Thursday, August 9, 2007 - 8:20 AM

COS 115-2: Secondary compounds: A search for consistency and clarity

David C. Haak1, Joshua J. Tewksbury1, and Doug Levey2. (1) University of Washington, (2) University of Florida

Plants, microbes, some fungi, and an increasing number of marine invertebrates have unique, but arbitrarily defined, dichotomous metabolic pathways referred to as primary and secondary metabolism.  Myriad chemicals are lumped under the umbrella "secondary metabolites" and yet some have major roles in the growth, development, and/or reproduction of plants.  We argue that the artificial distinction made between primary and secondary metabolites obfuscates their ecological and evolutionary roles.  Based on data from a literature survey, we show that terms such as venom, hormone, pheromone and toxin have clear and consistent evolutionary and ecological connotations, and are rarely obscured by the context of secondary metabolism.  We show that attempts to create similar clarity in the literature surrounding secondary metabolites, such as the adoption of the term "plant-defense compound" have had limited success (ca. 1 article/month for the past five years), whereas the production of articles containing the term "secondary metabolite" is ca. 200/month for the past five years.   Additionally, we demonstrate that a large body of literature (ca. 20,000 articles) has been developed searching for the "cause" or cost of single, or a few secondary metabolites rather than investigating the ecological or evolutionary tradeoffs in producing the diverse array of chemicals with which organisms can engage their continually changing environment.  Fruit chemistry, where the evolutionary and ecological fitness impacts are perhaps clearer, offers an excellent case study on the importance of looking beyond the false dichotomy of primary/secondary metabolism to understand the maintenance of metabolic tradeoffs.  Here we use wild chilies (Capsicum chacoense) and the production of capsaicinoids in the developing fruit as a case study.