ABSTRACT
Leiden/Amsterdam Center of Drug Research, Leiden, The Netherlands
I. INTRODUCTION
Metabolism is the closely coordinated series of enzyme-mediated reactions in a living organism such as a plant (1). This complex biochemical network consists of metabolic pathways in which compounds that have direct importance for the vital functions of an organism are synthesized and utilized. This collection of processes is defined as primary metabolism and the compounds involved are primary metabolites, e.g., sugars, amino acids, fatty acids, nucleotides, and the polymers derived from them (polysaccharides, proteins, lipids, DNA, RNA, etc.). Furthermore, in all plants more specialized biochemical pathways exist, known as secondary metabolism, in which a wide range of so-called secondary metabolites are produced, e.g., alkaloids, anthocyanins, flavonoids, quinones, lignans, steroids, and terpenoids (2). Secondary metabolites play a role in the interaction of the plant with its environment, such as attraction of pollinating insects via color and scent and protection against predation by herbivores and insects or against infection by microorganisms (2-4). These secondary metabolite pathways are often restricted to an individual species or genus and might be activated only during particular stages of growth and development or during periods of stress caused by, e.g., wounding, attack by microorganisms, or limitation of nutrients. Many intermediates are utilized in both primary and secondary metabolism. Therefore, the dividing line between primary and secondary metabolism is not always distinct. These overlapping roles of intermediates cause a close interaction between primary and secondary metabolism. Therefore, the regulatory mechanisms in plants that provide control at the cellular, genetic, protein, and molecular levels of secondary metabolism are the subject of considerable research.
