plants potentially leading to the production of tailor made plant products (Stitt and Sonnewald, 1995). The introduction of novel functions or the repression of endogenous routes in transgenic plants enables the creation of metabolic configurations which may be beneficial as long as cellular constituents necessary for growth and maintenance are not seriously affected. Thus understanding the regulation of metabolic partitioning is a prerequisite for metabolic engineering. Metabolic pathways commonly contain several enzymes, possibly regulating the flux of metabolites through the pathway; furthermore, cross-reaction between different pathways must exist, adapting metabolite flux according to developmental and environmental requirements. In the past metabolic regulation has been studied by determining substrate concentrations and characterizing the biochemical properties of a given enzyme in vitro. Although detailed information about the properties of individual enzymes have been obtained, the approach has several disadvantages, e.g. enzymes may be regulated by several effectors and alternative pathways may exist (Stitt, 1995). The use of mutants or transgenic plants altered in respect of the activity of a single enzyme allows the study of the function of the target enzyme in vivo, thereby omitting the problems discussed above. Furthermore ectopic expression of foreign enzymes enables the introduction of new pathways, allowing the manipulation of metabolite concentrations and/or end-products.
