General Mechanisms of Plant Defense and Plant Toxins

Long before the appearance of flowering plants, early plants were infected by pathogenic microorganisms and challenged by herbivorous animals. Consequently, plants and animals evolved defenses and counterdefenses from the very beginning. Therefore, to cope with a huge diversity of unfavorable biotic conditions, plants developed several different defense strategies. In particular, defense strategies against feeding arthropods are highly diverse, including constitutive and inducible, direct and indirect defense mechanism. Among all types of defense, chemical defenses based on the synthesis and accumulation of a consistent number of natural bioactive compounds is a very successful and ubiquitously distributed strategy among the plant kingdom. Many of those compounds are toxic; others act as repellents or are attractive cues for organisms belonging to other trophic levels. Often, toxic compounds have specific targets; other compounds exhibit general toxicity. In such cases plants need to protect themselves. Within the plants’ reservoir of chemical defensive compounds, alkaloids, terpenoids, phenolic compounds, and many polypeptides can be found. Not only herbivorous insects but also mammalian organisms including human beings can be targeted by such plant-derived toxins, which will be demonstrated in selected examples.