Plants represent the major food source for nonauto-trophic organisms. Therefore, the sessile plants are forced to defend themselves against feeding mammals, insects, and microbial pathogens. Plants evolved a large variety of defense strategies, ranging from simple mechanical barriers to the production of highly complex and toxic compounds. A vast diversity of secondary metabolites from different biosynthetic origins is devoted to plant defense mechanisms.

Plant defense mechanisms are often classified into two main principles: Defense is either available permanently (constitutive defense) or it is upregulated upon recognition of a current threat (induced defense). An in-between strategy is the so-called activated defense in which storage forms of defensive compounds are instantly converted to defense compounds upon wounding. Additionally, some plants attract predators of their attackers using signaling compounds (indirect defense).

All defense strategies have advantages and disadvantages. A constitutively defended organism has to invest energy in the production of defensive compounds all the time whether attacked by an enemy or not. However, in an emergency, the organism can respond instantly. Activated defense requires the production and storage of defensive compounds similar to constitutive defense but defensive compounds are released only when the need for defense is recognized. An economic way to surmount this is to make use of storage compounds such as polyunsaturated fatty acids (PUFAs) incorporated in lipids that can be converted into toxic compounds quickly (see Defense Strategies of Marine and Aquatic Organisms). An organism relying on induced defense saves the costs of production of defensive compounds, because it only activates its defensive system after recognition of a danger. As a consequence, until induction of defense mechanisms such as de novo biosynthesis of toxins, the organism is not well protected. Therefore, a fast and reliable recognition system is needed to respond quickly to attack. This strategy allows specific recognition and adequate, differentiated response to various threats and thus leads to a more efficient defense than a constitutively expressed undifferentiated response.

This article aims to present the diversity of plant defense mechanisms and to demonstrate their fascinating complexity and efficiency. Due to limited space, many aspects unraveled by the contributions of many researchers can unfortunately not even be mentioned.

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