Plant metabolic engineering.

Metabolic engineering is generally defined as the redirection of one or more enzymatic reactions to produce new compounds in an organism, improve the production of existing compounds, or mediate the degradation of compounds. In highlighting progress in plant metabolic engineering over the past 25 years, it is first important to stress that it is in fact quite a young science in plants. Our knowledge of substrate-product relationships in plant pathways was reasonably well advanced by 1975 as a result of the application of radiolabel tracer studies during the previous decades. Attempts to use this knowledge to engineer metabolism in plants, however, first required the development of basic molecular biological technologies such as cloning, promoter analysis, protein targeting, plant transformation, biochemical genetics, and other areas of plant biology (described elsewhere in this volume). Despite this delay significant progress has been made since the mid-1980s in the molecular dissection of many plant pathways and the use of cloned genes to engineer plant metabolism. Although there are numerous success stories, there has been an even greater number of studies that have yielded completely unanticipated results. Such data underscore the fragmented state of our understanding of plant metabolism and highlight the growing gap between our ability to clone, study and manipulate individual genes and proteins and our understanding of how they are integrated into and impact the complex metabolic networks in plants. With an estimated 100,000 unique compounds produced in the plant kingdom, elucidating these metabolic networks is likely to be an exciting endeavor. The few examples cited in this article are meant to highlight common themes that have emerged in the field of plant metabolic engineering, to exemplify the advancements and limitations of current approaches and to provide a forward looking perspective of this exciting area of plant biology over the coming years.