Animal defenses against plant toxins

Plant toxins have been known for many centuries. Rotenoids (rotenone), alkaloids (nicotine, coniine, strychnine), terpenoids (ovabin and hymenovin) are among the classes of natural products which provide numerous toxins (3-12). Toxins in plants often have the role of feeding repellents. They appear to be synthesized by plants as a defense against insects and other animals. 

In animals, we can observe the analogous situation in that many insects and other invertebrates (especially those which are sessile and unprotected by armor), but also some vertebrates, store secondary metabolites for their defense which are often similar in structure to plant allelochemicals 1,4,12,16,17,28-30,494-496,503). In many instances, the animals have obtained the toxins from their host plants 4,12,15,17,27-33). Hardly any zoologist or ecologist doubts that the principal function of these secondary metabolites (which are often termed toxins in this context) in animals is that of defense against predators or microorganisms 1,17,28,494-496). 

Leopold and Ardrey (1972) have reported that humans have not been plant feeders nearly as long as the other animals, perhaps because they learned early in their evolution that many plants were poisonous. According to these authors, humans learned to detoxify plant toxins by cooking them only about 40,000 years ago. If so, humans have been carnivores throughout most of their evolution and have had less opportunity to evolve biochemical defenses against the plant toxins. This may explain why humans have lower detoxification capacity compared to other mammals (see Figure 9.1). 

Over 20,000 terpenoids have been identihed (1), and more are being discovered continuously. Plant terpenoids are important in both primary and secondary (speciahzed) metabolism. Their importance in primary metabolism includes physiological, metabolic, and stmctural roles such as plant hormones, chloro-plast pigments, roles in electron transport systems, and roles in the posttranslational modihcation of proteins. In secondary metabolism, the roles of plant terpenoids are incredibly diverse but are associated most often with defense and communication of sessile plants interacting with other organisms. Examples include terpenoid chemicals that form physical and chemical barriers, antibiotics, phytoalexins, repellents and antifeedants against insects and other herbivores, toxins, attractants for pollinators or fruit-dispersing animals, host/nonhost selection cues for herbivores, and mediators of plant-plant and mycorrhiza interactions (2, 3). 

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