Biosynthesis and Chemical Properties of Natural Substances in Plants

The number of known so-called “secondary metabolites” (also referred to as “natural products”) that have been discovered to date is increasing at a constant rate. Yet, it is not only plants (as described in this book) that produce these bioactive compounds; rather, other organisms such as bacteria, fungi, sponges, as well as animals, are also capable of synthesizing a plethora of these metabolites. Whilst some of these metabolites are discussed in Chapters 4 and 5, a large number remain undiscovered. Moreover, secondary metabolites often possess interesting pharmacological properties, and therefore their characterization is very important. It should not be forgotten that plants synthesize these compounds as part of their own survival strategies, typically as defense compounds or as signals for pollinators or symbionts. In addition, recent evidence has pointed to additional roles for secondary metabolites in plant development. Although the term “secondary metabolites” perhaps infers a less important role for these compounds than those involved in primary metabolism, this is not the case. In fact, many essential and nonessential compounds in this group are found in plants, and even so-called “nonessential materials” can play a role in a plant’s responses against abiotic and biotic stress. In this situation, the deletion of a biosynthetic pathway would cause damage to the plant, even if the pathway was not needed under favorable conditions. Interest in the secondary metabolites of plants was further increased when more sensitive analytical instruments became available, as well as genome sequence data for many plant species. Together, these tools allow the details of biosynthetic pathways to be investigated, an example being biotechnological manipulation. Notably, attention also began to be focused on the evolutionary aspects of secondary metabolite synthesis. In general, secondary metabolites occur as complex mixtures (Figure 1.1). Their biosynthesis can be influenced by a variety of factors during development, in addition to stress, which makes the determination of their complete pattern essentially impossible. Whilst secondary metabolites can occur in the tissues as active compounds, they can also be synthesized as inactive compounds that must be transformed into active products; such metabolites that pre-exist are known as “phytoanticipins.” Compounds that are biosynthesized under stress conditions are typically not detectable in unstressed tissues; when they are synthesized after the invasion of plants by various pests these metabolites are termed “phytoalexins.” 1